Imo000

DOHH, I did miss it!

Carl Fausett

Drnick: Imo makes some nice points about a SC headunit with self-lubricating bearings being easier to install, and I agree, which is why we use a SC with self-lubricating bearings in our Stage 1 and Stage 2 kits.

The pics above from our Stage 3 kit is different - and caution and ease of installation were thrown to the wind for max performance. We used a bigger XB-1a blower that required oil lines here.

The early Paxton is very compact with a small scroll like Imo says - but a small scroll also translates into lower CFM (cubic feet per minute) of air flow.

You mentioned sizing: we find that a blower moving 850 CFM is the right size for the 4.5 and 4.7 liter 928. Keep in mind how small in discplacement these motors are (262 cubic inches for the 4.5L). A bigger blower, like those that move 1200 CFM - simply swamps the small V8's and we can never provide enough fuel to keep them happy it seems.

But on the 5.0 L and 5.4L motors - we find that a blower moving 1200 CFM is right-sized and does well. We use the Vortec there with excellent results.

I do not mine sharing all this info - because I believe you will only step down the road a short way and discover that you cannot make your own for less than I sell them for. We supply complete kits with all the parts for as little as $3300 on 16v cars.

Carl Fausett

Imo - I just toured your DIY post. Nice job! Man THOSE are some beef-eater SC mounting brackets! Them bad boys will never fail

It's amazing how you discover - when you put proper belt-tension to your SC - how the brackets will bend and torque under the load. Most guys have not discovered that yet.

Anyway - wanted to say I enjoyed your pics and I appreciate the home-built process. Lot of work there.

drnick

carl, i bet your right! but i really dont like the way the snail mounts up high in the front - i think its ugly!! id like it hidden down low and out of the way, also i think the airflow path from there could be more direct into an air-air... so what would i find using a smaller head unit on the 5.4??

Carl Fausett

A smaller head unit that moves less CFM would be adequate throughout the bottom end and mid-range, but as RPM increases, the small blower will not be able to keep up with the inhaling of the engine. The physical symptom of this (Dr talk) will be the boost pressure gauge trailing off as rpm's get higher - meaning the engine is drawing more air than the SC can make up.

In my experience, the 5.4 L motors benefit best from blowers of 1000 CFM or larger, and in our kit we use a 1200 CFM blower on them.

Then - you are considering and air-to-air intercooler setup. Calculate about .5 psi per 90 bend for pressure loss though your intercooler system (some say 1 psi per 90 bend, but I think that's a little too aggresive). Just be sure that you have enough CFM and Pressure at the headunit to sustain those losses thru your AA intercooler system so you have enough left at motor entry to keep it interesting...

drnick

i see, and its at high rpm that im wanting to see the most power so i will need the larger head unit. carl its such a shame that your previous work was wasted... thats exactly where i think a supercharger should be mounted.

Carl Fausett

Well not wasted - we learned a great deal and it made our second iteration of the SC kit, where you can keep your AC, much faster. In the end, our Stage 2 kit with the AC still installed ended up producing more HP than the Stage 3 kit did without the AC.

drnick

very interesting. took a drive today with a guy in his 968 with a centrifugal, it certainly makes a lot of power! really interesting delivery too, at lower rpm its just like a regular car but it just drives up through the rev range with increasing urgency. no hit as a turbo would. strange the way the urgency increases as the revs rise, i can see why you would want to raise the boost to fill in the midrange even more. even though its made me curious to know what a positive displacement blower would feel like on a 928 i do think that the progressive power with a centrifugal would make it very driveable.

Imo000

Hmm this is interesting. I have these symptoms and at high rpms the pressure drops by a couple of PSIs, down to ~4.5. I had a feeling the SC pulley was too large and couldn't delive enough air at the top of the rpm band. My other theory was that it's maybe a leaking bypass valve. I just finished installing my home made intercooler (double Vovlo cores with custom end tanks) and was planning to swap the SC pulley with a smaller one. Perhaps that will resolve the boost loss. I have to find out what the max RPM of the SC is. I don't want to over spin it. That's one of the mian reasons the older Paxtons fail, the other is excessive belt tightness.

drnick

i might end up mounting the snail in the place everyone else does so i can put a drysump pump where the aircon compressor is...

Carl Fausett

drnick - that's an excellent location for a drysump, although on the other side from the oil cooler lines, if that matters to you. Dry sump pumps are often longer and smaller dia than a supercharger head unit - it fits well.

IMO - you are right to be concerned about over-clockling the SC head unit. I make quite a good business rebuilding Superchargers from kids that just keep throwing smaller and smaller pulleys on their head unit in an effort to get more. That's is me - www.powerdynerebuilds.com

You get more PSI - temporarilly - but you do not get the same increase in CFM. When the scroll/impeller set is at max CFM - your done. Spinniing it faster just cavitates the screw and you get nothing but a repair bill.

This is especially true with self-lubricated bearings. The steps to failure are: the impeller is over-spooled, the bearing on the impeller heats up and receives no oil from the motor to cool it (like an oiled SC does), the lubricating grease in the bearing fails at 272 deg F, the bearing starts to weld itself together, then the Paxton eats itself. The Powerdyne, on the other hand - eats the cogged belt that runs between the two sprockets so no expensive parts are ruined. But either wway - you are out of business.

The solution is a larger Supercharger that is capable of the right CFM for your CID, and then run it at slower speeds. Better than a small SC running at higher speeds.

Imo000

Carl,

Thanks for the info. I send an email to the company that took over the old Paxton line. Got this reply back "The max Impeller speed is 38,500 and the internal ratio is 4.4 to 1." Now I'll go and measure the crank diameter. Is there an exisitng formula to plug these number in?

Carl Fausett

Sure - I have the formula on my website under "Supercharger Pulleys" You plug in your crank diameter, and the diameter of your supercharger pulley, and factor in your intened shift point (6,000 rpm, etc) and be able to know what the impeller speed is at that RPM. You adjust the supercharger pulley according to what you fins out, and what RPM range you want to focus on.

Imo000

Carl,

Thansk, but I must be blind. I've looked all over your site and can't find it. I've checked under the SC kits, Accesories and a few other locations but can't find the formula.