Exhaust Building 101

[cchyper]

So what do you guys know about exhaust building?

I was talking to a sharp fellow last night about the smell of gas & exhaust systems and he was discussing a lot of things i have not thought about. IE: collector size to fit rpm range, resonator placement, etc.

whatcha got?

Thinking about MSDS header with at 2.5 inch exhaust, 18" resonator and a muffler of some sort. Building the exhaust from scratch.

[Brando]

Isn't the 944 already equipped with an exhaust that is 2.5" in diameter? 3" would be the way to go, if you had a 3.0 that was super or turbo charged. It's not so much the diameter of the exhaust tubing moreso the design of the collector/header/extractor. The 944 (non S2/not the 968) uses a tri-Y design. The header will be where you get the extra power. As for a muffler, keep it stock. Resonator... Glass pack?

[cchyper]

I might do a 3" its very possible, i have heard mixed theories. I do know that fordahl builds 3" for n/a cars. Its all about tuning.

http://www.nsxprime.com/FAQ/Miscellaneous/exhausttheory.htm

[AA_Ezra]

So does lindsey racing, I've heard mixed theores to, that the 3" is to large for the n/a something about not being enuff back pressure. And the stock piping is a little under 2.5 like 2.25 or something close to that. But i think the most important thing is the header. The after market headers have individualy tubes all the way down to the bottom wich they then conect to be once peice. Theory is that The exaust has a free flow without the worrie of the exhaust chambers without letting hot exhuast into the other effecting it's performance. Or something like that.

[Usmellgass2?]

Excellent article, very good reading, funny too, I took a few minutes to get my mind back on subject after he mentioned Paula.......ahhh Paula....ahhem ah where were we, oh ya. One good point he makes is why the try y layout of the stock na header is a great choice for a sanely driven street car, but then there is the rest of us a 4 into 1 with NO back pressure is the best way to make mo bhp. A 4 into 1 will also let a set of cams with a decent amount of overlap scavenge properly. Sadly we are still stuck with that damn AFM. Collector length is Very important and must be defined by a significant change in tube cross section. A good place for a resonator. I very much agree with his opinion about mufflers. One detail that I believe raises the necessary back pressure myth is a good exhaust will lean out the engine, if the engine can not be properly re-tuned it will not run well, so at this point we flatten 3 sides of the pointy end of a golf tee and start twistin away on the fuel quality switch. This could turn out to be a great thread, lets hear more opinoins and ideas!

[Dantilla]

I race my N/A against Fordahl, and we both have 4 into 1 headers, and 3" all the way back.

More top end, however a little low-end is lost as a result. Overkill on a street car, but a good setup for a car that is at full-throttle for a good portion of it's life.

An engine does not need back-pressure. It needs the right velocity through the system for proper scavaging. Think back to high school physics, and it's clear that too large of an exhast pipe slows down the exhaust gasses too much.

[cchyper]

Very interesting. Dan, i have heard your name before. Thanks for coming out with a solid answer. I would ask then .... is a 2.5 in the best way to go for a track day street car? Also i was thinking about one of those msds headers, is it worth it? Do i need to pay worth of the header in ceramic coating costs or should i just use it as is? I know & have heard that ceramic coating the headers inside and out really helps with directing heat. I wonder if msds coats the inside and outside of their headers for the extra 115 bucks. I will have to ask

header $227 + 115 ceramic coating (outside only i bet)

keep em comin

- chris

[yellowline]

Originally posted by Dantilla
An engine does not need back-pressure. It needs the right velocity through the system for proper scavaging. Think back to high school physics, and it's clear that too large of an exhast pipe slows down the exhaust gasses too much.

I don't know about these engines, but on small air-cooled Briggs engines, it's important to run at least a small exhaust. A 3 inch straight pipe is enough. Otherwise, you burn the exhaust valve- don't know if that would be a problem on a water-cooled engine.

[cchyper]

So whats the consensus? 2.5" or 3.0"? I dont think they make 2.75" very accessible.

[fast924S]

stock Im 90% sure is 2.25. I would stay there, and just go with a better muffler and header with a resonator. As noted before, if you go to big dia in the exhaust your gonna lose alot more low end power. Im running stock dia with a B&B cat back a Bursch header and cat sub pipe. I love it

[cchyper]

Fordahl does this - Direct off spec sheet.

1. Header, FM/MSDS race header. 4 into 1 wi. Ball con. $ 599.00
2. 3” race exhaust wi. 18” resonator and Magnaflow muffler $ 399.00

[cchyper]

What does your car produce fast924s? Have you dyno'd or got a gtech on the beast?

[fast924S]

She is all apart right now and has been for like 2 years, Working slowly on doing other mods. I did have a g-tech on her once and hit 200, but I dont know how correct that was, But I do want to get it dynoed once Im done

[cchyper]

Very nice... 200! out of a 2.5 n/a... You do have a lot of mods tho. Lots of things that take you out of qualifying for spec racing, but none the less very nice.

MY BIGGEST MISTAKE was not milling the head for compression. I have no idea why i didnt think of that earlier! Now i have a head installed and its stock I really should have thought of that.

[fast924S]

Or when you had the head off you could have swapped out to 88 10.2cr pistons. Yea I believe the MAF kit takes me out of spec racing.

[cchyper]

I actually bought 10.2 cr pistons, but the pistons i bought from Jwade were the wrong tolerance group. I have a 2 tolerance group 1 pistons and 2 tolerance group 0's. Anyone want any? I am selling them.

[cchyper]

back to exhaust building. I think that the best thing to do in my case is:
1. MSDS header
2. 2.5in exhaust
3. Resonator 18"
4. Muffler: Magaflow, Borla, Flowmaster, Edelbrock, something...

p.s no cat for the nice exhaust and then, for emissions, cut the stock exhaust down to fit the new header... the ol' jury rig switcharoo!

[Dantilla]

Originally posted by yellowline
it's important to run at least a small exhaust. Otherwise, you burn the exhaust valve- don't know if that would be a problem on a water-cooled engine.

All engines need some type of exhaust system. Look at nitro-fueled drag cars. Simply a bent pipe, but with nothing at all, they too would burn exaust valves.

2-1/2" is a great system for a street driven car. My setup is track-only. Even though the car is still street-legal (it still passes emmision testing with no cat!), I have to use less than half-throttle to keep the noise down. It's pretty rare the car is driven on the street.

[Razorback1980]

In the case of a nitro methane car, the pipes are for exhaust control not to protect the valves. After each run the engines are completely torn down and rebuilt from the ground up (in sixty minutes or less). One of the tasks that happens is to evaluate just which parts have melted...often times it's at least one or more pistons and valves. These engines run so hot that after the first ten seconds or so, the engines are dieseling and don't even use a spark to ignite the fuel and can only be shut off by cutting the fuel supply. The flames you see coming from the exhaust pipes at night are not actually coming from the engine. The exhaust gases are so hot, that it is burning the hydrogen in the air causing the flame. Valves get burned on every run and the heads are replaced between runs. While the extra set is off, they are inspected and burnt vavles are replaced. You are right the heating up and rapid cooling makes the valve susceptable to failure, but those guys don't care, they just replace them and go on. They just don't want those hot gases being blown straight on to their tires and catching them on fire as it would completely melt the rubber. Not fun to have a blow out at 330 mph. Fires around nitromethane aren't too much fun either. The fans seem to like them...the drivers have different opinions.

[gohan]

Be careful on the selection of Resonator, assuming using it as wave cancellation box. Also the length of collector extenstion.

http://superchevy.com/technical/engines_drivetrain/exhaust/0505phr_exh/index.html is a good article.