Mivec advantage? - Page 9

crcain · Nov 4, 2009, 10:04 AM

From what I think was originally a Hi-Boost cut and paste:

Quote:

Since the intake valve closes after the bottom of the intake stroke, closing the intake valve earlier gives you a longer compression stroke, and higher peak cylinder pressure.

But at higher RPMs the valves are open for a much shorter period - with the later intake opening there is not enough time for the cylinder to fill so the peak pressure actually drops off.

Ok the first part, advancing the intake yields higher peak cylinder pressure makes sense.

But the second part confuses me. I understand as RPM rises the duration in time the valves are open reduces. But how does this effect intake cam timing? With ignition advance it is easy, you start earlier to give the flame front time to build. But with valve timing I don't quite get it.

H Jensen · Nov 4, 2009, 01:31 PM

Quote:

Originally Posted by crcain

Ok the first part, advancing the intake yields higher peak cylinder pressure makes sense.

But the second part confuses me. I understand as RPM rises the duration in time the valves are open reduces. But how does this effect intake cam timing? With ignition advance it is easy, you start earlier to give the flame front time to build. But with valve timing I don't quite get it.

I could be wrong but this is the way I understand as to how it would work.
If you advance your cam timing your allowing your cylinder to start filling sooner and add the vacuum that's created when the piston is traveling down your filling the cylinder better. In other words (depending on your valve overlap) your at the end of exhaust stroke your intake valve just opens and your exhaust valve is closing causing scavenging/your pushing exhaust out with boost. now before your piston starts the intake stroke your already filling the combustion chamber with air/fuel and creating cylinder pressure, now when that exhaust valve closes and the piston is going down your still adding more air/fuel. Now wherever that intake valve closes either before BDC or at BDC you now have filled your cylinder with the most air/fuel you can. Compared to having your intake at stock timing.

UT_EvoX · Nov 4, 2009, 02:59 PM

Eh, pretty much. But having the exhaust overlap the intake allows the exhaust pulse to actually pull fresh air into the cylinder, reducing pumping losses. This works great under vacuum, but is not beneficial (and actually detrimental) once you hit full boost. At that point, your turbocharger will literally force compressed air straight through the cylinder while the exhaust valves are still open.

tephra · Nov 4, 2009, 06:12 PM

ok another topic.

Compressor Surge.

I retarded the exhaust cam by an additional 2.5 degreees. and all of a sudden my WOT "load" got all jumpy.. also the car felt a bit grumpy.

is this a sign of compressor surge?

gunzo · Nov 5, 2009, 01:21 AM

Quote:

Originally Posted by tephra

ok another topic.

Compressor Surge.

I retarded the exhaust cam by an additional 2.5 degreees. and all of a sudden my WOT "load" got all jumpy.. also the car felt a bit grumpy.

is this a sign of compressor surge?

depends on when it happens .. that is onset of surge ..

another clue to queue you in is the tubo sound changes .. like an approaching helicopter ..

And whatever Bryan says on dynamic boost

CrCain .. fuel is only injected near the end of the intake phase .. not at the beginning ..

tephra · Nov 5, 2009, 06:02 AM

happens after spool, i pulled out 1 degree of ex-retard...so ill do another log tommorow..

crcain · Nov 5, 2009, 06:34 AM

Quote:

Originally Posted by gunzo

CrCain .. fuel is only injected near the end of the intake phase .. not at the beginning ..

So it's unlikely there is much fuel in the charge during overlap?

Do we know the injection angles for the stock ECU or is that a mystery?

gunzo · Nov 6, 2009, 07:11 PM

Quote:

Originally Posted by tephra

happens after spool, i pulled out 1 degree of ex-retard...so ill do another log tommorow..

Remove the exhaust restriction my friend

tephra · Nov 6, 2009, 07:12 PM

yeah thats what I am thinking as well.

Hiboost · Nov 7, 2009, 12:19 AM

Quote:

Originally Posted by tephra

ok another topic.

Compressor Surge.

I retarded the exhaust cam by an additional 2.5 degreees. and all of a sudden my WOT "load" got all jumpy.. also the car felt a bit grumpy.

is this a sign of compressor surge?

Only used to happen in 5th for me if i exceeded 20 psi by 3400 in 5th. After I did some light port and polishing to my exhaust side and switched to a shorter runner/higher velocity exhaust manifold I haven't been able to trigger it yet and the car feels more responsive and pulls at 25 psi like it did at 27 psi before. I'll have to do some full logging tomorrow to verify but balancing more velocity and volume through the exahust side definitely can counter compessor surge as can mivec tuning where less intake advance and less exhaust retard will settle down the overactive spoolup.