Ok, so I was trying to learn how and why a lack of backpressure can hurt low-end torque, and I heard that it's because the head doesn't get hot enough because the exhaust gasses exscape to quickly, and therefore it doesn't flow as well until higher RPMs, when the increase in combustion is enough to heat the heads up.
My question is, if this is the case, Would it be helpful to rev the engine up to heat the head, thuse helping the low-end grunt? Also, this seems like a miniscule difference in temparature. Any information on the matter is appreciated! Thanks!

 

your running a chevy right they like to have wide open headers they run better with almost no back pressure.
fords make better power with back preasure.

i've been around the tracks and engine builders for years now and this is the first time i heard anything of this sort.
most of the time the reason u here drag cars rev at the line right before they stage is to clear the carbon out.because race gas in a car just idleing can foul a plug up fast if the tune is off some what and there tring to clean the plug of any debrie on the tip.

 

I thought they were just revving to get a good launch. Either way, why would a Ford or Chevy have different preferences/. I'm lookingfor really in-depth information.

 

Backpressure? Any backpressure is bad. What you have to worry about is exhaust gas velocity. How much backpressure do you think top fuel drag cars have? Very, very little. Don't you think if backpressure were needed, of all cars, top fuel dragsters would encorporate some?

Let's further discuss EGV (exhaust gas velocity). Wonder why some Hondas have a higher pitch that others? Maybe they have an aftermarket muffler on the stock exhaust tubing. It all comes down to exhaust size. Too small, as the case in most stock Hondas, will choke your engine. Imagine trying to blow through a coffee straw/stirrer.How long does it take you to run out of air? 45 seconds? 60 seconds? Longer?

Now let's look at the other end of the spectrum -- too large exhausts. Blow with the same blowing force as applied when blowing through the coffee straw, only this time blow into nothing, just air. Takes you, what 2 seconds to run out of breath. An exhaust that is too large will also choke the engine. Why? Well, because the exhaust piping is too big, the exhaust gas isn't compressed very much and therefore travel at a slower pace. Sometimes too slow. In this case the gasses get backed up in the system and choke the engine. Also normally results in a lower exhaust note.

Finding the happy meadium between the optimal exhaust gas velocity and pipe size can be a fine art, but it's only as hard as you make it. A good rule of thumb is minimum 3" exhaust pipe size with turbo cars, around 2.5" for n/a cars making at or under like 250hp at the wheels.

In conclusion, backpressure = bad, exhaust gas velocity = good.

In relation to your original post, the reason less 'backpressure' hurts low-end torque, is not because of the loss of backpressure, it's because the engine isn't flowing enough air at lower rpms to effectively move all the exhaust gas out of the system. The EGV is too low.

 

u will find on most high $ or serious race cars that u will see what they call a stepped tube header the reason for this is to get the tq of a small tube header and the power of a large tube header.the ones i had built for my old 88 camaro they started off 1 an 7/8 and about 8inches in they changed to a 2 an 1/4 then it went into a 4inche collecter.

 

Again, another example of EGV coming into play. Small tube near the block, resulting in higher EGV and low-end torque, large tube near the end resulting in lower EGV and top-end power.

 

Woohoo! That is what I said to Troux on AIM.

 

Thanks, dudes! I wouldn't have thought that EGV was important on non-turbo cars, althought it would seem the engine would still be able to move all of the exhaust gasses out of the combustion chamber prior to its next cycle.
So if I'm understanding correctly, all that matters is that the entire amount of exhaust gas is extracted from the cylinder, whereas EGV itself doesn't matter, it's just a sort of diffusion or 'sucking' force that helps to extract the EG volume, and a low enough EGV would leave excess EG trapped in the cylinder, and this is the "choke" that was mentioned? It seems like this would be a pretty significant power loss, but I've heard that any low-end torque lost due to an unrestricted (to any degree) exhaust is overcompensated for in the higher end, so if streetability isn't an issue, the widest EG exit is the optimum. Correct me if I'm wrong with anything here.

 

You can have en exhaust that's too large. If the widest EG exit is optimal, Civics would be running around with 4" exhausts. Even at higher RPMs their little engines still wouldn't flow enough air to push all the exhaust out of thesystem in a timely fasion.

The optimal size of the exhaust depends on the engine and how it's used. An autox car that needs pulling power out of tight corners might opt for a smaller exhaust diameter to retain low-end torque. On the other hand, a drag car that's only in the lower RPMs once, may opt for a larger exhaust to gain top end.

 

That's surprising. I've never done autoX, but I didn't think low revs were hit enough to consider it. Then again, I probably wouldn't feel like downshifting, either.