TYPES OF EXHAUST MANIFOLDS

Graham Smart, Master Bugatti Manifold Engineer, explains the different types of exhaust manifolds to help you make the right choice for your vehicle. This information is not directed at Bugatti owners of course but more modern cars from the 60’s to the present day.

Follow this information to maximize the performance of your car’s engine. Obviously, the fuel and engine management will need to be adjusted accordingly.

The exhaust manifold is the most important part of the exhaust system and the two basic types of exhaust manifolds are the 4 into 1 and the 4 into 2 into 1.  

4 into 1 Exhaust Manifolds

This type of manifold works better at high revs 6,500-10,000rpm or even higher. If your maximum is 7,500-8,000rpm it is borderline as to whether this would be the best choice. It will depend on many things, the design of the engine, the cam, efficiency of the head flow etc.

If the 4 into 1 type was used 27″-30″ would be somewhere near. Diameter of tubes will depend on how good the head flow is and the valve area. With a two valve head as a rough guide inside diameter of tube is as big as the port size if not slightly larger 1/58″-1/78″ may well be correct, this is the O/D diameter.

4 into 2 into 1 Exhaust Manifolds

This type of manifold, known as an interference type, is where one set of tubes effects the others.

PRIMARY DIAMETER will follow same diameter as 4 into 1.

PRIMARY SECONDARY RATIO IN A ROAD CAR

Primaries would be short 12″-15″ long. Secondaries would be approximately half as long again 18″-24″ and as a guide, 2 sizes bigger in diameter to primary diameter.

PRIMARY SECONDARY RATIO IN A RACE CAR

This is the interesting part, where you can make 2 manifolds in one.  If for example your 4 into 1 primary length were to be 30″ your primary length for a 4-2-1 would be 20″ your secondary length would be 10″ add them together and you have 30″ the same length as a 4 into 1. 

By making 4-2-1 manifold and keeping all pipes parallel to each other at the point where the secondaries fit, you could also continue the four pipes for 10″ and you would have a 4 into 1!  4-2-1 for “torquey” race tracks, 4 into 1 for fast tracks.

THE RACE 4-2-1

(works like both 4-2-1 and 4 into 1)

When you design a manifold your target is a gas speed of 350-400ft per second. If you have that, say at 4000rpm, your gas speed at 8000rpm could be 600ft per second so, where do you set it?

Obviously the diameter of your tube controls the gas speed.  Too small it will be too fast to soon, too big and it will be too slow this is why it will be a bit of trial and error (sorry!) The Renault Williams F1 engine gives 800ft per second but that’s another story! So err on the larger size tube 1/34-2″ diameter.

This is how a 4-2-1 race manifold works like 4-2-1 & 4 into 1. If you manage to get your manifold working as you want torque wise, the gas speed at 5000-6000 will be 400ft per second approx. At this point it will work as an interference type (4-2-1). But as the revs raise towards  8000rpm the gas speed will be so great that it mistakes the secondary pipes as part of the primaries, so suddenly you have changed from a  4-2-1 to a 4 into 1 and gain that top end power, but also at lower revs you have the torque, so you can have your cake and eat it!

 

“CAT”

If you have to use one, you have to put it at a point where you still have a good flow of gas. I presume these cats will be approx 100 cells per inch (standard cats are 350-400cpi) Place too close and it will be too hot and it will collapse causing drop in power, too far and it wont get hot enough. If your total length of manifold is 30″, whether 4 into 1 or 4-2-1, place cat at approx  15″-30″ from end of manifold.

This length will depend on diameter of system, but length must be a division or sub division of the primary secondary length of manifold that you design. No more power can be gained after first box or cat (low pressure area)power can be lost in a system that is too small, but not too large.

As the gas travels down the system it starts to cool quite quickly so it does not need to be as big as the front, but all of us usually keep it the same, the main thing being that the gas is traveling slower if diameter is maintained, so there is more time to silence the gases.

 

SYSTEM

For a hot or hotish 2 litre 16v 200-230 bhp, I would recommend 2/12-2/34 diameter O/D with 50% open perf.  15″ x 6″ diameter sounds fine with 2 boxes or cat and rear box 15 x 5″ or 6″ diameter.

 

ANTI REVERSIONARY PIPES

 “Sorry Folks…we are awaiting the diagram…check back soon”

 

As exhaust gas slug is pushed out of exhaust port it causes a vacuum behind it helping to pull new air/fuel mixture into chamber but you do not want fresh air/fuel going out of the exhaust port. This is where the reflected pulse comes in.

If your lengths are approx. correct as it goes into large diameter system the pressure drops and reflect wave goes back up the pipe arriving at the head port just as neat air/fuel mixture arrives.

The reflective wave holds the new mixture at the port until valve closes. Sometimes old gas can enter the port (not wanted) the anti- reversionary lip causes old gas (reflect wave) to create small turbulence in the port stopping old gas from going into the chamber.

I would recommend a custom made exhaust manifold be made for your vehicle, since the box specifications will depend on your requirements as regard to use & noise limitations. I would be happy to discuss your requirements.

 

This is how to contact Graham Smart:

Phone: 0034 952 115 981 

Mobile 1: 0034 665 259 334 

Mobile 2 : 0034 625 279 286

bugattimanifolds@gmail.com