4/71 pully sizes

karrionchris · Post by **karrionchris** » Mon Feb 08, 2010 10:55 am

this is probably aimed at topcat

just about to hopefully grab myself a 4/71 blower and slap it on a 4.6 rover v8.

with an extra headgasket my compression ratio will become ~7.49:1 this will be perfect but without having much experience in the game i dont know roughly what speed i need to run her at.
Has anyone done this same setup and can point me in the right direction.

Chops

kiwicar · Post by **kiwicar** » Mon Feb 08, 2010 11:00 am

Hi
How much boost are you planning on using?
Mike

karrionchris · Post by **karrionchris** » Mon Feb 08, 2010 11:02 am

i was looking at runing about 13-14psi

karrionchris · Post by **karrionchris** » Mon Feb 08, 2010 11:12 am

also is it more efficient to lower my static CR and run the charger faster or keep CR at 9.35 and run the charger slower??

topcatcustom · Post by **topcatcustom** » Mon Feb 08, 2010 12:02 pm

karrionchris wrote:also is it more efficient to lower my static CR and run the charger faster or keep CR at 9.35 and run the charger slower??

I was wondering the same actually! 7.4CR seems Very low, even if you are using a lot of boost. My CR is in the high 8's and will be running about 6 or 7 psi.

I think Big Al would be much more knowledgeable on this than me!

Regards,

bigaldart · Post by **bigaldart** » Mon Feb 08, 2010 12:53 pm

That much boost will require forged pistons for sure. Our 4-71 makes 16psi at 11% under on a 3.5. We have run that on a stock bottom end and it travelled about 100ft before the ring lands gave way, it was the last meet of the year and we had just hurt the good engine so it was worth a try, I don't know about the 4.0 and 4.6 pistons but all the early ones are just too weak. I would think something more around 8.5 on the compression to stop it being too lazy. As for boost 6-7 psi sounds about right. 15% under or maybe 18% depending on how good the blower is. Are the late pistons slotted for oil return or do they have drilled holes behind the oil ring. The drilled ones will be ok as long as you are conservative. At 15% under you should make an easy 300+ hp and it will be very driveable and a lot of fun. The torque when you stand on it will certainly make you grin. Tell me more about the combo. compression, cam and blower speed need to be matched to what the pistons can stand.

Alan

karrionchris · Post by **karrionchris** » Mon Feb 08, 2010 1:21 pm

well the static compression ratio is 9.35:1 on standard pistons. my cam is an erson hi flow IH 228. (dur dur lift lift = 228 296 .315 .504).

from what i gather as long as i have a fuel with high enough octane i can keep detonation down, so maybe mixing a little avgas with tesco 99 will help.

The carb i have is an 500cfm edelbrock 1404 going onto a jwr intake.
When you say 15% under, what does that refer to?

bigaldart · Post by **bigaldart** » Mon Feb 08, 2010 1:38 pm

Hi the figure refers to blower speed vs crankshaft so 15% under would be 4250 blower rpm at 5000 engine rpm. The 4-71 moves 277 cu in of air every rev, your engine capacity on a 4.6 is about half that per rev. So at 1-1 you will make around 14 lbs of boost. With your compression you will probably be fine at 15 under if you are conservative with timing. That cam should work pretty well, you need a cam for decent top end and let the blower take care of the bottom end. A stump puller for example would probably put you into detonation very easily. Don't be afraid of retarding ignition, when we had petrol in the mix we used as little as 22 degrees total advance, I doubt you will need to go that far. Steve Wells who drag races an Orange pop pick up ran a 4-71 at 1-1 on a 4.6 carburetted on petrol for a good few seasons, compression was down in the 7-1 zone though and he did run race fuel. I would worry more about piston construction than anything else, do you know what yours are like?

Alan

karrionchris · Post by **karrionchris** » Mon Feb 08, 2010 1:59 pm

they are standard cast pistons

3.720" / 94.5mm bore]

rings are

Bore:3.7"[94mm] Comp: 1.2mm/1.5mm Oil: 3mm

bigaldart · Post by **bigaldart** » Mon Feb 08, 2010 5:22 pm

Slots or holes behind the oil ring? I would be inclined to set it up at 15% under to start with and see how it goes. Better to keep the twin port manifold and modify a stock one, the JWR will fetch a bit of money and you will end up cutting a lot out of it to use the blower. The carb will be fine on top of the blower but make sure you have enough fuel pump. You will probably need to richen it a fair bit at idle.

Alan

karrionchris · Post by **karrionchris** » Mon Feb 08, 2010 5:40 pm

im fairly certain they are holes rather than slots. I have access to a milling machine so i can just make up a matching plate for the charger.

Another question. pop off valves. from what i gather they are built into the manifold to release dangerous pressure if the engine backfires. are they essential with the pressures im using and is that the main reason for using them?

topcatcustom · Post by **topcatcustom** » Mon Feb 08, 2010 8:29 pm

karrionchris wrote:pop off valves - are they essential with the pressures im using

YES! Is it really worth skimping on £10 when you could wreck your blower?

bigaldart · Post by **bigaldart** » Tue Feb 09, 2010 7:09 pm

YES!!!! to the pop off valves. There is a lot of fuel air mixture in the manifold and blower, if an intake hangs open for any reason its a bomb. We have had loads of backfires over the years, but with the plate we have never hurt the blower ( long pause to touch wood ). Make sure you have some backfire protection.

Alan

Paul B · Post by **Paul B** » Tue Feb 09, 2010 10:47 pm

Is there anywhere in the UK that makes blower drive pulleys, the 8mm toothed variety, or similar? Or any other blower type stuff.

Or is it down to the usual jobbie of importing from the good old USA?

I have a 6/71 I bought for my FE motor, and I'd like to install it myself, rather than simply order a bunch of parts from the USA.

Post by **Coops** » Tue Feb 09, 2010 10:59 pm

speak to kev the mole,
he can get them made i think mate,