Hey guys,

I just got my car back from the tuner... new D1-SC build. It's an 8-rib kit running the 7.5" pulley and 3.70" blower pulley. I'm running a Vortech bypass valve. The tuner says that we have a belt slip problem because the boost levels keep falling each run. They tighten the belt, get some boost back, and then it falls the next run. In three pulls it went from 14psi (610RWHP ) to 3psi.

I took it home with a conservative tune to try to figure this issue out. In driving the car, I get different results. Without making any changes to the belt, the car will give me different boost levels at each pull. I notice that in lower gears I'm more likely to get boost. In higher gears it sometimes even shows a vacuum at WOT and 4k+RPMs. On one pull in 4th, I pulled through about 4500RPMs and saw -6psi vacuum at WOT! When I do get boost, it seems to take a second. I'll punch it at 3500 RPMs in second gear and get vacuum for two seconds and then suddenly the car goes sidways because I got 10psi of boost. It never seems to be an instant hit when I peg the gas.

We have a smaller belt than Bob's matrix indicates on the car (just to be sure), and it's damn tight. I don't see any dust yet, but I'm not sure how obvious that would be. Alignment looks good, but I'm only eyeballing it, and I'm not sure how I would adjust it if it were off, anyway.

I'm beginning to wonder if this isn't an issue with the BPV or the vacuum line running to it. I haven't adjusted much on the BPV yet (because I lost the instructions), and I'm running a very small vacuum line that came with the boost gauge (I'd guess it's 1/8") from the back of the intake manifold to a T-fitting and from there to the boost gauge sending unit (screwed down near the fuse boxes) and to the BPV (under the driver side headlight).

Could this be a vacuum issue? Do I need bigger lines? A different feed? Do I just need to adjust the BPV?

 

Bump... I'm trying to offer as much info as I can on this. I'm a total newb, so let me know if you need more info.

 

I doubt its belt slippage. Bob at epp has said a belt should be fine/fully stretched after its first run-And you have already tightened this so it shouldnt be an issue, and I know of people running alot more power on the 8 rib setup without any problems. I would think you're right about the vacuum leak, but it wouldnt help to check your surge valve. How many things do you have hooked up to your brake booster line?

 

I don't think it's a vacuum leak, but a vacuum issue. The gauge shows almost -20psi of vacuum when decelerating with the clutch engaged. It also showed 14psi of boost on one pull.

I don't have anything hooked up to the brake booster line. It's all hooked to the smaller vacuum line coming out of the back of the intake manifold. I think this might be the issue. The line is 1/8" thick and probably five to six feet long feeding both the boost gauge sending unit and the bypass valve. I'm afraid this might be a restriction that's causing the problem.

Has anyone had this issue before?

 

If its the small line causing the false readings, then don't you think the pull with the 14psi and 3psi would have the same power output?

 

I think the issue is that the vacuum line is preventing the BPV from working properly. The boost gauge and the BPV are both fed from the same 1/8" line. If the BPV actually has a large volume that holds either vacuum or boost, then it might require a lot of air to go through the line to shift from vacuum to boost and visa versa. If so, then it might not work properly, and the boost gauge would suffer the same incorrect pressures in the line.

I'm going to test this theory eventually. Unfortunately, the car is down again because I broke the crank pulley. Once it's replaced later this week, I'll report what I find. I was just curious if anyone had seen issues like this or if they had any other suggestions.

I think the biggest clue I have to work with is that when I'm cruising at 4k RPMs and then stab the gas, it can take 2-3 seconds to make boost. It does it very suddenly, but it's very delayed. I've never driven a blower car, but I'm assuming it should be instantaneously giving me boost.

 

I don't see a problem with the 1/8" line feeding the boost gauge. You have a centrifugal supercharger, so I'm sure its different from a roots or turbo. If you meant to type BOV, my tial requires a 6mm inner diameter hose

 

BPV = ByPass Valve. It's vented to atmosphere, so it's basically a BOV, but it's still technically a BPV.

If your BOV requires a 6mm ID line, then that supports my theory. I'm really glad you posted that. 1/8in = 0.125in = 3.175mm. That appears to be the OD of my line, too. So, I may be WAY short of what I need. It's only half the size of what yours requires, and that's a lot less than half the flow capability.

By my math, a 6mm ID line will flow somewhere on the order of 3.5 times as much as a 1/8" line, and that's only the minimum!

 

The instructions for the Vortech Mondo and Maxflo are found here:

http://www.vortechsuperchargers.com/...ypass%20Valves

What's the line to the bypass valve made of? Nice thick, sort of stiff rubber? Or soft thinwall stuff? I would try cutting the boost gauge out of the circuit - use a piece of line to go just to the BPV. The line is cheap so you can experiment. How short can you get it if it just goes from your vacuum/boost source to the BPV?

Don't get too excited by the inner diameter of the line thing - the volume of air in the BPV is very small and does not take much air movement to actuate. I use the Vortech Mondo and small hose works perfect.

Lastly, is this a MAF setup or SD? Maybe the most important Q, since if you are running a MAF and an open BPV you are screwing up the mixture calculations when not in boost (BPV open).

Check thoroughly for dust - it can be pretty small and flow backwards but if you look around you will find it in nooks and crannies.

Jim

 

The vacuum lines were the issue. I capped off the 1/8" vacuum line coming from the back of the intake and put a T fitting in the brake booster line. I ran 1/4" I.D. line to another T fitting feeding the boost gauge and the bypass valve. Now, I have instant boost, and it's consistently reaching 10+psi. The belt is much, much looser than before, too. It was definitely NOT belt slip.

Hope this helps someone else if they have the same issue.

 

It's good to see you got it figured out. Which air filter do you have? If you have the standard ProCharger air filter, try making a run with the filter off. If you pick up several pounds of boost then you need a larger filter. Bob

 

To be honest, I'm not sure. I bought the kit from Speed Inc, and the intake pipe wasn't the typical air box. It's a conical K&N filter, and it fit on the end of the truck intake (which I bought from you ). In any case, I'm making good boost now. I might try a pull without the filter just to see what it does. If the boost increases, I may get a bigger filter. I'm not concerned about the power because the car is already STUPID fast. However, I am concerned about the filter getting sucked inside out.