[rtanner]

ok i been keeping it secret that besides my pontiacs i actually own a chevy also, but now i need you guys help so secret is out, ive got 454 8-1 compression, square port heads, 290-300 540 540 cam on 114, going in a 3400 pound camaro turbo 400- 2500 stall, im ordering a weiand street 8-71, should do what 6-7 psi at 6k, so heres the question, i have 2 good 4781 holleys thats 850 mech secondary, the car will be cranked and idled for my buddies regulor, it will get driven maybe 20 cruisin stop and go up to 40mph a couple times a month, however it will go to the track regularly, and get run as hard and as many times as possible, my question is besides accelerator pumps, what kind of internal mods are done to blower carbs over n/a carbs im very familiar w holleys been rebuilding em since 1977, so i just need expert guidance in this area from the best group of guys around thanks!

[Airic1]

If your putting the carbs on a Roots style blower, just bolt them on and tune. You just need to watch the fuel curve and timing, so if you can get to a dyno then do it. I run two 4150 holley 800's and did not do anything special internal to them.

[Torment]

Some folks boost reference the PV but I've never bothered. For the combo you note I'd put 82 jets in the front and 87's in the back with the PV plugged. I like to keep the PV on the primary side for a street car. Notched floats are a good idea if they're side saddle mounted and you get the car hooking and leaving hard. You'll have to tune the discharge nozzles or 'squirters' as you would with anything - likely 40+ in the front and 35 ish in the back. Other than that I didn't do anything different to mine.

[rtanner]

Thanks guys that's a big help also easier than what I thought!

[Tom Vaught]

Doing a remote PV signal for the front PV is very easy to do.

You want the PV signal to be below the supercharger at the intake manifold.

Tom Vaught

[BruceWilkie]

Originally Posted by Airic1

If your putting the carbs on a Roots style blower, just bolt them on and tune. You just need to watch the fuel curve and timing, so if you can get to a dyno then do it. I run two 4150 holley 800's and did not do anything special internal to them.

Many will tell you to boost reference the power valve to manifold vac/pressure. Thats really not a must always do kind of thing. At WOT your power valves open as soon as the vacuum signal at the pv is below its rating. As long as the pv sees vacuum below its rating it will be open. It would take a seriously too small a pair of carbs to read say 7 inches of vacuum below the carbs on a blower AT WOT! Under that condition if the pv was a 6.5, it would close and you'd likely suffer a lean burndown.

THAT SAID It may still be good to reference the pv to manifold vac/pressure because there will be cruise conditions at part throttle where there is enough load and rpm (like going uphill) to have some boost and need for enrichment yet cruise vac below the carb could be higher than your pv rating in this mode! Re-routing the pv vac source from below the carb to below the blower is the safer way and allows lean clean cruising at light load and sufficient enrichment under any load below your pv rating. (provided your jetting and pv channel restrictions are sufficiently "adjusted" for best lean cruise (jet only pv closed), no flat spot transition from lean cruise to load (pv rating) and best power (jet and pv open).

I've assisted tuning a few and 4779, 4780, and 4781 carbs were all used successfully. I did need to increase the pvcr a few thou greater in all of them. I never kept the log of changes I left that to the owners. I did reference the boost to the pv vac channels. One of the combos I did had two pv's per carb. Like NA otherwise as far as tuning. I always kept total metering area as square as possible. (area of the jet plus area of the pvcr) If no power valve on secondary the jet alone would be as close in area as the combined area of the primary jet and pvcr. The 4780's I did were on a 6/71 350 sbc. the 4779's initially were on a 402 BBC 6/71 later used on a 454 6/71. He upped to an 8/71 and 4781's a few years later and I helped on those as well, same mods. None of them had radical cams. The specs were close to yours on the bbc's and the SBC had a Comp 270 magnum hyd that gave a nice lope sounding like the bigger cammed BBC's. When tuned none of them surged like you sometimes hear.

