TECH AND FAQ
Written by: Peter "Dragon" Cochetas
Below are some of the more commonly asked questions here at DRE.
These answers and questions are the sole property of Dragon Race Engineering Inc. and any attempt to copy, distribute, or publish this information, either in whole or in part, without written consent from Dragon Race Engineering Inc, is prohibited by law. Copywright 2004 Dragon Race Engineering Inc.
The answers come from 12+ years designing, tuning, and building high performance GM vehicles. If you have a question you'd like an answer to email
How much power can my car handle?
What parts can I bolt on without re-programming my computer?
What parts can I bolt on and keep my warranty?
What is computer Tuning and How is it Done?
QUESTION: How much power can my car handle?
ANSWER: I get asked this question daily and the answer is complicated. Let me address the basics of why one part is stronger than the another.
THE FIRST RULE: Not all parts are created equal. Factory parts are required to survive extreme tests but even being subjected to the industry standard "500 hour" test (engine is held at max power for 500 hours continuously), doesn't ensure the strength of these components. Additionally, factory parts compromise strength for cost. Aftermarket parts are generally stronger (and more costly) than factory parts but they too have their limitations. Knowing each parts limitation is a matter of research and experience.
THE SECOND RULE: An engine or vehicle is only as strong as it's weakest part. This is where most projects get hung up financially and operationally. If you build and engine with a 1500HP billet crank, Forged 2032 Pistons, and a Pro-M block, but you put your stock cast I beam rods in the motor, you've just built a $20k motor that won't be reliable above 350HP. Of course even if you built the engine out of the most exotic billet unobtanium parts you still can break the motor and that brings us to the third rule.
THE THIRD RULE: An engine or vehicle will only last as long as it is taken care of, including proper driving. This one kills more motors/cars/drivers/parts than anything else. This is also the most overlooked aspect of car building. However, this usually isn't a bad driver problem, but rather a problem being honest with one's self about how you're going to drive your car. If your planning on 6000 rpm clutch bombs on sticky tires followed up by rev-limited shifts, you'd better give yourself a strong driveline and some headroom on those valves and valve clearances. That being said, a car can be built and go very quick when driven this way. It is a matter of building the vehicle for how it will be driven. The important key to assessing that correctly is to try and step outside one's ego and be honest about how the car will be pushed.
OTHER THINGS TO CONSIDER: A good machine shop is worth it's weight in gold when it comes to engine longevity. A dollar spent now can save you thousands later --- save up and do it right the first time!
MOST IMPORTANTLY: Take care of your equipment. High output engines and cars are like thoroughbred horses.... treat them well and they'll win the derby every time.... mistreat them, neglect them, or overwork them and you'll be embarrassed, poorer, and have nothing more than an overpriced decoration left over.
QUESTION: What Camshaft should I use?
This question above all others is the most difficult to answer. The real problem is that if you put me and 4 other of the best engine builder's in the room with an engine and asked us to come up with cam specs for that engine, you'd get 5 different answers. What will really boggle your mind is that for a given application, 3 of us would be right, and the other 2 would have probably misunderstood the application.
There are several questions you should ask yourself to arrive at the answer when it comes to camshaft selection.
1. What am I going to use the car for? The answer to this question will most likely give you your target RPM band. For a street car that sees some drag time, maybe you're willing to sacrifice some bottom end torque for top end power. If this is a road track car what rpm's are you planning on using in and out of the corner and how often do you want to shift. If you want to shift less you'd better concentrate on average torque output rather than rediculous peaks. Here are some general rules of thumb.
Street car occasional racing 2000-6000 RPM
Race car road course 3000-7000 if you have a budget, 4500-9000 if you have unlimited funds.
Drag car 3500-7800 if you have a budget, 5000-11000 if you have unlimited funds.
Stoplight Warrior 1800-5500 RPM, add a turbo or centrifugal SC to increase your RPM potential.
Obviously, with RPM comes increased wear, cost of parts, decreased longevity, and did I mention cost of parts? 500 hp at 5500 rpm is cheaper in a big block than it is in a naturally aspirated 6 cylinder turning 8500. This is known as the cost per RPM modifier.
