Exhaust Back Pressure

Post by **alanworland** » Mon Jan 05, 2009 11:21 pm

Following on from one of my other topics about cylinder heads, the thread has sort of re directed and has got me interested in back pressure (sad i know)
When back pressure is referred to, what sort of pressures are we expecting at the point where the gases enter the main exhaust pipe? (mbar or lbs/in2?)
I am considering doing some tests on my existing system for use as a comparison after any modifications are carried out and would like to be able to measure 'back pressure'

Post by **mike.perry** » Tue Jan 06, 2009 9:21 am

Too technical for me I'm afraid.
You might need to try the main website, there are a few engine tuning enthusiasts on there.

Post by **PSL184** » Tue Jan 06, 2009 9:47 am

Around 2 psi is considered optimum but remember this will vary with enine revs. The job of the exhaust system is to remove the gasses as fast as possible. Most normal exhaust systems are a compromise as you will get higher pressures the faster the engine is spinning. Most comon misconceptions result in people fitting big bore systems which actually slow down the release of gasses (garden hose principle). Specialist systems (such as BMW's, Jag's etc) use a 2 stage silencer with a flap which shuts down half of the silencer at low revs and opens it up at higher revs in order to balance out the pressure throughout the rev range. Most of the work I have done on exhaust systems related to V8 and V12 engines but I did some research for 1275 A series engines and found that 1 3/4" with a single silencer gave a good balance. Not sure about sidevalve engines, if thats what you have fitted, so can't help with them.....

Post by **alanworland** » Tue Jan 06, 2009 7:04 pm

Yes I have the original sidevalve fitted - the 2psi you refer to, would that be the sort of pressure I might see at the point of entry to the 'exhaust system' ie straight after the manifold?
and if so I guess this would be at or near maximum power delivery?
I am planning to measure the existing pressure (thats why I need to know what I might be looking for) then, after removing the baffles in my silencer (or fitting another) note any performance changes which might be related to backpressure.

Post by **MarkyB** » Tue Jan 06, 2009 7:26 pm

There needs to be enough back pressure to stop fresh mixture being blown out of the exhaust.
Probably depending on the overlap of the camshaft.

Half way back from the south of France my exhaust cracked around the flange where it joins the manifold.
Couldn't find a way to repair it so soldiered on.
Sounded great, like a low flying aircraft. But the fuel consumption was terrible

Post by **PSL184** » Tue Jan 06, 2009 7:36 pm

Yes, the back pressure needs to be measured with a flow meter at the point of entry to the exhaust system and as you say, at peak flow, which is maximum power delivery.....

Post by **alanworland** » Tue Jan 06, 2009 9:52 pm

Thanks for all the info folks!
I shall make up a thick flange to put in the manifold/pipe joint so that I can run a pressure gauge from - just to see whats going on.
Please also see posts on - Twin Carb exhaust size which is related to this one.

Post by **DAVIDMCCULLOUGH** » Sun Jan 11, 2009 9:32 pm

I have the rear section of a 1000 exhaust and silencer fitted to the front pipe of the sidevalve, would this make it slower as I think the 1000 exhaust is slightly larger.

I see that club spares have sidevalve exhausts for sale at the moment, I may have to invest in one!