Primary jetting was pretty close to stock on all of them. Pretty much just opened pvcr .003" at a time and increased jet by id to get to same metering AREA at wot using the jet # to jet id chart from a holley catalog. Preliminary jetting way fat then adjusting jetting down to "safe" plug coloring then adjusting cruise jetting and pvcr id to match best jetting for power. (long before a/f meters were affordable) Tedious but worthwhile. Drivability was very good and the plugs stayed clean around town or in the pits. BTW I never had to mess with air bleed changes but it did take some secondary throttle plate adjustment to get proper idle/mix screw range and transition between idle and pump shot to my liking then adjusting pump shot etc.. I like to be able to go smoothly from idle in gear and relatively slowly to say 2000+ rpm with pump cam temporarily removed before messing with the acell pump. (simulating light off idle load you may encounter in traffic where the pump shot hasnt moved out the nozzle yet)

I'll follow this post with the rest of the article I've attatched plus a jet size to id chart.

[BruceWilkie]

Heres the rest

[69Goat1]

Mine has a wicked blower surge with 2 HP 1000's on it... I like it...

[rtanner]

Thanks, now that is helpfull info, and gentlemen this is why I recomend this board to my mopar and chevy buddies, the experience pool is over the top on this board!

[BruceWilkie]

Originally Posted by 69Goat1

Mine has a wicked blower surge with 2 HP 1000's on it... I like it...

Alot of guys like that sound but it can be a nuisance in a street car. Most often that surge can be tuned out or tamed considerably. FWIW I found by accident I can get a surge like that without a blower if I misadjust the dual quads on my wifes car. The vacuum gauge goes crazy and tach bounces between 400 and 1200! I could put a gear drive on and it would sound like I had a blower on too.


I highly suggest using power valves (at least on the primaries anyway) especially if any street use. Why wash out your rings from excessively rich mix for light loads. It keeps your plugs clean too! Thats nice especially when youve been in staging lanes a bit long and it wont start well or one of the plugs doesnt clean up during your burnout. I'm sure we have all seen the guy who gets staged then stalls at the line or has to be pushed out of the burnout box only because he fouled his plugs. I'd be very embarassed myself. Performance means more to me than "cool" sound.

[69Goat1]

I have street driven mine considerably and have found no issues at stop lights, burnout box or anywhere else for that matter - only issue so far has been a huge wheelstand and flattened headers. Car starts and idles immediately even in weather in the teens with no warm up... I do have Power valves FWIW and it still sounds cool as hell

[BruceWilkie]

Be sure you understand total metering area and metering area for each barrel. Basically area of a jet and area of the power valve channel restriction.

Basic math to find area of a circle: one half id squared times pi.

example .092" id jet and .059" id pvcr = (.092/2) squared then times pi (3.14) = 0.00664424
then (.059/2) squared then times pi (3.14) = 0.002732585

combine the two results(add) = 0.009376825 = total metering area.

You may find it handy to convert that total back to an equivalent jet id if you want to square up the secondary (no pv) metering area. Metering area .009376825 divided by pi (3.14) then square root times 2. or (.009376825/3.14) sq.rt = 0.054646591 then x 2 = 0.10929" A #94 jet is .108" a #95 is .118".

So if your primary and secondary have the same venturi and throttle blade diameter and a #79 jet (.091) and a pvcr of .059" gave best primary results then a #94 sec jet(no pv) would be very "squarely" jetted. If the primary venturi was a bit smaller you MIGHT find the # 95 jet at .118" the better jet choice for the secondary with blocked power valve.

BTW in my previous post I forgot to mention that after setting my idle I prefer to disconnect my secondary linkage and get my primary jetting set real close.(preferably a tid rich at this point) With a blower I found just getting up into the midrange with some boost(preferably using high gear) to be sufficient. Too high the carbs go rich from restriction. Once close there, I reconnect the secondary links and determine my secondary jetting from there, using metering area as a guide plus, determining if the secondaries have larger bore throttle or venturi. Again starting a tid rich. Test and read the plugs or better use afr meter. drop all jets 1 size at a time till you are where you want to be. With a blower I suggest you stay on the rich side.

One more thing 4160 side hung floatbowls (pirated off 1850 type carbs) worked good on the SBC and the carbs were mounted sideways. If I were to do it again I'd modify the bowls to use seperate feed lines or a larger diameter transfer tube, though we didnt have supply issues on the 350. Carbs were also mounted with the feed line side to the rear. QuickFuel currently sells notched floats for side hung bowls and jet extensions being used in normal orientation. The 350 was in a AMC Gremlin and had serious traction problems. But it had a cool factor especially in 1978 on the street.