chainreaction

I need some help.
I bought a bridge-port 12a to go in my sand rail and need to know what I should set the timing at. I have a crank trigger system that is non-adjustable....
I do sand drags with the car which weighs 638#. It has no transmission.... Ie: the motor is mounted sideways, has a Micro-Belmont snowmobile clutch driving a jack shaft, which runs a chain-driven rear spool. That clutch is tuned to engage at 4200 rpm's. The motor has a Racing Beat intake, which has been ported, and a 650 Holley 4-bbl which I built. I built my own header with a Racing Beat header flange and 2 1/4" id pipe, 30" long direct flow. I have a hybrid crank trigger system, adrenaline crank pick up with a MSD-6al box, 2nd gen coil feeding the lead plugs only. With my old stock 12a I had it set to idle at 3400 rpm's (so the carb was just into the secondaries) Upon launch it would jump to 8,900 rpm's within about .5 seconds and stay there for the length of the track. I ran a 150hp shot of nitrous with the stock engine. Was running 1.7 60' times with a best et of 4.59 seconds at 74.6 mph in 300'. Roughly 4 mph short of the SSDA national record.
I plan to use a degree wheel to lay out, drill and tap the two bolt holes for mounting the pick unit on the motor plate.

chainreaction

peejay

Somewhere around 22-25 degrees would be a good starting point. Some run up to 30-33 degrees but I have always seen power plateau around 22 degrees, meaning there were gains up to 22 but going beyond that saw no improvement. (I've run 27 degrees on stock ports with good success, for what it's worth) Part of this is rotor housing dependent, part of it is fuel dependent, and a lot of it is load dependent. If you're really screaming the thing at 8900 instantly with no passing through peak torque (which will happen around 6000-7000rpm) then you could stand more timing since you won't ever see full load at peak torque.

jamie k

What is your jetting

Shainiac

iTrader: (7)

You're also leaving some power on the table by not collecting the header and taking full advantage of scavenging.

j9fd3s

iTrader: (3)

i'd start around 18 btdc, and then work your way up, let the engine tell you what it wants, we don't know.

my experience is similar to pj's, power peaks around 20-22, and there are small gains by running a little more, like 25-27, but your safety margin goes away too

chainreaction

Thanks guys. This was very helpfull.
I usually run 66 jets in all 4 corners, both squirters are 31's and the power valve is blocked off.
Any other suggestions are welcome.

chainreaction

This is with the timing at 22 degrees BTC

chainreaction

I have a follow up question relating to the n/a 12a bridge-ported motor.
The motor came with no rear counter weight on it. i have tried 3 different 12a counter weights and no matter what, it shakes like crazy when I start it up. It doesn't come out of it when you raise the rpm's. I have broken two starters and a coil from the shaking. It shakes so bad it aerates the fuel and it don't like to keep running. The last time i felt a motor shake like that was when a guy sold me a 427 chevy crank that he thought was a 454 crank. Int vs ext balanced.
The guy i got the motor off has been no help in figuring out what the motor was built with. He has told me I need to go back to a distributor ignition, and also that I need to tear the motor down and put the front counter weight off my old motor on this new motor! Would someone possibly have used a 13b counterweight to start with in balancing 12a Bridgeport motor?

peejay

There's a possibility that it has lightened rotors. There are only two different 3mm seal 12A counterweightings, pre-'82 and '83-85 (heavy and light).

I have heard of people putting Series 5 flywheels on 12As but the part that gets rebalanced is the flywheel, not the rotors.