The Dragon's cost formula:
Cost = (Amount of Horsepower Required x RPM modifier)/cubic inch potential x Size modifier (a number that defines how much you've decided to push the CID of the standard engine of that size) x 1.5 (because you'll spend way more than you thought you would) = get a better job that requires too much of your time or sacrifice something (usually the latter)
2. What limitations are you going to apply to the performance of the combination. Will it be boosted? If so how much? Will you run nitrous, even after this combo has gotten "boring"? How much nitrous is your ego going to be capable of? Is the rest of your car going to survive under this much power. One of the most important limitations you must apply is whether you're going to re-lash valves, meaning solid or hydraulic. Unless you've lived under a rock for the last 10 years, you realize that a roller cam is the ONLY way to get the most out of your motor. Piston relief, head milling, gasket thickness are also limiters as they directly affect how much duration and lift you can safely run. In the world of LS1's or Gen III SBC's this has become the most limiting factor in what camshaft gets put in a motor when all out max power is the goal. Probably the biggest limit to ridiculous cams is brake vacuum and idle speed, if you're going to drive in rush hour traffic it would be wise to have power for your brakes. When you can put this on paper and stick to it, you'll have a rough Idea what you should be looking for out of maximum camshaft numbers, LSA, intake centerline, and maximum durations.
3. Here's where the science comes in. How do you manipulate what is mathematically possible down into numbers that will work well for your combination. Here is a VERY rough list of rules to help you out:
A. Start with idle speed and use the max intake duration possible to still maintain good brake vacuum at that idle. Your asking - Ok, how do I do that? Sorry, but most of this is experience related here. However, if you have a hydraulic roller you can expect brake vacuum to be stable enough up to about 234 degrees duration at .050, for an idle of about 900 rpm. For every 100 rpm lower in idle subtract 2 degrees duration or vice versa. Ramp rates really affect this so a really steep ramp rate will allow you more duration with better vacuum, whereas a shallow ramp rate will allow you less duration. This is the reason you WANT a roller camshaft. This is also the reason for steeper and steeper grinds these days - the XE-RTM Lobe on Comp CamsTM Camshafts actually uses a lobe design that attempts to create a square wave valve opening. NOTE: Steeper lobes means more valvetrain noise, it is not abnormal for an extreme hydraulic roller to almost sound like a solid cam at idle these days.
B. Get out your calculator and start doing some crazy, crazy math to determine valve to piston interference or go to our piston clearance calculator and just type in some numbers. No worries if you screw this one up it just means the possibility of a junk head, junk cam, or junk pistons or best case scenario 10 hours putting deeper reliefs in your pistons. As I mentioned before this has really become a key part of the process on the later model Gen III motors because of their perfectly flat piston design, shallow valve angle (a very good thing), and ridiculous head flow potential. When you know your interference at a given lift you can begin to manipulate your maximum lift potential. Don't forget to take into count the max potential of the springs and style of cam your running (These two are interconnected because spring pressure is related to lift capacity and RPM capacity, and spring pressure also directly affects the spring in the hydraulic roller lifter - Valve float often occurs because of the loss of integrity of the lifter). So now you've got a max duration and lift combination, and in the case your using flat top pistons with no valve reliefs, you've also begun to notice that you must already begin to trade lift for duration or vise versa.
C. Know your heads because they are going to be your best guideline here. Assuming you are not also trying to tackle a professional port job on your own - this I assure you is better left to the Pros. We've got 12 years in R & D and flowbench time to know what works and what does not. No book, no matter how well written will allow you to accomplish the same results the first time out. Don't get me wrong, with some solid research and practice on a junk head, you can have a decent pair of heads in probably 60 hours worth of time that could equate in performance to a pro's stage 1 or 2 head, but how much is your time worth?
