Can a 6-71support a 540 ci engine?

[MicronX]

I was talking to a friend on the West Coast about a new
engine for his drag car.We have most of the geometry worked
out for a 4.50 stroke 540ci engine but his class only allows
a 6-71 blower.The class allows the blower to be overdriven
65% but this engine also has to be able to turn 8000 rpm.
I have been trying to find some info on the 6-71s and what
their flow limits are,but so far I have gotten very little
information.I deal more with naturally asperated engines
so this has me at a stand still.Any info would be great.

Thanks

 

[NickB]

The 6-71 is a positive displacement blower. It's airflow is proportional to the speed it's turned at. However, as speed increases, efficiency will drop, and it will heat up the air as well. I've easily found the displacement of the popular Eaton blowers. It should be available for the 6-71 too.

 

[patdaly]

At least to my understanding, the designation 6-71 came from a Detroit Diesel with 6 cylinders, displacing 71 CI each, for a total of 426 CI. Those things ran at less than 4500 RPM ( I am not exactly sure what the RPMs were, but I dont believe they even went to 4000 ) What I do know is a modern SBC can easily outrun a 6-71, so it will probably act as a choke on a well prepped 540. What HP/TQ targets are you shooting at? I would be very surprised if the 6-71 would support much more than 700-750 Hp. Is the limitation on the rule a 6-71 case size? If so would a Lysholm style screw compressor be legal?

 

[MicronX]

This engine is for a AA/GAS car.It currently has a
427 ci BBChevy and runs low 7s at 2400 lbs and is turned
at 9000 to 10,000 rpm.We believe that if that the larger
engine will actually be easier on parts and provide a
major power advantage over the smaller displacment cars
in the class providing that we can get the 6-71 to feed
it.As far as screw compressor,its illegal because this is
a nostalgia class and the screw compressor type blower was
not used in the gasser era.

 

[TheBlacksmith]

Just for info, the 6-71 Detroit Diesels did displace 426 CID (6 x71) but were pretty well maxed out at 2400 crankshaft rpm. I think the blower was driven about 1:1.

Blacksmith

 

[patdaly]

Ok, I doubt the 6-71 is really going to feed enough air, however, it may still work. What fuel are you allowed to use? If you run a load of Nitro, what is the max. percentage? What I think may help is since the Nitro carries O2 with it, the Blower may help pull more fuel into the 540. A long shot, but otherwise I think you may not see any advantage from the 540.
I assume you are stopping at 540 because of limitations regarding Vintage looking heads or block?

 

[evelrod]

In the 50's we never had a blower that ran with teflon tipped rotors but instead had outrageous clearances built in to keep things turning a race speeds. Overdriven blowers were just coming in and it all worked pretty well then. My guess is that with the better blower it will work equally well today. You say it is a 'nastalgia'(540 cubic inch!!!) class? Well you are already a couple of seconds quicker than I ever was, but then again I ran A/Gas supercharged automatic (before autos were the IN thing).

Patdaly---Doubla A 'GAS' ---no exotic fuels. And the 6-71 WILL work OK even if an 8-71 would probably be better. Rules are rules.

Rod

 

[patdaly]

Whoops, I dont know why my brain was on AA/Fuel...I clean misread the Gas. The 6-71 might physically work, but the old 427 might just eat the new, bigger combo alive. I have no direct Roots experience, but huffing is huffing, and my combo, a 489 inch BBC with a centrifugal is a perfect study. While researching it, I ran thru the braintrust, including Jim Oddy ( man, I wish I would have listened...)They all swore up and down I would make more power with a 427/454 combo than with my 489. It boils down to how much manifold pressure you can build with a given size airpump. In my case, given the limitations of ported heads and the 1300 CFM of the blower, I could expect only 15 PSI out of the 489, whilst I theoretically could have been at least 20-22 PSI with the smaller CI. For each combonation, the Hp added per PSI boost increase will vary, but typically you can see from 20-50 Hp/PSI.
MicronX, I suggest you either get ahold of Craig Railsback at BDS Blowers in California or Jim Oddy at Oddy/Perfection engines ( I dont remember if he relocated to Fla. or if he is still in NY.) Either of those guys have forgotten more about Superchargers than I will ever know!

 

[evelrod]

I am with you on the centrifugal blower being the hot setup. I used the Roots because that was just about all that you could get for cheap (1950's) I got dusted by a small block roadster with a Latham running 4 Carter side drafts! Also got beat by a Paxton/Mc in a Studebaker, of all things. Oh well , I was only 18 or so. Best I ever got out of the 47Ford/55Olds combo was low 12's in 58-59 @ 4000ft alt. with a set of 'Bruce' slicks and a locked 4.10 Olds rearend. I have had street cars that will do that today but, don't get me wrong, that Ford would beat just about anything ANYBODY had around my area.

