A turbo'd car with a CAM question

[Froggx]

yeah, when it comes to 1.6 rockers, you can't beat the stock full roller rockers on the gen 3 motors

edit: well you can, for about $200 or something more

[blackbombshell95]

well i'm using a 3100 plenum with 3400 heads. don't the 3400 heads have 1.6 rockers?

[Froggx]

I'm pretty sure all the 3400 heads have 1.6 ratio rockers, I know mine does at least, it's a 2000. Someone feel free to back me up on this as I like to get parts confused.

[dbtk2]

All 3400 heads, as far as I know, have 1.6 rockers. If they are roller fulcrum, they are 1.6, thats the easiest way to tell.

Shawn

[Doward]

Just thought I'd through my .02 in...

The talk of more exhaust vs intake duration is only valid if your exhaust backpressure is significantly higher than your intake boost pressure. THEN is reversion a serious problem.

In all honesty, if you work to have minimal backpressure before the turbo, then you can set up your cam just like a N/A cam, for the most part. Still err on the side of less overlap, but for proof - just look at my dyno, with a mild 204/214 @ .050" and .422/.444" lift cam (1.5 rockers)

[dbtk2]

Yeah, you don't need a very big cam to make power, when my STE was stock, with the stock chip, and boost turned down to 6psi, it put out 182whp/217wtq. That is with a restrictive exhaust, and crappy fuel and timing tables. My trap speeds increased by almost 4mph with the chip (from 90.7 to 94.5), so that made a huge difference in power and especially torque. And that was on only 10psi still, and still with my T25. And the stock Gen II cam is crappy for lift and duration, lift is only .394 intake/.410 exhaust, and I'm not sure about duration but its like 182 intake/192 exhaust, so its pretty shitty nonetheless.

Shawn

[Froggx]

Just thought I'd through my .02 in...

The talk of more exhaust vs intake duration is only valid if your exhaust backpressure is significantly higher than your intake boost pressure. THEN is reversion a serious problem.

I wasn't aware our motors were taking boost pressure that would make the exhaust backpressure not significantly higher than what is going in. Don't most street driven turbo cars have a turbine inlet pressure (aka exhaust manifold) to intake manifold pressure ratio of 2:1 which translates into 10 psi going in is equal to 20 psi in the manifold?

[dbtk2]

Yes, the ideal exhaust pressure ratio should be about 2:1 on a boosted engine. So, really, reversion should be a problem with a lot of exhaust duration. At least that is how it makes sense to me.

Shawn

[fivepointhustla]

So would an h272 be good or bad for a turbo'd 2.8?

[dbtk2]

An H272 wouldn't be as good as a H260. It has too much valve overlap.

Shawn

[Froggx]

Yeah, alledgedly a 0 overlap cam would be optimal for a turbo motor, until you get to high rpm's, where a small amount of overlap is more acceptable as momentum and flow of gases and such keeps everything going in the right direction once the motor is going fast.