No Boost (vf supercharger)

[M5Yates]

I don't seem to be boosted any more.
It idles fine, but has lost power (well, back to stock, gasp).
Parked and revving engine, intake manifold has vacuum (lots).
The car has an intercooler and the piping seems to be tight.
I changed out the bypass valve to the original Bosch.
I cracked open the coupling at the charger while at idle and it seems to be working.
Can there be a leak that lets all the boost escape?

Thanks,
Bill
1995 318ti w/ S50

[pbonsalb]

Check the idle air valve hose. Since the intercooler is an add-on, recheck all of its piping. Check the couple at the TB. You could have a split that opens up only under boost but is OK at idle.

You could have old plugs with big gaps that are causing spark blow out under boost.

Check belt tension to make sure belt is not slipping under load. For the VF, this would probably mean the accessory belt tensioner.

Make sure your blower pulley key has not broken, allowing the pulley to rotate on the shaft.

[M5Yates]

Check the idle air valve hose. Since the intercooler is an add-on, recheck all of its piping. Check the couple at the TB. You could have a split that opens up only under boost but is OK at idle.

You could have old plugs with big gaps that are causing spark blow out under boost.

Check belt tension to make sure belt is not slipping under load. For the VF, this would probably mean the accessory belt tensioner.

Make sure your blower pulley key has not broken, allowing the pulley to rotate on the shaft.

Thank you for your suggestions. I took the car to the track today (COTA) and did get some boost. Maxed out at 3 pounds, 4 & 5 gear, 6800 rpm. Looks like I'm getting 1 pound starting around 4500. The boost gauge is new, so I have no idea what pressure was before intercooler was installed. I'll have time to pressure test later this week and check for other problems.

[badata1]

What size pulley are you using? If it's still using the stock size pulley with the added air/air intercooler then you might only see 3psi. I believe 3.6" is the stock size.

[pbonsalb]

That is true -- the intercooler adds a restriction and the boost must be raised to over come it. Not sure what IC the PO used or how restrictive it is or whether he changed the pulley to compensate. It is possible that if you have a pulley designed to give 6 psi with no IC, you are getting only 3 psi with the IC though I would expect less loss.

Centrifugals make little to no low end boost. They are crank driven and make more and more boost with rpm. It is exponential. You should get roughly the square root of peak boost at half peak rpm and peak boost only at peak rpm. So a 9 psi kit makes 9 psi at 7000 and at most 3 psi at 3500 rpm.

[95AWM3]

I am glad you got most things going and figured out, this is actually my old setup. Before the air to air with the 3.6" I would see 5-6 psi at redline like you should. I never had the 3.6" pulley on when I did the intercooler though, I always had the 2.62" pulley on, which resulted in 12psi at redline. It is important to note that the impeller is upgraded as well, from the Sci to the SI. Slap that 2.62 and gates blue belt in there and see what she will do!

[M5Yates]

Slap that 2.62 and gates blue belt in there and see what she will do!

Not with the stock VF software!

The Vortech SI propeller upgrade is supposed to add 2+ psi. My Vibrant 12810 intercooler is spec'd at a 1.5 PSI drop. I should be seeing 5+ psi.

I think the belt must be slipping. I only get boost with wide open throttle above 5000 rpm. Nothing with part throttle.
I'm still worn-out from three days at the track. I'll dive in Friday after work and maybe put the original pipe straight from charger to throttle body back on to see what it reads. After that, I think I have the right parts to test for leaks. I'm not sure how to test for belt slippage.

[badata1]

Test for leaks and see what comes up.
I have the same setup with the same intercooler and saw a 3 psi drop
I doubt you have any belt slippage with the 3.6" pulley unless your tensioner is absolute trash.

[M5Yates]

Test for leaks and see what comes up.
I have the same setup with the same intercooler and saw a 3 psi drop
I doubt you have any belt slippage with the 3.6" pulley unless your tensioner is absolute trash.

Thanks. Brand new OEM tensioner.
I plan to use the caps that came on my new MAF and stick an air hose connector in the middle.

[pbonsalb]

I doubt the Si trim adds 2 psi at very low boost. Probably does at high boost.

[M5Yates]

I doubt the Si trim adds 2 psi at very low boost. Probably does at high boost.

that would be 2 at 6500 rpm and should be enough to make up for most of intercooler pressure loss.

[M5Yates]

I do plan to put on smaller pulley and upgrade software and injectors. But think it needs to be running right at low boost first.

[pbonsalb]

I don't think it will add 2 psi at red line when you would otherwise be making only 4 psi at redline.

[M5Yates]

I don't think it will add 2 psi at red line when you would otherwise be making only 4 psi at redline.

Stock, this is a 6psi setup. Add 2 for the SI upgrade. Subtract 3 for intercooler. So I should be seeing 5 and at least some psi on the way up.

[pbonsalb]

I don't think that at this low airflow level the Si trim will give an extra 2 psi. At higher flow levels, probably. I would guess if 6 psi is stock you will get 6.5 psi with the Si trim. Get a proper tune and run a smaller pulley.

[M5Yates]

My leak test bubbled badly in several places. Including from the welds of a custom pipe (that I paid way to much for). I put the VF charger-to-throttle body pipe back on to see the boost without the intercooler - 7.5 psi at 7000 rpm. The AFR was 13.5 at that point.

When the JB Weld on the pipe dries, I'll put the intercooler back in the loop and see if the boost goes higher than before.

[pbonsalb]

That is a good test. At least you will know the pressure drop from adding the IC and piping. There was a guy on here maybe a year or so ago who had a supercharged 540i that to which he added an intercooler and measures pressure drops. He found surprisingly high pressure drop. Then he tried 1 or 2 more intercoolers and got the pressure drop down to about 2 psi. I think his final intercooler was a treadstone. But even within a supposedly quality line of IC, some may flow better than others for the intended use.

[M5Yates]

Pressure is up to 4 psi with the intercooler piped back in. No time to run another pressure check for a couple weeks. I assume there are more leaks.

I am changing the pulley from 3.6 to a 3.33. The current tune should handle the extra boost.

[M5Yates]

VF said to use a 2145 belt with the 3.33 pulley. I've seen others recommend a 2135. Stock VF is 2160. Anybody with a 3.33 know what they are using?

[pbonsalb]

So 3.5 psi pressure drop from the IC and piping (7.5 psi - 3.5 psi = 4.0 psi). Not ideal, but also in the range of what some others have experienced from what I recall reading. You can spin the blower faster to compensate and still make decent power without overspinning the blower.

[1000hp]

VF said to use a 2145 belt with the 3.33 pulley. I've seen others recommend a 2135. Stock VF is 2160. Anybody with a 3.33 know what they are using?

Measuring with a soft measuring tape will get you close. Measure the route with the tape, laying the tape where the belt will set with the tensioner cranked back. Subtract an inch from that length to compensate for the thickness of the belt vs. the thin measuring tape.

But it sounds like you've either got a restriction or a boost leak. A restriction will cut boost big time. If you have anywhere in the charge tract where the diameter of the pipe decreases, it will restrict flow. As little as a few millimeters can cost you precious psi. That, coupled with the fact that you're using up limited available air flow to pressurize that air/air intercooler, will lose you quite a bit of pressure.