Hello All,
What would happen if a person mounted two SN units to an engine? Would the boost double? Certainly the CFM would. Anybody have any personal experience on the subject. It must have been done in the past, anybody know of any articles that had actual dyno tests. If the VR4 conversions don’t get produced, I was thinking this might be an option. Mostly bench racing, but curious.
I have used Twin VR4s before. If you search it might still be around from about 5 years ago. I had pictures and I lost them from a hardware crash then this site went down and might have lost them also. You double your cfm which really helps move a large engine along.
Speedytang,
Yes, I read through those posts, interesting. Did you ever compare the motor with one VR4 vs. the 2 VR4s?
I would imagine that with double the cfm the boost would have to rise too. Or would the resistance from the boost/pressure make the ball drive slip? Is that why the VR4's use higher pre-load on the ball drive.
Thanks
bottom of page: http://cobraautomotive.com/horsepower.htm
Z. Ray
'65 K code
Zray,
I've seen that picture before. I think they are the folks who built a Side-Oiler with twins that they installed in a 67-68 Shelby Mustang. It made like 583hp all motor then 6## rhp with the blowers.
http://www.mustangmonthly.com/featur...iew/index.html
You mean something like this. This is my car photographed at the last Shelby American Convention.
Twin blowers do not give twice the pressure. Speedytang is correct. The boost pressure is the same but you have twice the volume of boost air over the throttle plates ready to tip in. A vintage setup like this is very raw - no grace. No blowoff valves, no pressure recycling, no modern fmus. Light touch on the throttle gets you home alive.
As they say ''It's like a dragster on ice''.
-Aggressor
Last edited by Aggressor; 06-08-2010 at 05:22 PM.
Nice!!!
And a Cross-Ram no less, very cool.
I had twin VR4s and the motor made over 800hp at the crank at 14psi. The problem you run into on the street it becomes a on and off switch. I built my last Twin VR4 for a Speed Boat.
Speedytang,
Can you expand on "on-off switch"?
Twin VR4s do not act like a pair of 'SN' units at all. When you push the gas/pedal it is full boost NOW. I would see 1psi then 14psi in less than a second. Your pushing so much air so fast that the boost jumps and not climbs.
Very interesting, did you have an intercooler for these 2 units?
Did you get full boost also at low rpm? As you have double cfm, do you need to make the blowers spinning very fast?
But it should put a lot of stress on the front of the crank, right?
Full boost was by 3k rpm on a boat out-drive. I used a 7" crank and 3.75" blower pulley. Never used a intercooler on any VR4 install for myself but have for others.
What the max impeller speed of a VR4?Originally Posted by speedytang
With such pulleys, it mean +- 6600rpm by rpm, it was not spinning very fast. You never tought swapping for baddest pulleys or was it enough?
No, with a 7" crank pulley and 3.75 blower pulley at 3,00rpm the impeller would be spinning 24,800rpm. I'm sure the boat engine would pull 5-5,500rpm, so the impeller speeds would have been 41-45k rpm or more.
Math:
Crank pulley/blower pulley * 4.44(internal stepup ratio of supercharger) * engine rpm = impeller rpm
I will post a bunch of pictures on Monday - I need to run to a meeting. In short... While at Paxton, we ran blowers in series - meaning double the boost but not so much on airflow - naturally you get a little more. We also ran the blowers in parallel which is what all of you are asking about. When running in parallel, the blowers double the flow. Based on how they are run, they can double boost or stay the same. Explained...
In this case (above) if each carb were to feed only 4 cylinders each then each blower would "in theory" only be feeding a 213.5" engine (half a 427) Those blowers at 42,000 impeller speed would make roughly 10lbs of boost at 720 cfm. In a perfect world the twins are good for 900 to 1000hp. In fact in 1982 we ran twin SN60 blowers on a 383 Chevy and made 875hp at 9 psi. We then went to twin VR4 superchargers with 4" inlets and made 1127 at 12psi.
We also tried pumping both blowers into one big box, enclosing a single 950 Carb. The blowers would go into surge and fight each other. Ball drives are like Japanese fighting fish. They may look pretty but they don't play well with others
Interesting to note: I ran twin SN superchargers on a 502 and they were fine together. We surmised that the big engine could digest all that the 2 blowers could put out. Motor made 560 n/a and 745 at 5psi. That works out to be like 90% efficient.
When my father did the first twin blown car for Shelby -
They were set for 6psi - they say they were set higher but it is not true. At 6 psi they made 620hp at the crank. The cars ran 131 in the 1/4 mile. The worst pass ever recorded was 11.86 at 115. That car had 3.31 gear and a bad C6 slipping all the way down the track. In perfect condition, everything fresh with 3.55 gears, shifting at 6200 vs 5500 normally. They ran 10.7 at a very traction limited 131 mph which works out to only 427hp to the tires at 2500lbs. You decide:D
I can tell you - I have never driven a scarier car than the twin blower 427 Cobra. Fun scary but scary. It was impossible to keep in a straight line as delivered from Shelby.
Ask your questions and I will do my best to answer on Monday
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