optimum inlet length - cammed / itb gti6

[jammapic]

Hi all,

I am playing with inlet lengths on my GTi6, throttle bodies and cams. Myself and Wayne Schofield found that we've made a lot more power with longer trumpets and even more length using spacers behind the filter housing.

So far we're using the standard Longmans manifold, 50mm of spacers and 90mm trumpets (120mm don't fit inside the filter).

We made even more power by using 75mm spacers, but then the filter housing fouls the bonnet.

How do people get to optimum length inlet? I did purchase a sachell inlet manifold about 2 months ago but it hasn't turned up yet, I'm led to believe that points the bodies down a bit more.

Also, we've been trying to tune out the resonant frequencies in the torque curve by using two different lengths of trumpets on 2 banks of cylinders with some good results. Has anyone gone down this route before?

JP

[Rippthrough]

That's pretty common for 'bodies tbh, using slightly longer trumpets on the inner cylinders compared to the outer usually widens the torque curve a touch.
Until you start chasing more RPM's and wilder cams your inlet is just pretty much going to be as long as you can actually get under the bonnet.

[Sack]

Hi,

I tried to work tho out for my fiesta. The only online calculators I could find were for plenum inlets but i figured it would be a good base start point. Based in a 2000cc engine with a 295 inlet cam it worked out at 35 cm from valve to trumpet edge.

Out of curiosity what kind of gains did you see?

[miles]

The longer inlet sticks the bodies upside down which makes a huge difference in room, Trumpets make a huge difference too as you have found

[welshpug]

Trumpet shape makes a difference too, I remember sandy comparing a few different shapes on Colin's new engine and finding some extra peak but loosing a good chunk of midrange with the same length overall IIRC!

[Craigus]

I think that Longer inlet manifold should make more torque and shorter for more high end, dont know if its the same with throttle bodies though

[Sack]

That is the theory yes but in practice most of the time the 'short' inlet length is still longer than can fit in most cars.

[Kezzieboy]

Same with exhausts, too; short exhausts (manifolds, at least) for peak power, longer for a flatter torque curve.

[Rippthrough]

Sort of, but you go up between resonant waves so you can go longer and suddenly find a shitload of topend appears again. The manifold on the buggy is incredibly long - but it's still optimised for around 4-5krpm.

[jammapic]

Ripp, that's what I'm finding - you smooth out the resonance in certain places, and somewhere else gains or loses massively!

I've gone with cyls 2 and 3 running a slightly shorter setup than 1 and 4. This seems to neutralise most of the resonance whilst still keeping decent area under the curve.

JP

[jammapic]

OK, looks like we've nailed it... more information to come - but 220hp so far and totally smooth torque...

[jammapic]

OK, so backtrack a tad...

I talked about using staggered trumpets to move harmonics in the torque curve, and make for a smoother power band and more area under the curve.

But, they say the proof of the pudding is in the eating, so here it is.



Here, you can see two dyno plots overlaid. The one which makes peak power at slightly higher RPM is with the shorter trumpets, and the one with power at lower RPM are the longer trumpets.

You can see the shape of the curves are identical, but higher and lower RPM peak power. This is normal for longer and shorter trumpets. This is the key here.

When we have two of the trumpets long, and two short, we effectly find a "middle ground" in the curves. This is helpful as we fill the troughs with power, but we do also lose our peaks - hence the peak power is marginally lower.

What's important though, is area under the curve. Clearly, the more the better, and by filling the troughs we make the car smoother and more drivable.

Below is the outcome of the staggered trumpets:



We actually did find a little more power with some more ignition advance and leaning the mixture out a touch, but the shape of the graph remained largely the same.