Lagavulin

I am sure it is because the belt is slipping; it is hard to initially judge how tight to make the belt. Judging by the rwhp numbers, the car was probably doing less than 7 psi; an 8 psi GT should do 435 rwhp.

Good job; another MURF928 SuperShark is on the road!

Jim R.

Lag ,

Those numbers you are estimating are closer to what I expected. Is it also possible the knock sensors are faulty and retarding the timing six degrees besides lower boost? I have some interest in this as well, since I plan on SC'ing my GT very soon. I am trying to schedule a dyno for next week to baseline the car. I have the last dyno from 2000 at 292rwhp and 289 rwtq bone stock. Hopefully those numbers will be consistent.

Jim R.

Lagavulin

It's possible, but I think it's more likely that the belt is slipping and why the power drops off like it does.


Those are strong NA numbers which means you'll have higher supercharged numbers as well.

Carl Fausett

Lag is right on the money - I just finished looking at the video from the Dyno Shop last night, and at the very end of the pull you can hear the belt squeel.
I never heard that from inside the car.

I had a thrown my video camera at a kid that was gawking at the beautiful 928 and asked him to video a couple of the pulls. I can hear the belt squeel on only one of the 2 piulls he taped.

I agree with Lag - it's hard to know how much to tighten a new SC belt - you want to go a little easy on the bearings in a new supercharger until they bed in a bit. Also - the rate of acceleration on a dyno that produces belt slippage will never be duplicated in the real world.

From Andy: 1) the Murf kit does not require a hole in the WWasher reservoir.
2) the kit does not require R&R of the intake manifold.
3) the kit does not require replacement of the knock sensors.
4) the kit does require removal of the stock 24lb injectors and replacement with 30 lb injectors that are provided.

"Non-Invasive" in medicine means not entering the body. In engine mods, it means not enteriing the engine. The Murf kit and the 928M kit are non-invasive, true bolt-on kits.

Remember that there is not "one right way" - there are a variety of ways to get more HP out of any motor: Boring, stroking, nitrous, turbo, roots supercharger, twin-screw SC, centrifugal SC, axial SC, fuel additives; and they ALL WORK if designed and engineered correctly. Each method has its pluses and minuses and unique characteristics.

It is not neccesary to put down another's product because it is different than your own. Point out the differences and leave it at that.

worf928

Make sure that you get the actual raw data from the operator - not just the pretty picture. That way you can plug the data into Excel (or other similar) and apply your own transformations rather than rely on what the operator is doing to make a pretty picture. It will also make it easier to do direct comparisons later - not only with your post-surgery shark but with other sharks from different dynojets. If you ask the operator should be able to supply your with a printout of the raw data.

Gretch

I can attest that the tighter the belt, the higher the pressure. The pulley Tim provided me makes 6 to 7 psi with the belt "snug" and I have seen as much as 10 psi when I cranked down on the tensioner and got the belt "gootentight" per Tim's instructions to me over the phone.

BrianG

Back in the day, we used to run the GM roots-type S/C with a toothed belt like the 928's cam timing belt, onlu about twice as wide. Why do these 928 S/C kits not use toothed belts to get around the slippage issues, guys?

Gretch

"We like the option of induced slip" ?????

BrianG

you young whipper-snappers got this all wrong......

Us fogies perefer our slippage at the asphalt-interface.....

Carl Fausett

Brian - some do use a toothed or cogged belt, like the Albrecht kit in Austria.

Generally, a centrifugal SC does not have near the drag as a positive-discplacement blower like a Roots, and a cogged belt is not neccesary.

The downside of a cogged belt is that they produce vibration and we like to avoid that where possible.

On the Albrecht system where they use a cogged belt, to change the pulley ratio to run a higher or lower boost, you must change out the CRANK pulley each time, the pulley on the head unit stays the same. Weird.

I have 2 customers in Great Britain that I feed with parts to keep their Albrecht system running, so I am pretty familiar with them.

To say there is a belt slippage "problem" is to overstate it. When properly tensioned, I have no belt slippage on any of my SC cars or Tims installs. In this case, I had a new belt on a new SC at the dyno, and I undershot the initial belt tension a tad. That's all.

Gretch

That is a compliment man......I am well past a half a century old.....

My Supershark has plenty of slippage at the asphalt-interface......so much so that I went to 10 x 18 Hoosiers just to "hook up".....I like the option of being able to lighten the boost for every day driving, and tweak it up for "special events"..

to each his own..............

Tim Murphy

I think I may have the solution to the belt tensioner. This is a die spring with 360 lbs force at 70% deflection. The one in the picture is on the car that I am testing out the stage one kit. Sorry for the fuzzy pic, I was obviously focused on the oil spot on the floor.

BrianG

Tim, that looks like a GREAT idea. A dynamic tensioner with controlable preload !!!

Simple, yet elegant!

Gretch

Tim, It is an "upgrade", right??????

Old & New

Brian,

It is also true that Vortech designs accomodate both types of drives; the standard unit utilizes a ribbed belt, and their "heavy duty" unit utilizes a cogged belt. I believe the gears in the standard unit would not fare well if driven with a cogged belt.