If you've begun to manipulate lift for duration, head flow numbers should be your guide here as to which way to go. If you don't have that problem yet because you've already got enough piston relief, then you need only adjust this lift number down if your heads are hitting what is called reversion. Reversion is the point (lift) at which the head begins to develop more turbulence rather than more airflow, this can appear as a plateau or dip in the flow numbers. While this is only negligibly controversial, you should not put your motor into a territory where it is trying to utilize this turbulent air. So as a rule of thumb take the reversion point and add .010 to it for your intake lift. Again, this changes with ramp rate, but the idea is that your cam spends almost no time at it's peak lift, meaning your port will never revert because the air does not have a chance to equalize in that short period of time in the short turn radius portion of the port (the leading cause of reversion). What you're really doing is adding .020 to the real reversion point (actual reversion takes place earlier due to the oscillation of the intake valve)
D. Now you need to address your exhaust valve duration. This is a question of relationship as well as airflow potential of your exhaust system. Know your exhaust to intake airflow ratio on your heads. If we assume 75% is the target, and we know that your heads flow 80%, we've got an exhaust potential that exceeds our need. In that case we need to decrease exhaust duration to artificially create a better scavenging wave in the exhaust (this is widely regarded as the most important factor in exhaust design). Of course headers have a HUGE effect on this because most scavenging and pressure waves propagate from the collector area which is directly affected by tube diameter and length (do you feel the math coming on again?). For arguments sake lets say the following guidelines define the optimum header diameter for a full length header on a V8. You can back calculate these numbers to determine the appropriate diameter for 4cyl, 6cyl, 12cyl, etc.
250-325 CID 1 5/8"
330-415 CID 1 3/4"
415-500 CID 1 7/8"
500-550 CID 2"
550-600 CID 2 1/8"
ADD 1/8" for every 50 CID over 600
For every 500 RPM over 6500 go up one primary size
These are rough guidelines that are obviously affected by many factors including materials, power adder, use, etc.
Now that you know your potential you can make a reasonable stab at exhaust duration. In general a single pattern camshaft makes more average power on a properly set up N/A combination. However, given that you have probably run into some constraint already, exhaust duration should usually be manipulated accordingly. It is not recommended to run any less than 4 degrees less than intake duration (reverse split camshaft), you should reduce primary diameter or put metal back in your heads (I know it's not possible, but you now know why your head work was so important). You should also not increase the duration of the exhaust more than 4 over the intake on an N/A motor. You're power adder is the next thing to address if you've got turbo, supercharger, or nitrous your motor has a LOT easier time getting air in than it has getting it to come out. Increase your exhaust duration 2 degrees for every 100 hp extra air your motor will see on the intake. This should be limited to a maximum of 10 degrees over the intake duration. If you have a turbo car, don't go too far with this or you'll start shoving too much exhaust back into the combustion chamber (this is a great way to increase detonation potential in your motor). If you've got a turbo motor this number will also be able to manipulate EGT which is important if you want to keep from liquefying your exhaust valves.
E. Ok it's back to the calculator now, as well as referring to your head flow numbers. Exhaust ports rarely exhibit reversion so you're probably not worried about that but based on your exhaust flow requirements, you may need to manipulate the lift higher or lower than your intake lift. Generally follow your duration increase/decrease as a guide. For every 2 degrees you've deviated from the intake duration manipulate your exhaust lift by .005 up or down accordingly. This will give you a rough estimate of what to run on exhaust lift and duration. Remember to account for spring capability.
F. Now it's time to manipulate Lobe Separation Angle (LSA). LSA is simply put the difference in angular degrees between the two lobes peaks. By widening this you can stabilize airflow down low and possibly increase brake vacuum minutely, but the name of the game here is knowing your car. Low LSA's 108,110 can produce better top end overall power but will idle high and produce worse vacuum. They will also produce more emissions and run less efficiently down low. It would seem that widening the LSA is the way to go but that is not necessarily the case. A wide LSA can increase the "peaky-ness" of the camshaft and can trade a lot of power under the curve for what you get back in idle potential. If you've got too much intake duration and too much LSA you can rob the cylinder of its ability to scavenge from the exhaust which is essential at higher RPM's but also tends to drive down power in the midrange due to increase reversion from the piston's movement in the cylinder. Whenever possible try to run a 110-112 LSA on a manual trans car for the street, if you don't get enough brake vacuum, decrease your durations to bring it back. On an automatic try between 112-114. A GEN III can tolerate up to 3 degrees more LSA because of head design and unusually above average exhaust potential. If you have a forced induction car LSA can allow you to increase your boost number. Careful, not all boost is good boost. Trying to back too much of this air up in the cylinder can really hamper the ability to get the spent exhaust out which can cause your motor to fast approach nuclear meltdown temperatures. In general add 2-3 degrees more to LSA for forced induction. For nitrous, the air is not entering under pressure, and while you don't want half your nitrous traveling down the exhaust manifold while both valves are in an overlap situation, you should run the same LSA as you would N/A. The final word on LSA is that it can have an effect on piston clearance depending on durations and type of piston.....back to the calculator.
