4 replies to this topic

[hhhh]

I was reading Vizard last night about manifold port runner diameters and his recommendation was for up to 1-1/4" 'at and near to the cylinder head face' for 850s to 1100s and 1-3/8" or larger for engines up to hot 1400s. So I measured my 1-3/4" SU no-name water heated alloy manifold and found the runners to constrict down to about 1.1" at their narrowest which seems to be a little restrictive for a modified 1275 engine. Comparing to a Weber DCOE manifold or likely even twin 1-1/2" SU manifolds, it's a whole lot smaller.

 

Is this typical? Wouldn't it be of great advantage to open this out to minimum 1-1/4" all the way through? Otherwise I can't see how a 1-3/4" carb can have any advantage over a 1-1/2" on this manifold.

Edited by hhhh, 23 December 2017 - 05:16 PM.

[Turbo Phil]

If it's only 1.1" diameter I'd definately open it up. A lot of manifolds were made so suit smallbore motors, or were simply done in a smaller size with enough material available to open them up to the required size.

Phil.

[hhhh]

Thanks Phil, that was my inclination. Now I'm wondering if some venturi effect is desirable or whether to take it right out to 1.375"? I think I'm going to pull the one on my 998 to see if it's bigger already to save me some trouble. I never liked the idea of the same manifold being used for both 1.5" and 1.75" carbs since it's a 36% increase in cross-sectional area.

[Earwax]

good spotting.

 

from recall, maniflow do two different IDs for their twin 1.5s.  and i think general consensus was stick to the smaller opening(36mm vs 38) unless in full on race mode.( but i think their metal vs cast versions dont have pipe narrowing)

 

from my limited guesses, i would be keeping a venturi and roughness to aid speed of entry and fuel air mix, but no bench testing or hard evidence to back this up, but remember, it is easier to take out a little more meat in the second take than try and put some back from going too big.  Also be prepared to adjust mixtures to get the optimal out of the new set up..... I think you will be pleasantly surprised... maybe take some before and after acceleration times to assist prove the seat of the pants feel..... Next thing you know you will be modding the bottom bellmouth of the SU..... again a worthy addition , but in my case kind of wreaked havoc with low idle and probable emissions ( but i don't have MOT to worry about)

[Turbo Phil]

I would make the Venturi/choke point in the cylinder head just before the valve seat. The manifold acting more like a plenum.

Phil.