Centrifugal supercharger question

[My427stang]

Jay, or anyone else that has experimented with centrifugal superchargers, a few questions

- Is there boost at idle?
- Do you limit boost on the top end with any kind of waste gate or is it all determined by pulley speed?
- What is the max static compression you have run?

Thanks

[jayb]

Looking at blowers now, Ross?  Those things are great, and if you happen to pick a Vortech blower I can help you with the mounting and pulley arrangements, because I went through all that stuff with my supercharged engine.  Answers to your questions are as follows:

- On my setup there was no boost at idle; boost didn't start coming on until around 1500 RPM, and then really didn't hit until 2500+.  However, the engine cooling requirements at idle were substantially greater than with a naturally aspirated engine.  I would guess this is because the blower heats up the induction air so much, and also because of the greater load on the engine to turn the supercharger.

- I was always looking for more boost, so I did not limit it at the top end.  Most people will just change pulley sizes to set the top end boost value.  I was able to get 17 pounds out of my Vortech V7 Ysi supercharger, with the HTD belt drive.  With the 10 rib belt, the supercharger would only deliver 11 psi due to belt slip.  So that is another way to limit the boost 

- Since I was going for big boost, and had a carburetor, I held my CR to 8.75:1.  If I decided to switch to EFI, I may have gone higher...

Hope that helps - Jay

[plovett]

That's interesting because I've read that you can actually run more boost/compression with a carburetor than with fuel injection.  Even ATI says that.  I don't know personally because I've never tried it back to back.   I've never even run a supercharger.   I do think centrifugal blowers are especially cool for a street/strip car.   More efficient than a positive displacement blower and less heat than a turbo, probably (maybe?) better mileage than a naturually aspirated 550-600 cube monster, plus the incremental boost seems ideal for a traction limited scenario.

I loved reading about the blown Mach I, Jay!  I'd like to run a blower myself someday.   I hope this thread keeps going.

paulie

[My427stang]

It wont be for a bit, my engine is currently 10.7:1 and although its a stable block, its a .047 over center oiler, I'd like to reblock at the same time I drop the compression, plus I need to spend my free time wrapping up the truck

That being said, in a pretty good place right now and considering selling my Harley due lack of use, so my timeline could be pretty quick if I decided it to be

To be honest a secondary plan was to buy a used Viper and hang a huffer off it, but that just seems frivolous.  A blown 505 inch EFI FE seems much more conservative

[fe66comet]

The main concern is always what you put under the top end. A 390 for instance will not hold up to big boost. Start off with a cross bolted heavy web block, forged crank and H beam rods. That is anything over 6 or 7 psi and 600 HP is pushing the limit. After that a 428 with after market caps and studs.

[My427stang]

Mine is a 489 inch .047 over 427, H beam rods, lightened SCAT crank, Diamond pistons.  Despite being a seasoned and trued 427 block that I have owned and ran for a long time, I just don't think it'll love living above 700 hp.

It'd probably make it, but if it didn't what a mess

[plovett]

You considered putting some Hardblock in that .047 over 427 Ross?   Sounds like you're planning an aftermarket block for your blower build which would of course be even better.  Expensive though.   

A blown Viper would be fast, but they sure sound funny.   

paulie

[My427stang]

I am not a hard block fan on the street, at least not the way I use the car. 

The reason I was asking the questions I did is because I am attempting to keep torque up at very low RPM cruise too.  Although I wouldn't build a fresh one with high compression purposely, I did wonder how 489-505 inches would behave in 5th gear at 1800 rpm with 8.75:1 and a little more cam vs the current 10.7:1 and a relatively mild cam.  Having boost down low should offset that

That of course made me wonder, what if I had no or little boost down low, but could control it up top enough to run the compression I have?  Of course I looked all over the net and haven't seen ANYONE recommending that much compression, so my guess is that the cost offsets the gain with only 6-8 psi of boost

[My427stang]

Well I did find this link this AM

http://www.blowerdriveservice.com/techcharts.php

It would lead me to believe that somewhere around 2-4 PSI is about all it would handle with the current compression LOL 

For the cost of a blower, the gain of 2 psi would certainly be deeply into stupid poser status 

I just need to finish up the truck and get planning on a big one.  Maybe do some swapping around and put the 489 in the pickup, sell the fresh 445 after the summer, and start from scratch

[falcon428]

Yes but the "stupid poser status" is so popular in my area.  :-).  Most of those cars around here are left-overs from the "Pro Street days".

