PBFred - 04 November 2009 07:52 PM

It's funny that you mention a Trans Am (in a different context though). I'm not a mechanic, even though I sound like one, but I brought my '94 T/A back from the dead last summer. A blown head gasket, which costs about $4k to get fixed. Since I've been out of work for almost 2 years now, I decided to tackle the project myself... the most I could get for it was $100 and a free tow. So I figured I had nothing to lose, and plenty of time to work on it. Almost every mechanic will be impressed if you tell them you replaced the head gaskets on an LT1 engine in a Firebird or Camaro... basically because there is zero room to work on the engine. They even make special 1/2 height sockets just to be able to get the spark plugs in and out because of the lack of room. Book time for replacing the 8 plugs is 6 hours! Yes, that is how hard these engines are to work on.

I really did my research, and learned an amazing amount about engines. When it comes to the LT1 engine, I now know more about them than about 99% of the mechanics out there, especially because I understand the computer tuning side of it too. I've actually argued with mechanics about certain things, like they don't believe I can turn off the EGR valve(smog stuff) via the computer. Too many of them are stuck in old school (pre-computer) mode and even then, most haven't played around with all the computer tuning you can do. You pretty much have to go to a dyno shop to find the mechanics that do know the computers. And when it comes to performance upgrades, most mechanics know the basics, but they haven't really done their homework for the more technical stuff.

So my T/A is now pushing about 350 HP (stock is 270) and I can say this accurately without it being dyno'd based on the fact that my stock injectors are being max'd out. So until I replace them, I wouldn't gain much by adding headers except moving the torque band around some (I do have a high flow cat and mufflers). So injectors and then headers are next of my list to upgrade. But like I said, I am jobless, so it won't happened any time soon. But based on what I have installed, like higher ratio roller rockers, and basically all the "bolt-ons", I predict that once I do, I should be pumping out close to 450 HP flywheel... and this on a street legal car. Granted, I do have to reprogram the computer to pass smog. A 14.7 AFR is ideal for smog, but 12.2 AFR is ideal for street driving power.

One thing I should have mentioned in my post above is that a 10 psi boost (from a turbo or supercharger) will bring VE numbers up to around 200%. So the engines aren't truly performing as a typical boosted engine, but more like a mildly boosted engine... and no where near a race car boosted engine. Keep in mind that the higher the VE, the more fuel that is needed to keep the same AFR. So that is why boosted engines are usually a lot smaller.

Another thing they do is actually have valve over-lap... having both valves for one cylinder open at the same time. Seems stupid, doesn't it? In reality, because of the air flow creates a "flowing" effect, the exhaust rushing out pulls in the air and fuel, and vice-versa. Thus over-lap helps improve VE if done right too. There is a true art to getting these things correct. And as I mentioned, they only work good at certain RPM ranges. But stuff like this is why there is only a handful of teams building engines.

A Trans Am or GT1 (SCCA sanction) is the closest thing to a CUP car. They share many things, a Carb'ed, pushrod engine, live rear axle and weigh in at 2800 lbs. Though the suspension is a departure from a Cup car.
When TA raced on the same weekend, they were large differences in laptime, to the point that TA times were mentioned as lap speed (The TransAm series never did that before) and the Cup times were in lap time. At Sears Point for example the pole time was 4.5 seconds faster than the pole time for Nascar(2003)... same thing happened at Watkins Glen....
In all fairness, the Trans Am car can weigh less, have 5 speed hewlands and was never designed to run ovals....

LFingar - 26 April 2008 04:50 PM

LFingar - 26 April 2008 12:11 PM

thewelderdude - 13 April 2008 09:07 PM

one reason there is a small difference between the speed of a restricted motor and an unrestricted motor is that as the speed increases, the amount of power needed to accelerate it increases exponentially.

Hate to argue with you, but that reasoning makes no sense. Run it through your mind again and I am sure you will see what I mean. Because the power demands rise exponentially the lower power engine should have nowhere near the speed of the higher powered one. The plate engines, with the current drag figures and mandated gearing are topping out at around 7500 rpm, or so. Even with the same gearing the unrestricted engines can make around 1500-2000 rpm more. That's a lot more speed. That's why NASCAR went to restrictor plates in the first place. At the rest of the tracks having to slow for the turns currently keeps the speeds in check. Well, almost. Wasn't it Greg Biffle who recently turned some incredible speeds in testing at Darlington?

Hey, Welderdude, I owe you an apology. I misunderstood what you were trying to say. To create a big speed difference would require a lot of additional horsepower. Of course, that is exactly the situation that exists between a plate and non-plate engine.

The horsepower (Hp) requirement is related to velocity (speed) to the third power. I do not have a reference, but I can explain how that relationship is derived with basic physics. Drag force is related to velocity squared in the turbulent regime. At very low speeds in laminar flow the drag coefficient becomes a strong function of velocity, but racing takes place in the turbulent flow regime where the drag coefficient is relatively constant.

The drag force is related to velocity squared because it is primarily caused by the pressure effect on the body caused by the kinetic energy of the air colliding with it (Bernoulli equation). Horsepower is defined as energy per time. The type of energy we are concern with is work, so it is the rate of doing work. Work is defined as force times distance. Therefore, horsepower is force times distance divided by time. Distance divided by time is velocity, so Hp is force times velocity. Velocity squared (drag force) times velocity yields a relationship of velocity to the third power.

The result is that it takes 8 times the Hp to double the speed. If we take a hypothetical car that requires 100 Hp to go 100 mph and 800 Hp to go 200 mph, then the following chart holds true.

Speed (MPH) -- -- -- Hp Required

100 -- -- -- -- -- 100
126 -- -- -- -- -- 200
144 -- -- -- -- -- 300
159 -- -- -- -- -- 400
171 -- -- -- -- -- 500
182 -- -- -- -- -- 600
191 -- -- -- -- -- 700
200 -- -- -- -- -- 800

Note that this is the required Hp to counter the drag force. Any Hp being put out by the engine beyond the drag requirement is available to provide acceleration. For example, a car with a 200 Hp engine will have the same potential for instantaneous acceleration at 100 mph as a car with an 800 Hp engine will have at 191 mph. This assumes they have the same weight and the same hypothetical body used for the above chart (each one has 100 Hp in excess of the drag requirement). The word instantaneous is important, because the higher speed car will use up its excess power much sooner as it speeds up because the incremental speed causes drag at a higher rate.

Obviously, the relevant issue is Hp at the rear wheels. Mechanical friction can affect the analysis. Other things that create friction such as under inflated tires and disturbance to the air flow from wind can also have an affect.

Drafting changes the effective drag coefficient and projected area of the cars, but it does not change the basic relationship. Bump drafting is so very effective because the following car has much lower drag than the lead car and therefore has more excess Hp available for acceleration. Drag coefficient remains a linear effect so it can be very important. Anything that affects projected area is important (e.g. fender damage).

RPM is not a relevant issue so long as the car has the proper gearing to take advantage of the available Hp. Since the restrictor plate is essentially reducing torque by reducing the density of the air/fuel charge, it stands to reason that the restrictor plate will reduce the RPM at which maximum Hp is achieved. Since Hp is related to torque times RPM, lowering the torque curve has a multiple effect.