hot and cold exhaust

[Doctor slow]

I have a twin exhaust on my spitfire 1500. It's attached to a single pipe, and a stainless manifold. I've removed the two silencers, cleaned the joints, and refitted the them.  After a recent long journey, I checked the anchorages, and in doing so, touched both tailpipes. The left one (viewed from the rear) was just warm, the right one burnt my hand (as, I suppose, one would expect it to). I don't think that there should be this difference in temperatures. should I be worried? Car is running well, apart from some popping one overrun. Any opinions?

[johny]

I think maybe one silencer is more restrictive than the other so the majority of the exhaust gases take the easiest route. Two silencers on a 1500 is a bit of overkill really and especially with a free flow manifold the back pressure will be lower hence the popping. This can make the mixture weak and push up full consumption...

[Clive]

I would agree that one box may be partially blocked (or the Y piece)

Poppingn on overrun not related, although more obvious as less silencing. As to back pressure, I have never understood the worries as it can't effect cylinder filling, to me zero back pressure, or better still, tuned manifold to "suck" the exhaust out, is only a good thing reducing wasted energy.

[johny]

I thought the manufacturers set up (cam, carb etc) is done based on a certain back pressure and if the exhaust gases escape too rapidly then at the end of the exhaust stroke when the inlet valve opens fuel/air mix can be pulled through past the closing exhaust valve. This unburnt fuel then burns in the hot exhaust and produces the pop....   

[Doctor slow]

Thanks for that. I know that there are no obvious obstructions in the "Y" or the silencers. I wondered if this is a "swirling" problem. If the down pipe enters the long pipe at an angle is it possible that the gases swirl in that (wide bore) pipe and just happen to be on  the right hand side as they hit the "Y".....? Certainly at idle, the output from the tail pipes appear to be equal (purely subjectively, of course).  Agree about the back pressure with the popping. Also from previous experience, any unburnt fuel hitting free flow manifolds can cause popping. Not unduly worried about at the moment. Grandsons think it's cool( not literally!)

[johny]

Of course the other possibility is that one silencer gets better cooled than the other?

The only thing with the unburnt fuel is that it hurts its not doing some useful work like making the car accelerate quicker😁

[JohnD]

No afterburner effect?

 

[NonMember]

1 hour ago, johny said:

I thought the manufacturers set up (cam, carb etc) is done based on a certain back pressure and if the exhaust gases escape too rapidly then at the end of the exhaust stroke when the inlet valve opens fuel/air mix can be pulled through past the closing exhaust valve. This unburnt fuel then burns in the hot exhaust and produces the pop....   

Well, possibly in theory, but you're unlikely to get enough to ignite, in reality. In fact performance engines are deliberately set up to pull through inlet mixture so as to purge the incombustible exhaust gasses.

Popping on overrun is nearly always an indication of misfire. You don't notice the misfire itself because the expected torque output is very small (negative if you're talking flywheel torque) but the unburnt mixture gets vented into the very hot exhaust manifold and ignites there instead.

[johny]

31 minutes ago, NonMember said:

Well, possibly in theory, but you're unlikely to get enough to ignite, in reality. In fact performance engines are deliberately set up to pull through inlet mixture so as to purge the incombustible exhaust gasses.

Popping on overrun is nearly always an indication of misfire. You don't notice the misfire itself because the expected torque output is very small (negative if you're talking flywheel torque) but the unburnt mixture gets vented into the very hot exhaust manifold and ignites there instead.

Yes of course in performance engines youre not worried about wasting a bit of fuel.

My motorbike was brand new (2007) when I changed the standard exhaust to a more free flowing one and straight away it was popping and banging - cant see its misfiring....