[jayb]

That's interesting because I've read that you can actually run more boost/compression with a carburetor than with fuel injection.  Even ATI says that.  I don't know personally because I've never tried it back to back.   I've never even run a supercharger.   I do think centrifugal blowers are especially cool for a street/strip car.   More efficient than a positive displacement blower and less heat than a turbo, probably (maybe?) better mileage than a naturually aspirated 550-600 cube monster, plus the incremental boost seems ideal for a traction limited scenario.

I loved reading about the blown Mach I, Jay!  I'd like to run a blower myself someday.   I hope this thread keeps going.

paulie

Well, that's interesting Paulie, I would have never guessed that.  My theory that you can run more compression on a supercharged application with EFI is that you would have better cylinder to cylinder fuel distribution than you would with a carb, and so the possibility of leaning out one cylinder and detonating is much reduced.  I also thought that I read somewhere that some of the more recent factory supercharged offerings are running up at 10.5:1, or maybe it was a Mustang race class, and this was also with EFI.  The only thing I can think of that would be an advantage for a carb setup is that the fuel would have more time to cool the charge, compared to EFI.  But since I've never run that kind of compression with a blower, I can't say it won't work with a carb...

[jayb]

Yes but the "stupid poser status" is so popular in my area.  :-).  Most of those cars around here are left-overs from the "Pro Street days".

LMAO at "stupid poser status"!  We get the Car Craft Summernationals every year in Minneapolis, and there is a LOT of that going around...

[plovett]

I am not a hard block fan on the street, at least not the way I use the car. 

The reason I was asking the questions I did is because I am attempting to keep torque up at very low RPM cruise too.  Although I wouldn't build a fresh one with high compression purposely, I did wonder how 489-505 inches would behave in 5th gear at 1800 rpm with 8.75:1 and a little more cam vs the current 10.7:1 and a relatively mild cam.  Having boost down low should offset that

That of course made me wonder, what if I had no or little boost down low, but could control it up top enough to run the compression I have?  Of course I looked all over the net and haven't seen ANYONE recommending that much compression, so my guess is that the cost offsets the gain with only 6-8 psi of boost

Now I think I understand where you're coming from.   I don't think you can do what you want with the current compression ratio. 

You ever read about the old Mcculloch variable ratio superchargers?  They had a variable speed pulley set up.   They had a higher ratio at low rpm for more low rpm boost.  At higher rpms the ratio was lower to keep the boost from going too high.  I don't know exactly how it worked.  It seems like it was an arrangement of springs working against centrifugal force (inertia) and oil pressure.  Paxton later had a variable speed supercharger with the ratio change made internally in the supercharger.  Something about moving the planetary balls around for different contact points???   Hee hee.

I realize this stuff isn't helpful.  Just shooting the shite so to speak.   Old airplane engines often had multiple speed superchargers as well.  They would have lower ratio for use at low altitude to avoid overboosting the engine, then at a certain higher altitude they would shift into another gear to run the supercharger faster  to maintain similar boost.  The Germans even used a hydraulic coupling to the supercharger to make continuously variable ratio.   These were all centrifugal blowers, though turbos were sometimes used to feed the centrifugal blower.   Like I said probably not helpful.   

If you want boost at very low rpm it seems like a turbo might be better as it's boost is more load sensitive than rpm sensitive.  You could dump the "excess" boost at high rpm without the huge power loss of turning a mechanically driven blower.  Maybe? Or if you're using fuel injection then maybe a helical screw blower might be low enough in height to fit under a scoop.

Or maybe just use very large displacement motor like a 572-604 with 385 series architecture.
 
paulie

[plovett]

Well, that's interesting Paulie, I would have never guessed that.  My theory that you can run more compression on a supercharged application with EFI is that you would have better cylinder to cylinder fuel distribution than you would with a carb, and so the possibility of leaning out one cylinder and detonating is much reduced.  I also thought that I read somewhere that some of the more recent factory supercharged offerings are running up at 10.5:1, or maybe it was a Mustang race class, and this was also with EFI.  The only thing I can think of that would be an advantage for a carb setup is that the fuel would have more time to cool the charge, compared to EFI.  But since I've never run that kind of compression with a blower, I can't say it won't work with a carb...

Yes, I believe it's a heat of vaporization deal with the carb having more distance/time to cool the charge.   I don't know if modern supercharged cars run a lot higher compression with EFI.   The naturally aspirated cars sure do though.   

Ponder this.  Would those methanol/water charge coolers be as effective if they were injecting at the cylinder head?  I dunno.

paulie

[fe66comet]

Really most superchargers are happy at a 5000 rpm limit anyhow. They become inefficient after that and turbocharged becomes a better choice.