SWCDuke

This question has been asked many times and I finally came up with enough valid test data to make a reasonable analysis.

Attached is a pdf of three '65 L-84 lab dyno tests. The engine is basically a "stock rebuild" - .060" overbore, with OE replacement 30-30 cam and forged, 5.3cc dome pistons. The heads received a valve reseat, but no "massaging" for improved flow, so they are representative of typical Flint-machined heads. Gasket thickness is not verified, but as is typical I am currently assuming that it has a thick composition gasket which would place the CR at something close to 10.25:1 assuming nominal deck heights, which were not measured.

Download the pdf and we can compare. Look at page three, but ignore the annotation about headers and mufflers. This configuration had the OE manifolds routed into the 3" dyno exhaust system with no mufflers. Correction is STP so this engine is representative of the configuration and SAE gross test procedure that Chevrolet used to establish the advertised SAE gross ratings.

Note that the 321/332 torque/power ratings are 92/89 percent of the advertised 350/375 HP ratings that can also be expressed as actual SAE gross torque/power being 8/11 percent lower than the advertised ratings, which is typical of the era. Sorry guys, that's the way it was back then. Honest ratings did not come into effect until the new SAE net procedures were first implemented in 1971.

Massaging the heads would have improved torque and power by close to ten percent, which would about achieve the advertised ratings.

Now let's look at the two tests on page 1 and 2. These were back to back tests within about five minutes and the exhaust system consisted of the OE manifolds and full OE sidepipe system that routed to the dyno cell exhaust system with no downstream mufflers. The A/F data does not appear to be valid, so ignore it.

Sheets 1 and 2 torque/power average about 291/293 or about 91/88 percent of sheet three and the only difference is the addition of the sidepipes, so sidepipes alone knock down torque/power by 9/12 percent.

Now we can get some insight into how restrictive the sidepipe system is compared to the under-the-car exhaust. I have dyno data for two "327 LT-1" configurations. One started life as a '65 L-79 and the other as a '65 L-76. Both were tested with OE exhaust manifolds and measured CR averaged about 10.4:1. Both have the ...461 OE aluminum manifold, OE Holley, and LT-1 cam with nicely massaged heads. One was tested on a lab dyno at 356 GHP and the other on a chassis dyno with under-the-car exhaust at about 270 with SAE air density correction. Using 0.85 driveline/tire efficiency that would be about 318 SAE net at the flywheel, which yields a 0.89 net/gross conversion factor.

About half of this net-gross difference is the lower air density correction of SAE net compared to STP used for SAE gross, and the rest is the exhaust system and front end accessories. Of the latter, only an alternator and clutch fan that does not tighten absorbs only a couple of horsepower, so from a practical standpoint we can say that half the net/gross difference is air density correction and the other half is exhaust pumping loss.

Given the above the under-the-car exhaust system pumping loss is about 19 HP or about 5.3 percent.

Sidepipes ate up 39 GHP on the L-84, which is about 11.8 percent, so we can conclude that sidepipes cost about double the parasitic pumping power as the under-the-car system, which is significant!

Duke

Mossy66

When I built my big block, I was able to break it in and do some testing on a dyno. With the dyno shops headers and mufflers it made about 500HP. Then we took off the headers and installed the complete side pipe exhaust including the manifolds. It lost 100 HP.


Gerry

SWCDuke

..some additional info. The "327 LT-1" tested on the lab dyno that started life as a '65 L-79 was tested with both OE 2.5" manifolds/OE front pipes and 1 5/8" dyno headers, both systems being routed into the 3" dyno exhaust system with no mufflers. All data uses STP correction so it is effectively SAE gross. CR averaged 10.35:1 across the eight cylinders and 100LL avgas was used to ensure no detonation during the tests.

The system engineering and testing was documented here on the CF with extensive discussion back in the early/mid 2000s. Search for "327 LT-1" in the thread titles and threads started by Dave McDufford, Then search for threads by ghostrider 20. That was the 327 LT-1 (started life as a '65 L-76) that was tested on a Dynojet chassis dyno. Both engines were essentially the same configuration.

With manifolds the best lab dyno torque and power from multiple tests was 338 lb-ft @ 3700 and 356 GHP @ 6400, 45 degress total WOT spark advance.

With headers the best torque/power from multiple tests was 358lb-ft @ 4400 and 369 GHP @ 6500, 41 degrees Total WOT advance.

Headers/open exhaust increased peak torque by 5.9 percent and peak power by 3.7 percent compared to manifolds/open exhaust.

Since exhaust back pressure negates some of the wave dynamics effects, I expect in the car with the under the car exhaust the difference between headers and manifolds would be cut in half.

These tests show that the OE 2.5" manifolds and 2.5" under-the-car pipes/mufflers are very efficient - probably the lowest restriction OE exhaust system of the era.

Duke

Critter1

Duke.. I tried to explain a lot of that to the folks over at the NCRS site several years ago but I don't think any one understood it.

Bluestripe67

Duke, That's some very interesting data, thanks for posting it. Can you speculate if a % of HP is gained by port matching, runner smoothing and using Hedman side exhaust headers. Also, in the data sheets it shows the oil pressure being a continuous 61-66 lbs. What oil pump was used? Dennis

capevettes

Interesting data Duke. I was aware that the side exhaust costs you some power but the sound is so good it's worth it in my opinion

4 Speed Dave

Making a general statement that side pipes rob big power is very mis leading to many people. In today's world there are many different types, i.e. outside case diameters and inside free diameters of side pipes that can be purchased. Now on the original OEM sidepipes that have an effective internal free diameter of 1 3/4" I can most certainly understand how they would take power away. Lets be honest how many people still have the original sidepipes on their cars after 50+ years. However in today's world of where you can get side pipes in 3" O.D. and with a clear 2 1/2" clear inside opening through the pipes the loss is no more or less than a 2 1/2" under car setup. The devil is in the details.