G. Last thing to decide on in Intake Centerline (ICL). It is commonly accepted that if you've chosen the right cam it will make the most power advanced 4 degrees. This is a misnomer because many cams are considered "straight up" at 4 degrees advance. In fact rarely is the cam ground advanced or retarded from that 4 degree point. When most people refer to advancing or retarding the cam, they are speaking in relation to this point. An advanced cam will behave as a smaller cam would have on the same motor and a retarded cam will behave as a larger cam would have on the same motor. This is a great tuning tool for the end user, because it can be manipulated with the timing set. There are some things to be conscious of though. Too much advance can have a profound effect on your exhaust potential as well as your piston to valve clearance on the intake valve. Too much retard and brake vacuum and idle quality are gone. Generally, have your cam ground standard (4 degrees advance) and then leave the ability to manipulate it later with your timing set. ICL varies with LSA. 4 degrees advanced on a 114 LSA would result in a 110 ICL. I've dealt with a lot of cam grinders, and their standards can be different, so it is best to ask your cam grinder exactly what the ICL will be ground to so you can get it the way you want. One of the tricks to drive up the power band on an LS1 is to grind the cam retarded, which could just be accomplished with the timing set but allows really big durations without interfering with the pistons which allows those monster heads to breath.
F. Well you now have a ROUGH estimation of camshaft you should be running. What you've done in reality is define the largest camshaft possible given your scenario. The trick now is to manipulate the numbers down to match what can be realistically expected from the intake components, driving style, and designed use. If it's a street car drop the durations about 4 degrees to give yourself some margin. For a street/drag car or dyno queen you may be closer than you think.
A final note: although this will get you in the ballpark, I have glazed over some pretty important material as well as 60 years of valve timing/pressure wave/port angle/air velocity/mass air potential/compression ratio/VE/thermodynamics/fluid dynamics/curtain area/etc/etc research in order to give you a rough overview. While a professional will be able to really tell you what you should run for a given combination this info will give you some basis to keep up with the conversation.
ABOVE ALL ELSE, REMEMBER IT TAKES A GOOD CAM GUY YEARS AND EVEN DECADES TO GET THIS ART DOWN. IF YOU'RE GOING TO PICK THEIR BRAIN YOU'D BETTER BUY THE CAM FROM THEM. GOOD CAM INFORMATION CAN SAVE YOU HUNDRED'S OF DOLLARS AND DOESN'T GROW ON TREES, I SEE WAY TOO MANY TUNERS AND PROS GETTING BURNED EVERY DAY DONATING INFORMATION. YOU WANT THAT TUNER TO STAY IN BUSINESS DON'T YOU?, AND YES YOUR CAM DOES MAKE A DIFFERENCE!
QUESTION: What is the difference between Rear Wheel Horsepower (RWHP) and SAE Horsepower (Standard factory rating)?
SAE net HP refers to the factory standard established by the Society of Automotive Engineers, that defines the horsepower generated by a motor using all the standard accessories being driven during testing. This method is different from the gross horsepower ratings used in the 60's and early 70's. The gross horsepower number will always be larger than the net horsepower for a given motor. As much as a 15% difference in power output has been recorded but this is dependent upon the accessories being driven by the motor.
