MG-Cars.info

Welcome to our Site for MG, Triumph and Austin-Healey Car Information.

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB GT V8 Factory Originals Technical - Exhaust Bore

My 4.6GT is making 250 rwhp. My exhaust system is 2 1/8" bore. Would I release more hp if I went to say 2 1/2". Im using the RV8 manifolds and a single system with a single straight through 6"x 20" tailpipe.
Car used mainly for trackdays.

Any thoughts

Mark
Mark

Mark, hard to say for sure, but usually larger is better due to reduced back pressure. There is a point of diminishing return where gas velocity drops off too much with too large a pipe. Ideally you could find out the flow capacity of a certain pipe size at a given pressure and then calculate how much capacity your engine would need at your maximum rpm.
Bill Young

I just installed 2 1/4" pipes on my 4cyl B, and the flow improvement really helped the throttle response. I can't see why it wouldn't help you as well. My 2 1/4" bore for a 4 cyl would equate to between a 3 1/8 - 3 1/4" bore for a V8. I'd say that a 2 1/2" single pipe would be conservative for a track car!
Jeff Schlemmer

I run a single 2¼" on my V8, it works great but my throttle is light anyways with the alloy flywheel and pressure plate.
Jake Voelckers

Thanks for the replies.
Think Ill try and cobble together a 2.5 system and see.

Mark
Mark

2-1/2" duals is going to be a bit large unless your are looking for unrestrained top end performance around redline. Back in the days before cats the factory hi-po option on medium sized v8's was a 2-1/4" header tube and a 2" tailpipe. With a big block you would have a 2-1/2 and 2-1/4 system. I'd guess on the 215 it would have been 2" and 1-3/4". The step in size was because by the time the gasses made it through the mufflers the exhaust pulses had pretty much mushed together, cooled a bit, and didn't need as much room. I also think maybe it widened the powerband a bit. Usually low restriction mufflers was the only change made to the system even when replacing it because it was well tuned to the engine, resonating in the midband right around cruise speed, and increasing torque and economy. Shortening the system moves the resonant frequency upwards, as does enlarging the tube diameter, meaning that torque increases happen at a faster speed in the rpm range, so the serious engine builders would shorten and enlarge the system for higher rpm power, but for the average hot rodder that only helped if you also increased the redline, or at least the speed where you spent most of your time. It's not just a matter of back pressure, even behind the headers it is a tuned system.
Jim Blackwood

Yup - I'm with Jim.

Look at it this way - how big is the engine half you need exhaust for (treating the banks as separate)?

Right - 1750 CC. Just how big do you think you need for a low revving 1750 ;-)

I thought my Lamborghini exhaust looked small until I realised it was really a sub-2 litre straight 6 on each side and thought about a GT6 engine - which hardly needs huge exhausts.

Obviously if you run both sides into one pipe, you'll need a 2.5 or a bit more.
Bill Spohn

Im using a single system.
My current system has a diameter of 52mm and area of 21.3 sq cm.
So basically ive got 2, 2.3litre engines trying to push exhaust through an area of 21.3 sq cm.
If I devide this by 2 then it works out to an equivelent of 2 pipes of 37mm diameter which seems tiny!
Theres got to be an optimal size for a single pipe.
Just need to find out what it is.

Mark
Mark

Mark, the sizes I mentioned were also the sizes used for single exhaust on non-performance applications. Apparently the tuned frequency was considered more important than the increase in flow that would have been allowed by a larger tube and this is where backpressure begins to enter the picture. If you are using a 2" tube all the way back I would suggest increasing the size of the header tube to 2-1/4" which will improve flow and may not affect the tuned frequency too much. Should work fine for an engine under 5L. in a performance application. Remember, the MG is a good bit shorter than a '70 american sedan so less restriction from length, and the redline on the engine is probably 500-1000 rpm lower than on the 1800.

Or, you could compare the area of those two dual pipes to what a single pipe would be using the 2" and 1-3/4" sizes and see what you get, probably closer to your 2-1/2" size but of course the resonant frequency will be considerably higher with this.

Jim
Jim Blackwood

This thread was discussed between 04/05/2006 and 18/05/2006

MG MGB GT V8 Factory Originals Technical index

This thread is from the archives. Join the live MG MGB GT V8 Factory Originals Technical BBS now