This might be a stupid question, but just bare with me. I have a 1/2 bridge ported 12a w/ all emissions removed. I am (my father is actually) attemping to put on a Holley 600 (not modded) on the car. Can you run Vac secondaries? (vac pods are disconnected) are do you have to run mec. secondaries?

Thanks

-Ryan

 

1/2 bridge, I would go w/ the mech. secondaries 1st. May need some rejetting but should work fine. Then of course you could use an Edelbrock and......nah I'm just being a wiseGuy!

 

my RB SP setup is mech secondary's and i'm BP too. works fine for me. although i think this carb is holding me back probably time for 48DCOE.

 

Ryan I'm running a 12 bridged too. with holley 600cm with 67 on jet its a sinlge pump, and on the sencodary's a put a screw to hold the butterfly and when you goes full throtlle it's opens, check that ans see if work . also you need to get a good fuel pump and run 6 to 7 p.i.s I'm using a red holley pump, fuel regultor too,. thanks Good look

 

Bridgeports run such low of a vacuum due to the amount of overlap I would hate to try and run vac operated secondarys. My 1/2 bp normally runs 8-10", opening up the throttle only makes it worse. If you must run a Hollry, make it a mechanical one.

 

Thanks for all the info guys! I am running mech. secondaires now. It's running great, just breaking up top end (timing issues). What timing do you guys run on the 1/2 bridge ports? B/c I know my car hates the stock timing for the 12a. Also, my Holley 600cfm doesn't allow me to change the jets on the secondaries (stock holley). Other than changing the jets on the primaries, is there anything I should do to get optimal performance out of the motor??

Thanks!

-Ryan

 

Good call with the Mech Secondaries. The next mod could (should) be more spark! If It were me, I would convert to an FC coil for the leading ignition powered by an MSD 6A.

I would run the Trailing stock with an MSD Blaster II coil.

Then I'd adjust the timing, and adjust the leading trailing split. I usually do this step by feel, then fine tune it, by feel.

Doesn't hurt to have a Wideband, or at the very least, a Narrowband to get a general Idea where the A/F Ratio is at.

 

IMO if you're thinking about turning a Holley vac secondary carb into a mechanical, just buy a double pumper and do it right. The fine tuning you can do on the secondary side with a true double pumper is worth the money.

The only time I would run a vac secondary carb is on a heavy car with an automatic.

 

Depending on your Elevation versus Sea-level, you could run an 850 Double Pumper. They are a dime a dozen off old Ford trucks.

 

FYI you can swap the secondary metering block on vac. holley for one which will allow changing the jets. Mine had a high rpm miss and needed to this mod. after I hooked the ignition to a scope and found the spark pattern was okay. Summit racing has about the best price on the metering block. Call the tech support for the part number. You'll need to test figure out which jets you'll need for your engine combo. One problem I had was without secondary pumps, the engine would bog due to a sudden drop of vac. when I did a quick WFO and the secondarys opened up without a the extra shot of juice that double pumpers provide.

 

Since the trailing fires at different times, running an MSD would require 2 of them, unlike the the leading which fire both the front and rear at the same time, twice per combustion event, the 2nd fire being used as a wasted spark. For the 2-3% gain in hp, it's not worth the extra money and the stock, or an upgraded stock type coil will be fine. MSD does make a replacement for the stock type coil.

Running an 850 DP would be a waste of time/money and kill performance. That carb is simply way to big and will not pull the fuel needed through the ports due to the low vacuum that bps run. A 650 DP is more than enough of a carb for even a full j-bridge. Bigger is not always better when trying trying to pull fuel into the engine with the low vacuum and the overlap that a bp has. My 1/2 bp will pull a little over 10" Hg, occasionally 12" on hard decel. Open it up for full throttle, it can easily drop to 6". That low of vacuum, along with the overlap is not conducive to pulling fuel through the carb.

I did notice that when I installed the dual Weber's DCDs, the vacuum went up slightly because the runners are separate all the way up to each of the 4 butterflys which did reduce some of the overlap that kills vacuum.

 

You dont need to run an MSD on the Trailing ignition because its just waste spark. Your correct, you do need two, but its not nessesary. An MSD Blaster coil will be sufficent.

As for the carb size, it depends on your elevation. Me, being at least (cant remember) 2500 feet above sea level, I found a 650 DP worked way better on a stock 13B over the RB 465 (or whatever it is). Theres a few guys around here that have upped to waaaay bigger carbs and gain significant power.

Thats why I said based on elevation. I mean, if I were to bring my old car to Sevenstock, it would have choked and probably died.

 

Thanks for the responses guys! Again, what timing do you all run on your 1/2 bridge ported motors?? Thanks

Ryan