A
Anonymous
Guest
Generally speaking this is true, right? If so, has anyone ever figured out the optimal runner length (talking custom intake manifold)?
8) that depends on the rpm range you are talking about, longer runners allow for more LOW rpm torque, shorter runners move the torque peak up the rpm scale. the amount of torque an engine produces though is dependant on many things, like compression ratio, cam lift/duration, port size, shape, length, valve size, stroke length, bore size, etc, etc. if you build two identical engines, the one with the longer intake runners will usually have a lower rpm torque peak than the shorter runner engine.
twentyover
Well-known member
There's a number of web sites out there addressing intake runner length. I'm looking at 17 inches on my application (not a smallsix), and IIRC the third resonance occurs about 4700 rpm, so I would expect a torque increase at this rpm. Progressively smaller increases are avilable at the 4th, 5th, etc resonances.
There's a bunch of 4ag Toyota guys down in OZ building steel tubing intakes who've published a spreasheet on runner length, Plenum length, and rampipe size.
If you're really interested in numbers, go to the megasquirt forum and ask about tunned runner length. Most probably get a response there.
There's a bunch of 4ag Toyota guys down in OZ building steel tubing intakes who've published a spreasheet on runner length, Plenum length, and rampipe size.
If you're really interested in numbers, go to the megasquirt forum and ask about tunned runner length. Most probably get a response there.
There's an optimal length and diameter to increase torque at a particular rpm. above or below that length the power starts to diminish at that rpm.
It would seem that you would want to optimize that length at the cam's torque peak, but when doing the tuning, you may want to bump the curve somewhere else.
It's another game of diminishing returns. Try sucking a drink thru a regular soda straw. No problem. Now try sucking a drink thru a straw thats a lot longer....it's a lot harder because of the increased mass and drag that you encounter. Same happens on runners too long for your application.
It would seem that you would want to optimize that length at the cam's torque peak, but when doing the tuning, you may want to bump the curve somewhere else.
It's another game of diminishing returns. Try sucking a drink thru a regular soda straw. No problem. Now try sucking a drink thru a straw thats a lot longer....it's a lot harder because of the increased mass and drag that you encounter. Same happens on runners too long for your application.
Air, air and fuel, no matter. It has mass, and if the volume of each intake pulse is right for the camshaft and characteristics, then it will work with any thing from a foot to half a foot, with an inlet runner diameter equal to the area of each port at the gasket face.
Most limits are package based.
Two important things.
1) Longer runners generally help mid range torque, but only if your fuel delivery system has isloated runners (port on port or so called IR systems).
2) Secondly, the runner volume must be a certain % of the engine volume to provide inertail ramming. Air gets compressed, and volumetric efficiency goes up. It mildly supercharges the incomming air, and this increases the amount of air fuel mix in the combustion chamber.
The 'inertial weight' of each slug of fuel air mix is based on its volume and it's ability to accelerate. It compresses as it banks up at the intake valve.
On little A-series engines, seven inch intake runner works great with a 45DCOE carb. On a siamese 2 intake port engine feeding four cylinders, it got 1.4" ports. Intake runner volume is 21 cubic inches, and it was 25% of the total engine capacity
The 413 Max Wedge had twin 4-bbls, and 13 inches of intake runner, and it had dynamic mid range torque with eight runners about 1.8" in diameter, about 60% of the total volume of the engine.
The old X-flow 250 EFI engines got small 1.4" ruuners, 12 inches long, for brillaint low end torque, about 150 cubes of inlet runner for 60%.
Eg GT40 or Aussie 5.6 Windsor 250 engines had very short intake runners, and large volume. The stock 5.0 HO lost power above 320 hp, and even the GT40 couldn't flow 400 hp with ease. The 250 Windsor intkae plenumb had short stub stacks, and was based on the later Boss 260 and 290 intakes, and can flow well over 430 hp with ease. Very little length, and very large stub stacks that look like old GT40 289 Weber carb manifolds.
Most limits are package based.
Two important things.
1) Longer runners generally help mid range torque, but only if your fuel delivery system has isloated runners (port on port or so called IR systems).
2) Secondly, the runner volume must be a certain % of the engine volume to provide inertail ramming. Air gets compressed, and volumetric efficiency goes up. It mildly supercharges the incomming air, and this increases the amount of air fuel mix in the combustion chamber.
The 'inertial weight' of each slug of fuel air mix is based on its volume and it's ability to accelerate. It compresses as it banks up at the intake valve.
On little A-series engines, seven inch intake runner works great with a 45DCOE carb. On a siamese 2 intake port engine feeding four cylinders, it got 1.4" ports. Intake runner volume is 21 cubic inches, and it was 25% of the total engine capacity
The 413 Max Wedge had twin 4-bbls, and 13 inches of intake runner, and it had dynamic mid range torque with eight runners about 1.8" in diameter, about 60% of the total volume of the engine.
The old X-flow 250 EFI engines got small 1.4" ruuners, 12 inches long, for brillaint low end torque, about 150 cubes of inlet runner for 60%.
Eg GT40 or Aussie 5.6 Windsor 250 engines had very short intake runners, and large volume. The stock 5.0 HO lost power above 320 hp, and even the GT40 couldn't flow 400 hp with ease. The 250 Windsor intkae plenumb had short stub stacks, and was based on the later Boss 260 and 290 intakes, and can flow well over 430 hp with ease. Very little length, and very large stub stacks that look like old GT40 289 Weber carb manifolds.
FastRonald
Well-known member
8) I am still planning the removal of the cast intake on a 200 head. My plan is to have three short and three longer intake runner on alternating cylinders. Why? to see if it will balance the power curve smoother.
I like the engines that have variable runner length. The Audi 4.2L V-8 has variably length runners. At low end it opens all the louvres (or what ever seperates the sections) to get more torque and at high end it uses a shorter run for HP. Pretty cool (though more things to break).
Slade
Slade
It might balance the torque curve, but more likely what will happen is you will get two "bumps" in the curve instead.
And remember that too long is worse than too short in most cases.
And remember that too long is worse than too short in most cases.
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