Why Do Long Runners Make More Torque at Low RPM?
This is not a scientific article, these are my thoughts on why short runners lose less power at the top end and why long runners make more power down low.
This article is not going to cover pressure waves, if you want to read about pressures waves here is a forum post.
Layman’s Explanation – Why Short Runners Make Torque Low Down
The runner is presenting the air in a very digestible packet. The shape of the runner will be close to the shape of the port in the intake manifold. The lower the RPM, the less turbulence in the runner because we are not forcing/sucking the air from a large shape (plenum) into a much smaller runner.
Runner Capacity.
As RPM increases we are taking air from the runner faster and we are taking more of it. The resistance/restriction at the interface between the plenum and the runner starts to build.
This might give the idea that the bigger the runners the better, under all conditions, but we know this is not true. The higher the RPM the shorter the runner should be.
What is going on?
The interface between the plenum and the runner is not the only factor. As we are demanding more air through our runner, the speed of the air is increasing. As the speed increases the friction between the runner wall and the air entering the engine increases (exponentially?) causing less efficient cylinder filling. This is at least part of the reason long runners become less ideal at high RPM.
The solution for high RPM is to reduce the resistance on the runner walls and the most obvious way to do this, is to reduce the surface area of the runner, ie make it shorter.
Optimising the Compromise
To recap where we are. Long runners work at low RPM because the resistance at their entrance is less and the shape of the air entering the engine is optimal. Long runners become less than ideal at high RPM because of friction between the runner wall. Having said that, runners can get pretty long before they start giving up torque to short runners.
Tapered runners seem like they could be a good solution. They give the runner bigger volume, reducing speed and hence resistance on the walls. And the transition from the plenum to the runner is less of a resistance factor because the air speed differential is less.
I think the ideal inlet runner will be straight and of maximum length until we see torque start to drop off. And at this point we introduce a taper into the runner to reduce the wall/air resistance and the resistance between the plenum and the runner.
I don’t think there is a hard rule, parallel runners and better than tapered for example, the best could be a combination of both.
(Under all circumstances having a bellmouth on the runners is a definite plus.
Short Runners vs Long Runners – Comparison
A dyno comparison. Normal runners vs medium and short. More comparisons here
Phrasing Things Differently
Is it correct to say longer runners make more torque at low RPM? Or is it more accurate to say that the short runners/velocity stacks are not as bad as long runners at high RPM?
Is this a distinction without a difference? Possibly, but I think it could help us understand the problem more clearly.
If it’s a case of the prefect world being long runners with no wall friction, then reducing friction on the walls should be the goal for power everywhere in the rev range. And if this is the correct route, increasing the taper as flow increases may be the best compromise.
