What is the frequency Kenneth ? A rethink on the LCB theme part1

I have written a bit about my effort in simulating a 5 porter and a reverse 7 porter ( with proper exhausts and shared intakes)  and the effects of pipes on the power delivery.

The 5 porter has a lot of problems, not the least of which are the shared exhaust ports. I know that there has has been a lot talk about charge robbing and such, but the exhaust has a much larger impact on mass flow than the intake, however well tuned it may be.

Wave Tuning is frequency matching and if you look at the sketch below things should become slightly clearer.

Note that the pipe coming from 2&3 aka Centre Branch is a secondary where as the pipes from numbers 1 and 4 are primary pipes.

The centre Branch has twice the amount of pulses as the nr 1 and 4 branch. Not exactly rocket science but still IMHO a salient point.  The LCB has a LONG centre branch and it works quite well. Why does it work ? Well that is a bit of a mystery. Well we all sort of know that it works quite well, and that the values in the big yellow book are pretty hard to improve upon, even though they are by now 40 plus years old. But why does adding that bit of pipe work ?

If you look at just the centre branch and forget about the other cylinders what would be the sensible length compared to the outer branches ? The two outer branches are single cylinder engines ( forget about the intakes for now because that will make your head hurt) and the middle pair is either a twin with primaries that are tuned to 5.000.000 rpm ( at 12mm long) or it is an angry single cylinder running at twice the speed of the two outer singles.

If you look at it like that the CB of the LCB should if anything be shorter, not longer as it has twice the pulses !? So while to overall power might be helped with going LCB, the inner cylinders are paying for it by not  having an ‘optimal’ (hey it is an A-series) tuned pipe at higher rpm. The simulations strongly suggest that the inner cylinders are indeed not pulling their weight at higher rpm’s.

Before someone comments on the rpm’s and usable power. Yes you are right, normal road engine speeds are from 1000 to say 4500 99% of the time and maybe 4500 to 6500 for 1% if you are a very sporty driver. To be honest if you want a fast road engine there are plenty of proven ways to do that. It is basically a 240 to 270 cam (sw5/sw7/RE13/piper255HR or what Graham Russell has come up with lately)

Add a decent head and LCB and a hif 44 or twin Hs4’s and you’re set, the hard part is not to build out 15 bhp at assembly . It has all been done, but lets face it you are not going to break any records power wise (without a turbo) when you do not rev past 6000. For circuit racers in a mini or a sprite/midget, especially those racing on long North American circuits, the rev counters will only sporadically go below 4000 , and that is probably a when you bodged an up-shift. So for this use the equalising of power between cylinders is getting quite useful. Now, how could one try and achieve this ?

Provide each cylinder with what it likes. There we already run into a few problems. What it really likes is a intake and exhaust port all to its self, but that is not going to happen unless you use an X flow head . The intake situation is basically what it is and adding a lot of manifold length seems to work well there.

On the exhaust side, the ends have all the luck, and at least the centre cylinders have two mirrored cylinders sharing the same exhaust port quite far apart pulse wise. As a consequence it is probably ok to treat the centre branch as a single cylinder but at double the rpm. Hence the length of the centre branch will be not longer but shorter as the frequency will be higher.

In the next part the effect of this idea will be illustrated by simulated data.

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