8/71 Overdrive/Efficiency

What would be the optimal overdrive for peak efficiency on a black & white stripped 8/71 on alky? Obviously we want to spin it as fast as possible for most boost, but don't want to go too far where it's heating the charge and sapping more power to negate the extra boost.

I know it depends on many things and will change at different RPM's, but as a rough guide?

combo is:
517ci
Rodeck 481X block
Bac-Man Billet Heads
8/71 Danekas/Capps Blower
Enderle 110 pump
8 injectors in the hat, 8 in the ports
Super Mag 3

to go in a 122" altered, 3 speed B&J, say shifting at 7000, top end at 7500?

 

We run a Kobelco K-9 8-71 HH on a 510 inch hemi on 20% nitro. I spin it at 65 over until I need the 70 over. When we tried lowering the ratio to around 50-55 over the car slowed down. I know a lot of people we race with run a 14-71 at the maximum 35 over and swear the cooler air charge benifits them, but we run just as good with our little blower.

 

In my experience I have never spun my blowers too fast. I've dyno tested LF HH K-8's 8-71's up to 68% overdrive at about 7000 engine RPM and I just kept gaining HP.
Remember when gathering opinions that the real numbers you should look at are blower RPM's not overdrive %.

7500 engine rpm @ 1.60% over = 12,000 blower rpm
8500 engine rpm @ 1.41% over = 12,000 blower rpm

In this example its the same blower speed, but if you soley went by overdrive it would really through you off.
If I can spin them 13,500 RPM, I do!

 

I totally agree and should have included that in my previous post. We turn about 8400 RPM on the 1/8 and never spin the blower over 63% on the 1/4 with about 9200 RPM.

 

Hello Jack..how have been feeling? good to see ya back on track we have been running alot last year..finally got into the 5's 5.98 in the 1/4. If you have time go to our web page and check it out...Dave CHROME Lowe
www.myspace.com/daveloweswr

 

Dave,

We are putting the car back together and my son is going to drive from now on. It has been a long year of healing, but I am doing pretty good now. Good luck this year.

Jack

 

thanks guys, in the senisble range of overdrive's we can run it'll be all good, don't wanna kill it but it's there to work

 

I would think that it would depend on so many things that it could never be answerered so generally. Whats the helix of the rotor ?the design of the rotor? the inlet and outlet size and style and mods ?are the rotors set back? does your injector have a center divider? how big an engine and how fast do you spin it?

Mabe an inlet temp guage would be a usefull tool for you.

I know a guy that runs an AJ 481x witha 8-71 superman that slowed his blower down and gained a lot in speed , et and reliabilaty!

 

overdrive

call JR at RBS he can answer any questions the 14 will do the trick and last twice as long due to spinning it half as fast the strips will not go away as fast and lower intake temps

 

standard helix rotor, not sure on inlet and outlet size (will be stock as it came from Danekas) no mods, no setback, injector doesn't have a centre divide (but it may get one) it's an Enderle 'ribbed' bugcatcher

engine is 517, max rpm would be 8000-8500 I'd say, it's on stud mounted rockers not shaft, but got titanium valve etc

Inlet temp gauge sounds like a good idea, any recommendations?

 

I'm curious to know the brand also, although I don't think it would change my answer.

The old timers used to say 38% OD was max on a blower like that. I question that because they didn't take engine RPM into consideration, but,,,
7000 engine RPM x 1.38 OD = 9660 rotor RPM

If I were you I would start with about 10,000 rotor RPM.

my $.02

 

thanks for your help, what would you think as maximum rotor rpm?

the blower is a DC (Marc Danekas) all fresh; clearanced, bearings, strips etc

 

I am not familiar with a DC at all. The RPM is limited by how much centrifigual force is applied. (so it doesn't blow up, although I've never heard of it happening)
Modern day billet rotors are being spun to 14,000 rotor RPM.