gtaguy

I have a 1987 trans am GTA that i just put a supercharged 383 into. I am running AFR 190 heads and the accel superram. The blower is a paxton SN2000 H/O. i am supposed to be seeing 6-7lbs of boost, but I am only seeing 3lbs. I have talked to a lot of people and some say that tuning could be the problem. Is that true? i am running the stock computer and it does need a lot of tuning. Others have said that the tubing supplied with my autometer gauge isn't a good enough size and that it might not be reading accurately. i have never heard of the tubing being a problem. Should i change it to -2an line? Could that be the problem? Is there anything else that i should check?
Thanks!
-Matt

Willie

iTrader: (1)

Matt,

I believe I know what's going on. First of all, Paxton's SN series superchargers are designed for stock engines. You're far from stock. That means you'll produce much less boost on a modified engine. Case in point: Me!! My SN compressor was capable of 10.5 lbs boost on my stock 305. After a cam and head change, the very same compressor was only capable of 6 psig boost. That's because the intake was dramatically "opened up" -- less restriction means less boost. With AFR heads and a Superram, you've opened up the intake as much as possible. I'm not surprised to see you're only getting 3 psig boost.

Unfortunately, there is no simple solution to this dilemma. The SN compressor is only capable of spinning to 38,000 rpm. Given a step-up ratio of 4.4:1, a 5.75 inch crank & 3.25 inch SC pulley, the supercharger is redlined at only 4,861 engine rpm.

The only way to increase boost is to increase impeller speed. Yes, it is possible to install a smaller diameter SC pulley, but you will severely lose upper rpm's -- your redline will be much less than 4,861 rpm's!!! So this is not a viable solution with this SC. This is the VERY reason why I decided to go with ATI. Both Vortech and ATI have similar SC redlines -- much higher than the SN -- on the order of 65,000 rpm's.

Matt, I believe your combo will not produce any more boost than what you're presently getting. By replacing the Superram with your stock plenum might help, but I don't think your boost would increase by much. Your only solution is to go with ATI or Vortech. Sorry for the bad news. Hopefully, I'm wrong but I don't think I am.

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Willie

Supercharged 1987 305 IROC-Z, Daily-Driver, Emissions-Legal.
12.57 @ 111 mph.
12.04 @ 114 mph (50-hp nitrous).
http://members.optushome.com/au/downunder1/rides/willie/willie.html

1987 "20th Anniversary Commemorative Edition" Z28 Convertible -- Super Chevy Show Class Winner, 1998.

[This message has been edited by Willie (edited December 14, 2000).]

gtaguy

Thanks for the reply. Would stepping it up like that really lose 4psi? I would have figured a 1 maybe 2psi lost.
-Matt

Willie

iTrader: (1)

Well in my case, I went from stock heads to ported/polished Corvette aluminum. I also had the stock peanut cam and stepped up to a Crane 2032. Here are the figures:

Stock Dur @ 50: 179/194
Crane Dur @ 50: 214/220

Stock Lift: 350/384
Crane Lift: 482/496

Stock LSA: 109 deg
Crane LSA: 112 deg

This equates to a substantial increase in flow and volume. But the interesting fact is that I gained approximately 1.1 seconds and 10 mph with the stock setup (with vs. without SC) and with the modified setup (again with and without the SC). So this leads me to the conclusion that boost is a meaningless figure. You cannot determine how much power you'll gain based on an arbitrary boost number. It depends on how well matched the SC is to the engine -- how much flow both can support.

Equally as important is to match the supercharger and engine redline rpm to occur at the same time. By doing this will allow the supercharger to obtain maximum boost at (or near) your engine redline.

Willie



[This message has been edited by Willie (edited December 14, 2000).]

gtaguy

I see your point. I only wish I had known this before I got the blower. Thanks for your help.
-Matt

Willie

iTrader: (1)

Me too. When I rebuilt and modded the engine, I gained usable rpm. My redline (engine alone) increased to 6,000 - 6,200. Imagine how I felt when I could not rev past 4,861 rpm because of my supercharger. I then tried an experiment. I calculated that a larger (4 inch dia) pulley would allow me to match supercharger and engine rpm redlines to occur at the same time. But this wasn't without a compromise. I knew I would lose boost simply because I was slowing the SC down substantially. I paid almost $100 for this custom made pulley only to find out I could not get any more than 2 psig boost. It was at this time that I KNEW the SC was not compatible with my engine. I mean, what could I possibly do? The only solution I could think of was to start over (sigh...).

