Siamese ports vs non siamese ports

[Mettler]

I've started a thread about this because I do not want to hijack the other gentleman's thread.

Here's my (somewhat informed) opinion regarding the issue.

Firstly, the airflow is split in both heads before going to each intake valve, it just splits earlier in the siamese head. The effect of this I don't know, but I'd wager it's not adverse in either scenario.

A smaller diameter intake will make the same volume of air flow through it at a higher velocity than through a larger diameter, up until the volume of air exceeds that which the port is capable of flowing.

This means the siamesed intake ports on the VH45 will allow it to have slightly faster intake air speed at lower RPMs, greatly benefitting low end efficiency, torque and throttle response... whereas the VH41 heads will allow for a higher maximum airflow volume, better for higher RPM.

This, coupled with the longer stroke of the VH45 makes it the ideal setup for one of these engines, whereas the slightly revvier VH41 happens to have a more ideal setup for its own operational powerband.

If you're wanting to pump out 600hp+ from a VH motor, I would tend to lean towards the higher flowing head myself, though with forced induction there would be no major noticeable difference unless you were boosting more than 20psi or something !

Now lets look at another Nissan engine that has a similar thing...

R31 RB20DET (first generation of these motors), with NICS intakeR32 RB20DET (second gen), with ECCS intake

The NICS setup has two separate intake runners going to each cylinder, whereas the ECCS has one larger one.

The NICS engine demonstrates exactly the characteristics I discussed, with the better low down response & torque, but it runs out of puff at around 5500-6000RPM. Granted it's got a tiny little turbo, but this same effect is still noticeable when compared to the ECCS engine when both are running the same size bigger turbo.

The ECCS engine on the other hand, is commonly modded to rev out to 8000RPM+, because the higher flowing manifold & heads allow for it.

In short, there won't be that much difference on a VH engine, they're both pretty damn awesome, but I'd be inclined to think the best possible setup would be a Y32 VH41DE engine with the VH45 crank & rods to give it the extra 400CC displacement, granting it good head flow & metal chain guides.

But that's just me Thoughts ?

[sijoko]

Don't forget that the 94-96 VH45DE have the oval ports and also the metal chain guides.

[mtcookson]

Some Q guys on here mentioned to me in some older threads that the dual port VH has a sort of dual power peak, which you can definietly feel when driving the Q... and it feels awesome.

If you have more low end power with slightly less top end power, I think for most setups the dual port VH would be much more fun. It sucks having to run the hell out of an engine up high to get anywhere.

My MR2 for instance... I have to downshift to pass anyone whereas my Maxima with a lot of low end torque, I can keep it in 5th and pass people like its nothing.

If going for a mostly street driven vehicle, I would go for the low end power any day. If going mostly race, then I'd stick for the high end power.

[elwesso]

To be honest, there isnt that much of a difference.. The dual peaks are kinda cool, but on the 94 VH it feels about the same... Just onepeak in the middle instead of 2.

[mtcookson]

Happen to know if there are any dyno charts comparing the two on Nico?

[Ezekial]

dual for NA where air speed is importantsingle for turbocharged

who cares really ... its a 4.5 litre with 32 valves, DOHC and VVT ... its going to make power no matter what

[sijoko]

The non-siamesed ports would ultimately make the most power, but it would be at very high levels of airflow where the siamesed ports would start become restrictive.

If the engine is turbocharged, then the difference between the two would be even less due to the pressurized intake.

I think the main reason that Nissan changed the port design was for better emissions.

[mtcookson]

I think the main reason that Nissan changed the port design was for better emissions.

That's what I thought as well.

[defrag010]

I've had alot of time on the flowbench with alot of heads with similar ports, and like someone said the non-siamese will take advantage at higher airflow levels at high lift because of the larger volume of the runner.The individual siamese port VH45 head is VERY similar to a cosworth vega head - the farther away each valve's airpath is split away from the valve the less turbulence it will have.

Solution = take a siamesed head and bore the intake runners and accompanying lower manifold runners out to equal runner volume of the non-siamese head and you will be able to get the high-airflow/high-lift flow numbers up to the same

[Q45tech]

The 18" * runner volume [34.25 ci] is equal to the cylinder volume on a 90-96 Q, the 2% loss/gain between the two styles.

Notice that the 2 ports in the siamease system are not perfect symetrical....the valve diameter 1.5" and the cam lift 0.390 makes for a unique curtain ratio.......too large for low rpms

*As measured from valve lift midpoint thru heads to the plenum

[defrag010]

the siamese ports are symetrical ? They are perfectly round and straight and even with each other.

picture

[Q45tech]

Measure them to the thousanths of an inch, overlay the perfect intake gasket and look from the intake runner side, measure the lower intake runner ovals vs the gasket.

Looks good but measures differently will fool you.

Measure the runner floor anti-reversion ramp. All it takes is a few thousanths to create the desire effect to spread the peaks out.

[klh6686]

I hate to be that guy that brings a thread back from the grave, but I'm very interested in this topic and would love to hear some more input. Actual facts would be great and not just i heard or w/e. I have both heads and am trying to figure out which ones I want to run.

Here's the two gaskets overlayed:

[hurrication]

Mettler or Defrag010, do either of you have flow and velocity numbers for VH heads?