Rear Wheel Horsepower (RWHP) or Rear Wheel Torque (RWTQ) is the torque or power recorded at the wheels. By measuring the power at the wheels or, more specifically, the contact point between the tire and the dyno, you are measuring the power output of the engine with the losses occurring in the transmission, torque converter, universal joints, differential, etc. Generally, the engine horsepower can be back calculated from the wheel numbers. Commonly accepted losses through the drivetrain are 16% for a manual trans and 22% for an automatic trans vehicle. This varies widely though and is only a rough estimate of loss. Many cars have less or more due to widely varying driveline design. A 4WD truck eats at least 25% through an automatic transmission. Conversely the 5th and 6th generation Corvettes lose a lot less due to their rigid drivelines that lose very little in the transfer of power to the differential.
SIDE NOTE: No dyno is consistent to another dyno or another dyno brand. A dyno is relative measurement mechanism. While entirely consistent and repeatable to itself, a dyno (engine or chassis) is not currently calibrated to a national standard. With no national testing standard or limits, a dyno's output is calibrated by the respective manufacturer. Most manufacturers have consistent standards to which their dynos are calibrated, but most manufacturers do not share calibration information. The standard of the performance industry is the DynojetTM because it reads the highest of all the manufactured dynos. It is also possible to manipulate the base calibration of the dyno, which allows the user or operator to manipulate the outcome of the dyno's results. Generally, you should consider a dyno number a measure of the relative power output, relative to what other vehicles produce on that dyno and relative to what your vehicle produced on that dyno prior to modification. It is always recommended that you comparison test on the same dyno every time, otherwise you're really just guessing. If you want a dyno record find a DJ dyno, but just don't claim later that all 500hp cars run 14's at the track because the track is calibrated perfectly every day!
QUESTION: What parts can I bolt on without having to reprogram my computer?
Unfortunately, this varies. For the most part a mass air flow vehicle can tolerate most bolt-ons with moderate success. Some vehicles, due in part to manufacturing tolerances or wear, do not tolerate even the littlest modification. Long tube headers are usually the exception to this tolerance, as considerable fuel and timing changes are necessary for optimization with a header. That being said, it is not uncommon for a vehicle to tolerate, although not optimally, headers without any changes. See how inconsistent this is?
The reasons for the varying amounts of success bring us back to how the vehicle is determining fuel air mixture. Again, a mass air flow (MAF) vehicle uses the MAF to measure incoming air flow and adds fuel in proportions pre-determined by the factory. If you increase the amount of air entering or leaving the motor, the computer adds fuel accordingly. Unfortunately, different cylinder pressures (power levels) can require different fueling and especially different timing. Generally if you keep power within 10% of the factory power levels, your vehicle should tolerate the bolt-ons without any major problems. If you have a speed density computer system, you've got a vehicle that will not tolerate many modifications at all. These motors are calibrated using specific external air measuring devices that are then crossed to a given manifold depression and static pressure in a table in the computer. This means any variance outside this table or values that do not correlate correctly can and will move the air to fuel (A/F) ratios radically around. This results in decreased performance and often detonation if the mixture is lean. If you have a speed density vehicle you can still put an air filter, intake setup, and some ported intake components in but any changes in the motors VE (volumetric efficiency) should be addressed with computer tuning.
The bottom line is that you should have the tune at least checked by a professional on a dyno for any modification, but you'll have to temper those results with the fact that the tuner is probably hoping to sell you a tune. On the upside their isn't a good tuner that I've ever known that can't give you more power for your buck than any other modification (nitrous excepted).
QUESTION: What parts can I bolt on and keep my warranty?
This is a sticky question because it often depends on your dealer's interpretation of the warranty. In general most bolt- ons that are external to the engine do not void your warranty. However, any part you replace voids your warranty on that part, but this is usually offset by the fact that the part manufacturer has a warranty and the fact that aftermarket parts are usually a lot stronger than the original.
Going inside the motor: Believe it or not, there are things that will not void your warranty here. You should check with your dealer first, but a lot of dealer/factory offered replacement parts will not void your warranty (hot cams, lt4 heads, ls6 heads, etc.). Factory designed race parts (c5r heads, racing cams, etc) are probably not covered.
If you read your warranty carefully, any failure of your vehicle during the warranty period is covered except in the circumstance that it can be proven that a given modification caused the failure. Having spent some time doing failure analysis, I can tell you that this is not only cost prohibitive but also not completely certain. The translation: even radical mods like superchargers and turbo's don't technically void your warranty unless a cause and effect relationship can be established which is not realistically possible for most failures. This begs the question: Do I want to go to court over this?
