(Sorry. Posted this in teh ‘For Sale’ section by accident)
Hi there,
Just wondering is anyone has put in a twin (tail pipe) exhaust
into their Elan?
Would there be any performance advantages?
Eg. 4 → 2 exhaust from engine to atmostphere (via silencers
of course) rather than the existing 4 → 2 → 1.
I’m aware that the stock exhaust is rather narrow bore for the
spec of the engine. (Someone mentiond to me that the stock
system bore is really engineered to be on engines up to 100bhp)
Thanks in advance,
Peter 1973 +2S 130/5 (I guess it’s asthmatic!)
The twin cam in all states of tune will benefit from a bigger exhaust than the original Lotus design. Bigger diameter headers and a bigger tailpipe and the minimum muffler restriction you can get away with.
No real need or benefit and potentially a torque spread and power loss for a 4:2 system as you loose some of the pressure wave reflection benefits from a 4;2:1 system. Just fit big 4:2:1 headers from someone like TTR or make your own and get a 50mm tail pipe and you will be fine up to 180hp.
The best muffler system for a late plus 2 for road use I believe is a small straight through muffler just begind the gear box. This fits nicley in a spyder space frame chassis as it fits in the cut out in the backbone tunnel floor at that point and then a large as possible straight thorugh muffler in the orginal location under the boot. Also wise to stick some heat shield between the body and the muffler if you set it up with tight clearances to avoid cooking the fibreglass.
I have just fitted a large bore system to my S4. This is a 4-2-1 design with a main bore of 2.25" (57mm). There is certainly an improved spread of torque across the rev range. The engine also revs up much easier reaching its rev limit with feeling “breathless”. Good job I fitted a rev limiter I plan to give it a run on the dyno soon and I will post the results.
This subject has been on my mind for about 35 years. Originally I pondered it in relationship to ground clearance, the exhaust being so low, could two smaller pipes be run instead of one, how about four?
Rohan you’ve begun to answer my dilema. I call it “back pressure” and I think you call it “pressure reflection wave benefits.” I’m aware of it but don’t understand it fully when it comes to the design of the engines. What determines optimal pressures? Is it different for a Stromberg versus Weber or even Dellorto(sp) version? Would a straight pipe with no muffler provide maximum HP? Do the engine designers take this all into effect and design the muffler accordingly? Signed, Perplexed
I know what you mean about the ground clearance The new large bore system is pretty close to the ground, it will be a challenge getting on and off the ferry this summer. I was also having similar thoughts about running 2 pipes from the header (manifold) to the rear box in order to provide some more ground clearance.
The flow of exhaust gas (and inlet air and fuel) is not a smooth flow but occurs in pulses driven by valve opening periods.
For the exhaust when the high pressue pulse that is created by the exhaust valve opening reaches the end of the pipe or the join with another pipe that creates an expansion you get a low pressure pulse reflected back up the pipe towards the engine.
By having the correct length of pipe you can get this low pressure pulse to arrive back at the cylinder just before the inlet exhaust valve closes and help scavenge residual exhaust gas out of the engine.
The length of pipe required depends on the system design and the cams you are using. For a twin cam and a 4 to 1 system you need around 26 to 30 inch headers and you will find there is just no room for this. A 4:2:1 system needs around 13 inch primaries and 13 inch secondaries and this is more practical to fit in.
A similar but reverse process can occur with the inlet tracts and carbs where the low pressure pulse created by the inlet valve opening is reflected back as a high pressure pulse that increases the amount of fuel /air mixture pushed into the engine.
I have been mulling over putting a decent after market silencer on my S4 SE for years, I already have branch manifold into a single lager diameter pipe to a Cooper S silencer, not pretty nor great sound, I am looking for one that fits with good flow, but also a good but not raspy sound.
Can anyone recommend and or suggest a silencer manufacturer? I do NOT want one of the modern oil drum size silencers!
Quote Would a straight pipe with no muffler provide max. H.P.
Some time ago I had the muffler torn off my Lotus when giving it some welly, I stopped and walked back to get it only to find it gently steaming and hissing in a pile of horse shit. I put it in the boot and drove on, the car felt utterly gutless until I refitted it. So no ,I do not think a straight through pipe gives max H.P. but hot horse shit smells.
John.
My plus 2 had a hole in the manifold/downpipe at one stage and the lack of backpressure was enough to cause it to backfire on overrun. Unlike most production cars of the time where less backpressure is always better, with the Elan, I think it was all part of the design.
It fits in the S4 nicely but a bit ugly when viewed from the rear. I simply added a straight pipe tip to extend it out beyond the bumper. It sounds great to my ear. The system begins with large primaries (forgot the size exactly) to a 2" pipe straight back to this silencer.
I don’t know where you are located but Jeg’s is a mailorder speed shop that may well ship internationally.
I think its perfect. My buddy says it’s not loud enough and I can cruise for a couple of hours without it bothering me. But loud enough to be noticed with a nice throaty sound,(probably attributed to the 2" pipe). At that price, you can’t afford not to experiment!
I plan to give it a run on the dyno soon and I will post the results.
The new large bore system is pretty close to the ground, it will be a challenge getting on and off the ferry this summer. I was also having similar thoughts about running 2 pipes from the header (manifold) to the rear box in order to provide some more ground clearance.