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Feb 11, 2021 20:26:44 GMT |
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is it true that is it bad to have the exhaust port the same size as the manifold? Ive heard all different stories. Ive heard you need a step to the manifold, but dosnt that slow the speed of the flow down?
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Feb 11, 2021 20:55:08 GMT |
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Bearing in mind the manifold gasket is often used as the template for material removal when porting I'm not sure where you got that from.
Sure some gas flow guru will be along shortly.
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Needs a bigger hammer mate.......
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goldnrust
West Midlands
Minimalist
Posts: 1,623
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Feb 11, 2021 21:20:03 GMT |
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I believe the theory is that you should have a step up in diameter, so the manifold is larger than the exhaust port.
This essentially won't make any difference to flow out (as the gas goes from small to large). But if there is a returning pressure wave, from either another cylinders exhaust valve opening or from the tuned length of the tubes in the exhaust manifold, then as that pressure wave hits the step (this time from large to small) this creates a flow restriction and reduces the amount gas that would back flow into the cylinder and therefore reducing the amount of exhaust that could get back into the cylinder and mix with the fresh intake.
I believe this sort of issue would be more apparent with a long duration cam with large overlap.
That's what it says in the book I've got on tuning (https://www.amazon.co.uk/Four-stroke-Performance-Tuning-Graham-Bell/dp/0857331256). How much real world difference it makes with a comparatively standard engine... who knows!
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Last Edit: Feb 11, 2021 21:25:23 GMT by goldnrust
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xbl
Part of things
Posts: 863 
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Feb 12, 2021 13:49:54 GMT |
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The step up (only need a very slight one) has an effect on the turbulence at the crossover between head and manifold, the theory is the turbulence creates a swirl effect to draw the gases away from the valve train.
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Feb 12, 2021 17:22:33 GMT |
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right ok then, i have ported my head to match the gasket, then the cast manifold is ported to match.
The car is mapped on a rolling road, and when i drive the car i have found it dips in power between 3500 and 5000. I have no idea why it did that but from what ive been told recently the cast manifold could be pulsing wrong and causing problem pressures etc.
The engine is a volvo b230 it has a 300 deg camshaft, it is massive duration so this all makes sense now.
Looking at the bmw m44 manifold, that is tubular, not equal lengths, it is like 32mm primary too compared to the volvo would be 45mm. But the M44 one is by the looks just what they could get to fit. I also had a m44 that was 2000cc and did 180 bhp, i know the manifold must work.
The standard manifold for the s14 engine looks horrific imho, but that obviously worked too.
So how important is the equal length of the manifold? I'm really tight on space
thanks
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