Practical Performance Car magazine approached us recently with an invitation to assist them with an exhaust feature they were thinking of running, well here it is.
HOW TO BUILD A PERFORMANCE EXHAUST MANIFOLD - PART 1
YOU CAN USE A HACKSAW. YOU CAN WELD. BUT DO YOU THINK YOU COULD MAKE A PERFORMANCE EXHAUST MANIFOLD?
KEVIN LEAPER THINKS YOU COULD
In the pages of PPC each month we feature modified cars that range from mildly modified to mildly mad. Sometimes they’re high tech engine swaps with turbos, bespoke fuel-injection and four wheel drive, sometimes they’re period tuned with multiple carbs, big valve heads and lairy camshafts. But the unifying theme is they’re all trying to go faster. Each of these builds encounters its own problems but one area they all have in common is the need for a decent exhaust manifold. And that’s often where the problems start.
You think you’ve struck lucky and find an aftermarket manifold that will fit your engine transplant. Well it may fit, but the chances are it’s not going to be designed to match your particular engine’s performance or your driving style, so you look at the options of having a bespoke system made. This involves trailering the car to the workshop, digging deep for the expertise and ending up with something wonderful, but not something wonderful that you’ve made. How about the third option then – you make something beautiful? Read on for what is and what isn’t possible.
Theory and Design..........................
Forget you’re a bloke for the moment, put your hacksaw down and turn the welder off . If you spend a little while first getting to grips with the theory and then making a design, you’ll save time on the fabrication and produce a much better manifold.
It’s going to be hard to explain exhaust manifold theory without calling it a black art, so let’s get that out of the way first. Anything that anyone calls a black art is just something where the variables are too huge to calculate. It doesn’t mean we can’t understand the principles and use guidelines to make a good stab at the answer.
First off the exhaust manifold can only lose engine power – open primaries will give you the best power readings – just look to dragsters with their open pipes that aren’t restricted by the noise police. So when we’re designing a manifold what we’re actually trying to do is reduce these losses. The only time you make more power with an exhaust manifold is when you take off a crap one and bolt on a good one. And a good exhaust manifold is one that presents an opening exhaust valve with as big a pressure diff erential as possible.
The bigger the difference between the pressures in the cylinder and the exhaust primary, the quicker the exhaust gasses will flow out. Gas speeds in the exhaust are in the region of 500mph, while a sound wave is also produced as the valve opens that travels at... the sound of speed of course (770mph). Now understanding the complex relationship between these two waves will give you a degree in clever bastardness, but it’s not critical for making your own manifold if you understand the next bit.
As we’ve already mentioned, all your manifold can do is minimise the loss of power, but the clever bit is to reduce to a minimum this loss at the most useful part of the rev range. You can’t have an exhaust manifold of fi xed dimensions that produces more power from 0-8000rpm. It’s only going to have sweet spots and where these sweet spots are depends on four things; primary pipe diameter, primary pipe length, the design of the collector(s) and the confi guration. By altering these parameters it’s possible to alter the position of peak power by as much as 10%. Now that might not sound like a huge gain but it can be the diff erence between needing an extra top gear or having to change gear mid corner. Ask the top Formula 1 teams – who regularly change manifolds for specifi c circuits – how useful that can be.
The four parameters.......................
The first consideration is what type of manifold does your engine require. If it has four or eight cylinders you have a choice of either a conventional 4-1 or 4-2-1. If you’ve decided on a conventional 4-1 design this will give you the best power available at the top end of the rev range whereas a 4-2-1 design will achieve better mid range power with a little less top end power. Some of the more modern, highly strung motors (especially bike engines like the Hayabusa) will only give any meaningful power gains when using a 4-2-1 design. It depends on the engine and how you’re going to use it.
The parameter with the biggest effect is primary pipe diameter. The thing to remember here is the smaller the primary diameter, the faster the gases travel through it: the larger the primary diameter, the slower the gases travel through it. If the primary’s too small the engine will develop its power very low in the rev range, sometimes at the expense of top end power. If the primaries are too large, the result is all the power at the top of the rev range, sometimes at the expense of bottom end power.
Clearly, the primaries are very important and choosing the right size also depends on how you actually use your car (eg: racing or road).
When it comes to choosing the ideal length for your primary pipes, it’s a similar case as with the diameter, short primaries tend to provide power at the top of the rev range, whereas a long primary will do the opposite. Having said all that, the ideal primary diameter and length is in most cases unachievable. For example, a Jag XK engine develops its best bhp fi gure at 6250rpm with a primary length of 500mm, but grab the next gear and the engine drops 2500rpm and would need a primary length of 700mm to be in the power band. So there’s no true ideal length primary pipe.
The next parameter we can alter with good effect is the one everyone talks about and that’s equal length primaries. In truth these can vary by 5% without altering things much, and in reality probably do vary by 5% in many ‘equal’ length systems as measuring pipes accurately with doubles curves isn’t easy (the best way is to fill with fluid and measure capacity).
The final, often neglected, parameter is the collector (whether it’s a 4:2:1 or 4:1 system).
A well made one will maintain high gas speeds and assist in scavenging and actually pull the gases through the engine. The emphasis here is on maintaining velocity so larger, lower revving engines require a shallower merge angle and smaller higher revving engines require a steeper merge angle (F1 engines have extremely steep merge angles). For most applications the merge angle varies between 12-20˚.