Exhaust sizing

[JamesH]

Hello all,

After too many years repairing (and bringing up children) , my Locost chassis is back in one piece, and I need to think about the exhaust.

I have a nice shiney exhaust from a motorbike, I think an R1, but not sure. (all measurements are from memory, but the proportions are about right) This has a 2" input diameter, but 1.5 exit diameter. My manifold, IIRC is 1.75" diameter. Its a 4-1.

Engine is a 1300 crossflow, about 85hp.

Now, given that the R1 (or even the r6 which is an alternative option for where the muffler came from) is much higher HP than my Xflow (>110hp), can I use this muffler? I will be dropping the overall size of exhaust down from 1.75 to 1.5, but since the exhaust was designed for much higher HP is that OK?

James

[Zico]

Can only give you a link which may help..

http://www.superchargerperformance.c...ger-calculator

[JamesH]

Been taking a quick google around, and from various equations and calculators on the net, it looks like for about 90hp, I need a 1.5" final pipe. Now that looks just about right for the silencer I have, so looks like this could be a good option. If its a bit pants, I'll try and find a 2" bore motorbike silencer - certainly cheaper than getting one custom made (the 1.5" one was £20 off Ebay).

Still, any further thoughts welcome.

[phoenix]

My calculations suggest the bore of the tail pipe/silencer should be 1.875" for 11000 rpm peak bhp/red line. Any smaller than that and you may find it is too restrictive at the top end.

Oops, sorry. That was for an R1!

For a 1300 xflow @ 7500 rpm the bore needs to be about 1.75". For 7000 rpm you need abourt 1.7"/43mm bore.

[JamesH]

Originally Posted by phoenix

My calculations suggest the bore of the tail pipe/silencer should be 1.875" for 11000 rpm peak bhp/red line. Any smaller than that and you may find it is too restrictive at the top end.

Oops, sorry. That was for an R1!

For a 1300 xflow @ 7500 rpm the bore needs to be about 1.75". For 7000 rpm you need about 1.7"/43mm bore.

How did you work it out? I saw some math from Vizzard that said 2.2 x required HP gives CFM, and a straight pipes flow 115CFM/sq inch. I dropped that down to 100 for a silencer.

Not sure how you can have such a simple calculation though, given varying pressures, exhaust speed, RPM etc.

[phoenix]

My calculations take account of engine speed and gas speed, which it seems Vizard's method does not and, from what I have read, obviously should. The back pressure is related to velocity and increases to the point of becoming a problem, as explained below.

There is a widely accepted maximum advisable mean speed of about 300 ft/sec or 91.4 metres per second for the exhaust gases in the pipes, beyond which pumping losses will increase which will affect power available at the flywheel. As you are combining gasses from four cylinders that has to be taken into account as well - of course only one cylinder is on it's exhaust stroke at any one time but some of the gasses from the previous firing remain in the exhaust so have to be considered as well, which is why your exhaust is not the same diameter as you manifold primaries throughout it's length.

Although a pulse will travel through the exhaust gasses on opening of the exhaust valve at the speed of sound, the actual speed the gas will achieve is related to the engine speed, bore and stroke. Although the exhaust gasses are expanding when the valve opens, the expansion will not cause the mean gas speed to be faster than mean piston speed, and any back pressure before the piston reached BDC doesn't cost you power. Maximum piston speed will occur much later in the cycle than when the valve opens - in your case about 73 degrees before TDC by which time the exhaust valve has been open for 140 crank degrees or more, so there will be relatively little gas left in the cylinder, but what exhaust gasses are left need to be pumped out of the cylinder into the exhaust system by the piston, which of course takes some power away from the flywheel. The idea is to minimise that loss.

On your engine at 7000 rpm the mean piston speed is 14.7 metres per second. With a 80.97mm bore that means you need primaries with an area of 827.7mm^2 which is given by 32.5mm ID pipes to achieve a mean gas speed of 91.4 metres per second. Once merged at the collector, an area of 1471.1mm^2 is required which is given by a 43.3mm ID pipe.

Of course a smaller pipe would be ok if you are using lower revs, or if you want better low down torque and don't mind losing out at the top end. (A smaller pipe will have a higher gas velocity at a lower engine speed, which will improve scavenging, but back pressure will rise and cause losses at higher revs.)

[JamesH]

Thanks Phoenix. Much appreciated.