Triple Carb Manifold Short or Long??

[Gav240z]

Ok I got a set of Triple Mikuni 44's recently see my gallery if you like. They came with what I believe is a short manifold so the runners look a little more bent than the long runners of a long manifold.

I'm wondering what the opinion is on these manifolds and how the length affects performance. I would believe that the longer runners would increase the torque of the engine but the shorter runners look a little restrictive with the bend in them.

However having read a few articles on porting and air turbulence this might be the aim of the short manifold. To slow the air just enough and reduce the turbulence before entering the combustion chamber.

The manifold itself is a Mikuni item it actually has a Mikuni stamp on it.

So what's the verdict what do the experts think?

[rzola]

Longer runners give you peak torque a lower/mid RPMs. Short runners are for high RPM race engines. For a street car the longer set up may be better for drivability.......

[jmortensen]

If you see the two side by side the short manifold has quite a sharp curve and funelling effect to it compared to a longer manifold. Based on this I chose to use a longer manifold, and so far I have told several race engine builders about my choice and they've all said the longer manifold works better. The problem is that you won't have as much room for an air cleaner, velocity stacks have to be shorter, and the linkage has to be adapted to work. If you're not racing just use the manifold you've got.

[Steve Parmley]

Australians get more room for tripples and air cleaners with the brake booster and master cylinder on the other side of the engine bay.

Here is a website with some Mikuni info and where I buy parts for my 44's. http://www.wolfcreekracing.com/carbid.html

[HS30-H]

Gav,

Steve has touched on something that you might not have thought of. The shape of your Mikuni inlet manifold might have a lot more to do with packaging issues, rather than being dictated purely by efficiency........

Mikuni made some inlet manifolds that were expressly for the LHD market, and I believe that these placed the carbs higher in the vertical plain in order to give more clearance for a tubular exhaust manifold on an LHD car.

I've also been told that some inlet manifolds for the L-series engined Skylines displayed these traits, because the Skyline engine bay was narrower than that of the Z.

But I wouldn't worry about it if I were you.

[jmortensen]

Australians get more room for tripples and air cleaners with the brake booster and master cylinder on the other side of the engine bay.

Here is a website with some Mikuni info and where I buy parts for my 44's. http://www.wolfcreekracing.com/carbid.html

Actually with a Cannon manifold and Mikunis, my main issue was that the air cleaner just barely clears my strut tower bar clevis, which in my case is welded to the strut tower. Even if that weren't there, you still couldn't use a really long stack because there wouldn't be enough room between it and the strut tower to finagle a deeper air filter on there.

[Gav240z]

Well I did buy these from a guy in the US on ebay . So that might be part of the explanation. I've seen a few Canon manifolds around from time to time at a fairly cheap price. If I can get one cheap I might grab it just incase however as Alan said I don't think I'll stress too much over it for now.

[EricB]

random related question...

if you have a short manifold with long velocity stacks VS a long manifold with short velocity stacks such that the length from cylinder head to tip of velocity stack is the same on both setup...

does it make any difference in performance?

curious about that one...

what do you all think?

-e

[jmortensen]

That's a good one Eric. I don't really know an answer other than to say that the closer the carbs are to the head the sharper the bends and the more severe the taper from carb outlet to the port size has to be.

I've heard some really sketchy reasoning behind why having more air/fuel already mixed in the manifold was better, but they sounded so hokey to me at the time that I didn't commit them to memory.

[EricB]

I remember talking about something related with Alan a while back when I was comparing just how short the S20 manifolds were (crossflow head of course) in comparison with our regular cylinder heads... In some of the period pics the camera angle of the pics is such that you could almost believe the carbs are bolted straight to the cylinder head hahahahahaha.... just kidding.

-e

[jmortensen]

Just took a look at one of Alan's photos. Judging by the perfectly straight intake runners I'm guessing Nissan designed the S20 head for Mikunis (duh!). Unfortunately this is not the case with our L engines, but I suspect that on the S20 you could put a very large stack on the end of the carb, and that might potentially enable you to get more air through the carb and into the engine.

[HS30-H]

In some of the period pics the camera angle of the pics is such that you could almost believe the carbs are bolted straight to the cylinder head hahahahahaha.... just kidding.

-e

Yeah, the inlet is pretty compact on the S20, especially when you consider that it is full of coolant passages too. See attached pic of an end-on view for comparison. The point ( in relation to Gav's question about his Mikuni L-series manifold ) is that there is not 'step' down from carb to head - it is just a ( short ) horizontal plane.

.....but I suspect that on the S20 you could put a very large stack on the end of the carb, and that might potentially enable you to get more air through the carb and into the engine.

The stock airbox on the S20 ( and the remote filter - which was mounted in front of the radiator support panel ) was pretty big. The air box had integral airhorns for the stock 40PHH Mikunis, and these air horns were attached to the cover of the air box rather than its base plate. I think this was done to ease servicing; it would be a real struggle to get the air box cover over the stock airhorns.

Nevertheless, I have had to address this issue with my project car's engine - as I am using Weber 45DCOEs instead of Mikuni 40PHHs. I reckon I can get the airbox cover over the 45mm high full-radius rampipes I hope to use, and will therefore be able to use the stock filter box too. It will be a tight fit, but I think it can be done. I think longer rampipes would be better, but for sure they won't fit under the modified stock airbox.

The factory race cars were fitted with injection systems that used very long trumpets. The inlet manifolds for the ( sliding throttle ) injection system tipped the inlet UP at an angle - presumably for packaging reasons.