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98c43amg
08-05-2003, 06:35 PM
Supercharging: Boost vs. Backpressure (a.k.a. When Less Means More)

That's the title that got me...

Check out the September issue of Super Rod magazine. This is not a magazine I'd normally buy. But the article's tout on the cover caught my eye, especially now as I'm thinking of supercharging my car. I recommend anyone considering supercharging, regardless of manufacturer, give it a read (there's no online copy to link to as superrod.com seems to be a work in progress). Interesting theory brought to reality.

Here's the "Reader's Digest" version of the Super Rod article:

o More boost does not translate into a linear increase in power due to the efficiency curve of the supercharger (we knew that) and BACK-PRESSURE of the engine (most of us probably didn't know that; pardon if you do ... you can stop reading now ;) ).

o Less Boost pressure does not necessarily mean a decrease if power. ...what?!

o The article was using a roots-type blower and their car started at a boost of 5.5 psi rising to 8.5psi @ redline (not that that's supposed to happen). Noting this rise in boost against RPM, it was explained it was mostly attributable to the reduced efficiency of airflow of the stock engine at higher RPM (more restrictive). That reaction to the back-pressure pushes up the boost pressure (since there's a decrease in air-flow), thus charge temperature, thus an increased risk of detonation.

o I'll quote from the article here, "Since the Roots blower is most efficient at low boost levels, it stands to reason that every attempt should be made to keep boost levels at a reasonable level". How do you avoid that rise in boost pressure? Run lower boost? That reduces power.

*** The major point of the article is a better way to accommodate safer charge temperature and run the car safely at pump gas yet still crank out the power is not to reduce the pulley ratio, thus blower's airflow, thus power, but rather increase airflow of, and through, the engine. ***

The observant will recognize increasing the airflow of the stock, un-blown engine will increase its power anyway. That's true. But the additional benefit you recognize with a higher flowing engine once blown, is the surprise, and the point of the article.

o The magazine performed this experiment. They supercharged a stock 302 ci engine and took measurements as a baseline. They kept the pulleys on the supercharger constant, but installed
a) more aggressive cam
b) freer flowing exhaust.
c) freer flowing heads

o Result was something like 50 more horsepower of the supercharged engine. You'd probably expect this even if the engine wasn't blown; maybe not 50 hp, maybe so, probably close at least. But here's the important part. A REDUCTION of the boost level by up to 2 psi at the high RPM range; plus the boost level was now close to flat over the whole RPM range up to 5000 rpm, after which it increased just about a pound.

To quote the article again, "Two psi of boost pressure can mean the difference between running premium unleaded pump gas and running race fuel. A 2-psi drop can easily cool the charge temperature by 20 to 40 degrees. ... remember that the best supercharged engines begin with efficient normally aspirated engines...".

Now, in my particular case... In that an AMG car already is touted to have a pretty freely flowing exhaust, and hi lift cam [don't know about how great the heads are though] it makes me feel a lot better about what I'll get if I supercharge it. I also found this article especially timely in that it reinforces what I've been reading from Adam of HPS. It's not boost pressure that's important rather it's the air-flow. I'm now much more confident their setup at 4 or 5psi is very safe; especially in that an AMG car having an increased air-flow over a stock engine should keep the rise in back-pressure at high RMP in check.

I'm of the opinion it's always a positive thing when multiple independent sources reinforce the same argument.
:D

Denlasoul
08-05-2003, 07:26 PM
Thanks for the info, I'll be sure to check that out too. If you want more information try the book Supercharging, Turbocharging, and Nitrous Oxide by Earl Davis and Diane Perkins-Davis.

The book is a good read, and goes into a fair amount of detail about the three options. About $20 at your local Barnes and Niz-obles.

DLS

speedybenz
08-06-2003, 10:18 AM
98c43amg,

Thanks for the write up. Very nice.

Jeff

Koolvin
08-06-2003, 12:48 PM
nice!

HPS
08-06-2003, 01:09 PM
Thumbs up! :cool: