killav

I read a very interesting article this weekend about utilizing a wastegate designed for a turbo application, and installing it in the pressure side of a supercharger system. This allows for a much more aggressive boost curve, while allowing the excess boost pressure to slowly bleed off to the atmosphere after a certain boost pressure is attained.

The car in question switched to a 4" pulley and a different compressor wheel, and along with the 44 millimeter wastegate, managed to pick up 80 ft/lbs of torque in the meat of the curve where most street driving is done without losing any HP up top. I was impressed.

The downside is, you are going to be generating a lot more heat, and will also be getting close to spinning the supercharger to fast so you have to be careful. The article even mentioned that the manufacturer (of the supercharger) frowned upon this practice. But still, if kept within reason, this seems like something the Murph Kit could really benefit from.

Thoughts?

Ian928

I think Carl has a valve designet for just that

http://www.928m.com/parts/boostlimitervalve.php

killav

Ian,

Not sure if that works the same way, but its close. I did a search and found this article, not the one I read in the bookstore, but same principle.

I also did a search on R-list before posting this and didn't come across a thread right away that talked about it.

http://www.musclemustangfastfords.co...l/viewall.html

killav

From the article:

"The curve looks like a turbo car," Spetter says. "It may not be as aggressive as a Kenne Bell or Whipple Cobra, but if you compare this graph to another one of my customer's Cobras, you can see that Grundman's car crosses past the Cobra at 4,000 rpm. From a roll, this Saleen would be all over a Cobra." The Whipple Cobra made 614 rwhp at 17.99 psi of boost (on race fuel), while the Saleen knocked down a best of 655 rwhp with a wastegate regulated 15.02 psi. Spetter couldn't find a similarly prepped turbo car that had been saved in the Dynojet program, but he said a turbo street car would have a boost curve that looks similar to Grundman's Saleen."

killav

I would gladly shift at 5500 rpm to keep the supercharger speed down, if I could have an extra 80 lb/ft of torque under the curve......especially for street driving.

FredR

Mike,

There is nothing wrong with doing this providing you do not exceed the design rpm's of the s/c unit. You will of course be absorbing more parasitic losses at high rpm's for no benefit, indeed whatever you currently produce at the chosen boost pressure will be reduced at the wheels by the additional losses driving the s/c for which you get no benefit.

On the other hand you gain lower down the range where the s/c does nothing at all until about 3k or so. Now, if you can translate the curve to get boost at 2k rpm [auto stall speed] what you now get at say 3k rpm it may well make the centrif s/c route more interesting to auto owners. Once you have some 450 bhp on tap do you really need more?

It might sound like a steam train when the valve lifts at 5k rpm or whatever but who cares and how often are you likely to be up there?

Fred

soupcan

Yes

Lizard928

I agree with Brian, but the one thing a turbo car can do vs a SC is have two (or more) boost settings to vary how much power is on tap at any one time. Makes it handy for every day driving, or for that extra bit to pass someone on the track. You know like a pesky beetle (997) turbo that won't move over!

FredR

You have an intercooler so you already have more than 450 BHP!- probably more like 450 rwhp?https://rennlist.com/forums/graemlins/drink.gif

On your avatar what is that purple anodized thingy? Something to do with s/c lubrication?

Regards

Fred

soupcan

That's my version of an adjustable boost controller...

soontobered84

Holy Crap!

RFJ

Hey soupcan,do you have holy crap,any idea what the hp would be out of that thing?

killav

This was exactly one of the things the article mentioned. Combine the wastegate with any good electronic boost controller, and your boost settings could be highly adjustable, on a fixed pulley centrifugal supercharged car. Tuning is the only obstacle on a 928, because you would have to make the mixture fat enough on the highest boost setting to be safe, which would make it pretty rich when you dialed the boost down. It could be done, but it would take some careful tuning, and I don't think the boost PSI spread could be very wide, maybe 4 PSI or so.

Does anyone know off hand the current supercharger gearing in the Murf Kits? Is it close to being maxed out RPM wise already?

Z

There are other more efficient ways to change the boost vs engine RPM rate with a centrifugal superchager besides bleeding off air that you compressed for no good reason. One of them is by varying the supercharger to engine RPM ratio as the engine's running. One manufacturer has used this method with a two speed transmission between the crankshaft and the supercharger. Another method is much simpler, cheaper, and is used in the design of the Murf928 supercharger kits.

There are dyno and data logging results showing measurable boost and power increases with Murf928 centrifugal kits on 928s at 1,500 RPM already.

That would only be the case if the engine management configuration wasn't adequate for the car at the higher boost setting in the first place. If it was, and you lowered the boost, you'd just be in different and lower load cells of the fuel map, which would be tuned for the lower boost. With a proper configuration and tune, there'd be no need to change any of the tuning when boost levels were changed.


Centrifugal equipped cars have infinitely variable boost and power settings up to their maximums. They're controlled by the gas pedal.

killav

This is true. I was thinking more about the car running a little rich by dialing down the boost a lot, and thereby being in a cell in the middle of the map somewhere where the boost was significantly lower than before. I am just thinking out loud here. With a fixed pulley supercharger, it shouldn't matter but I don't know. I think as long as you tuned for max PSI, then anything less would probably run ok as long as you kept things within reason (not dropping to 2 PSI for example from 11 PSI max),

The bottom line is, according to the article that I read, is that it works. If you are looking for much more torque under the curve and don't mind all the heat and wasted boost at the top end that must be release to the atmosphere that is.

I'm still interested to know what RPM the Murf kit blowers are turning at present in the stage III kit.