Is this manifold going to be on a race motor, where performance counts? If so, judging by your lack of knowledge, you're going to be better off either buying one or having someone else make it for you. Fabricated manifolds are extremely touchy and won't run for shit if you miss by the slightest amount, either in the plenum or runners. I'm not trying to be an ass, just some facts for you to digest.
Even if it's for a street motor that just has to sound good, your welds better be top notch, or it'll crack right away with the weight of the carburetor(s) and vibration from engine and driving.
I bet Andrew from Australia can back me up on this one.
i've been known to be wrong Sometimes:), but i must agree with mike. If you dont do it all right it might not work. Im sure if u got to a good machine shop they might be able to fab one up for u. Well just my 2 cents.
I too will agree with buying an aftermarket if you can't calculate and control the volume to match in each runner. If you get unequal runner sizes and velocities that thing will be junk and won't run. You'll never get the carb tuned if that thing isn't right. I've seen em' made of of aluminum plate, but Sonny's Automotive here in town built them and I trust his mountain motors for what they are known for, "HORSEPOWER".
I'm sure someone could suggest a type of material and filler for that application if you still want to give it a try.
John Wright
Mike is right, your best bet would be to buy one off the shelf if available. If not then you really should look at a fabricator with experience in manifold design particularly if it is for a normally aspirated engine. I have worked on inlet manifolds but only on cars with forced induction where flow rates aren't as critical and in manifold design I always defer to those more knowledgable than myself. I've seen some beautifully made drag race manifolds but I wouldn't even attempt trying to make them as I'm not an expert in that field.
If your engine is a 4 cylinder with carbs or injection hanging off the side then the inherent vibration problems will quickly show up design problems.
V8's are a better balanced engine generally so vibrations aren't so much a problem and generally carbs/injection are pushing down on the manifold rather than hanging off to the side.
I give you a quick example of just how little things make a big difference.
We do some work on a racing Dodge Viper V10 and the owner sent the throttle bodies away and had a lip removed from the front of the throttle bodies to effectively remove what they thought was an air restriction. They had them air flow tested before and after and they certainly gained extra air flow but when they put them back on, the car wouldn't idle properly no matter what they did. The lip they had removed was there to slow the air when the butterflies were closed to allow good idle properties.
Everything they did was good in theory and the air flow tests confirmed this but in practice it failed.
The best advice would be to seek out the experts and talk with them of your requirements.
Regards Andrew.
Andrew,
Good illustration, and informative reply.
John Wright
These new imports have all sorts of parts available these days. I'm sure what ever make he's working on has a performance manifold to fit his 4 cyl. Their getting great horsepower from those little motors and combined with the light weight vehicle, they really run. :)
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