C2 Exhaust System 327/Powerglide

[Paul L.]

I am contemplating changing the exhaust system on my 1967 with 327/300 and Powerglide. The various catalogues note differences between 4-speed and Powerglide cars. I thought this might simply relate to 2" vs 2 1/2" but in reading Noland Adams' book he speaks of different shaped exhaust pipes between the manual transmission and automatic cars. What is the difference?

[William C.]

Re: C2 Exhaust System 327/Powerglide

Auto trans is wider and the exhaust has to accomidate the extra width so has extra bends.

[Paul L.]

Re: C2 Exhaust System 327/Powerglide

Thank you for the reply. Basically that means that a 2 1/2" exhaust will not clear the Powerglide.

[William C.]

Re: C2 Exhaust System 327/Powerglide

That is correct.

[Paul L.]

Re: C2 Exhaust System 327/Powerglide

Thanks again. I now feel more confident in ordering.

[Patrick H.]

Re: C2 Exhaust System 327/Powerglide

If you really wanted a 2 1/2 inch system, you could swap your exhaust manifolds to 2 1/2 inch openings (If I recall, yours are 2 inch) and then call Allen's or Corvette Central, and see if they would custom bend a 2 1/2 inch system for a PG car. There's not a lot of benefit of 2 1/2 inch exhaust if your exhaust manifolds are still 2 inch diameter.

Patrick

[Duke W.]

Re: C2 Exhaust System 327/Powerglide

The 2.5" exhaust system IS a benefit with 2" manifolds, but it depends on how much top end power the engine makes. For a 250/300 HP engine w/PG, I don't think the bigger pipes would be worth much on an unmassaged engine.

At close to 300 net HP that a massaged SHP engine can make, a 2.5" exhaust system generates about 3 psi backpressure, which is very good for a reasonable road exhaust system; 2" pipes increase the backpressure to about 4.5 psi, which is above the 3 psi maximum that I like to see on hi-perf road engines.

Two inch manifolds on a 2.5" system are a little more restrictive, but it's really not terribly significant.

Backpressure increases with the square of exhaust mass flow rate and exhaust mass flow rate increases a little more than linerly with power. As OE built the 300 HP engine makes approximately 225 net, so maximum exhaust flow is less than 75 percent of a massaged SHP engine, which means exhaust backpressure on the same exhaust system configuration is about 50 percent less, so larger than 2" pipes gets into diminishing returns relative to cost on an OE medium performance small block.

A 2.5" exhaust system is just right for a high output SB, but is restrictive for a BB. That's why the C6 Z06 has 3" pipes, and dual path mufflers. There's no way the engine would make over 500 net HP unless the exhaust system was well engineered and backpressure was limited to no more than 3 psi.

Duke

[Paul L.]

Re: C2 Exhaust System 327/Powerglide

Patrick,
I would be very happy with a 2" exhaust. And yes the exhaust manifolds are that size.

But allow me to explain where things are at the moment. When I bought the 1967 in 2003 it had an Allen's chambered system (see pic). I tolerated the noise for a year but it was too much. I then went with Midas turbo mufflers. But they talked me into leaving the chambered pipes in place to save a few dollars. Sounded OK as the Allen's pipes were like new. But I did have concerns about back pressure. The Midas mufflers were hooked up with the chambered pipes. Afterward I noticed the Midas mufflers had a center inlet, not outside, so they stick down too far. Being new to C2s I did not know the difference. I do now. And it irritates me. Live and learn in this hobby.

I must admit the car runs very well despite my concerns about back pressure but I would like to have a proper, not bastardized, system put on. The Midas mufflers are polar opposite from chambered: they are passenger car quiet. So I have been thinking of either the Corvette Central Magnaflow 2" or Paragon's similar with "off-road" mufflers. I would like a little more noise but not in Flowmaster territory. So that's where things stand. The 1967 is just a sunny, Sunday afternoon cruiser. I would appreciate any advice.

[Paul L.]

Pic of Muffler Position

Hanging down. I painted them black to draw away (somewhat) from that flaw.

[Paul L.]

Re: C2 Exhaust System 327/Powerglide

Duke,
Thanks for that reply. The engine is very mild and I think 2" will be fine. As I have posted before it is a 1962 327, 250hp, 4-barrel, passenger car block with Powerglide. Just a Sunday afternoon ride. It's the chambered exhaust pipes that I worry may be be restricting flow even more than an open 2".

[Duke W.]

Re: Pic of Muffler Position

Walker makes a set of Dynomax Turbo mufflers for the C2/3 that have proper front and rear nipples and hangers, so they are "bolt on" once the exhaust pipe is cut at the proper point, but I'm not certain if they are made for both 2" and 2.5" pipes

You should be able to find out on the Dynomax web site.

Duke

[Patrick H.]

Re: C2 Exhaust System 327/Powerglide

Paul,

Given the power (or lack thereof) of your motor, I doubt that you are having a significant power loss with your current muffler setup. I use the same setup on my 71 (which has more power) and have not (subjectively) noticed any loss in power. Also, I really love the sound.

If yours is like mine, it's a 2 1/2 inch chambered system anyway, so the change in power to go to a conventional 2 inch system may not exist.

But, if it's just a Sunday cruiser, from my point of view I'm not sure it would be worth the expense.

Patrick

[Joe L.]

Re: C2 Exhaust System 327/Powerglide

Duke-----

1969 Corvette big blocks, including L-88/ZL-1, originally used a 2" exhaust system.

[Duke W.]

Re: C2 Exhaust System 327/Powerglide

Yes, and here are some ballpark numbers. The C2 2.5" under the car exhaust system generates about 5.5 psi backpressure on a massaged (all OE other than the normal street prep head work) SHP BB. Double the backpressure for C2 sidepipes!

The 2" '69 under the car system will increase SHP BB backpressure about 50 percent to 7-8 psi. I don't have any data on C3 side pipes to make an estimate, but it is probably no less than the 2" system.

Add a huge overlap L-88 or ZL-1 cam to this much backpressure (with manifolds or headers - doesn't make much difference with high backpressure) and you have a recipe for a poor idling, torque shy engine that doesn't make impressive top end power.

Most do not understand the SIGNIFICANT effect that a vehicle's exhaust system has on the engine's actual installed output, and the more overlap the cam has, the more deleterious high exhaust system backpressure is.

And as we all know, the manifolds and OE exhaust system for L-88 and ZL-1 were installed at the plant merely as a convenience so the cars could be moved around under their own power until delivery to racers. Chevrolet assumed that the racers who bought them would install a proper racing exhaust system. Chevrolet provided basic racing exhaust system design guidelines, and valve timing of the L-88 and ZL-1 cams was designed specifically for the racing exhaust system they described.

Duke

[Paul L.]

Re: C2 Exhaust System 327/Powerglide

Wow, I am quite taken aback by the replies. I may just sit tight with what I have!