Any thoughts in minimizing boost lost in cam switch-over?

[Monster Hui]

I'm not sure if anyone has address this question before.. but i just came back from a quick dyno tune session with the Trust S/C on my car, one of the first thing the guys noticed was the boost drop at the vvtl-i engagement point (i'll be posting the dyno graph with the boost curve later). So the manifold is losing the build-up boost (stock pully, 7.5psi) back down to 5.5psi at ~5900rpm. And that's WOT... think there's a way to minimize the drop?.. guess our stock cams are not really meant for force induction :p

[deltaB]

Isn't that actually GOOD? The air is making it into the engine instead of being hung up in the intake manifold.
dB

[Gravel]

I'm not sure if anyone has address this question before.. but i just came back from a quick dyno tune session with the Trust S/C on my car, one of the first thing the guys noticed was the boost drop at the vvtl-i engagement point (i'll be posting the dyno graph with the boost curve later). So the manifold is losing the build-up boost (stock pully, 7.5psi) back down to 5.5psi at ~5900rpm. And that's WOT... think there's a way to minimize the drop?.. guess our stock cams are not really meant for force induction :p

The problem with the charger is that it is a roots design, albeit one with twisted rotors. These chargers do not generate any pressure within the charger, instead they just rely on air 'backing up' at their outlets to compress the air they're pumping. When the cam change over happens, there is a sudden change in the rate air is flowing out of the manifold, but no real change in the rate at which it is coming in - therefore the pressure drops. There's not much you can do about that I'm afraid, but have a look at your dyno graphs, or just feel how the acceleration changes around lift - it's as smooth as a baby's behind so it doesn't seem to be having a negative effect on performance :)

[Smaay]

you might be able to tune that out if you have a PFC.

[Jesse IL]

you might be able to tune that out if you have a PFC.

Yep. You could probably retard the cam slightly where you're getting the pressure drop. At this point, you're probably blowing the boost right out the exhaust port.

[Gravel]

Yep. You could probably retard the cam slightly where you're getting the pressure drop. At this point, you're probably blowing the boost right out the exhaust port.

Hasn't Korben007 got a PFC on his Greddy setup - I don't remember seeing his dyno charts, but he did report having more power than with the emanage...

[monkeywrench]

A Power FC could solve this problem. WIth a Supercharger you should be able to lower the lift point. You can see that by the fact that the engine obviously starts moving a lot more air when the cams are switched as they are now. You should see minimal change in boost pressure if it's switched at the optimal point which I've seen be as low as 4k depending on the forced induction. Tuning the VVT is critical to getting this right as well.

[Gravel]

A Power FC could solve this problem. WIth a Supercharger you should be able to lower the lift point. You can see that by the fact that the engine obviously starts moving a lot more air when the cams are switched as they are now. You should see minimal change in boost pressure if it's switched at the optimal point which I've seen be as low as 4k depending on the forced induction. Tuning the VVT is critical to getting this right as well.

I am pretty sure that Lotus' supercharged 2ZZ-GE which uses an identical charger does indeed have a much lower lift point - it might even be 4k.

I still can't figure out whether or not the fact that the blower doesn't supply pre-compressed air will make the system behave rather differently to a turbo setup. :shrugs:

[jlitman]

A Power FC could solve this problem. WIth a Supercharger you should be able to lower the lift point. You can see that by the fact that the engine obviously starts moving a lot more air when the cams are switched as they are now. You should see minimal change in boost pressure if it's switched at the optimal point which I've seen be as low as 4k depending on the forced induction. Tuning the VVT is critical to getting this right as well.

I am pretty sure that Lotus' supercharged 2ZZ-GE which uses an identical charger does indeed have a much lower lift point - it might even be 4k.

I still can't figure out whether or not the fact that the blower doesn't supply pre-compressed air will make the system behave rather differently to a turbo setup. :shrugs:

Yep -- 4000 RPM for the Elise S/C kit... this discussion has me really tempted to experiment with Nolat's lift controller. I'll have to speak with my tuner about this and see what he thinks.

Actually, take a look at this -- Monster's boost curve

http://img.photobucket.com/albums/v693/monsterhui/CIMG4708.jpg

If the optimal cross over point is where boost is first lost, would that suggest lowering lift to, say, 5800 would be about optimal, or might power be made going even lower, as in the Elise kit? :confused:

[Monster Hui]

i mean if the boost is able to keep building up, even to just the full 8.5psi instead of the max 7.5 that i saw with the dyno... lowering the lift may allow you to have more boost and a steeper incline of power :)

i mean the boost curve didn't come to any flat lines

[jlitman]

i mean if the boost is able to keep building up, even to just the full 8.5psi instead of the max 7.5 that i saw with the dyno... lowering the lift may allow you to have more boost and a steeper incline of power :)

i mean the boost curve didn't come to any flat lines

Ah, I see. But why does it drop sharply a few 100 RPM before the cam profile switch, or am I misreading the graph? :confused:

[Gravel]

Ah, I see. But why does it drop sharply a few 100 RPM before the cam profile switch, or am I misreading the graph? :confused:

It doesn't seem to hurt the power curve though :shrugs:

What I can't understand is how you can fix this - when the valve lift increases, there will be a sudden jump in airflow out of the inlet-manifold. As the supercharger produces no pressure internally, but relies on back-pressure at its output, if the flow in the inlet manifold suddenly jumps up, surely this must lower the back pressure? :shrugs:

As others have said, you can change the valve timing with a CAMCON, but you can't contol the lift-level - you are in lift or you are not. So can you control the flow enough with the VVT bit to let you flow just a tiny bit more than when you're not in lift, but with the high-lift cams engaged? :shrugs:

[Korben007]

i got this issue somewhat worked out on my setup. Monkeywrench racing is right. I played with the cam switchover point down a bit. not sure i think about 5500rpm. it definately smoothed out the boost curve quite a bit. Also tuning the VVTi as well helped with smoothing out the powerband. I didnt play with it much as that wasnt my main goal but i know that if i lowered it some more and played with the cam timing that it could be taken out more. i think i barely loose a pound of boost at the cam switch. but is recoved within a few hundred rpm again, and actually with the powerfc i have the redline raise to 8600rpm at that point boost starts to creap up to just over 9psi. Im sure if the RPMs kept going up so would the boost. One thing to note is the cam switch over barely affected the powerband. the vvti was much more effective in tuning out a couple flat spots, but you have to do both together you have to switch them both to match cause there is a jump in cam timing when you hit lift. so you have keep that occuring at the same point.
I would spend more time fine tuning but I dont wanna spend anymore money on this old motor. It has a weak piston ring at this point and im trying to be conservative with the timing at what not. Woohoo to 170k and boost. still making about 210whp. When this engine goes im putting a built motor in and a little smaller pulley. and im gonna get alot more agressive on the tuning.

[jlitman]

i got this issue somewhat worked out on my setup. Monkeywrench racing is right. I played with the cam switchover point down a bit. not sure i think about 5500rpm. it definately smoothed out the boost curve quite a bit. Also tuning the VVTi as well helped with smoothing out the powerband.

So, the overall effect was more usable power across the rev range, but the top hp and tq was not affected? Can you tell us a little more or share a dyno graph? :)