The other consideration is that most dealers want to make money in their service department, and while they don't make as much profit by doing warranty work, it still nets them profit and work. Given the choice between having work and not having work, they'll usually choose work. If they think they'll get you to pony up the extra bucks to make the job really profitable they're probably going to try and get you to pay for it rather than warranty it where they're stuck with a set quota from the manufacturer. You're best bet is to establish a relationship with your dealership over time and then they'll be more than willing to help you out when it comes to modifying/warranting your car.
WARNING: MOST DEALERSHIPS DO NOT EMPLOY MECHANICS WHO ARE FAMILIAR WITH MODIFYING VEHICLES, ESPECIALLY WHEN IT COMES TO POWER. THIS MAKES DEALERSHIPS BAD PLACES TO GO FOR PERFORMANCE. I HAVE YET TO INTERVIEW A DEALERSHIP MECHANIC (TECHNICIAN TO BE POLITICALLY CORRECT) THAT I WOULD TRUST TO WORK IN MY SHOP. GOOD TECH'S DO EXIST AT DEALERSHIPS BUT THEY ARE RARE!
QUESTION: What is computer tuning and how is it done?
The million dollar question. I will be releasing a book soon on this subject which will be available in our store.
The basics: Computer tuning is nothing more than manipulating the 1's and 0's in the computer that control the fueling, timing, and airflow of your motor. All of these values are separated into different tables or Maps in the computer. Each table or map can represent up to 4 variables and the associated fuel, air, or spark. Knowing what tables to modify, where to modify them, and how to modify them is a matter of experience, knowledge, and root engine theory. Older computers use removable chips, commonly referred to as eprom's (electronically programmable read only memory). Older chips may be upgraded to eeproms (electronically erasable programmable read only memory) to speed up the process of re-flashing the chip. Newer computers have a permanently soldered eeprom embedded into the computer. While this chip can also be removed, with some difficulty, it is possible to reprogram it without removing it, hence the electronic handheld programmers we see today.
To tune a modern computer you need a hardware package capable of extracting the information from your cars computer. For chip cars this means a pocket programmer or similar eprom reading device and for later model vehicles this means an electronic interface (ls1 edit, hp tuners, superchips, hypertech, etc.). Usually this hardware is also the hardware you'll use for the actual reprogramming of the vehicles parameters.
There are two classifications for the hardware involved in tuning. One, does not allow end user input (hypertech, superchips, diablo, tech2, etc.) other than simple parameters like tire size and rev limit. The other, allows nearly infinite control over the factory's programming (ls1 edit, hp tuners, tunercat, etc.), and are often considered "professional" tuning devices (although a lot of latitude should be allowed in the word "professional"). Obviously these are targeted towards different markets. If you don't want to tune your car yourself and want to rely on somebody else's preprogrammed knowledge, you need not invest in professional programming equipment. There are variations of the non professional versions that allow more or less end user manipulation. An example of this is the Diablo programmer, in which the user has limited control over fuel and timing. Although more powerful than a preprogrammed chip in the hands of the right user, even the Diablo's capability pales in comparison to the professional programming equipment.
Once you have the hardware, you'll need a software package that allows you to edit the data you've extracted from the computer or chip. The software you obtain can be anything from shareware or freeware found commonly on university web sites dedicated to engine control experiments, up to multi-thousand dollar packages that outline every parameter in the smallest detail. With the exception of the older programmable chips, the hardware usually comes with the software necessary to tune. Depending on your knowledge and experience, you may choose to use the existing software or upgrade to something more powerful. When tuning a Ferrari for example, neither the hardware or software exist in prepackaged, easy to use combinations. A relatively simple electronic reader must be fabricated or an expensive universal reader must be used and $5000 worth of software must be utilized to interpolate the vehicles existing data correlated to the factory parameters....a very expensive proposition just to tune for a new camshaft or supercharger. Nevertheless, if it runs by computer it is reprogrammable/tunable...but cost can get in the way.
Now you're saying to yourself: Ok, but how do I actually manipulate the numbers in the tables?
