: Boost primer

08-12-09, 08:43 PM
A friend of mine sent me a link to a thread on this forum. It was quite long and I tried to wade through it, but it quickly deviated from the topic of the original post.

There was, however, one thing I wanted to post up about, but I decided to do it in a 'general' post, rather than in response to a given thread. Why am I posting anything at all? Because you are Cadillac owners, V racers and not rice farmers (emphasis on rice).

Boost is a measure of atmospheric pressure inside your intake manifold. It is only indirectly, and sometimes quite incorrectly used as a measure of performance. What is really needed is a measure of cylinder pressure, but we don't have one.

A supercharger (turbo or blower) is an air pump. It exists solely to push more air (and, therefore, oxygen) into the engine than would occur in a naturally aspirated engine.

If you increase the ability of your supercharger to pump air into the engine and make no other changes to the engine, you should see an increase in boost levels and an increase in performance.

If you increase the ability of your engine to breathe freely, but do not increase the ability of the supercharger to pump air, you may not see any increase in boost. You may actually see a decrease in boost. But you should see an increase in performance.

To understand this, open your mouth and breathe in and out. Now cover your mouth with your hand and breathe in and out. Harder to breathe with your hand over your mouth? Notice how your cheeks puff out? That is boost. Nothing changed about your lungs ability to pump air, but boost increased when your hand was over your mouth. What you changed was your freedom to breathe. Remove the restriction, the engine breathes more freely, but boost levels drop.

Generally, anything after the supercharger that allows the engine to breathe more freely (e.g., ported S/C housing, larger runners in the head, larger valves, cam with increased duration and lift, headers, x-pipe, cat delete, freer flowing mufflers, cut-outs) may not increase boost. In fact, boost levels may drop off, even as dyno numbers go up.

Obviously, if you increase the ability of the supercharger to pump air and the engine's ability to breathe at the same time, all bets are off. Boost may increase, decrease, or stay about the same. That is why boost is a poor measure of performance.

It is a bad idea to get into a Pl$$ing contest about boost numbers. I recommend that you leave that to the rice farmers.

One final comment. As you modify your car, make absolutely sure you put it on the dyno to check your A/F ratio. I have dual widebands in my car, but I still go to the dyno first thing after making any major changes to my car. Yep, the first time it goes WOT is on the dyno. As a rule of thumb, external mods (headers, catback, etc.) are safe without a tune. Anything internal (heads, cam) you need a tune. Finally, changes to the S/C seem to result in exponential increases in performance, rather than the more linear increases seen with other mods, which makes it more imperative that you have a dyno tune before ever going WOT.

Enjoy the car, folks. It is one sweet ride and probably the nastiest 4 door (and soon, 2 door) GM has ever built.

P.S. -- I don't claim to be a tuner, I don't consider myself a boost guru. I'm just a car guy, a lifelong lover of GM performance, and I seem to have a better than average understanding of the internal combustion engine. Based on what I have seen and experienced, the above is true, but I do not have a degree in any relevant engineering field.

08-13-09, 09:53 AM
Well done.

08-13-09, 01:33 PM
Very informative. Thanks for posting.

08-13-09, 07:37 PM
If I push hard enough on my cheeks, I get what sounds like an exhaust leak.

Would this condition be considered back pressure or a back fire?
Maybe just bad fuel...

08-13-09, 09:45 PM
Remember that boost gauge measures pressure ratio and what you are truely concerned with is DENSITY ratio.

This is why compressor efficiency and intercooler efficiency play such large roles. Higher compressor efficiency means less heat for a given rise in pressure: therefore higher density ratio.

Same with intercoolers - an intercooler lowers the air temperature, which once again, increases density ratio - with the sacrifice of a pressure loss through the intercooler (nothing is ever free).

Compressor maps are on Eaton web site (and posted elsewhere on this forum) - you can plot where you are asking compressor to operate and calculate the the density ratio based on inlet temperature and efficiency.

Not to mention that lower temperature induction air is less sensitive to spark knock which further increases the power potential.