Exhaust Manifolds

[david.g.chapman]

Spring is in the air and I have been out and about in my +2 - it's going really well. I have even fixed my wheel balance problem - but I had to move weights around after some high spreed runs to do it.

However, I was wondering if I could easily extract a few more bhp out of my engine (how many times have we all thought that!).

I have strombergs with richer needles than standard (B2Y), CPL2 cams, and some porting to the low compression head. The exhaust maniford is the standard later 4 into 2 into 1 gas flowed type, but with the narrowest bore. The spark plugs are the right colour. The induction filtering is completely standard except for an additional 20mm hole in the bottom of the nose air cleaner body.

I have not taken any measurements, but I reckon by the feel of the car that I am getting about 120 bhp.

Would just changing the exhaust manifold for the larger bore type (fitted to big valve engines) be worth while to improve engine air extraction?

Or is this a red herring?

Dave Chapman

[steveww]

First of all there is no magic bullet, no one tweek is going to give a big increase. However you have correctly identified that the standard exhaust manifold is a bit on the small side. If you are going to the trouble to fit a new one then go for a real big one. The TTR option is the best but very expensive. Paul Matty also do one, you also pick up one second hand at Donnington. I would also look at the air filter and fit something larger here to. These tweeks should be worth 5 - 10 bhp and a fatter torque curve as well.

[alaric]

Hi

According to the performance calculator link in my recently started thread, going from 120 to 126 bhp with a 850kg rear wheel drive will gain you about 0.2 sec 0 to 60mph. I think it hardly seems worth the bother for that.

An alternative would be to trim 40kg from the weight of the car.

On my own engine I've left the top end standard for now, so that I can drive the car in standard trim when it's first on the road. I'd like to push the power up to 150 at some point, and will probably approach QED for one of their head reworks - they put in slightly hotter cams and rework the head. It's a standard upgrade so their quote should be accurate. According to the calculator going from 120 to 150 will put your 0-60 at 6 secs from 7.33. I know these aren't the actual numbers that are achieved, but it's a useful indicator of what sort of difference you could expect.

Is lightening the flywheel ever successful with a twinc? I did this with a mini some years ago, and it was fantastic.

Sean.

[rgh0]

Dave

I agree with Steves comments. A bigger exhaust will definitely help a little even on a relatively standard engine. Changing the standard air filter may help but the dyno tests I did a few years ago now with the standard filter on versus off on a 140 hp Weber engine showed no measurable difference in output .

cheers
Rohan

[david.g.chapman]

Thanks for all your replies.

I will think about replacing the manifold - it's a bit corroded at the bottom anyway. The Y piece will also have to be changed - after that I am not sure.

Incidently, further back I have a straight through silencer bomb from MG spares feeding a standard stainless back box from CN. I have also fitted a home made baffle in the 2 inch tail pipe made up from 1.5 inch pipe closed off at the exhaust outlet (holes are drilled in the pipe to the same surface area as the straight pipe between the bomb and the manifold).

This set up seems quieter that the old mild steel exhaust I had, without the bomb. Its especially good at motorway speed. I have also fitted two additional mountings to the chassis to take the the weight of the bomb. Ground clearance is still OK at 4.5 inches under the bomb. I think I got the idea of the bomb from you Rohan - nice one!

I have tried removing the baffle but there is no great increase in power.
I might try it again though when I get the manifold...

Dave Chapman.