Why does the N/A 3800 engine lose low end torque if you put too big an exhaust on it? On typical V8s, putting dual exhausts and headers on increases the whole power band. What are the physics behind the power loss on the smaller engines? Is it because the exhausts are already free flowing enough??

It is what'* called the scavanging effect. On bigger V8'*, you can get the exhaust too big. I did it on my 502. We went with a 4 inch dual with only glasspacks for slight noise control and lost over 18% from crank HP...that'* a 10% loss over what is acceptable. Quick fix on this - we cut out a 2 foot section 18 inches back from the end of the headers, welded in reducers and a 2.5 inch pipe and gained 13% back.

Think of it as a funnel. If you pour water into a straight pipe, it comes out as fast as it went in, but not smoothly. The funnel will smooth out the flow and the outgoing water actually drag the rest of the water out. Turn the funnel around and you can force a whole motherload of water through that hole, but there will be a limit because of the hole size. If you figure this...each exhaust port on the 3800 L36 is slightly squared and is about 1.75 by 2.5 inches, unmodified. This goes into a single pipe that is also slightly squared, but is now 2 by 2.25 inches. This goes (front) into a round (almost) 2.5 inch pipe and into a 3 inch crossover, then joining with another 3 inch into a 2.25 to 2.5 inch for the rest of the way, depending on your year. That'* about 27 inches (flat) into a volume of a little over 400 cubic inches (total length of pipe from heads to join). Since our mills are 231 cubes, that means there is a suction effect... scavanging. Change that to, oh, say 600, and you start losing the ability to produce power. You can only put so much water into that little hole. You'll get more HP, but that'* not what moves mass. It'* torque that moves mass and that'* why we can kill any 4 banger out there with a little effort.

Thanks, that makes sense.


Woo Hoo! I'm more than just a member!! I'm a SENIOR member!!

3/20/03 What'* the deal? Last night it showed I was a senior member... now today I've been demoted back to a lowly member...

Good writeup comp. Not bad at all for a bit-chaser.

Yes I must agree...VERY informative...But to reduce this to the common question?

Just what (in your opinion) should a '93 SSEi do to apply the stated exhaust flow dymanics to its stock system...i.e. what size(*) used to maximize torque without sacrificing hp??

Thanks..I am laying out my exhaust mod. plans...any help would be appreciated

Which is called the Venturi effect, which is how you make it scavenge. That'* why you will see 1500HP pro stock motors with collectors that go all the way down to 2 inches but that makes more power. But if you want more info on exhaust tuning, I put these files together but they are not for begineer'*, I suggest reading them a few times and you will start getting the jist of it.

http://www.angryducktofuracing.com/v...austtuning.zip

Alot will depend on how heavily you intend to mod the car. If you intend to do the traditional smaller pulley, intake, and maybe a chip, go ahead and move up to 3" diameter and a good high flow cat and muffler. You should be ok. To be SAFE, stay with 2.5". Cheaper, too, but still go with a new cat and muffler. This is the route I've chosen for my own car. 2.5" with good performance parts.

This brings up the question of wrapping the exhaust on your car with thermal exhaust wrap in order to increase scavenging. The objective being to cool engine bay temperatureswhile holding in heat for the exhaust which is supposed to increase the scavenging effect. I'd like to hear your view on this. You are well informed my friend. And this is something I'm working on as well, but with an NA engine.

Simple physics, my friend! Wrap your exhaust, the air gets hotter and expands. Heat rises or escapes faster than cold air, forcing the colder air behind it out. Don't wrap all the way to the cat, only to the join behind the engine. The cat will take care of the rest.

@ Complinitor. Thank you sir; you have just laid out todays project.