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

________________________________________

HDI Engine Tuning / Remapping Services - https://www.facebook.com/dervtech

306 GTi-6 Track Car - 200+ hp all N/A!
306 DTurbo Track beasty - 209.9hp - RIP

Ok, will do. Should be worthwhile. It seems that adding around 50mm to the length moves a peak by half a phase. As such, I suspect that adding 50mm to 2 of the trumpets should effectively (or near as damnit) should bring up all the troughs and smooth out all the peaks. Yes, I'll lose power in some places, but should gain the same elsewhere giving a smoother power curve. We will have to see I guess!

We are running at 140mm from the end of the throttle body to the end of the trumpet, and that seems to give a great combination of midrange grunt and top end power. I'm still on the longmans manifold though, which arguably is a bit too short for a perfect setup.

I suspect with a longer manifold, and the similar spacer / trumpet length we may start to lose power, but at that point we can take out some spacers and tune there.

JP

________________________________________

HDI Engine Tuning / Remapping Services - https://www.facebook.com/dervtech

306 GTi-6 Track Car - 200+ hp all N/A!
306 DTurbo Track beasty - 209.9hp - RIP

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.

________________________________________

HDI Engine Tuning / Remapping Services - https://www.facebook.com/dervtech

306 GTi-6 Track Car - 200+ hp all N/A!
306 DTurbo Track beasty - 209.9hp - RIP