clearrun

I am trying to get the best free flowing streetable exhaust system for my stroker. I just got off the phone with a "tech expert" from Flowmaster who siad CFM numbers do not matter for mufflers. He said that they do extensive dyno testing to come up with the best possible performance for their mufflers. He said that if CFM was the only thing that mattered we would all be running see thru glass packs. He started talking about pulses and turbulence and how hot gasses flow different than cold air, etc. etc. I asked if the distance of the mufflers from the cylinder heads did not have a significant impact and he said it did.

Bottom line is, does any one publish dyno data on mufflers or would it be meaningless without knowing exactly were the muffler was relatibe to the exhaust port or outlet of the headers? How do I compare mufflers in terms of performance or is it impossible ??

It doesn't make much sense to me to build high HP engines for the street and then choke them to death with an exhaust system.

Any suggestions as to were to find tech data would be appreciated. :confused:


[Modified by clearrun, 2:20 PM 2/19/2004]

SWCDuke

Exhaust system performance is EXREMELY important. For every psi of exhaust system backpressure, the engine looses at least 1 psi BMEP due to exhaust pumping work.

The Corvette 2.5 inch manifolds and pipes are quite efficient, but mufflers make a big difference. It's best if they are as far back as possible which the case.

Straight through mufflers are not necessarily the best, but flow tests are the best measure of exhaust system backpressure. Personally I don't like the sound, look, or constrution technique of Flowmasters. I consider the best aftermarket muffler to be the Walker Dynomax Ultraflow, but they are only available in standard sizes without the proper nipples and bracket to bolt to a Corvette. Walker does offer Dynomax bolt one Corvette mufflers, but they are based on the "turbo" design, which is not as efficient as the Ultraflow.

My hunch is that the Allen's "offroad" OE replacement should also be reasonably efficient, but I've never seen any data.

With the OE type manifolds there is no harnessing of wave dynamics, so the key issue is to clear the exhaust with minimum backpressure, and muffler flow tests are a good indicator of exhaust system flow capacity and how much backpressure they will build. The Engine Analyser computer simulation program allows the user to input exhaust system flow data and then computes backpressure, and you can simulate the difference between installed exhaust system backpressure and open exhaust. On a SHP 327 the difference between 3 psi backpressure and zero backpressure is close to ten percent! Of couse, there is a tradeoff between backpressure and noise, but the Ultraflow appears to provide the best sound attenuation for the least backpressure

For a high performance engine it's best to have an exhaust system that offers no more than 3 psi backpressure and the OE Corvette 2.5" exhaust system with low restriction mufflers should achieve this specification.

The effect of headers is vastly reduced with ANY backpressure, which is why they aren't of that much value on a street engine, especially considering the problems they can bring.

Duke


[Modified by SWCDuke, 3:24 PM 2/19/2004]

toddalin

:iagree:

I use the 2.5" Ultraflow and love the sound. And, being stainless, they carry a lifetime warrantee. Like the Duke, I also hate the continual drone of Flow Masters at cruise speed.)

I have Doug Thorley headers and all 2.5" mandral bent pipeing, including the tips. (BTW, 2.5" tips centered in the holes look far nastier than the whimpy 2" tips.)

My mild 327 makes 293 hp and 293 ft-lb at the wheels as measured on the dyno using 9.5 CR, camel hump heads, and a 222 degree 0.445" cam. This is 25-50+ hp more than the larger ZZ3 and ZZ4s are making, so I must be doing something right in the way of flow.

clearrun

Duke,
Thanks for the response. I now have a couple of engine analyzer programs. Where do I find flow data for exhaust systems / mufflers ??

Allcoupedup

Anyone have any data on 2.5" small block factory side pipes?

Brian

L79vette

Buy David Vizard's book, "Building Max Performance Small-block Chevys on a Budget" ( I may have the title a little off, but it's close). In it he discusses Ram's Horns, Headers, pipes, and mufflers. He has a whole section on building muffled exhaust systems that do not lose power relative to open headers. In the foreword, a friend of his relates a favorite Vizard quote: "I don't have opinions, I have a dyno!"

Vizard has a lot of respect for the Ram's Horns, he says that in a properly designed system with some manifold porting around the inlets they give about 65% of the benefit of headers relative to log manifold type exhaust, on a mild motor. But, I think mine is ready for the next step up.

