tomcat

Remember when I was proposing a Supercharger Shootout a while back? Check out the August issue of Hot Rod magazine. The article is called the "Battle of the Boost". I was so impressed I wrote a letter to the editor:

Congratulations to Hot Rod, author Richard Holdener and the participating companies for having the guts to do this testing and publish this article. I can't think of another word in the high performance dictionary that is steeped in so much mystique and misunderstanding as is supercharging. The risks and challenges of going through with such a comparison are many, but the consumer is well served when it takes place. You included comments in the article that explained the likely reason for every notable difference between the three different approaches. It's just too bad that a screw compressor company wasn't able to participate.

I would like to add to a few points that were raised in the article:

1) The wastegate and the relatively small turbine housing are the reason the turbo kit had such high boost and torque at moderate RPM. You didn't explain this when comparing the curves. If the centrifugal supercharger had a variable speed drive, which Paxton actually used in the early days, it would greatly increase it's midrange power. If the turbo used a larger turbine housing and no wastegate, it's power curve would look very much like the Paxton. You tested them the way they came out of the box, which is fine, but knowing why the results turned out this way is important.

2) The curves for the turbo and the Paxton are an excellent example of the relationship between power, boost, temperature and the power required to drive the supercharger. These blowers both make the same power when boost is the same. This tells us that their very similar design leads to similar efficiencies and similar air temperatures. It also points out that the energy needed to drive the Paxton is similar to the energy absorbed by the turbine/wastegate combination driving the turbo. This is a very interesting result, since turbos definitely have an advantage in peak power due to the energy recovered from the exhaust gases. Apparently this advantage disappears when you use a smaller, more restrictive turbine housing to pump up the midrange.

3) Finally, you ruled out the real slugfest by talking about insane boost levels and unsafe impeller speeds. I can sympathize with you not wanting to destroy your test engine, but this is what we as readers expect you to do! Seriously though, the true limit to power in gas engines is detonation. The next time you do a test like this, I suggest that you allow the tuners to make as much power as they can on pump gas. Arbitrarily setting a maximum boost level clouds the question of which approach makes the most power. You dance around this issue in the "Players" section, but you should face it head on.

All things considered, an excellent article. I enjoyed reading it (and reading between the lines).

Keep up the good work!

220BR

Well said. I hope it is published and the editors/writers add their comments.

Turbojack

Now I got to go out & get the magazine to read the article. Good post TC, I bet it comes up in next or months after letters.

Point #2, " is a very interesting result, since turbos definitely have an advantage in peak power due to the energy recovered from the exhaust gases. Apparently this advantage disappears when you use a smaller, more restrictive turbine housing to pump up the midrange. " Could it just be that the turbos are losing there advantage with the eff. going up on the centrifical's?

Iggy

Good going tomcat. Excellent observations. Hope it makes it to print.

I remember years ago there was a blower that was designed like the compressor section of a turbine engine. Built boost like there's no tomorrow but had the advantage of using less power to turn it and if the drive belt broke the engine would continue to run, the center section would spin as air was pulled into the cylinders. Wonder what happend to that?

tomcat

Turbojack - Without seeing the compressor maps it's hard to say but the efiiciency of the compressors should be very similar. The drop in the turbo's boost at high RPM is may be due to the way the wastegate operates.

Iggy - You remember that old supercharger shootout article in Car Craft or Hot Rod too? You're showing your age! Wasn't the name of that turbine style compressor Latham or something like that. I never saw one after that article.

JUSTONCE

may I debate one of your points if you don't mind? When you state that at equal boost they put out nearly equal power, thus each robbing similar power and putting off similar temp air charge. I believe it more so that it is a closely matched trade off, the supercharger draws more power for its drive than the turbo which kills very little power by exhaust restriction, but the turbo gives a much higher temp. air charge thus in total giving a comparable final #
please correct me if I'm wrong,
Martin.