11 replies to this topic

[mattcave]

Is there a rough rule of thumb for internal bore size for exhausts, based on expected power output? e.g. a 1 3/4" system works well up to 85bhp. For over 110bhp, need to go to 2"...

 

If I'm building a torquey road motor around the 90bhp mark, what size system should I buy?

[Turbo Phil]

Rather than basing it on size, the proper way of doing it is to flow test the exhaust. You need roughly 2.2cfm of flow per expected HP.
As I'm guessing you don't have a flowbench tucked away at home, I'd suggest the genuine RC40 from Minispares.

Phil.

[mattcave]

...which is 1.75" internal, right?

[Turbo Phil]

1.75" OD mate.

Phil.

[mattcave]

Got it. Thanks!

[Chris.Williams]

I just fitted one of those systems, twin muffler type, beautiful! Do it!

[Cooperman]

As a 'rule-of-thumb', and this depends on the rest of the engine spec, you can work on a 1.75" for a 1275, 1.625" for a 998 and 1.5" for an 850.

Also make sure the exhaust pipe is the same diameter from the end of the exhaust manifold to the final outlet point at the back.

For race applications, as Phil points out, a flow calculation to match exhaust gas velocities with peak revs is the best way to do it, but for road cars, which have a wider power band and varying exhaust gas velocities, the above figures are a good compromise.

[carbon]

For 90bhp road unit the twin box RC40 would work well. This system should be good for over 100bhp, Vizard recckoned up to 120bhp before it caused any significant power losses.

[Cooperman]

I have a twin-box 1.75" RC40 system on my 'S' and that is putting out over 100 bhp.

[minisilverbullet]

I have a 1.75" RC40 system on my k1100 headed 1380, I thought I may need a 2" system but the car made 107 Bhp at the wheels. 

[mattcave]

Right on. 1.75" it is. Time to put in a call to Maniflow to see what their shipping costs are to Canada.

[hhhh]

mattcave, you could try http://hunterclassicsportscars.ca/ in Victoria BC

for a quote. He's a dealer for the RC40 system and a he's a great guy to deal with.