Mike454SS

iTrader: (4)

I am trying to understand cylinder pressure better for my twin turbo 370 build and I was thinking, on a turbo car, when it's spooled and making full boost...as the transmission yanks the RPM back down for a shift (be it an automatic or power shifting a stick...which I probably won't ever do), does cylinder pressure spike during the shift? Is this when a head would start to lift and cause the engine to push water or damage the head gasket?

I'm not sure I fully understand when cylinder pressure is highest. I do think I have a decent understanding of whats going on for cylinder pressure through one cycle of the engine while it's accelerating in the middle of a gear, but not what happens when the engine is being forced to a lower RPM by the drivetrain during a shift. If anyone has any graphs, or has ever studied this, I'd really like to learn more.

white2001s10

iTrader: (4)

There is a load dump on the engine right after the shift.
The pistons slow down and there is more time for cylinder filling.
As long as your spark and fuel are correct for that point at shift recovery there really shouldn't be a problem with cylinder pressure spiking.
On a very aggressive tune there could be a problem with spiking.
A part in the chamber such as an exhaust valve or sparkplug overheating can also cause spikes at shift recovery from autoignition.

crashly

on any forced induction engine, the 'dwell time: of the piston @ tdc and down to about 12- 15 * atdc is the area where the most cylinder pressure is obtained.

on the gear change, the engine rpm decreases, and gets closer to max torque rpm..

@ max torque the engine is at max volumetric effiecenty....

if the cylinder pressure spikes, i would say the a/f ratio is not optimium... ( read too lean)
there shouldn't be a increase in cylinder pressure , just due too gear change.
yes, there will be MORE load on engine, more load creats more heat , etc, etc

i have some data from a dyno... some where, it shows the graphed line of how the cylinder pressure rises and falls due to piston position.

generaly speaking, a shorter con rod / crank combo , allows LESS time for the psiton to be at tdc,and less chance for excess , cyl pressure.

RednGold86Z

There probably is a significant spike, unless you have a blowoff valve. The turbo is still spinning much faster than necessary from the high RPM, and it won't slow down instantaneously (like a blower would). The wheel's inertia and still pretty good exhaust flow (slowly dropping input), and rpm dropping (quickly dropping the output) should cause a bit of boost pressure spike, and consequentially cylinder pressure spike.

gametech

iTrader: (1)

With a turbo, an auto trans, and NO blow off valve, boost pressure could spike. This would result in an overboost condition that would be the equivalent of running much more boost than intended. With any other type of FI, with a stick shift, or with a functional BOV, peak cylinder pressure will always occur at peak torque. This does assume, of course, that you aren't experiencing detonation. Keeping abnormal combustion (detonation) in check is key to making power without lifting heads.

chevy2808

i agree with redngold88z and gametech..it depends on if you have a bov or not, and if you do the pressure shouldnt spike

engineermike

If you don't lift the throttle on the shift, the BOV won't open.

The question, as I understand it, is ... is there a boost spike during the shift due to the momentum of the turbocharger wheels and lag in wastegate response.

Here's your answer... It depends.

A) If the turbocharger is near its limit, then at the top of the gear the compressor begins to run out of steam. To compensate for this, the wastegate closes to maintain boost. This speeds up the turbocharger, possibly a lot. When you shift, there will be a boost spike since the wastegate will take a little time to respond and the wheels have momentum.

B) If the turbocharger is larger than the engine needs, then you are operating on the "flat" part of the compressor curve. This means that the compressor can supply increasing flow as engine rpm rises, without actually speeding up itself (a difficult concept to grasp, I know). This means that during the shift, the wastegate doesn't have to respond nor do the turbo wheels have to slow down, so there is no boost spike.

Mike

DanO

There is a big difference between Cylinder pressure and pressue before the throttle plate..