I am starting out again in "Vintage" racing in a couple weeks at Buttonwillow and it's not the 'real old days'. What I infered about that 540 inch monster was that there NEVER was an A gasser with ANYTHING like that in the late fifties early sixties. If you ran an authentic nastalgia rig today, the guys wouldnt know how to keep it running !!! LOL


Rod

 

[MicronX]

Guys,thanks for the info.Bob decided to scrap the idea and
stay with the smaller engine.In the Nostalgia classes the
engines must appear to be period correct but internally
anything goes.I have a Nostalgia SS engine on one of my
stands right now that is 513ci.Believe it or not,some of
the factory team cars had engines this big in the 60s.They
would use them as "Hit" cars to take out the closest
competitor and then claim they broke to avoid inspection.
This would allow the other Legal team cars to advance
without any real threats from the other teams.

 

[patdaly]

".I have a Nostalgia SS engine on one of my
stands right now that is 513ci.Believe it or not,some of
the factory team cars had engines this big in the 60s.They
would use them as "Hit" cars to take out the closest
competitor and then claim they broke to avoid inspection."

LOL, kinda like the Hot Rod boys with their 489 inch 396/375 Hp back in 68 or so...They put Grumpy Jenkins on the trailer, then walked away.
Ah, for the good old days again!

 

[Sammy2]

Yes, a 6-71 can work on a 540 ci engine, IF......
you have a high enough static mechanical compression ratio and if you spin the blower fast enough.
Obviously you will be limited, but if you build the engine with around a 9.3:1 comp. ratio and run the boost at under 7 psi it will make plenty of power and not heat up the charge too much. The trick is to keep the boost levels down and the mechanical compression high enough to keep from loosing on the left of the curve.
high octane will be an absolute necessity.

I ran a 392 hemi many, many years ago with a 4-71, overdriven 33% with 9.5 to one pistons (before I learned to measure deck height) and it was very strong and stayed together well. The mooneyham blower needed re-stripping fairly often due to the high speeds.

 

[ShaunT]

You'd be much better off with a 4.250 stroke, and you should consider at least an 871. I would look real hard at a Whipple if I were you.

Shaun TiedeULTRADYNE Arl,TX(stiede@ev1.net)

 

[SteveHuck]

The Weiand and the GMC 671,s displace 411 CID and the BDS blower displaces 390 CID.

I have a 540 CID (4.5 bore - 4.25 stroke)with a BDS 8-71 (440 Cid) on it. At 18% overdrive she made 7Lbs boost and with a 25% overdrive she made 10.5 Lbs of boost. Im not sure and im sure someone will correct me if i'm wrong,but i think the heat is produced by compressing the air. At 65% overdrive or less with a 6-71 the boost levels wont be real high. At 10.5 Lbs the motor made 1024Hp and 820Lb Torque.

Call BDS and get there take on what you want to do. If it's Possible, i'm sure they have tried it!

 

[patprimmer]

You get a predictable and unavoidable heat build up as you compress the gas (boost). This is explained by the combined gas laws, being Charles and Boyles laws.

Roots blowers start to heat the air a lot more than is explained by this as the rotor tips go supersonic.

I have not done the sums yet, but I believe from anecdotal evidence by "Blower Experts" that this happens on a big body 6:71, and 8 through to 14:71s at just below 9000 rpm at the blower.

A roots blower is a positive displacement pump, so to get boost, you need to turn the blower hard enough to displace more volume of air at that rpm than the engine would use at it's rpm when the drive ratio is considered.

A big bore 671 theoretically displaces 411 CI per turn of the blower, and your engine probably uses 210 CI for every turn of the engine. With leakdown from the blower clearance. practical examples indicate a 6:71 on a 540 will only maintain atmospheric pressure in the manifold at 1:1 drive ratio. Normally aspirated raceing engines normaly have a slight depression in the manifold, so it is a slight boost in fact.

My sources say 25% OD should give about 10 lbs boost, which falls in line with Steve Huck.

My main source is of info is

http://www.kuhlsuperchargers.com

Regards
pat

 

[ShaunT]

If you run a solid flat tappett cam or a solid roller, you will make more power than a Hydraulic with half the boost. If you run the same boost, the gain over the Hydraulic will be HUGE. I am talking upwards of 50%+

Shaun TiedeULTRADYNE Arl,TX(stiede@ev1.net)