Drop in boost pressure at high revs??

[JMC 0]

OK folks, here's another question. When I get to high revs (above 5600) my G60's boost pressure suddenly drops (see the graph below), down to a very static 0.6bar. Now what could be causing this?? I thought boost pressure should climb gradually throughout the rev range (or is that just wishful thinking). Just for info, it's with a ported and flowed big valve head, running a piper 285 cam.

[Shaggy 0]

Sounds like a very similar problem i was having?? turned out to be a leaky water injection jet... could be a similar problem on yours?? although it may be a boost leak??

 

Mine did pretty much the same thing as yours, after i had checked / tightened all boost hoses and sorted the jet it has been fine ever since...

[flusted 0]

not the isv bleeding is it?

[JMC 0]

Thanks for the responses folks, keep 'em coming.

 

Mike - strange that it only leaks above a certain rev point though, as it's spending most of the time before then around 0.8bar, and then it's a real quick drop.

 

Flusted - ISV was my first thought, perhaps it's venting, thinking that the mixture is going lean?? It has been mapped, but the ISV does vent boost if pinking is detected, if I recall. Wonder if it is going too lean at the top end??

[JMC 0]

Has anyone else had their G60's rolling roaded, and do you see the power curve flattening off above 5000, with a big drop above 5600??...

[Shaggy 0]

JMC, your problem sounds pretty much the same thing that was happening to me! i dont think it s the isv venting although it maybe?? they only usually start to vent above 1 bar??

 

My boost dropped suddenly aswell, i know it may seem strange but it may be worth checking all boost hoses etc for leaks, also check the throttle body mechanism that controls the boost return butterfly??

 

you would think if there was a leak it would do it all the time wouldnt ya??, but some leaks only show up under a certain pressure and then just leak at the higher end of boost???

[JMC 0]

Cheers Mike, looks like it's time to get the hands dirty again :D

 

I came across a thread last night which (from beavis, I think) mentioning that chipped cars tend to show up the weaknesses of the old sensors, specifically the hall, knock, and co pot. Will have to look into this as well, I think....

[W3RKD 0]

Cheers Mike, looks like it's time to get the hands dirty again :D

 

I came across a thread last night which (from beavis, I think) mentioning that chipped cars tend to show up the weaknesses of the old sensors, specifically the hall, knock, and co pot. Will have to look into this as well, I think....

 

Jonathan - if you at no rice tonight i can have a check for you. I can also bring your bits up with me.

[iow_corrado_g60 0]

JMC I looked at my rr readout from stealth yesterday and funnily enough mine drops off at 5600rpm

[JMC 0]

Cheers Darren - see you at No-Rice.

 

iow_corrado_g60, did stealth give you a plot of boost vs. revs as well, and if so did that drop at the same time as the power??

[iow_corrado_g60 0]

no they didnt mate just weird how it was happening at the roughly the same revs as you mentioned!

[JMC 0]

no they didnt mate just weird how it was happening at the roughly the same revs as you mentioned!

 

There was someone on Dubforce that mentione that the ISV vents a small amout of boost above 5600, so that it's not on full boost when you hit the rev limiter. That would explain whats happening, and I've PM'd the guy for a few more details (for instance is this just with standard chips, or is it with custom mapped cars as well), so lets see if he comes back with anything. I'll report back if I hear anything, but if anyone else has thoughts on this, please chip in....