The V8 Owners Forum -

Difficult question this, but are my exhaust manifolds losing me power?
The manifolds are fabricated from 1.5" steel and have a couple of 90 elbows in each leg, down to a 4-1 collector. Primary length is around 18".
If I were to have a better set fabricated in stainless with swept bends and longer primaries again into a 4-1 collector would this give a noticeable power gain. I'm considering whether this is worth the money (about£800).
Engine is a 4.0L RV8 with a piper 285, flowed heads, MS and I regularly go up around 5500rpm when I'm havingf fun.

Kev

they will be, 90 degree bend are not good for flow

can you define what you class as elbows and swept bends?
Longer primaries move the power lower down the rev range so that might not be what you want with a 285 cam. That said it might fill a bit in where the 285 lacks but could also cause issues higher up when it gets in its stride. No doubting good headers pick up power but packaging gets in the way more often than not. An example would be F1 pipes, if your ever in the area of mike the pipe's shop have a look at his wall and you will see mclaren f1 headers there. They bend all over the place and the bends are as tight to the ports as you can get. Not because they are high performance, although they obviously work but because they fit the car. So if your bend radius isn't too tight I wouldn't worry about trying to pick up a couple of HP.

Is mike the pipe still around? i remember his ads in street machine and CC from many years ago. Where is he at?

Exhausts, your exhausts sound strange with very short primaries for a 4 into 1, I would be thinking 24" to 28" for the primaries, position of collector can be critical as well. Considering you dont rev the nuts off the engine your tube size is not too bad, My 3.9 will go to 7k all day long and I run 1,5/8th" primaries
For a tri Y 18" is about right, opening into 2" secondaries and 2.25" collector,
Ideally, straight out from the heads before bending is better, allowing the gasses to expand before restriction, the more gentle the bends the better, A general rule of thumb, the smaller the primaries, the better the torque, and bigger more power you will make at the expense of torque, same goes for tri Ys, better torque at the expense of power, 4 into 1 will make more power but less torque.
There is a lot of science that goes into exhausts, but there are some basic fundimentals to follow, and common sense will generally know what is right or wrong

HairbearTE wrote:Is mike the pipe still around? i remember his ads in street machine and CC from many years ago. Where is he at?

Mike Randal sold up about 10 years ago, the fella who runs Mike the Pipes in my opinion is nowhere as good as as Mike was but still charges the old style prices.

Here is a set of replica pipes as run on the 7 liter GT40 Mk2
http://www.gt40s.com/forum/gt40-tech-en ... er+project

Loads of tight bnds but they worked very well!

Ian

Have to admit it was a good few years ago that I had anything to do with him. I just assumed he was still around since his work was first class.

I'll post a few pics tomorrow Forgot to mention that 1,2,7,8 are longer than 3456 and the longer primaries are about 18" or so, I'm guessing that 3456 are about10".

I know that the design is far from optimal, what I'm trying to decide is if it's worth spending the dosh to get a decent pair made up. There is a guy locally who can make up a set out of stainless for about £800, he normally does bike zorsts but knows the theory. I guess I'm saying that if I'm only going to get a coupla ponies extra it won't be worth it. If I can squeeze a bit more torque and 15 ponies then I'll probably go for it.

Anyone recommend a good exhaust guy near sunny Stoke on Trent

Thanks for the input, it is helping to clear the fog.

Kev

http://uk.briefcase.yahoo.com/bc/kevin. ... %26.view=l

2 pics of the exhaust, what do you think?
Kev

Tony Law in Leeds (not a million miles away) does the best stuff in the GT40 world

He will need the car at his workshop to do the job but you wull not be displeased

Ian

Tony Law is one of the better builders in the country, and like Mike the Pipe knows how to charge

ian.stewart wrote:There is a lot of science that goes into exhausts, but there are some basic fundimentals to follow, and common sense will generally know what is right or wrong

I've tried to simplify the science here:

http://uk.blog.360.yahoo.com/blog-CxcM5 ... H7XAU?p=41

ramon alban wrote:

ian.stewart wrote:There is a lot of science that goes into exhausts, but there are some basic fundimentals to follow, and common sense will generally know what is right or wrong

I've tried to simplify the science here:

http://uk.blog.360.yahoo.com/blog-CxcM5 ... H7XAU?p=41

Interesting blog, but it doesn't recommend header diameter or length.
Kev

Kev wrote:

ramon alban wrote:

ian.stewart wrote:There is a lot of science that goes into exhausts, but there are some basic fundimentals to follow, and common sense will generally know what is right or wrong

I've tried to simplify the science here:

http://uk.blog.360.yahoo.com/blog-CxcM5 ... H7XAU?p=41

Interesting blog, but it doesn't recommend header diameter or length.
Kev

That's because the science is applicable across the board but you do get some pretty good clues to work on. ie: Diameter in relation to valve and port size. Length in relation to use of engine. From this you may be able to work out what would suit your application best.

Summarising the criteria.
• Header primary pipe diameter (also whether constant size or stepped pipes).
o Despite many ideas about pipe sizing, usually the primary pipe size is related to exhaust valve and port size. Just think of it as trying to provide the smoothest possible exit route for the hot gasses with either, nil, or only smooth changes of outlet/primary pipe cross section.
• Primary pipe overall length
o Primary pipe length is dependent on wave tuning. Typically, longer pipes tune for lower rpm power and shorter pipes favour higher rpm power.
• Collector package including the number of pipes per collector and the outlet sizing.
o Collector arrangement is obviously dependent on the number of cylinders, the engine configuration (V-8, inline 6, etc.), firing order and the basic design objectives (interference or independence)