Variable intake systems

[James Bruce]

Hi guys,

I am currently running my intake geometry through Ricardo WAVE to produce power and torque graphs. My understanding was that changing the length of your intake runners changes where the peak torque of the engine is found on the engine speed range. When changing runner length in Ricardo, my peak torque is staying at 2500rpm. Any ideas as to why the peak torque won't change?

[Owen Thomas]

Originally posted by James Bruce:
Hi guys,

I am currently running my intake geometry through Ricardo WAVE to produce power and torque graphs. My understanding was that changing the length of your intake runners changes where the peak torque of the engine is found on the engine speed range. When changing runner length in Ricardo, my peak torque is staying at 2500rpm. Any ideas as to why the peak torque won't change?
You won't necessarily change the peak torque location by tuning runner length. By selecting a length that has a "resonant peak" at a certain RPM, you are maximizing the volumetric efficiency at that RPM. Thus, you will increase the torque at that location, which may still not be higher than the maximum. What you should definitely see is a change in the shape of your torque/power curve.

Also, intake tuning is very much a second order effect in comparison with things like port geometry. So depending on the engine (which one are you using?) it may be difficult or impossible to straight up move the peak.

[Racer-X]

I agree with everything Owen Thomas said.

To add to that you may want to double check your model. 2500rpm for a torque peak is super low in the land of 600cc engines, that's more like what I would expect from a big block V8 or a Briggs. You can also run some hand calculations with Helmholtz and Ram theories and see where they put your local peaks.

Remember that intake design will help with local maximums and minimums but may not help on the global max.

[Warpspeed]

There are several factors that influence the torque peak besides just tuned lengths.

The most important are optimum flow velocities in both induction and exhaust, which correspond to cylinder displacement versus induction and exhaust flow areas. Valve sizes come into this as well.

If flow areas are made way too large, flow velocities will be low, and so will the inertia and resonant energy you are trying to tune.
If flow areas are made too narrow, there will be increased pressure and flow losses.
So there will be some optimum engine speed where cylinder filling and emptying will be optimum, producing highest torque.

Changing lengths can effect this, but you cannot shift the natural torque peak very far by simply adding or reducing lengths.
Another thing you need to think about is valve timing. The valve events need to be suitably timed to get everything else working properly over the engine speed range you wish to operate in.

[James Bruce]

Thanks guys, I need to map my valve timings as per the actual engine (KTM 500 EXC single cylinder for those who asked). Changing the volume of my plenum does yield greater maximum torque and power values, it was just the location on the engine speed range that has stumped me.

Is there a way for the system to distinct between a diffuser, plenum and intake runner since the only different between the 3 is tapering and diameter?

Also is there a way to identify how the restriction on the engine effects the wave resonance? (I currently get first and second pulses on my graphs).

[Warpspeed]

Your best bet might be to get a copy of very inexpensive "Pipemax" software to analyze your engine. It is one more very useful tool to compliment what you already have.

This will supply a host of information about optimum flow areas, volumetric efficiency, valve sizes, tuned lengths and valve timing which operate optimally over a given rpm range.

As you have already discovered, tuned lengths can move the torque curve around somewhat, but not radically.
There are several other factors that must also be made suitable for the rpm range you desire.

Something is holding the engine back, it could be the cam or the cylinder head.
Pipemax can give you a pretty good indication of where the problem may be.

[fukaynaedfu]

Cams timings affect a lot of the power and torque peaks. Also i suggest running the experiment setup in wave wherein you can define lots of constants. If you analyze the output graphs, you can as decide upon what suits your requirements.
Also are you guys running with stock cams? I believe cam opening periods on the stock are not well suited for restricted engines