Downsizing a 4bbl carb?

[SoCar72]

Don't know how many of you guys have read up on the Pony 4100 480cfm carb, but basically (due to the limited core supply for 480's) they have taken the 600cfm models and sleeved the primaries down to 1.00" to match the stock 480cfm size, and left the secondaries as the stock 600cfm size. Effectively making the carb flow something like 540cfm (which isn't the point here but merely an example).

Has anyone tried doing this in a home shop? At work, I have access to a lot of standard shop equipment, but no milling machines or the like that a sophisticated machine shop would have.

I have a Holley 390cfm that I'd like to downsize the primaries to better suit a 170. By my calculations, the 390's primaries should well flow into the 4000rpm band before the secondaries begin to open. I'd like to bring this down to the low 3000's which would be about 750rpm above my typical hwy cruise rpm.

I haven't pulled the carb apart yet to see what this may entail, so this is all hypothetical so far. I've been toying with the idea of sleeving with aluminum or copper, as well as UHMW or nylon. Thoughts on material?

Or maybe I wouldn't need to. I haven't heard of someone running a 390 on a 170, but it does seem a little big for a streetable build.

 

[rbohm]

8) remember that while the total cfm that the 390 holley flows is a bit much for a healthy 170, if you get one with vacuum secondaries, assuming one is available, the secondaries will not open fully unless there is a string enough signal to do so. also understand that the primaries are going to be smaller than even the small 1.08 2bbl autolite carb. if you want a 4bbl carb on the 170, the 390 is the one to use.

 

[SoCar72]

Good point about the secondaries. Mine is indeed a vacuum secondary carb.

 

[FalconSedanDelivery]

I understand the Volumetric effiency deals ( bin there done that ) but take it from a guy thats tried both sides , and this is not a bash on anyone as it takes years and money ( my case more years and less money) , its not all a math formula, all cars and combo's are different , and I see a very conservitive trend on this site when it comes to carbs , But what I have found is Holleys ( or a type of ) are the HP Kings and Autolites , Eldebrocks Carters all work but if its all out performance USE A HOLLEY , Case in point the 390 carb , I built a 225 Mopar slant 6 for a customer back in the mid 80's, mild combo, purple shaft cam, slightly oversise valves , offy intake , 390 holley , hooker headers AND a Recurved Electronic Dist !! ,this was in a 69 dart with a 3spd overdrive and 3.90 gears , it weighed 3350 with driver ( who was 6'4 275 ) anyway it ran 14.80s at 96 mph , played with the combo for weeks and that was the best , bolted on a 600 , YES a 600 and bam 14.20s at 98 mph , The Point! , try bigger carbs Especially Vacuum Secondary' types , and as for down sizing a production piece , Why bother the 390's work fine 0n 2.0 and 2.3 ford 4cyls ,a 170 with the good aluminum head will handle that fine , take a chance guys and you just might suprise yourselves

 

[Anonymous]

Summit offers a 600 cfm carb made by holley that has annular discharge nozzles.

It looks like the old autolite 4100, but uses the adjustable floats found in a holley, has vacuum secondaries & uses holley jets???

Might be worth trying. Outlaw

 

[SoCar72]

I had no idea that 390's were found on some 4 bangers.
That greatly boosts my confidence for use on a 170. I'll give it a whirl, when/if dream become reality.

Good note on the Summit carbs. Been wondering about the tune-ability and inner parts. I've been eying one as possibility for a future Bent8 build.

 

[rbohm]

I had no idea that 390's were found on some 4 bangers.

8) sure, how do you think they started some of them high compression 390's.......

oh wait you are still talking about carburettors arent you?

 

[Ronbo]

I had no idea that 390's were found on some 4 bangers.

I think circle track cars maybe? Not stock for sure. Can't imagine my Mom's old Chevette with a 390 on it :roll: But yes, if you look on the web you will find many 4 holes with 390s on them. Good luck in your quest.

Ron

 

['68falconohio]

I had no idea that 390's were found on some 4 bangers.

I think circle track cars maybe? Not stock for sure. Can't imagine my Mom's old Chevette with a 390 on it :roll: But yes, if you look on the web you will find many 4 holes with 390s on them. Good luck in your quest.

Ron

And 500cfm holleys. My buddy raced 4cyl class on dirt and he ran a 500cfm holley to be competitive. Should've been way too big but it worked better than a 350cfm.

 

[SoCar72]

8) sure, how do you think they started some of them high compression 390's.......

oh wait you are still talking about carburettors arent you?

:rolflmao: That's funny...took a moment...

Buddy of mine built a monster Ford BB, 557cid I believe with 14:1 for drag racing. It took a gear reduction starter and 2 honkin' batteries to turn the beast.

With all the positive feedback on the 390cfm, I'll let it ride as stock before I consider resizing it.

500cfm on a 4-banger Good God!

 

[mach1 mark]

Because you are on the subject of modifying the Holley or the Autolite 4100. I have a question: can you change the Autolite 4100 w/vacuum secondaries into mechanical secondaries. Has anyone done this, is there a kit. Photos if you have them. Or a link to look at. Thanks.

 

[rbohm]

8) there is no kit to make a vacuum secondary 4100 into a mechanical secondary carb, and while you could do it, the action would be compromised at best and the carb would never work smoothly. even if you got the throttle plates to operate properly, you would never have the needed accelerator pump shot to cover the hole created when the secondary throttles opened up.

if you want a mechanical secondary carb, buy one, dont make one.