Long runners vs. short runners....

[Vtec 4 Life]

A friend of mine told me that engines equipped with short runners on the intake manifold produce high end horsepower, where as long runners provide low end torque. Can anyone help clear this up for me by explaining to me how this works. Any help will be appreciated. Thanks in advance.

[curtis73]

Sure... your friend is right. It has to do with the mass and velocity of airflow in the runners. Every engine parameter has an RPM where it works best which is why its important to match them together. As RPMs increase, the velocity of airflow will increase in the runners. Once it reaches a certain velocity (actually around 300 mph) it will start becoming restrictive. So, as the velocity increases, the more inertia it carries with it. Once the piston reaches the bottom, the column of air will use its inertia to cram a little more air in there before the valve closes.

A long skinny runner will hit that peak velocity early in the powerband and use its inertia to cram extra air in the cylinder down low in the RPMs where torque is favored. Above those RPMs it starts choking off flow and restricting high RPM power. A big short runner will reach that peak velocity later in the RPM band, meaning you have to rev the engine higher to get it to get any inertia behind the column of air. This also means that short runners can actually remove some low end torque from the combo because there is very little velocity in the incoming air.

There are other factors involved but they're mild. When the intake valve closes, it sends a pressure wave back up the intake tract much like when you slap your hand over a tube. If you tune things carefully, that sound wave will be headed back to the intake valve at the same time its ready to re-open. Long intake runners (like Tubas) develop those harmonics lower in the RPM range. Short intake runners (like flutes) develop those harmonics higher in the RPM range.

[2.2 Straight six]

so what about cars with forced induction, which favours them best?

since they don't rely so much on intertia, and have the turbo/supercharger to force air in, is there so much of a difference? i'm guessing this will also relate to turbo boost pressure etc..

[UncleBob]

Originally Posted by 2.2 Straight six

so what about cars with forced induction, which favours them best?

since they don't rely so much on intertia, and have the turbo/supercharger to force air in, is there so much of a difference? i'm guessing this will also relate to turbo boost pressure etc..

off boost the same applies as a N/A engine

On boost, its all out the window. Doesn't really matter.

[2.2 Straight six]

Originally Posted by UncleBob

off boost the same applies as a N/A engine

On boost, its all out the window. Doesn't really matter.

i see.

Thanks,
Moose.

[slideways...]

on boost, generally the best thing you can do is lower backpressure. theoretically, more volume in the exhaust manifold slows turbo spool just like more volume between the turbo and the throttle body. but more volume = less backpressure(with an ideal design). so just like in a n/a motor, basically its all about balance. fortunately, most header companies have already done the research for you. but the ebay crap headers wont. so dont get those. taking these things in mind you can tune your car to run like you want.

[Vtec 4 Life]

I getting the grasp of things now. I'm curious on one thing though. I now understand that the long runners design will help yield more low end torque. However, taking the Z32 300zx n/a with the vg30de motor, how is it that not only does it have some good low end torque, but if you look at the specs is produces some decent high end horsepower?

[curtis73]

Its all a trade off. They probably selected a runner length and diameter that supported it somewhere in the middle so that it provided the least penalty possible at both ends of the spectrum. If you lengthened the runners you'd probably see a small boost in low RPM torque and a loss of high RPM power.

The other reason is that its EFI. On a carbureted car, the intake carries air and fuel which has much more mass. On a port EFI car, the intake carries only air, so the runner length has slightly less tuning benefit than on a carbureted car. It still affects it but not quite as much.

A good example is my chevy LT1. Its cammed for pretty low RPM, but the intake runners are very short. The engine made massive amounts of torque, so the designers didn't mind giving up a little, and the fact that its dry flow means it was less important than the packaging constraints of the engine bay.

[Vtec 4 Life]

Ok I get it. So basically Nissan did their homework when designing. I just noticed that the vg30de motor also has variable cam timing (I think they have it as nvcs or something like that). Could they be a big contributing factor?

[UncleBob]

Originally Posted by slideways...

on boost, generally the best thing you can do is lower backpressure. theoretically, more volume in the exhaust manifold slows turbo spool just like more volume between the turbo and the throttle body. but more volume = less backpressure(with an ideal design). so just like in a n/a motor, basically its all about balance. fortunately, most header companies have already done the research for you. but the ebay crap headers wont. so dont get those. taking these things in mind you can tune your car to run like you want.

the problem with the ebay headers isn't the flow. They actually flow very well.

You should stick to direct injection pontification

[slideways...]

Originally Posted by UncleBob

the problem with the ebay headers isn't the flow. They actually flow very well.

You should stick to direct injection pontification

my point was that the companies that make em dont do research into flow characteristics and such, like most name brands do. they just copy and produce. your right though the problem with ebay headers is they are junk and crack/warp.

[UncleBob]

Originally Posted by slideways...

my point was that the companies that make em dont do research into flow characteristics and such, like most name brands do. they just copy and produce. your right though the problem with ebay headers is they are junk and crack/warp.

They copy and produce. That is true. Thats why they flow well.

The only thing wrong with them is a) poor welding and b) material composition.

Unlikely to warp. Could crack. Some parts will rust.

FITTING on the other hand, is another issue.

[curtis73]

Originally Posted by Vtec 4 Life

Ok I get it. So basically Nissan did their homework when designing. I just noticed that the vg30de motor also has variable cam timing (I think they have it as nvcs or something like that). Could they be a big contributing factor?

Yes... I forgot about the variable cam timing. That is a huge factor. The most important cam timing event in this discussion is intake valve closing. Since they have controlled the IVC event, they can broaden the torque curve significantly.

[Vtec 4 Life]

Oh ok, I knew it had to play a big role. Does the VG30DE motor also come with a variable intake induction as well? I read somewhere that it did, so please let me know if this is true. Thanks.