Greetings people,
I have a 2006 r53. Alta cai, Wosner forged pistons (slightly oversized), 16% + 2% pulleys, detroit tuned bypass valve and one ball exhaust. At 5600 rpm i have 16psi then at 6500 rpm i have 14.5 psi and then at the red line i have 17 psi. Any idea why this is happening? I mean shouldnt the boost supposed to gradually increase?
Thanks in advance.

 

Is this consistently the case? Are you data-logging or just watching the gauge?

I wouldn't sweat a drop of 1.5psi near redline, though the jump back up is weird. Also, why even rev past 6500RPM?

 

I am data logging, I think its not normal since there is a drop in boost and then it increases again. Maybe i have high boost at low rpm which is not normal either. Is there a problem when i have 16 psi at 4000-5000rpm? I always thought that the boost should start low and then increase as the rpm goes up, and not starting up high then low and then high again. Isnt that the case?

 

Slightly oversized as in, raised compression?

Are you tuned? If not that is where I would start, as well as running the injectors to compensate for such mods.

There are multiple supporting mods required to run raised compression,especially in a boosted engine, and if you don't have them in place you will cause severe damage.

These cars make power higher up in the rev range, high RPM is where SC cars shine. Especially on the Dragon where you need the extra few hundred RPM you get from tuning the car. Being able to rev high and engine brake into the corner is much easier than shifting into 3rd then downshifting into 2nd, controlling revs, as well as braking all while trying to look out for the guy who is crossing the double yellow.

 

Yes , ive got slightly raised compression. Since the pistons are the next larger size than the stock ones. Can you give me examples if the supporting mods required? Yes the car is tuned.
Thanks a lot for the quick replies.

 

any modifications to the head? it is very strange to see a blip like that, usually it just tapers off since your leaking somewhere... but to come back....


as for boost, you should have all of it around 3.3k RPM and hold it to redline. and most of it before that point

 

Stock rebuilt head. Im trying to find possible solutions to the problem. Might be the belt slipping, boost leak, compressor close to end of his life? But the weird part is that low point in boost and how it just increases after

 

You don't need to live at 7000+RPM in order to engine brake into a turn and still have power coming out. Using the The Dragon as an example is a bit inappropriate, anyhow.

Back to the supercharger, specifically a very small, Eaton roots style: These things are sooo far out of their efficiency range at both engine and blower RPMs that high, it's not even funny. 17psi and a 14,000rpm blower speed puts this thing in outer space on it's factory efficiency island map.

Anyways, the boost increase after 6500rpm pretty much eliminates most of those causes in my mind, with only the belt slipping seen as a possible contributor. Not sure why it would slip then grab though, when you're steadily increasing load.

 

http://imgur.com/d1YSkXP

The grey line is the boost and the rpm the purple one

 

Check your belt and tensioner.

 

You have come to that conclusion after seeing the graph?

 

My Chrome isn't opening the graph, so no: it looks like your link is broken. Based upon your reaction, you disagree. When you are having loss of boost at high rpm in a supercharged car, start simple and check your belt and tensioner, and go from there.

 

I dont disagree, on the contrary. The belt seems the most logical explanation. The weird thing is that i have loss of boost at 6000rpm and then near the redline i regain the normal boost. Thanks a lot for your time and effort explaining

 

maybe after the slipping it heat up just a touch, then can grip again. I cannot see the graph either

 

http://imgur.com/d1YSkXP
Can you see the graph now?

The grey line is the boost and the purple one the rpm

 

most likely a belt slip. Tensioner and belt.

And did you get higher CR pistons when you overbored and put in the oversize ones?

Oversized pistons does not mean higher compression by default. you would have to order them in a higher compression ratio.

Oversized just means they are ... let's say .040 over bore. Which means they are .040 larger in diameter than stock and the cylinders are re-bored to .040 to match.

This helps to make sure the pistons fit the bore with the correct clearances as from the factory they err on the side of caution to make sure all the motors fit and compensate for tool wear etc. Also done to re-round the bore should you over heat the block and the factory combustion chamber goes slightly out of round (less than .040 so that the .040 overbore makes it round again)

 

it could also be your cam profile.

What cam do you have in the car?

It is not an abrupt change so it could be indicitive of your car breathing better up top around 6500 RPM. (lower boost isnt a bad thing. It could mean you have more flow going through the head -> less backpressure due to intake runner length being tuned for that RPM + cam selection + head work + exhaust back pressure (effects scavenging) and as you go past that optimal band you are getting out of the design range for that cam (the less technical term would be when a car "falls on its face after X RPM" ) and intake and exhaust setup, so you get more back pressure, which results in a higher "boost" pressure as you are actually flowing less air to the cylinders / intake event. ( less air mass)


In short.
Due to your current setup of intake, cam, exhaust and head work your best volumetric efficiency could be around 6500 RPM and your engine at higher levels may be less effecient at taking in fresh air and filling the cylinder resulting in "more boost" before and after when in fact 6500 is still flowing more air / intake event than anywhere else.

After all boost pressure is just a measure of the resistance to the air going into the cylinder.

(it is common to see lower peak boost values with a top end cam and high-flowing head setup)

 

It sounds like he has a stock head, though rebuilt. No aftermarket cam.

 

well that makes sense then as the stock cam "does not make power" after ~66-6800 RPM

-> as in it doesnt breath as well at 6700 rpm as it does at 6200 rpm.

Since the supercharger is still linked to the crank, it is pushing a larger volume of air / turn, but the engine is taking in less volume of air / turn causing a larger back pressure and higher boost readings

Could also be belt slip as while what I am saying works in theory, I am not sure as to the scale of it as I have never bothered with data-logging it (and I have not had a stock cam in quite some time )

 

That's a perfectly valid point, but the dip still doesn't quite make sense. I personally wouldn't worry about it, because as I said up top, there's little value in revving this motor with a stock head that high; it's just not productive.