badcoupe

What size are the runners and which intake do you have. Thoughts about it. Thanks

gt37vgt

a little bigger than the port in the head and no longer than 6" .its that simple

badcoupe

Gotcha take it you've done a little flowing or development, you always seems rather keen mate! Thinkin of fabbing one this winter, I have a spare to cut up/measure up the flanges etc. I have done some mild porting on the head as well as a 5 angle job on the valves. Thanks again!

gt37vgt

i havn't flow tested many but the sceince behind it and every highpowered turbo engine around tells the story.
basicly long runners give some magical harmonic boost on NA engines in the upper RPM's . to tune the manifold for low rpm magical harmonics, the runners have to be stupid long so it is just not feasable not for a turbo engine .. cropping the runner lenth increases flow alot in a turbo engine bigger planum volume also helps ...
the most strait forward way to do it is to cut the underside out of the turbo manifold and box it flat on the bottom leaving just the 90deg bend ..it would look like the runners are still long but the first 6" would be bottomless plenum .
the only tricky thing is to make sure the Throttle body doesn't point strait at one runner and ram more are into it than others ..

67King

What are you trying to do? I've got a model I can use to play with some things to tell you how to do what you are trying. The approadh I've always taken has been to start wiith a target RPM for peak power, size the intake valve from there (if possible), size the ports from the valve size, choose target RPM for peak torque, and set the runner length according to that.

The inputs to the Helmholz equation include air charge temperature, compression ratio, runner length, and runner diameter - this gives you an RPM at which you'll tune. Note that there is nothing in there related to pressure. The reason many turbocharged cars don't use long runners is that it is easier to just up the boost. Peak torque at low RPM is also where one is most highly prone to detonation. Another reason is that density IS a factor in the Reynold's number, which is a factor in peak flow.

Note that the design of an intake manifold regarding runner length has NOTHING to do with flow numbers. Runner length is all about tuning, which has nothing to do with flow.

I'll be happy to run some quick numbers for you, if you have some sort of specific goal in line.

944obscene

Here's the real question... Where does one WANT peak power and peak torque? I would have no idea where to target peak torque, but I imagine that you'd want the two to correlate. Meaning torque peaking, just as power picks up and peaks at some point before redline, complimenting the design of the car and it's use. Seeing as I'm going to be street driving my car, and running it at road courses, I don't want my peak numbers too high in the rev-band.

If I were drag-racing, which I'm not, then I'd want all my power up top, for a good build after each shift. But instead, I want a strong mid-range and beefy torque curve, to help me when I come out of turns.

So say I'm targeting 350-400whp. I'd honestly be done with engine modding after I hit this target, for a long time... So Estimating a figure of 350whp, and roughly 400lbs of torque, on a 60-1... What runners and plenum volume would compliment a torque peak around 4-4.5k and a power peak of around 5.5-6k?

67King

Alan, it all depends. I did some design work for a guy who is racing a BMW and his current RPM band is 6,000-8,000. As he makes peak torque at 3,900 RPM, and peak power at 6,900, he knew there was room to improve. So, we moved things up a little bit. He hasn't finished the engine, but he should be able to shift at 8,500 now. Peak torque is still a little on the low side, a bit shy of 6,000 RPM. But if you can turn the RPM, you want to.

On edit - this was a naturally aspirated engine. For a turbocharged one, the ability to turn RPM should also consider the flow capability of the turbo.

Anyway, here's a quick shot at what an intake you might want would look like. There are two situations, one is without constraint, the other is with a 45mm valve size. I highlighted those dimensions in yellow. Note that length INCLUDES the portion of the runner inthe head. I had to make several assumptions, but without very complicated models, this is a good estimate. If anyone has a head apart and can take some measurements for me, that would help improve the precision.

951_RS

This is assuming your car will actually make power at higher rpm's. If it is a turbo application and the turbo just can not produce enough pressure at that high of rpm then you are just losing power at a higher rpm compared to shifting back into the turbo's appropriate efficiency range. Case in point is a vw 1.8t which can rev past it's redline by a good bit mechanically, but the stock turbo just will not allow you to make any usable power once past 6500 rpm as it can not maintain boost pressure. Rpm does equal more power assuming your car can maintain some semblance of torque, but if not, well, then increased rpm is just useless. Not saying you don't know this 67king, as I'm sure you do, but it's the internet after all. Full of $0.02 that nobody needs =D

As for the OP's question about intake manifolds, a larger plenum will reduce your air velocity but ultimately increase max flow rate, as will the cross section of your runners. With a bigger plenum and cross section your idle and throttle response may suffer, but obviously if you're modding the car this much, a near stock experience is not what you're after. A good model to strive for is a plenum near or even sometimes larger than the displacement of the motor, obviously space constraints will control this. Having said that, velocity stacks for the runners inside the plenum can help with keeping the plenum larger and the runners longer without taking up as much space. Not sure if you plan on doing all that though, obviously as it is more complex of a design.
So really I'm not sure what I've said has helped in any way, because it just comes down to where do you want to shift the powerband to, if at all. There's no exact length or plenum size that is magically delicious unfortunately, lol.

67King

Thank you, VERY good, and imortant point. As as the owner of a Mazdaspeed6, I assure you that I know full well what happens when a turbo is undersized! I didn't think about going into it, in the context of the question asked. But for anyone else reading it, not thinking about a 350 horsepower application, I should have made that clear. Think I'll go edit that, now.......

badcoupe

I was mainly after runner diameter so I'd know what materials to buy, but this has been a great discussion. Most of my experience has been on ITB midget engines with mechanical alcohol injection. But I've been toying with the notion of making one for the porsche for the hell of it. I've got a few ideas just wondering whats already out there. I have a nice of velocity stack to put on the ends of the runners in the plenum already.

gt37vgt

having them flow the same is far more inportant than the size of them and is easy to do as long as you keep it simple . all the rpm harmonics runner lenth stuff is redundant on boost and you cant get it to work with a runner shorter than 13" below 3500 rpm