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Old 04-22-2004, 06:37 PM   #1
heffergm
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Default Solution to RPM "surge" under boost?

I'll do my best to explain this:

Under heavy throttle (not necessarily WOT), it's very obvious that when I shift when still making boost, I get RPM surge.

In other words, I'm on the gas making boost, I get off the throttle to go to shift, and even though I'm off the gas, as soon as I clutch in, my RPMs will rise a few hundred or so rather than dropping immediately.

The problem doesn't exist when shifting when not making boost (or when only boosting a few psi or so), so my theory is that the factory recirculating valve just can't "blow off" the extra boost quick enough between shifts.

Its become very annoying, even more so because revs in my S2000 drop so fast between shifts that you have to shift like mad to keep up.

I certainly don't expect the WRX to respond in the same manner as the S2000, but I'm wondering if there is a solution for this "problem" of rising revs between shifts when under boost.

Has anyone noticed with a fully or semi-atmospheric BOV that this problem is alleviated? I'd be curious to find out...
Although I'm not really interested in a BOV due to the problems it causes with running rich between shifts, I'm wondering if I might learn to live with it to solve my other issue.

Looking forward to any suggestions...
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Old 04-23-2004, 05:34 AM   #2
file_id
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Known subaru quirk. Try letting off the gas pedal earlier then push in the clutch.
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Old 04-23-2004, 08:43 AM   #3
mouserWRX
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Quote:
Originally posted by file_id
Known subaru quirk. Try letting off the gas pedal earlier then push in the clutch.
Yep, I have found that if you let off the gas for about a second before depressing the clutch, you're fine. Otherwise, just live with it, it's not that bad
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Old 04-23-2004, 08:48 AM   #4
heffergm
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Well, I agree, you're right... letting up on the throttle for what I'd consider an extended period of time (enough for the boost to drop) solves the problem.

What I'm curious about, however, is whether running a BOV that vents at least partially to atmosphere has any effect on the problem (either making it better, worse, or not changing it at all).

Anyone running an aftermarket BOV notice a difference?
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Old 04-23-2004, 10:04 AM   #5
ride5000
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i noticed a slight difference for the better when switching to a 100% VTA BOV. still occurs to some extent though.

the extra fuel helps prevent shift knock!

ken
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Old 04-23-2004, 11:54 AM   #6
Aegon
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I'm VTA and I will still surge if I don't let off early.

When it does surge I'll take my time on the shift and everything works out great.
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Old 04-23-2004, 01:31 PM   #7
Turbo9000
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I have heard that it does this to burn off extra fuel in between shifts here ...and a reflash could help. I have the same problem and I have a APS bov...a hybrid valve that recircs and VTA. One thing I notice is when the boost is low enough to recirc and I shift I do get the RPM spike. When I get enough boost that the bov will VTA the car will want to after fire instead of RPM spike.

edit: maybe not with a reflash...I think I read that wrong.

Last edited by Turbo9000; 04-23-2004 at 01:37 PM.
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Old 04-23-2004, 01:34 PM   #8
heffergm
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Interesting. In that case, perhaps some folks with a Cobb AECU (stage 1 or 2) would chime in so we could see if that helped at all.
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Old 04-23-2004, 01:41 PM   #9
mbiker97
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Replace your heavy stock flywheel whith an Exedy or some other significantly lighter one.

I don't think that the cable throttle models rev for emissions. I wouldn't think that is possible without throttle by wire.
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Old 04-23-2004, 01:51 PM   #10
Aegon
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Quote:
Originally posted by mbiker97
Replace your heavy stock flywheel whith an Exedy or some other significantly lighter one.
Flywheel weight would tend to carry momentum, not to add momentum. The only thing that will make revs rise in this scenario is fuel+air. A lighter flywheel will simply rev higher!
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Old 04-23-2004, 01:57 PM   #11
PKer
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I have a Cobb stage two and don't notice any difference. I think it is more a matter of driving style than mechanical problems.
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Old 04-23-2004, 02:00 PM   #12
mbiker97
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Quote:
Originally posted by Aegon
Flywheel weight would tend to carry momentum, not to add momentum. The only thing that will make revs rise in this scenario is fuel+air. A lighter flywheel will simply rev higher!
Well that's my thought (the mass part anyway). Since the flywheel is being accelerated by the engine, when you release the clutch only the pistons/rods/etc. is acting on the flywheel to deccelerate it. However since there is less mass in a lighter flywheel and the mass of the rest of the engine stays the same the flywheel should deccelerate faster.

Or disregard what I'm thinking because I haven't had physics since I graduated from college.
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Old 04-23-2004, 02:15 PM   #13
Aegon
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Quote:
Originally posted by mbiker97
However since there is less mass in a lighter flywheel and the mass of the rest of the engine stays the same the flywheel should deccelerate faster.
The flywheel will drop revs faster, just as it will pick revs up faster. The amount of time between | clutch in -> revs gain -> revs drop to RPMs at clutch in | will be the same for both flywheels, but the lighter one will hit a higher RPM.
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Old 04-23-2004, 04:32 PM   #14
TheJonesBoy
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Flywheel mass and rev limit are 2 separate things. Lighter flys in general will allow the engine to rev quicker, slow faster, screw up idle and engine balance, and accelerate quicker. Your rev limit is decided by reciprocating mass (piston + con rod mass, valve + lifter mass) and valve springs.

That little blip you are trying to get rid of is usually seen in fly-by-wire cars for emissions. Cable linkage cars like ours are not immune, as the idle stabilizer valve may open to allow a little air past the butterfly. I am actually trying to track this down too, as it tics me off immensely.
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Old 04-23-2004, 05:02 PM   #15
Aegon
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Quote:
Originally posted by Aegon
but the lighter one will hit a higher RPM.
All this means is that given the same amount of energy the lighter flywheel will spin faster. I'm not suggesting that the redline will be increased or anything along those lines.
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Old 04-24-2004, 04:02 AM   #16
Hey_its_Cole
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MIne surges like everyone else but doesnt seem to make a bit of difference on slow shifts or fast shifts. Maybe I just shift differnt or just don't notice but I dont see how it makes shifting smoothly a problem. I know it doesnt for me. I think most turbo cars behave like this.
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