I'll probably be selling my 2.5" Ram Horns and very nice 2.5" mandrel pipes this month sometime.


[Modified by L79vette, 5:50 PM 3/2/2004]

SWCDuke

If you have "Engine Analyser" the documentation has guidelines for exhaust system flow. For SHP/FI engines I used "streamlined manifolds" and 500 CFM flow, which I think fairly accurately represents the SHP exhaust system with low restriction mufflers. It creates no more than about 3 psi backpressure.

Duke

L79vette

Anyone have any data on 2.5" small block factory side pipes?
Brian

----------------------

No flow numbers, but many people report they are restrictive. Apparently the chambered sections pinch the pipe down to 2 inches. A 2-inch pipe has 36% less area than a 2.5-inch pipe! That in itself wouldn't be any more restrictive than a decent turbo muffler, but apparently all the little chambers make the flow turbulent as well.

Another thing to watch out for: The stock undercar pipes get crushed down for a couple of feet in front of the cross-member, apparently for ground clearance. This reduced the flow area a lot.

My car made 255HP at the wheels with 2" ram horns and stock pipes. With 2.5" ram horns, 2.5" mandrel-bent pipes, and admittedly also a better carb tune, it made 281HP at the wheels.


[Modified by L79vette, 5:56 PM 3/2/2004]

bcwaller

OK, let me hijack the thread a bit. I've been going back and forth for a year and I'm ready to make my decision, hopefully. My problem is my heads. They are Edelbrock Victor Jr. heads, which have angled plugs and raised exhaust ports. Finding headers that will fit is tough, and finding some that will work with sidepipes is virtually impossible. I estimate gross HP to be in the 450 range as well. None of my local trusted shops do custom headers anymore.

I had almost decided on Sanderson shorty headers going to the sidepipes as they were the only ones who said no problem. I'd love the Stahl setup, but they say that they will not work with factory sidepipe covers (they fit the C3 body, not C2). But, as it comes time to decide, I'm going back and forth again.

My options as I see them (in estimated order of price):
1) Sanderson shorties with factory sidepipes until I go and get a full custom header and higher performance sidepipes. Need to figure the right primary pipe size still, thinking of 1 3/4"

2) Find a long tube header that will work and have a shop fabricate an "S" bend to fit them to the sidepipes.

3) Buy the Stahl headers and higher flow pipes/baffles and then pay a shop even more to customize and make it all fit.

Opinions? Other great ideas?

valrico_stingray

L79 - I sent you an IM. I may want your 2.5 ramhorns. Chuck

Plasticman

Has anyone seen or used the new Hooker Aero Chamber mufflers? How do these compare with the Walker Ultra Flo and Flowmasters for noise at cruise. Understand that they do not have the flow of the Ultra Flo's (but more than equivlent size Flowmasters), but am concerned that the Ultra Flo's may be a bit too noisy at cruise. Also, to compound the "problem" of getting just the right sound, I have a crossover pipe (which tends to quiet the exhaust). Existing turbo's are just too damn quiet (especially at idle - I have a heard louder Suburbans)!

Further info available at:
http://www.holley.com/HiOctn/ProdLin...SHHM/Aero.html

As for what the affect exhaust system CFM has on torque and HP, look at the above website and review the charts at the bottom.

Any and all comments welcome!

Plasticman



[Modified by Plasticman, 10:33 PM 3/2/2004]

LT1driver

how would 3" exhuast benefit a mild street motor, would headers and 3" be a good step up from 2-1/2" systyem??? :confused:

L79vette

3" vs. 2.5" on mild motors:

Not according to Vizard or to http://www.headerdesign.com . The Header Design program actually recommends 2.5" collectors up to about 430HP. Most collectors are greatly oversized, according to exhaust design theory. This results in too low a velocity for best scavenging at RPMs we typically use.

L79vette

One thing I forgot to mention about the Ram Horn performance - to get anything close to header performance, you must have a merge detail in the exhaust, either a Y-pipe or an X-pipe. Haven't seen any figures on H-pipes, but it seems unlikely they could be anywhere near as good given the strange flow path. In the Vizard book he shows a beautiful Y-pipe he used on the dyno, looks like it is set up for a truck chassis though because it crosses under the engine. My X pipe is right behind the rear crossmember. I haven't done the dyno work myself to show the importance of the merge, but Brzizinski and Vizard have. Certainly my data supports the claims, though I made exhaust diameter improvements at the same time.