4-71 Blower on a 350 SB Questions...

[Bonermetal]

Hello Guys...
I am new to the forum so let me introduce myself an my Car.
I am 30, from Austria and drive a 1927 Ford tudor sedan with a 350 SB + 4-71 blower in it.

Kraftwerk.jpg

I would like to spin the blower a little faster and get more power at all and from the beginning (1500-2000 rpm).
At the mom. both pulleys have the same size and the blower is not under- or overdriven.
It is for Street use and I do not rev the engine higher than 6000 rpm.
Comp around 7:1. 750 cfm Edelbrock carb.
Unfort. I do not have a boost gauge installed yet.

Is 10 or 20% OD still ok for this setup?
Beside the smaller pulley would I have to change the metering rods / Jets in the Carb?
Any other advice's or comments?

Thank you very much!
BR Thomas

[mag2555]

You can spin it faster , say like 10% and then jet your Carb that same 10% richer on both the primary and secondary, or you can just run bigger then a 750 Carb as that will increase boost also.

[Bonermetal]

Ok so 10-20% OD should be ok.

Carb --> How much bigger? 850? 950?

How much of a difference does 15% OD and a good carb setting make when going WOT?
Clearly noticeable?

Thx for the answer.

[Bonermetal]

Another Question....do I have to re calibrate the ignition timing when running 3-4 psi more boost?

[mag2555]

How long have you had this vehicle, and did you build the motor?
Also keep in mind that the 471 Blower was made to feed a 284 cid motor.

[F-BIRD'88]

If the drive ratio is now 1:1 you should have plenty of boost now.
7 to 8psi approx..

There is a 2x4 carb adapter for that blower that allows you to run 2 carbs.
2x 750 edelbrocks is just right especially if you want to crank up the boost and drive ratio and power.
Yes you may need to adjsut the WOT jetting.
Also as you ad boost and drive speed you must watch the power piston spring on the primary rods.
As with jsut a single carb the blowr pulls HARD on the carb at WOT and can pull the power piston closed lean at the wrong time... Use a 10" rated spring set (single carb high boost or jsut swap to a Dual 750 carbs setup.

What cam is in this motor... You wnt a cam that lets the air in and the exhaust out of the blower motor so it makes more POWER as a result of more boost.
The wrong cam or a tiny cam will really restrict the engine power. It has to breath deep.
Yes ignition timing under boost may need to be adjusted depending on fuel octane, carb jetting, amount of boost, and engine true compression ratio.

There is a blower boost chart that shows the approx expected boost with the 4-71 blower on various engine CID sizes. It is approx. There are factors that effect this.
Boost is boost but you want increased engine power output,, not just more boost.
get a boost gauge ,,,You are limited on the practical boost by the quality of the fuel used.
Yes a true 7:1cr on a 350 allows some agressive boost ( blower drive ratio)

How did you arrive at the 7:1cr spec?
More info gets better specific advice.

[F-BIRD'88]

If this 350 engine has cast pistons tread lightly on adding boost on pump gas.
the cast pistons are strong and will take all kinds of boost under NORMAL combustion.
But added boost adds the likely hood of detonation. Detonation especially at WOT under BOOST eats cast pistons quick. ( top ring land failure.) You must avoid Detonation at all cost.
better fuel or water injection may be required. Keep it real if it has cast pistons.
Again tiny carb tiny cam restricts limits power output.
It is increased engine power output you are looking for not just more "boost".

[F-BIRD'88]

The old Weiand 2x4 carb to 4-71 flange adapter is #7161 Old stock
2 x4 afb carbs.

[Bonermetal]

How long have you had this vehicle, and did you build the motor?
Also keep in mind that the 471 Blower was made to feed a 284 cid motor.

Not that long and no I did not build it. Most of the information I have is from the previous owner. The other info I got/get by myself.
And yes you are right BUT It was build for a 284 cid 2 stroke diesel motor so you can actually double that value for a 4 stroke V8.

[thedynoguy]

I don't know how you can make any decisions without knowing how much boost you are making now. Without that info, you're shooting blind...