SWCDuke

"Head massaging" - pocket porting, port matching, chamber relieving with multi-angle valve seats, all other things equal will make 5-10 percent more peak power and another 500-1000 useable top end revs.

The range has to do with other configuration issues. For example, a restrictive exhaust system, like OE sidepipes compared to under-the-car exhaust, will yield less improvement because the higher parasitic exhaust pumping power eats up a good part of the increased indicated power improvement due to head flow improvement.

Head massaging on the seventies vintage single catalyst engines is basically useless because of the huge backpressure caused by the restrictive single cat.

Search for a thread started by me "Tale of Two Camshafts" for more info on the subject.

Late '63 to '65 mechanical lifter engines were equipped with nominal 55-60 psi relief springs in the normal standard volume OE oil pump, so those pressure readings are a little on the high side, but can be considered normal.

I have no data for the exhaust system config. you stated.

Duke

Bluestripe67

Appreciate you response. Can you address my last question of my post? Dennis

DansYellow66

I think most enthusiasts understand how inefficient the stock GM sidepipes are - but are many still running them? I would be interested in seeing a back to back test of 2-1/2 inch chambered core Allens or Sweet Thunder sidepipes compared with stock 2-1/2 inch undercar system.

4 Speed Dave

I believe Joe "65tripleblack" did with the 3" G.N.M. Sweet Thunder side pipes and the loss between open headers and the side pipes was very low, if I remember under 10 hp on a chassis dyno.

MikeM

I'd be interested in seeing a comparison between the two exhaust systems as OEM equipped. That means assembly line parts, muffleers and sidepipes included.

I realize that will never happen due in part to a shortage of assembly line parts.

The exhaust flow of the after market parts may or may not equal the performance of the OEM.

DansYellow66

Or in the case of big bore sidepipes - may exceed OEM.

63Corvette

Hi Duke, and glad to see you back on the Forum!!!
I will agree with every thing that you say, BUT:
For RACE motors, a radical camshaft (and high compression etc) REQUIRES headers....period!!! Think the Darrington bundle of snakes exhaust on the 4 cam Indy Ford engines. My 1969 L-88 picked up 110hp at the wheels with only headers (and rejet the carb....leaner). Modern Vintage Corvette legal, iron race engines, like Terry Gough, and Jerry Golnick, put 500hp to the wheels with a .030 over 327CID Chevy small block. Some modern race engines run 15:1 compression!!!
Your figures and conclusions are spot on for street engines, BUT, some street engines on this year's (2016) Power Tour were in the 1000HP class, so "modern" applies equally to both (reliable) street and race engines. Modern hydraulic roller/boosted/sprayed engines are much stronger than any RACE engines from our past

69ttop502

I realize the purpose of this thread is to compare the OEM exhaust configurations, and I thank Duke for posting those results. Very interesting information.

In my experience with the big bore Hooker type sidepipes, there is still a bunch of horsepower lost. On my 600 horse 502, I lost over a hundred horsepower with the Hooker mufflers although not the Max Flow. With a 3 inch glass pack I still lost nearly 30. Now I understand 65 now has a true 3 inch muffler in his. Also have to remember the Hooker style have way too long 45 inch primaries and that big bend which is far from ideal. In my case, to keep all my horsepower I had to have the car so damn loud I didn't enjoy driving it. Now I have a custom built header and 3 inch undercar system and enjoy the car more and am sure I am keeping more of my power.

SWCDuke

All I did was provide some data that allowed a reasonable conclusion about the difference in power between C2 under-the-car OE exhaust and OE side pipes with OE manifolds on a typical Corvette OE high performance road engine.

I made no conclusions about headers and race engines, (or aftermarket non-OE design sidepipes), but as you say race engines with very high overlap cams need headers and open exhaust. I've done system engineering work on race engines that required optimizing valve timing and exhaust system design, so I understand the underlying physics and engineering considerations.

As I have said many times before, road engines and race engines are totally different animals.

Duke

65tripleblack

My 327 made 414 RWHP @ 6554 RPM with open headers, and 406 RWHP with headers going through Classic Chambered 3" sidepipes.

C2Racer

I have wondered about the Hooker headers / side pipes on my 63 as I know they are not efficient. Have considered running it under the car instead and out in front of the rear wheels. The biggest problem for us doing this is clearance to get the car in and out of the trailers. My 69 has Stahl's on it which I hear are better. Completely different design than Hooker and shorter header tubing lengths.

Thanks Duke for the info.

Steve

Chambered

Depends on which aftermarket sidepipe mufflers you run or test. Light years difference - much improved options these days. Here's a photo of the inside of an OE chambered sidepipe muffler: 1-5/8" ID - measured it myself. I wouldn't run something that small on a 289 Mustang. No wonder the power losses were so excessive.

69ttop502

Steve, although I have no personal experience with the Stahl's, I have always heard they are much better than the Hookers. Much better primary length and I think they have a smaller collector as the 4 inch of the Hookers is way to big. I had a real highly respected exhaust guy do my undercar headers and exhaust, and he built mine as a 421 system with 2.5 choke merge collectors. Said it wouldn't be a restriction until well over 800 horsepower. Dispelled quite a few myths for me on exhaust sizing for sure. Amazing the tube sizes he uses, even on the many race cars he has done. Asks whether you want big Dyno numbers, or do you want to go fast lol!