2" vs. 2 1/2" exhaust

[Ryan Clark]

The 300hp cars with Powerglide were equipped with 2" exhuast. correct?
For a 327/300 What would the drop in HP be with a 2" exhaust instead of the 2 1/2".?

Thanks for any response
Ryan
#35188

[Ryan Clark]

Sorry, Posted new message, not response *NM*

[Duke W.]

Re: 2" vs. 2 1/2" exhaust

The SAE gross rating method used for the 327/300 was done with the engine in a dyno cell with the manifolds connected to generous sized pipes with an evacuations pump, so exhaust back pressure was slightly below atmospheric to insure that exhaust gas was evacuated.

Though the rating did not include losses from the on-vehicle exhaust system as the current SAE net rating does, theoretically the 2" manifolds should have cost a few horsepower, but it's not reflected in the rating.

Short of testing the same engine on a chassis dyno with both manifold/exhaust system combinations, one can only estimate the difference in power at the rear wheels.

An honest 300 gross should be about 200 at the rear wheels, and I'd take a WAG that the exhaust system difference would show up as about 10 horsepower at the rear wheels.

Duke

[Joe L.]

Re: 2" vs. 2 1/2" exhaust

Ryan-----

Yes, for 1965 300 hp with Powerglide were 2" exhaust. I don't know what quantifiable effect of a reduction from 2-1/2" to 2" would be.

[Ed Jennings]

Re: 2" vs. 2 1/2" exhaust

Unless you're driving around at WOT all the time you probably would be hard pressed to tell the difference. Note that in 66-67 all small blocks had the 2" exhaust, including the 327/350.

[Ryan Clark]

Thanks all for the info. *NM*

[Bill Stephenson]

Re: 2" vs. 2 1/2" exhaust

Ed,

-----The exaust manifolds on all 67 327s were,indeed,2 inch,but 350HP front pipes were swedged up to 2 1/2"shortly thereafter!...........Bill S

[Ed Jennings]

Re: 2" vs. 2 1/2" exhaust

I know my sidepipes are as you describe. I had a 65 with the 2-1/2" manifolds once, but I can't remember the size of the actual exhaust pipes. My 62 has the 2-1/2" manifolds, but the pipe size reduces to 2" or at least something smaller than 2-1/2" immediately after leaving the manifold. On a Sunday afternoon drive it probably doesn't make a whole lot of difference, but it would be nice to see one set up 2-1/2" all the way back. Then you'd know for sure.

[Dale Pearman]

Re: 2" vs. 2 1/2" exhaust

The 2 inch exhaust system is MUCH more desirable for the street driven Corvette unless you're interested in top end high performance where horsepower is important. Exhaust scavenging is tuned with this powerglide to provide torque, which translates as "snappy" acceleration and head snapping performance.

You need horsepower to go very fast BUT to do it QUICKLY you need torque.

Dale.

[Ed Jennings]

Re: 2" vs. 2 1/2" exhaust

Are you saying that the increased velocity of the exhaust gas through the smaller pipe creates a scavenging effect?

[Duke W.]

Re: 2" vs. 2 1/2" exhaust

Unless you have a "tuned header" there is very little beneficial wave dynamics action in the exhaust system. With the production manifolds the primary wave is dissipated in the volume of the exhaust system and little in terms of a rarefaction is reflected back to the exhaust port to aid in scavenging.

With an individual header pipe, a fairly strong rarefaction is reflected back when the primary wave reaches the end of the pipe, and the "organ pipe formula" suggests a 36" inch length for this rarefaction to appear at the exhaust port at about 6000 RPM as the valve is closing.

Exhaust system back pressure is a function of total exhaust system volume, so, in general, it's best to have large pipe and muffler volumes behind the header. A small diameter header pipe is best to maximize the primary rarefaction wave amplitude, but if manifolds are used, the lowest overall back pressure is achieved with large volume manifolds. This is why the 2.5 inch manifolds and pipes are best - throughout the rev range. I can't quantify the difference, but it increases with increasing revs because of the square relationship between flow and backpressure i.e if flow doubles back pressure increases by two squared, which is four. The ratio of the areas of the 2.5 inch system to the 2' system is 1.56, which suggests a 36 percent reduction in backpressure at the same flow rate.

Large volume pipes and manifolds will genally increase the amount of perceived noise with the same muffliers, and I believe this is the reason the 300 HP/Powerglide combination had the two inch manfifolds and pipes. I believe Chevrolet assumed - correctly - that Powerglide customers were more interested in the Corvette as a luxury grand touring car, rather than a hairy-chested sports car.

My SHP engine definitely makes more power with the cutouts open, proven by quarter ETs and MPH back in the sixties. Either way - cutouts open or through the production exhaust system there is no effective wave dynamics, but the cutouts reduce overall back pressure from about 3 psi at high revs to essentially zero, and for every psi reduction of exhaust backpressure, BMEP increases by a like amount.

Duke

[Dale Pearman]

Re: 2" vs. 2 1/2" exhaust

That analysis and theory sounds great BUT this ole boy has had a few lessons in the school of try it and see what happens. Drag racing, land speed records, road racing and racing in general benefits from larger exhaust systems.

I seriously doubt if a man with a powerglide is going to enter any of the above.

The exhaust side of the engine is CRITICAL to torque production. A flat power band is the superimposition of the intake side with the exhaust side. You can lower the point of peak torque by fiddling with the exhaust manifold and exhaust pipes. Remember the old tri-y headers? For street engines a large diameter header is just plain STUPID. I can attest to this through experience. When I removed my $268.00 Brezenesky enlarged ram's horns (guaranteed to make 18 extra horsepower) in favor of stock manifolds I noticed a vast improvement in bottom end torque and neck-snapping performance. Again the goal here is NOT RPM (horsepower) It's low RPM torque. I've learned this the hard way.

GM engine engineers wanted torque and a wide, flat power band for the 327's with hydraulic lifters. Dyno runs confirmed their choice of exhaust size and exhaust manifolds. The over-all package was what counted then as well as now. I seriously doubt that these engineers were unaware of the availability of larger pipes while all the high-school Harry's knew better.

Bigger isn't necessarily better. I've witnessed dyno runs with small intake valves making more power than larger intake valves.

Dale.

[Dale Pearman]

Re: 2" vs. 2 1/2" exhaust

I'm saying that the over-all engine package was a tuned engine for great over-all performance and the result of many engineer's and technician's labor.

When you start fiddling with something that affects performance then basically you're re-engineering the engine. I've tried most of my life to improve on the work of these guys and after all these years I've finally realized that they mostly got it right in the first place.

Dale.

[Ed Jennings]

Re: 2" vs. 2 1/2" exhaust

That may be part of the explaination as to why Chevrolet sold about a zillion 327/300's. It was really all the engine most folks needed in a Vette, Chevy II, or Chevelle to drive on the street. Good power, (read torque), smooth running, no bad habits and good reliability, all for just a few bucks more than the std engine.

[Dale Pearman]

Re: 2" vs. 2 1/2" exhaust

Best engine GM EVER made!

Dale.