mazdaspeed7

There you go.

j9fd3s

you know they used like miata headlight motors on the 787b intake trumpets right? it would actually be more fun to use a ricola horn



mike

White_FC

mazdaspeed7, just wondering why you choose to do the bridge on the aux runner as opposed to the normal secondary one? was there any reason for this?

88IntegraLS

The objective of an aux-bridge ported engine is to have higher low-end and midrange torque than a half or full bridge ported engine, while having more top end power than a street port.



I concur with Mazdaspeed7 that a port of large duration needs a longer runner for a given RPM than a short duration port. A peripheral port is not an bridge ported sixth port, let's be clear about that. Just because the R26B has tuned intake runner lengths for X rpm does not mean that an aux-bridge ported engine will need an identical length to tune for a given rpm.



As the port opens it creates a negative pressure wave which acts like a rubber band - the open port pulls on the air in the runner which 'stretches' for an instant before it gets moving into the port. After it begins moving and flowing into the port, it will have built up kinetic energy in the direction of the port as the 'rubber band' of air contracts in reaction to being stretched at first. This contraction will happen at a given time relative to runner length. Since an aux bridge probably opens earlier than a PP, it begins pulling on the air in the runner earlier than a PP would. And since it probably closes later, it will also need the contraction of this stretched air mass to be complete at a later time than the PP, hence the longer runner for the same RPM.



One thing which I have not seen mentioned yet is that good runner design features a tube which gradually gets smaller in cross section as it approaches the port. For all the stock S4 manifold's shortcomings, it actually does come equipped with this design trait.

White_FC

There is no way an aux bridge motor would open earlier than a PP, and I seriously doubt it would close later than a PP.... maybe if It was a full on J port that cuts WAY into the water jacket it might. but definatly not just an aux bridge port.



I understand a PP isn't a bridgeport, but I fail to see how this would change the intake wave THAT much...



I'd go so far as to say that the port timing of the r26b quad rotor would have an even longer duration than an auxillary bridge port motor. All of this doesn't change the fact that to tune your manifold for better performance the higher the RPM you want to make max power at the shorter the manifold has to be, assuming the same port timming of course.

I'd love to see some real figures of how much the port timming affects the tunned length of a manifold, would help me greatly when it comes to making my new manifold in the next few weeks

88IntegraLS

Well good luck then. A few of us have tried to explain the reasoning behind the principles which we adhere to when modifying our engines, but that wasn't enough for you. You will be the one to produce the figures then. I do recall, however, that on the graph supplied with the R26B engineering paper, that power peaked and then dropped off as a function of runner length for a given rpm. But who cares. Go ahead and do your thing.

j9fd3s

well the runner length also depends on what you're aiming for. on the 787b they are going for a broad power curve and max fuel economy, NOT peak power. ive seen with my own eyes the 787 starts long at idle and gets short at high rpms, you may find however that you like the power/tq a long runner gives you, its also possible that a more radical pp motor wants a shorter intake vs a side port. they do on the exhaust side



mike

mazdaspeed7

Didnt I say that before?

White_FC

Far out guys, all I was asking for was some proof of a relationship between inlet duration and runner length. Since your all well versed in this subject I was just (wrongly) assuming that you guys had some relationship worked out that you can go off and tell me for an engine with X duration you should theoretically have a runner of Y length.



88IntegraLS, of course the power curve peaked and then dropped off on the 787b, thats why they made the variable intake, so they could have this peak continue over about 4K RPM or so..... All engines have this HP peak, I can't see what this has to do with the price of eggs in china.



Either way, my inlet track will be about 5inches long, but I would still love to find out how much longer/shorter it should be if I change the port timing, that'd be great.

mazdaspeed7

Tell me what the port duration is, and what rpm you want it tuned for, and Ill tell you the optimum runner length. And since I already have the port duration, tell me what rpm you want each of the port pairs tuned for(primary, secondary, and auxillary), and Ill tell you those 3 lengths.