I lifted this from CorvetteForum.com I hear alot of us are pitching the OEM design pipes, and that is a good thing....... Sidepipes suck... -------------------------------------------------------------------------------- I was recently contacted by a '65 L-76 owner who felt his 225 RWHP @5500 was low, and I agreed. His engine is configured very similar to Mark Johnson's engine, which measured 278@6500, so 225 represents a nearly 20 percent reduction. A series of compression and leakdown tests indicated more variation than one would expect for a relatively fresh engine, and one cylinder definitely appears to be a little weak, but not enough to drop peak power 20 percent. Peak torque of 270 was good - a little better than Mark's 263, but Mark's fan was probably consuming 15 lb-ft, and the other owner reported that the fan clutch probably did not tighten during the tests. There was one major "external" difference. The car in question has OE sidepipes and Mark's car has the under-the-car system. So I decided to see if I could come up with a flow model for the sidepipe system and analyze what effect it has on backpressure and power output. The Engine Analyzer simulation program allows the user to model exhaust systems by stating flow rate at 1.5" Hg (20.4" H2O) depression, which is the same depression used to rate four barrel carburetors. Using some engineering intuition I came up with a model of 250 CFM per side, 500 total, and this jibes with test data. For example, Mark's 278 RWHP converts to 327 SAE net at the flywheel using 15 percent driveline loss, and EA's prediction is 324 so they correlate within one percent. Using some good verbal descriptions of the OE sidepipe system I modeled it as a 1 3/4" pipe, which is probably giving it the benefit of the doubt, and the flow ratio at constant depression is proportional to the area ratios, which is a function of the square of the diameters ratio, which is 0.49. In other words, one side of the under-the-car system flows as much as two sidepipes. Imagine a SHP engine with a single exhaust system! Plugging 245 CFM total exhaust flow into EA increases exhaust backpressue on Mark's engine at 7000 revs from 3.3 to 9.5 psi - nearly triple- dropped peak power ten percent, and knocked 500 revs off the useable power bandwidth, reducing the upper boundary from 7000-7200 to no more than 6500. It's still not enough to account for 20 percent loss, but I'd be willing to believe that the sidepipes account for 15 percent loss and the slightly weak cylinder 5 percent, and this is supported by the peak torque data. It's always been "heard on the street" that sidepipes are more restrictive than the under-the-car-system, but until now I had no quantification. Now I know, or at least have a pretty good idea. If you want sidepipes and high peak power - forget it, they are mutually exclusive. I always felt that the under-the-car system was good. It may not be a wonder to behold visually, but it was very well engineered and works very well. Limiting backpressure to only about 3 psi with modest sound level is indicative of good engineering. Large free flowing manifolds, generous pipe sizing, and mufflers located as far back as possible is a textbook description of a good exhaust system. Now if someone could only do an accurate reproduction of the OE off-road mufflers, so we could have that great sound... Though the under-the-car-exhaust is very efficient on a SHP small block, it's going to be restrictive on a SHP big block generating about 5.5 psi. Backpressure increases with the square of exhaust mass flow, so the sidepipe model yields about 16 psi backpressure on a SHP big block, which places exhaust backpressure in the range of seventies vintage Corvettes with the single bead bed converter, and your big block may be lunch for Mark's small block. The other bit of insight is that if your exhaust system flows 80 percent or better (at 1.5" Hg. depression) of what a properly sized carburetor flows, then you've got a good system. So 500 CFM is good for a SB, but a high revving BB needs 700 CFM - 3" pipes, bigger mufflers... sorry, there's no room! My model for the C6/Z06 is 750 CFM. It would not be that hard to test complete a exhaust system - one side from head pipe to tail pipe - on any flow bench that is suitable for testing cylinder head flow, but until customers start demanding real engineering data, they'll continue to get sold a bill of goods based on "look", "sound", and all sorts of other marketing foo-foo. Duke
I know Terry removed his side exhaust,I think the gains he made from that change only,will be quite noticable this weekend....... :eek2:
Good writing is: not loosing the audience Ok Donny.....I glazed over after the first paragraph. :laugh: What's the Readers Digest version? o No:
Cliff's notes version: 1. Sidepipes suck 2. Not worth testing to get data until enough people request/demand it ($$$) 3. Under car system is probably better 4. Corvette writers are quite verbose. 5. Sidepipes look and sound cool, but suck.
I'll take the enhanced audio of sidepipes over the "out the back" deal anyday! ..........and how much is it really going to slow it down?? .20 maybe? Heck, I'll cut the 'em just before the elbow on friday and run it wide open so we'll get a real world comparison! Surely there's an exhaust specialist in attendance this weekend!
