The V8 Owners Forum -

What size primaries should i be looking at on my ole gal?
at present the primaries are 1 1/2" should i be looking at 1 5/8" ?
the down pipes are 2" and so is the rest of the system (twin)

In Des Hammill's excellent text, for the 4 into 1 manifold....the eight 1/1/2" primaries join into two 2" downpipes, which when used in a twin system maintain this diameter (2") all the way to exit.

Ron.

is that for a stock v8 or a tuned one mate?

Get hold of Bells 4 stroke tuning and calc from scratch for your application.

Using some assumptions on the max power rpm and the cam timing it comes out at 1.61" ID and the rest of the system at 2.175" ID

Based in a 3.9

If you can provide RPM figures for max torque and max power, CC,and the exhaust valve opening BBDC I can calc it fully.

edit

What I also cant find is a correction factor or equivalence for using Nitrous as the extra fuels should mean more exhaust - so may need to be taken into account

edit/

Cheers

Andrew

Bell's book is pretty good having seen the results of a v8 exhaust based on them.

I would expect that the sizes for the nitrous would involve pretending the engine was supercharged... It should be possible to calculate the approximate equivilent capacity increase somehow.

I made some with a kit from custom chrome.


4 into 2 into 1

1 3/4" primaries

2" secondaries

2 1/4 collectors

full duel 2 1/4" system right to the back with a link pipe as close to the headers I could get.

It was very tractible on a hot cammed 4.5



Mark

Gut feel would say 1.5 inch is fine
GT40s ran 1.5 headers on the 5l cars

then 1.75 inch headers on the7l cars

But given today you could probably get better flow out of heads this may change.

But then Most rover heads are nit known for the great flowing

Ian

Tony, Im running 1.5/8ths, tri Y style, into 2" then into a single 2.5

At a guess, 1 5/8 into 2.5" collector would be more than ample.


I wouldnt see any need to go bigger than that on a Rover V8, unless making silly power.

I ran all my best times on my LS with 1 3/4" manifolds, into 3" collectors.

thev8kid,

Here is some more info.

Primary size 1/1/2 for standard to lightly modified 3.5 to 4.6
The largest to use for the above is 1/5/8

Racing engines 3.5 to 4.6 featuring gasflowed heads with larger ports always use minimum 1/5/8 primaries upto 1/3/4 feeding into 3" main pipes for 4.0 to 4.6.

Hope this helps,
Ron.

while we're on the subject have you guys seen these:

http://www.v8enhancements.co.uk/

I'm impressed he got all that pipework in there....

also what sort of lengths are you guys running? By my reckoning it should be about 40" for the primary obviously this will change a bit with different cams and revs but I'm assuming 6K and a Kent 218

Matt

Interesting what he said and the gains

The numbers he quotes are about typical for a cross over system - as mentioned he has a lot of pipe in there but there is little other way of doing it in a front engined car

A bit easier in the mid engined GT40

You need to cross 2 and 3 on each bank onto the other exhaust to get the full 180 degree cross over and correct pulsing.

Ian

I'm running 1.5 inch fairly short primaries into a 4-1 collector that I have opened to 2.5 inch, then into twin 2.5 inch free flow mufflers. I'm a bit stuck for space in my bay so thats the best I could do. Seems to work ok at the mo. Albeit, not too pretty to look at.

Mine is 1,5" primaries (unequal length due to space restrictions), 4-1 design, 2" dual system with X-typ Jag silencers in the back. I had a feeling that I might need bigger primaries but had a word with a local exhaust specialist and his opinion was that 1,5" primaries are big enough to about 350 hp.

i think 1,5 primaries are ok to use with 3,5 or modifield 3,9 only..
1 5/8 is ok to over 4,6..

Here is my new setup for 1 5/8 egualenght SD1 5200cc headers..