evil 944t

Thanks for posting that Russell. Its easy to take things out of context, on the web.

Rogue_Ant

Why?



Rogue

rberry951

As RPMs go higher and higher the engines ability to push all of the exhaust out of the combustion chamber in the decreasing time allowed diminishes and you reach a point where all of the exhaust is not exiting it is then mixing with the new air/fuel coming in on the next stroke which may cause detonation. But just the pressure itself of the old exhaust being in the chamber with the new air/fuel coming in creates a pocket of pressure in the combustion chamber that is higher than the head can hold in, and the head lifts, and this is bad news. It can break/stretch/warp studs, warp the head, damage valve seats, break piston rods, all kinds of nastiness.

But the bottom line of 'why' is that the amount of 'air/gas/exhaust' that can be pushed past the exhaust valve in the alloted time the valve is open is finite, and that time decreases as RPMs increase.

Regards,
Russell

rberry951

Ain't that the truth...

I like to be as clear as possible, and I never mind restating things for just that reason. No amount of tuning in the electronics can make your engine stronger, it is what it is and if you push its limits it will fail prematurely.

Regards,
Russell

toddk911

Glad you clarified as I understood it that you certainly would not drive around shifting at 7k, but was under the impression the motor was ok to run to 7K once in a while, and/or on the dyno for a couple of pulls, etc.

Also, I thought the only issue with high/increased rpms was an issue with the lifters.

So is there really that much jump in head pressure from 6,500 to 7,000???

Rogue_Ant

Contaminating the intake charge, thus becoming more susceptible to knock I understand...

It seems to me that there would not be enough pressure from resident exhaust to cause the head to lift itself. Simply because if there is an excess of pressure from the exhaust then the intake draw would be a minimum, and the dynamic CR would remain fairly stable...


Rogue

rberry951

7K once in a while is different from riding every gear up to 7K, in 3rd it's not that big a deal, but in 4th/5th with greatly higher load your RPM becomes a significant factor.

Regards,
Russell

rberry951

"Head lift" happens because of the combination of boost and RPM. So yes, if you were just sucking in new air/fuel on top of the unexpelled exhaust that would be the case, but on a turbo car you are forcing in air/fuel through the intake valve after not expelling all of the exhaust and that combination of factors creates more pressure in the combustion chamber than the stock head/studs/gasket can withstand. And while it may not 'blow' the head gasket completely, it can do worse things like force oil/fuel into coolant, coolant into oil or worse into cylinders and the damage may not be immediately evident.

My whole point being that just because your A/F is okay at a gazillion pounds of boost at 7K RPMS, your stock engine just won't hold up to it forever, it will fail.

Regards,
Russell

Brian Morris

So there's more pressure in the cylinder at 7K rpm even though the torque has dropped significantly?

--Brian Morris
89 951

Rogue_Ant

It won't hold up because of knock or exhaust backpressure?



Rogue

special tool

Do you have experience with head lifting at high rev/boost on a 4 cylinder porsche, Russel?

rberry951

Nope, I've personally never lifted a head (that I know of). I have several friends with race shops who have shown me the results, the stretched studs, exhaust carbon between head and block, coolant in cylinders causing wall scoring and the like. You are probably better equipped to address the 'why' it happens in more detail than I have conveyed thus far, I'm stating my understanding as taught to me by various engine builders. I've never heard of it happening on an engine with upgraded head, bigger valves, studs/gaskets, but have heard of the bottom end eventually dying from spinning high rpms under extreme boost pressures with big turbos.

My understanding being, if I can explain it better, that for each stroke the exhaust valve open time is shortened as RPM increases and therefore reaches a point at which it is no longer open long enough to flow out all of the exhaust. This point will vary depending on the amount of inherent backpressure of the exhaust system/turbo being used. Then on the next intake stroke new air/fuel is being forced in on top of the remaining exhaust causing inconsistent A/F for one thing, and a higher internal pressure rate inside the compustion chamber on the compression stroke.

If my understanding of that function is in error feel free to correct me, I don't know everything!

Regards,
Russell

special tool

I agree with you.

toddk911

So it is certianly load based? As you mentioned 4th, 5th being more likely to have an issue than lower gears?

This issue of "exhaust forced back into the chamber" came up in another thread in regards to turbo's with a small hotside and running a lot of boost.

I thought that this could be the cause of a lot HG failers as opposed to just being too lean, at least in regards to the stock turbo. The pressure itself.

Or, that exhaust/air that gets forced back into obvioulsy makes the chamber instantly very lean, hence there is detonation and the HG goes.

??????

rberry951

My only point was specific to the open/close time of the exhaust valve at high rpm, where you won't really have as much load, but a factor of that open/close time not being long enough to expell all the gas around the valve area. What you are talking about here is purely load/backpressure. When you have the wrong turbo sizing to an extreme that exhaust is trying to push through at a VOLUME that the turbo can not flow it ends up pushing against it, creating HUGE backpressure and hence going back into the cylinders.

But what you're stating in that scenario is correct, but if your turbo is matched size wise to your displacement that won't be an issue unless there are other exhaust restrictions. In either case, no matter what the reason for the exhaust not getting out, you risk a lean condition and detonation.

Regards,
Russell