This is a matter of some debate amongst the professional tuners but here is a rough outline of what I propose all novices do if their going to tackle this on their own. This is not the method I use, because the method I use relies heavily on past experience and knowledge with hundreds of cars and hundreds of modifications. Expect to spend 5 times the amount of time a professional would for 85-90% of the results. What I have outlined here may seem vague at times, and uses a lot of commonly referred to terms. An expanded in depth version, including equations, pictures, rules, engine theory, maps, tips, tricks and explanations are in my book. That being said, lets take a stab at tuning your vehicle.
1. Get a dyno or similar performance measuring device. A dyno is preferred but if you must blast up and down your street with a G-Tech, I don't want to know about it.
2. If you've got access to a Wide Band O2, use it but don't rely on it religiously because every combination is different and you'd be surprised how far off that $700 sensor can be from what the "experts" will tell you is right.
3. A mandatory is a scanner or similar device that can tell you knock retard, timing, fueling (BPW), oxygen sensor values, and air flow rate (MAP or MAF or Both). Without a scanner you'd better have been tuning cars for 20 years electronically and get lucky.
4. Run up a baseline, provided knock retard does not exceed 5 degrees. If knock is excessive, check your vehicle for sensor problems and mechanical problems then retard timing half of the knock and add 15% more fuel in the rpm band with the knock. If you're saying: Wow that's radical. Well, yes it is, but I'm assuming a novice is trying this and has no knowledge basis for making lesser corrections. Keep repeating this until you get a full baseline. If you can't get a reasonable baseline, you've either got a mechanical problem or way to many modifications that you've done at one time. Move on to step 5 to begin tuning and get a run as soon as you can do it safely.
5. Address your major mods logically and individually. Here is a list of the basics and what to do with them:
CAMSHAFT: A camshaft will necessarily change the VE of an engine at different RPM's.
Speed Density: If you have a speed density computer you should rescale (relative to the original points) a map that loosely would follow the torque curve you'd expect from that cam. Don't go over 100 % and scale backwards relative to your torque peak. This is a very difficult thing to do for a novice and requires some real research into you cam and head choices. It is always better to err on the side of rich so add a couple of percent accross the board. The computer determines fuel by calculating the ratio of fuel to air at a given RPM and MAP value, if you tell the computer the motor is more efficient at that RPM your telling it that it is getting more air and hence, needs more fuel at that RPM. Try not to deviate to far from the factory here. If you've really gone overboard with cam selection 7000+RPM on a V8, just flatten the top of the VE tables to 100% and work backwards from there. If you've got a late model GM, you'll notice the VE is a number not a percentage, well your in the wrong table (you've got a MAF car), and these numbers should be treated in respect to the relative efficiency of the original combination. IMPORATANT: These 3d tables are the inverse of a torque curve, do not directly plant a curve on the table because the table slopes from top to bottom and left to right in most applications, the torque curve goes from bottom to top and left to right in all cases. Just pay attention to the calibration point on the scales and you'll be all right.
MAF: Go to your power enrichment table and add fuel in small percentages that loosely mimic your expected torque curve. The MAF guys have it easy here because this is usually a simple linear table. If you've really cammed your motor a lot over the factory go to your VE tables and multiply the values below 1200 rpm by 90% to account for startup fueling. By telling the computer that your car is less efficient at lower rpm's it makes it easier on startup because it adjusts fueling not to flood or overlean the startup mixture. The VE tables on a MAF car are primarily used for MAF failure and open loop operations. Don't make big changes 10% at most because the MAF measures (fairly accurately) the incoming air and adds fuel according to that reading. What your telling a MAF car is here is the amount of fuel you should be adding proportional to the airflow your seeing. The factory is usually deliberately rich in this calibration (~12.5:1 A/F on Naturally aspirated cars)
Idle: Cams love to mess up idle so for now bump your idle to where you think it should be and then add 100 just so you don't get hung up on this at the beginning of the tune. When it comes to the end of the tune you'll have to manipulate your low RPM timing as well as idle speed and low rpm VE tables to make this just right.
HEADS: Heads almost always increase VE across the band but can decrease it (low air velocities on race ports) and usually display progressive increases.
