hacker-pschorr

Why not just tune the upper end for the added boost???

I'm running the supercharger on my car at max impeller speed at redline for my engine. Bolting on a wastegate as the article describes would only lower my HP upstairs and do nothing for the torque down low.

Such a setup would dramatically slow my car down.

If you are really worried about the engine at those boost levels, re-tune for less power upstairs and let the boost flow. When are we going to get past the mentality that some magic amount of boost kills these engines?
Bad tuning destroys engines.
Also, the highest risk for detonation is at peak torque, not HP. People are setting themselves for a false sense of security by limiting the HP upstairs thinking it's "safer".

This is a lot of work and wasted power when all you need to do is drop a gear if you want to go faster.
One of the most exciting cars I get to drive on a regular basis is my fathers 355GTS. Talk about no torque down low.........and it doesn't need it.

dprantl

+1. It does not make sense. A supercharger is driven by the crank, and the wastegate is not going to make it spin any faster. All it can do is bleed off boost to make the pressure in the manifold lower, which means less power. If the car is tuned properly, it should never need such a system.

EDIT: Sorry, I didn't fully read that post. So you put on a smaller pulley to get more boost, but then put a wastegate on to bleed the pressure off at the top end? Why not just take advantage of the higher top end boost as well. If you exceed the limits of the supercharger at high RPM, then it's like you are limiting yourself to lower RPM on purpose.

Dan
'91 928GT S/C 475hp/460lb.ft

killav

The reason is that the boost level was somewhere in the 20 PSI range on this particular car after switching the impellar and pulley, so it had to be bled off to maintain 15 PSI on the top end or it overboosted this application. In return, this car was getting much more PSI down low, around 4 PSI more than the previous set up "under the curve". So the HP stayed identical at peak RPM, around 600 HP, but the torque and HP increased significantly down in the mid-range.

So, the curve acted more like a turbo car, peaking out around 4500 RPM, and maintaining constant 15 PSI to red-line. The current Murf kits or any other supercharger kit for that matter do not do this. They max out at red-line.

Edit: This modification would give better streetable midrange, without overboosting the car. I'm not advocating everybody run out and do it, but it is something I would be looking into doing if I had chosen the supercharger route.

killav

The supercharger can't be maxed out already in order for the wastegate mod to work. That is why I am interested to know what the current Murph stage III kits are spinning at.

hans14914

If someone were to use an intelligent boost controller in the aforementioned scheme, it would be possible to use the same controller to toggle the coding plug mode (with a small relay based circuit) so as to have two completely different maps in the LH/EZK.

killav

Hans,

Looking forward to seeing you at Third Coast!!

I recently installed a manual three position, double throw toggle switch and wired this to my coding plug and three LED lights, yellow, green, and red for the three different maps I am running. It works great. Using an electronic boost controller for this function would be down right cool.

hacker-pschorr

That's because this "mod" is based on a giant misconception that boost alone somehow kills engines.

The problem is, like I said before, this goes on top of another misconception that the most "dangerous" area is high RPM. Meanwhile you have engines that are detonating like a diesel at 2,500 rpm and people wonder why the head gasket let go.

If someone is concerned about making more power upstairs than an engine can handle, de-tune the motor. If they don't know how to do that, pay someone who does.
Z hinted at another method for limiting boost upstairs, I'll let him elaborate on that.

soupcan

I can see how this would be helpful trying to build as much power under the curve on an engine that is rod/crank/block strength limited.

Ketchmi

The way I did it on the 630rwhp 89' was to install a second BOV, disconnected from any vacuum or boost source that opened mechanically at 16psi. (just spring/washer pressure) The engine was tuned for up to 16psi and without taking it to a lower altitude to tune it for higher boost I felt this was a safe addition. With the Vortech V7-Ysi it has plenty of boost midrange, enough to roast the tires at will in first or second with an auto, an upgraded LSD and 295/35-18's. I would need to datalog to see exactly what boost it's hitting at 3000rpm, pretty much as soon as it hits boost it hits redline.

FredR

A Murf stage 3 with nitrous on top of it?

Questions:
1 What does that thing turn out?
2. How many tires do you get to the gallon?
That thing must be manic- hope your gearbox can handle it.

With respect to the original post I kind of assumed the blow off valve is something you might do to a stage 1 type of kit to get more streetable use without resorting to intercooling etc etc.

Regards

Fred

anonymousagain

Hacker, the point is not to limit use of the higher <available> boost, nor that high rpm is an engine killer. The point is to allow tuned max boost to arrive earlier than max rpm, to promote a higher avg hp under the curve.

Lizard928

If you wanted a torque/power curve of a turbo, why not just get a turbo?

killav

I did!!

I have to say I have never ridden in a Murphed car, and it probably doesn't even need any extra down low torque, but, there is always somebody out there who wants more.

killav

This pretty much sums it up. Having the ability to maintain constant boost pressure after a certain RPM, just like a turbo does, with a belt driven supercharger. To me, this is the best of both worlds. You get better performance down low like a twin screw, and the higher effeciency of the CS design for high RPM horsepower.....

soupcan

Just talked to one of our local mustang racers and he has this setup and swears it works as advertised.
He installed the wastegate to lower the boost so he could run on the street with pump gas and was really surprised at the increase in lower rpm boost.