4agze change in manifold pressure

[TooF]

ok lads heres an odd issue.

my engine 4agze aw11 bigport with SC12 running custom 180mm crank pulley always used to make around 13-14psi at low speed but when you were really giving it stick (no doubt overheated intercooler too) it used to whip up to 17pis without stress.

now a few weeks ago i had a headgasket let go on me. call this gasket failure as a result of too much boost or poor manufacture or just plain bad luck i dont really care.

whats odd is now its all back together im only pulling around 12psi low speed and when i give it a bit more of a stomp it hits about 13-13.5 tops.

my initial thought was a leak somewhere but i cant hear any leaking and all the hoses seem nice and tight. my thought given its an AFM engine any leak would be causing it to run overly rich and generally like a dog. which it doesnt it idles like a dream and runs just fine.

now heres a possible thought. is it possible that last time i put the engine back together i accidently put the timing belt a tooth out such that the inlet cam was out a notch resulting in poorer cylinder filling and hence higher manifold pressure ?

seat of the pants dyno says it feels pretty much the same performance wise possily a little down on power but i cant really say for sure. if its down it isnt by heaps.

[30psi 4agte]

If it was a tooth out i think it would run pretty average> so i dont think its that.

[011]

If there doesn't appear to be any change in performance using the seat-of-the-pants-o-meter, could there possibly be some sort of leak in the boost gauge's manifold pick up? Would you have touched it at all in the process of taking the head off, etc?

Just a possible thought. I've learned to examine the smaller things before the bigger things, after the last couple of gremlins with my automobile...

[McFly 11]

ok lads heres an odd issue.

my engine 4agze aw11 bigport with SC12 running custom 180mm crank pulley always used to make around 13-14psi at low speed but when you were really giving it stick (no doubt overheated intercooler too) it used to whip up to 17pis without stress.

You got 17 PSI from a 180mm pulley? I have the Diabolique/NST 180mm pulley and I never noticed that much boost at high RPM (although I admit I wasnt looking at the gague.) NST themselves specify 13.5 @ the manifold with their 180mm crank pulley mod on their site which I what I have seen regularly.

(see http://www.nonstoptuning.com/pKitToyMR2GZ.htm)

I run a rubber vaccum hose to my boost gauge from the manifold (via ABV mod) around the rear firewall and thru the grommet near the battery. A long pathway for the hose, which wont help accuracy but there arent lots of options. This also reminds me... It is easy for the boost hose to kink along the way too which affects the reading of boost gauges so an optimistic boost gauge bing throttled is not too far fetched... but you probably already checked that

Its not along the cam tooth issue but im *inexpert* in that area

[JBs]

Hey mate as you know I'm no AW11 genius But it wouldn't be the ABV isn't bypassing properly anymore would it?

[TooF]

i only ever hit 17psi when real into it ie to 3-4-5th gears. it would never hit that high in lower gears.

my first thought was abv but it is hooked up exactly the same way it always has been. ie: direct to manifold signal teed into the FPR line.

the boost guage is run just like mcfly has described also with a rubber hose.

when lifting the head i didnt remove the inlet manifold so it isnt that gasket.

im kind of at a loss as to what it is...

[DX20VT]

Anything changed in the exhaust side?, as it affect's airflow and boost by large amounts.

[TooF]

other then bolting the manifold back onto the head after removing it no.

[feral4mr2]

didnt leave a rag stuffed in the one of the i/c pipes? i've done things like that before.....

[TooF]

nope but again wouldnt that result in an increase in maifold pressure given the PD blower would move the same ammount of air but less of it would be getting in...

although i will pull it apart on saturday and check.

[Ben Wilson]

17psi is huge for a SC12, if you're really pulling those sort of numbers, the coating on the blower won't last long...

Bear in mind, I made ove 130kW with 10psi.

[TooF]

this im aware of

my odd thought is it possible that instead of moving the Xcfm to produce 17psi my engine was only injesting 0.75*Xcfm as a result leaving me with a higher manifold pressure. im just curious if having the cam timing out would casue this.. my understanding would be that it could cause this issue. although sprisiingly not with any serious performance loss.

im just throwing around options as to why its changed suddenly.

[Ben Wilson]

That make s a sort of sense - Theoretically, the supercharger not the motor will be the power producing device.

If the cam timing was out before, there is a possibility that the motor was actually running badly, but the supercharger was compensating for it.

I doubt that this would really happen, but it's an interesting concept.

[TooF]

im just trying to put together a theory as to why last week it was whipping up 17psi and now only 13psi with the only part changed being a headgasket and headbolts.

id be quite happy with the 13psi and knowing "oops i stuffed up putting it together before" being the reason for th 17psi but im just worried there is something esle wrong with the syste,m

[feral4mr2]

17psi is huge for a SC12,.

i was getting up to 17psi with the 175mm crank and 120mm map sc12 in the lil feral. made that auto boogie.
pity the s/c has stuffed bearings.