I honestly feel you're experiencing the same problem I did. Hopefully, someone else out there can concur or come up with a different angle..... Good luck.

Willie

gtaguy

I am going to have to save up now. I think that I am going to convert to a serpentine setup. Will one from a 92 TPI work? Then I am thinking of running a 15-20psi D-1b aftercooler kit from ATI. I know that my bottom end can take it and I will probably only see about 12psi anyways.
-matt

IROCKZ4me

I do concur Willie. I've got an old SN92 I got back in....you guessed it '92. The redline for the ball drive is lower than the IQ of a rock. I have visions of Prochargers dancing in my head. I wish they had been around back in '92

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• 355 cid
• AFR heads
• Arizona Speed & Marine hydraulic roller cam w/ AFR hydra-rev kit
• modified SLP runners
• TRW forged pistons/ceramic coated
• fully balanced
• Edelbrock headers/ceramic coated
• SLP cat-back
• Paxton supercharger
• Nitrous Express nitrous oxide

My IROC-Z
EFI Performance Club on Yahoo

Willie

iTrader: (1)

Absolutely!! Even a serpentine setup from a V8 TBI car will work. I took mine from a '92 305 RS. There is only one support bracket that's different between an TBI and TPI and it's still available thru GM.

A word of caution. Start low on the boost and work your way up. I decided to go with a D1SC (self-contained) unit. I loathe the thought of having to punch a hole in the side of my oil pan, therefore my decision was easy. All the D1 kits come with a three-core intercooler.

Willie

cy Z28

Just remember, that boost is the RESISTANCE to flow. If you open up the engine, you will lose boost. I think enerybody else has covered it. Just hought i should mention that.

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Cody "cyZ28" Young
'84 black on black Z28

355, 5-speed (on the way out. What goes back in, ooohh baby!!)

gtaguy

>A word of caution. Start low on the boost >and work your way up.

Why can't I put out the full boost to begin with? I have never heard anyone say that before, but since I am new to the supercharger world I thought I would inquire. I know that it is rated at 15-20psi, but once again the engine is opened up a lot so what should I be actually seeing on the gauge?
Thanks for all of your help!
-Matt

Willie

iTrader: (1)

Matt,

If you decided to start at 9 psig, I would say that would be fine. But you're intending on running 15 psig initially. That's awfully high. With supercharging, there's a LOT of tuning to be done, especially in the area of air/fuel ratio. If you run lean, you're prone to inflicting severe engine damage -- can you say "hole in piston"?

My recommendation is to start at 9 psig (or thereabouts), tune the engine (using a dyno to do this is a excellent method), then up the boost to 12 or 15 psig. Do it incrementally. I just recently ordered my ATI and wanted to start at 15 psig. I was told I could "explode" my engine, even though I was previously running 6 psig boost with a 50-shot of nitrous. ATI's recommendation to me was to start around 10 psig, which I'm going to do. An engine is a terrible thing to waste, then to later say, "I should have done it that way."

Good question. Boost is an irrelevant figure -- it means nothing. As stated in a previous post, it is the resistance to flow. If you decrease resistance (i.e. cam & heads), boost values decrease. From my personal experience, this does NOT mean you lose power. I did not. What ATI does is take information about your engine and how much boost you want, then designs the system to meet your goal. There really is no such thing as a generic 15-psig kit. Each kit is designed for a specific engine. However, as ATI has told me, designing for a specific amount of boost is not an exact science. The only way to determine how close they get to the design is to try it -- trial and error. (Similar to torque converter stall ratings.) Once you've tried it, ATI can send you another SC pulley to adjust the boost level. In conclusion, you will eventually get the boost you're asking for on YOUR engine.

Willie