maybe I'll use this as an excuse for putting the cams in... but not until after one more track day of running mid 11s!

what are your cam gear settings again if you don't mind me asking? I don't have gears but would you suggest them?

 

I'm using 0/-2, and that setting is working very well. I suspect it would work well for you also, but even 0/0 is going to be much better than what you've got now.

 

sounds like surge to me from what is happening although I really don't know what the compressor map for that turbo looks like...compressor map is an easy way to find out for certain if you can't data log.
I think it is probably just bov flutter though.

in response to another post, how would not having surge ports make the spool up quicker?...I would have guessed the opposite, having never seen data of one vs. the other

 

How can you be certain by looking at a compressor map unless you had your head flow tested?

 

well you will need some data logging(boost) and a few ballparked assumptions (VE being one of them)...you can see with the boost pressure and airflow (mass or volumetric) where you fall on the compressor map...if you are past the surge line, well...I would assume he's surging.

 

I think that one's volumetric efficiency is quite a big factor to assume.

It's like having a graph of X and Y...and you're guessing the Y values and then making judgements on it.

 

yes, but it can be calculated (hence ballparked) from given values of the car, such as hp/tq at a given rpm with numerous other variables...such as air temp, boost pressure (need a compressor map for air flow at temp), air fuel ratio, etc...

 

why didn't you search first?

there are many types of surging. a turbine based surge is whent eh turbine is the bottleneck and the overall engine delta moves towards unity to the point that there is a resistnace in the acceptance of more mass flow. this is not common but can be produced with bad compressor to turbine matching. basically compressor is too big for turbine, can't cram anymore air in choke choke buck buck is what you feel. so putting a 35r wheel on a stock turbine would do this.

the other type of surge is what the compressor housing is machined for. it's when the housing moves TOO MUCH AIR and the fins of the compressor blade stall. this stalling causes a momentary free floating of the wheel, what i mean by free floating is that the wheel is moving under almost no friction, it moved so fast that the air cannot grip onto it and that being the case it moves without viscous effect (kind of) this of course only happens for a fraction of a second after which the air grabs the wheel and gives everything a nice yank. this is bad.

the housing is machined such that there is a reduction in pressure gradient going into the compressor inlet. those holes fill up with air and fire it backwards, this makes sure that the air is never moving too fast as it comes into the compressor. does this reduce mass flow... it rather babysits mass flow... it makes sure your mass flow stays intact with the compressor so it can continue to flow, otherwise it's just mass... no flow.

if you understand this coarse definition then you'll see that ONLY large compressors will have antisurge housings. there is no need for small turbines to have such a device because for one cannot move fast enough and two will never flow enough. the flow enough part i'm fudging because the flow doesn't dictate the compressor wheel speed, but let's say you'll never come to a case where the flow will seperate from the wheel when the wheel is small. why? because the smaller the wheel the smaller the radius of the wheel this means that the speed of the tip of the blades is slower than a bigger wheel going at the same rpm.

just about all turbos go the same speed... flippin' fast. so small radius wheels don't have high tip speeds, but big wheels do. so you'll see the flow seperate from the big wheels and that's bad.

 

turbo's have a large variance on speed...generally the smaller the faster they turn and small turbines can have surge as well, it is dependant on both housing with respect to the size of the engine (and exhust temps capable i.e. rotary).