Eshaust system tests

[evelrod]

I had not , until this last week, done much work on exhaust systems v cam timing since the mid 80's. We just spent three days with a four cylinder ~185hp(155 at the wheels) @ 8400 rpm, 1594cc Lotus twincam engine on a Clayton chassis dyno testing just exactly that.
What a shocker...After hours and hours of fabrication and testing exhaust headers and pipes/mufflers and three separate cam setups (what a PITA) the highest power/tq was with a $40 Dynomax 2.25", 20" long turbo muffler...by 2+ hp and almost 2 ft/lbs tq. ! The tests included all different configurations including several open pipes of differing dia and length. We tested several high dollar "racing" mufflers, too.

Acoustic tuning (original Cosworth and current computer programs) pointed to a 54" overall pipe, try y headers at 13"x 1 5/8" primary, 1 3/4" x 13" secondaries, 28" x 2" pipe....However, the ultimate power ended up being the headers feeding into a 96"x 2.25" dia pipe into a 20" Dynomax muffler and a 12" x 2.5 inch tail pipe. I am totally surprised that the results came out this way. Now I must rethink some of my basic ideas...Exhaust tuning is so much 'cut and try' anyway, especially in combination with jockeying the intake and exhaust cam timing setups by as much as 5 degrees advanced/retarded in combination with the various open exhaust and muffled exhaust systems.

Throw it up and see what sticks. I'm interested. This thing is quiet enough to be street driven without fear of noise violations!!!!!!!!!

Rod

 

[GregLocock]

If it is a muffler for a turbo installation then it may have less back pressure than your normal one, as they are often straight through perf tubes with rockwool in the can.

I don't suppose you measure exhaust back pressure directly?



Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Lou Scannon]

Interesting topic. Thanks for sharing, Rod. My guess is, if the Dynomax 2.25x20" wasn' the lowest average backpressure setup, then a freaky pulse tuning issue must have put it on top. If that's the case, and you can figure out what the magic tuning configuration was, then realize it with reduced average backpressure, there's a little more hp to be found in your combo.

 

[evelrod]

Sorry, we did not have a manometer set up for exhaust work.
The $40 muffler is a single pass "S" configuration with no actual baffles. We were going to try the Dynomax race muffler, exact replacement for the std. but with a 2.25" diagonal straight through, oval can, 4x12x20 centre in and side out but it was not in stock and we did not get it (still haven't got one)in time for the scheduled dyno time.

I am a bit disappointed with Flowmaster as even their high dollar 'race only' can was way down, even below a straight pipe. Second best muffeled power pull was with a 2" dia pipe and a 14" Walker Dynomax at some 4 to 5 hp down. Third was 64" open pipe 2.25" dia. some 10 hp down. Best open pipe was 158" (stuck WAY out the back) down 2 hp.

Best was with a Flowmaster collector, suitably modified for this application, two 1.75" into a 2.5" collector oval 'box' 10" long tapered to 2.25" and on out to the muffler out back. Long 'open pipe', both 2" and 2.25" were this configuration, minus the muffler (wanted to test a 'megaphone' but, no time). The 'side pipes' were with a conventional "Y" of two 1.75" welded an mated to either a 2" or 2.25" pipe, varying from 54 to 64".

Computer graphs showed any 'tuning' glitches, mostly in the short 'side pipes'. Mostly big tq fluctuations in and around 4000 rpm (not all tests this low rpm) and 5500 to 6500 range. Hp peaked early in all open pipe tests and jetting was universally rich going from muffled to open tests. Big range of jetting in the 45 DCOE 152 (Spanish Webers)...40 chokes 150m-200a-f2 and 50f9 idle---Worst power overall to 36 chokes 135m-160a-f2 and 55f8 idle---Best power
All tests corrected to statistical norms, whatever they are.
We are at 1300' AMSL and air temp was 80f. Engine oil and water temp were 180f.

I do think, based on careful study of fuel graphs that I can pick up a little by going a bit bigger on my air jets.

Of course the last test was with the cams set In 106 ATDC Ex 110 BTDC for no power gain, indeed a loss in tq, so it's back to In 100 ATDC and Ex 110 BTDC which was the highest 'average' power with the long ex pipe and the Dynomax.
It may well be the quietest vintage racer on track!

Incidently, we did similar tests to the 1380cc, 111hp (at the wheels, same dyno) Mini Cooper in '04 but with typical results, muffler made about one hp less but tq was down 12% over a straight pipe...Mini is LOUD!

Rod