"Jesus (And Sidepipes) Is (Are)Just Alright"

[Joe C.]

(Copyright: Doobie Brothers/Capitol Records, 1973)

Here is some data relating to RWHP for a high horsepower (L76)1965 Corvette:

A 13.866% gain in peak horsepower over the same car, tested on a chassis dynamometer was realized on the same car, within 30 minutes on the same day. The results were obtained with the stock GM (NOS) sidepipes connected, versus disconnected (open ramshorn manifolds).

Joe

[Michael H.]

Re: "Jesus (And Sidepipes) Is (Are)Just Alright"

No surprise there. Not going to make any decent HP numbers running the exhaust through open manifolds only. Wonder why they didn't run a test with regular mufflers or off road exhaust.

[Jack W.]

Re: "Jesus (And Sidepipes) Is (Are)Just Alright"

because it would have showed that the supposed "high performance" sidepipes actually ROB a significant amount of power, vs the undercar exhaust that was provided for the same engine. Sad but true. I still love the sidepipes on my 65 L76 though . . . . .

[Duke W.]

Re: "Jesus (And Sidepipes) Is (Are)Just Alright"

I can only give an estimate based on EA models and chassis dyno data, but I estimate that Mark Johnson's "327 LT-1" that I reported on in July loses about six percent at the top end with the under the car exhaust versus zero backpressure.

The 278 SAE corrected RWHP equates to about 327 net at the crankshaft versus EA's prediction of 324 net with 3.2 psi backpressure @ 7000, and with zero backpressure SAE net increases to 344.

A 14 percent gain equates to to about a 13 percent loss, which would be about 300 net on Mark's engine. To drop power to that level I had to reduce total exhaust flow rate (@ 28" H20) from 500 to 300 CFM, and backpressure increased to a little over seven psi. With a well set up L-72, the level of backpressure will DOUBLE!!!

Based on this, C2 sidepipes are more restrictive than my exhaust system model for '62s.

Duke

[Joe C.]

Re: "Jesus (And Sidepipes) Is (Are)Just Alright"

Michael,

That was my test. I would have liked to test vs. a set of N11 mufflers, but I could not obtain a set. It would have been impractical for me to do such a test, in the same dyno session anyway. The dyno operator allowed me to drop the sidepipes between the 2 pulls.........a matter of removing 6 nuts (5 minutes)

It would be helpful if somebody with a stock L76, similar to mine, could run the same test with standard undercar exhaust, or N11 system in place.

It sounds like you are saying that open ramshorns make less power than with a low restriction exhaust system in place. I can't see why that should be so! In the old days, I believe that SAE gross horsepower was obtained on a test dyno, on a BLUEPRINTED engine, less accessories, with open manifolds.

Joe

[Michael H.]

Re: "Jesus (And Sidepipes) Is (Are)Just Alright"

Joe,

You have mail.

If the best HP were developed using manifolds only, or no manifolds at all, then the header industry probably wouldn't exist today. Getting rid of the spent gasses in a cylinder is a LOT more complicated than just opening a valve and allowing the gasses to escape. The head pipe and it's specific length are designed for the "pressure waves", not the exhaust gasses. The pressure wave is what makes these things function. An exiting wave (pos) begins at the moment that the exhaust valve leaves it's seat and travels the length of the head pipe at roughly 1670 FPS. When it reaches the open end of the tube, it reverses sign and returns up the pipe as a negative sign, or pressure. The wave reaches the end of the tube long before the exhaust gasses even begin to move out of the chamber. The timing of this trip to the end of the tube and back is critical to the efficiency of the tube, or header system. If the tube is too short, the system doesn't work at all. These pressures, pos and neg, are responsible for moving the spent gasses. Clearing the chamber is not completely a function of combustion chamber pressure. There's a lot more to it.

In a street system, such as that of a Corvette with iron manifolds, the primary pipe is the exhaust pipe. If that primary is too short, most of the wave time is lost as the wave returns too quickly. In a factory side exhaust system, the chambered pipe kills the wave. The first chamber in the pipe acts as an open end of a primary tube and reverses/returns the wave too quickly. The under chassis system takes advantage of it's long exhaust pipes and wave timing is much more efficient.

Exhaust design is a very complex subject and most people don't quite understand how it works, or why it works. Primary pipe design/length is a lot like the length and design of intake manifold runners. The length has a dramatic effect on HP and what RPM ramge it works in. The physics of all of this is way too complicated to explain here. I'm sure there are books on the subject.

[Duke W.]

Re: "Jesus (And Sidepipes) Is (Are)Just Alright"

With manifolds, there is effectively no harnessing of exhaust pressure wave dynamics, so whether there is a pipe on the manifold or not is not going to make much difference in power. Backpressure is effectively zero with a pipe or no pipe and there is no wave tuning.

Same with the full exhaust system, but then you have backpresssure, which increases approximately with the square of engine speed at WOT.

Duke

[Michael H.]

Re: "Jesus (And Sidepipes) Is (Are)Just Alright"

Anything short of individual primary tube headers is going to eat power for more than one reason. Back pressure is just one of these reasons. Iron manifolds with at least some exhaust pipe will see some function of wave pressure assistance, even though the manifold itself would some what disrupt the wave. Even with a full exhaust system, there is some benefit from wave pressure. We proved this many times.

Few years ago, Honda motorcycle engineers learned that their racing machines actually developed more HP when the exhaust was routed through a muffler that was at least 24" down stream. It had nothing to do with back pressure.

There's even a slight downside to the pressure wave (sound wave) from the exhaust that affects the air stream that's entering the carburetor. These waves are the secret to HP. I spent a lotta GM's dollars playing with all of this a few decades ago. Gets VERY deep.

[Jim T.]

Re: "Jesus (And Sidepipes) Is (Are)Just Alright"

A week or so ago there was dyno testing of horsepower with different headers done with a GM crate engine on Saturday's Horsepower TV show. The TRI-Y header design were the best of the ones they tested on this show.

[Michael H.]

Re: "Jesus (And Sidepipes) Is (Are)Just Alright"

Not surprised. A properly designed set of Tri-Y's will partially eliminate the problem created by the two cyl's on the same bank that fire one after the other (8-4 & 5-7) 90* apart in the firing order. Much better than iron manifolds without the hassle of 30" primary headers.