Speed Density: You want to do the same thing as with the cam, but you should be adding VE progressively as RPM builds. If you know the ceiling on your heads (Max flow rate), you can calculate potential VE from cubic inches and RPM. Using the potential VE from your heads, with some regard to your previous cam selection, you can effectively build a base VE profile for your heads and Cam combination.
MAF: Go back to your power enrichment tables and add accordingly relative to your air flow potential in the heads. Calculate your potential VE (described in Speed Density) and use it to correlate increases in fuel at those rpm points. Again, little changes because the MAF understands the extra air flow.
If you've killed your intake velocity with a big race port, go back and take out 5-15% progressively from 4000 rpm down in descending fashion. This will allot for the decreased efficiency of your combination at these RPM's. Be very careful here because, while some heads do trade low RPM power for high RPM potential, most often these monster ports still outperform the stock combination by a ton even at low RPM. In the absence of surety here, avoid this modification until it is clear later that it needs to be addressed (grossly rich at the lower RPM's).
Blower/Turbo: By now you've become accustom to the differences in the Speed Density vs MAF applications, so I'll spend less time explaining the differences. For a blower or turbo you need to multiply your fuel curve by the amount you expect to be boosting the motor. To really extrapolate a good estimate as far as increased fuel needs you should do some math here. The most basic mathematical principle is simple:
Given ~14.7 psi is atmospheric pressure at sea level (you may need to rescale if you're at altitude) and you'd like to add 5 psi to your motor, you'll add the two together resulting in 19.7 psi. This is the total pressure entering the engine. Divide this by the original pressure of 14.7 and you have your incoming pressure ratio which directly correlates to the amount of extra fuel required for that mod. In this case 19.7/14.7 yields 1.34, which is 134%. You should add 34% at the RPM point at which you expect the motor to make 5 psi. Repeat this math for ever 200-500 rpm interval and you'll have a basic idea of the extra fuel needs. Remember, a centrifugal style blower or turbo increases airflow and boost relative to RPM so plan accordingly. Heat is also a big factor here but requires much more in depth math and engine theory than is covered in this FAQ primer. If you have a MAF car make tiny adjustments (I'm going to beat this in). A good rule of thumb is to add half the percentage (34% in this case) to the existing percentage in a MAF car's table. Example: Current curve says 1.135 is the fueling value for 5000 rpm at WOT (this loosely approximates a number of 113.5% of the standard fuel ratio). If we add 34% in boost we'll add .34/2 to the original number which results in 1.305 in the table. Tables vary as much as the values in them so you'll have to make a judgment on what the factory table is telling you about the fuel curve.
6. Adjust your timing. Timing will effect the output air fuel ratio in the exhaust so changes here usually necessitate fueling changes as well. More timing will read leaner (provided you don't have a lean misfire or detonation) and less timing will read richer. Timing should increase smoothly with RPM. If your timing curve has large spikes or shelves in it you do not have a good timing curve. Timing decreases with cylinder pressure, meaning big boost or nitrous requires less timing to extract max power. Contrary to popular belief, retarding timing does not always reduce power and it actually often increases power output on modified motors. Always start conservative with timing and work up from there. The better the combustion chamber design and the higher the CR, the lower the timing number needs to be for max power. SBC's can use up to 40 degrees total timing but efficient LS1's only require 21 degrees total timing on gasoline.
The number one misconception is that black tail pipe emissions always means rich. In actuality, especially with computer controlled cars, black usually means detonation (timing retard makes rich) or lean misfire (fuel ratio is too lean and cylinders are actually failing to light effectively - common on boosted apps.). By decreasing the fuel in either of these instances you'll just make the problem worse. Knowing what black cloud is truly rich versus what black cloud is a misfire is a matter of experience.
7. Repeat steps 5 and 6 as often is necessary to arrive at a stable maximum power output. Remember, timing is relative to load beware of fringe timing on a dyno because street load is bigger than dyno load unless you are doing load testing on the dyno.
8. It's not a recipe for the perfect tune, but if you want to tackle this project yourself, these guidelines will keep you safe. Rule #1 richer with less timing is safer!
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