I'm am about to modify my Holley 4160 carb to install it on my 4-71 blown SBC. I understand how to "boost reference " the power valve on a Holley carb, but will vacuum secondaries still work with a blown motor?
You probably don't need to boost reference the power valve. Vacuum secondaries will probably work OK, most guys run a double pumper...because you can.... what size carb? what size engine? how much boost?
Its a 350 sbc with 9:1 compression and 4 to 6 pounds boost (estimated) and a 650 Holley single pumper, Comp cam (seat to seat duration 262 intake, 270 exhaust) in a '28 roadster.
Could probably use a bigger carb....650 is about right for the engine without a blower. 800 would be closer to what it needs.
Holley makes 780's with metering blocks specifically for blowers. Don't go more than 6# with that compression and you might want a timing retarder. As for the power valves just make sure they're half of your total vacuum at cruising speed. I run dual 780's no choke, with power valves. The newer carbs have bypasses to save your power valves but you can never be too safe. Lots of secondary jetting. Lean is awful with a blower. You would much rather be rich on the carb side. Best of luck to you. I believe it's the 3310-1 model I'm running, been beating the same carbs for 14 years. Do you know what LSA the cam is, plus I've seen a few comp cam failures in the last year or so. Maybe look in to a lunati voodoo with a LSA of 112-114.
Blowers also like a wide lsa. I would go minimum of 112*. Even at 6# you're talking 40% hp increase with a roots blower. I hope I'm not discouraging you, that's not my intention at all. I just don't want you to melt down your engine or chase a terrible idle situation that can't be fixed without the proper parts. Boost referenced power valves are quite simple also. Especially on a single carb set up.
[/ATTACH] View attachment 2677975 This blower installation is more about nostalgic appearance than it is performance. To me it is another high dollar fashion item like a Halibrand quick change. I'll probably never drive it like it was a race car so I don't care too much about the performance. I'd just like it to be reliable and not over heat. This engine has a couple thousand miles on it and I'm locked into it for now. I can change carbs now if it is necessary. If I change cams it will have to be when I replace the melted pistons.
Cant speak for the holley but I have an Edelbrock 750 on a 327 with 9to1 compression with a 471 that is street driven and reliable as can be. As a matter of fact my wife just drove it 200 miles each way to the Meltdown drags. I'm not a big fan of Holley carbs but I've used vacuum secondary Edelbrocks on 2 blown motors with no problems. As already mentioned a boost referenced timing retard box will make life much easier and probably save you from hurting the motor. I can and do run mine on 89 pump gas most of the time on the street.
If you don't buy a "blower" carburetor or at least the metering plates, be carefull, you may have to open the idle mixture and high speed passages some. The idle more thAn the high speed. I've known a couple of people that have had problems like this. It's why both Holley and Edelbrock offer blower versions of their carburetors. Mike
Just asking...Would a 4-71 be large enough to make much boost, at the ratio in the pic, I'd be supprised if it got to 5psi..yes/no?
Sorry for the late reply. You are right, probably about 5 pounds boost. It is intended to look early 60's and even though it is not apparent in the picture, it is slightly over driven. It is also a very snug fit.
Yes it do look tite...In my opinion you have a proportionate package, nothing overwhelms..Wonder how long those belt labels will stay lined up..
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