[Bonermetal]

If the drive ratio is now 1:1 you should have plenty of boost now.
7 to 8psi approx..

There is a 2x4 carb adapter for that blower that allows you to run 2 carbs.
2x 750 edelbrocks is just right especially if you want to crank up the boost and drive ratio and power.
Yes you may need to adjsut the WOT jetting.
Also as you ad boost and drive speed you must watch the power piston spring on the primary rods.
As with jsut a single carb the blowr pulls HARD on the carb at WOT and can pull the power piston closed lean at the wrong time... Use a 10" rated spring set (single carb high boost or jsut swap to a Dual 750 carbs setup.

What cam is in this motor... You wnt a cam that lets the air in and the exhaust out of the blower motor so it makes more POWER as a result of more boost.
The wrong cam or a tiny cam will really restrict the engine power. It has to breath deep.
Yes ignition timing under boost may need to be adjusted depending on fuel octane, carb jetting, amount of boost, and engine true compression ratio.

There is a blower boost chart that shows the approx expected boost with the 4-71 blower on various engine CID sizes. It is approx. There are factors that effect this.
Boost is boost but you want increased engine power output,, not just more boost.
get a boost gauge ,,,You are limited on the practical boost by the quality of the fuel used.
Yes a true 7:1cr on a 350 allows some agressive boost ( blower drive ratio)

How did you arrive at the 7:1cr spec?
More info gets better specific advice.

Thank you for your answer!
This is my winter project so I will work on the car the next months during European winter and come back with questions and more information about my engine/ Cam / curent amount of boost.
Well yes the engine runs good and I can not complain but you know there is always the need for more power
We get 95 and 100 OCT fuel at our gas stations. According to the boost table with 95 OCT, 7:1 comp, 20% OD would generate around 10lbs boost at 6000rpm and still be ok with no detonations. I almost never push the engine that hard...
Bottom end power is more important for me than power above 5000 rpm...
I want spinning tires without using the clutch (it is a 4 speed manual)

Unfort. I am restricted with the space above and behind the blower. See pic on my first post.
BUT other option....
Would one bigger Carb like a 950 or 1050 cfm make driveability on lower rpms worse than two carbs or does the extra boost with 20% OD compensate this with a one carb setup? Thx!

[KnightEngines]

There is some not so great info posted in this thread.

Shoot me a pm & I'll give you my number to call me, you can make quite a lot more power but sticking a bigger carb on it is not gonna get you much.

Little blower spinning hard = heat, which will cost more power than it gives & push you further from the sweet spot.
Far better to make better use of what the blower can comfortably deliver than to try to get it to do something it's not gonna be happy doing.

[Bonermetal]

There is some not so great info posted in this thread.

Shoot me a pm & I'll give you my number to call me, you can make quite a lot more power but sticking a bigger carb on it is not gonna get you much.

Little blower spinning hard = heat, which will cost more power than it gives & push you further from the sweet spot.
Far better to make better use of what the blower can comfortably deliver than to try to get it to do something it's not gonna be happy doing.

I totally agree with the part of spinning the blower too hard = more heat and not more power. But remember I do not often rev it that high and not for a long time. It is not for racing. Only cruising around and painting black stripes on the street from time to time so i think the 10-20% OD should not be too much.

[F-BIRD'88]

I don't have the 4-71 boost/Cid chart but working with the larger 6-71 chart and a bit of math it looks like if you want 10psi boost on a 350 Cid you will need to spin that blower at about 25% over..

The current boost @1:1. Should be about 5 psi +/-.
These are approx.

[F-BIRD'88]

When you do drive the small blower faster the boost will come in quicker too.
Yes most of the gain of 2x4 carbs vs the 750 will be above 4500 rpm.

When you do up the drive to 25% over you may well need to reduce the wot spark timing under boost.
27-31 degree ish.
Use slightly cooler non projected tip spark plugs.
If you don't use a spark retard box you will need to
Re-curve the diz.
More initial or locked out with vacuum advance connected under the blower. And less total.
The carb will need jetting and the pp step up spring mustbbe right.. try a 10" rated pair.