Thunderbolt ignition to Davis Unified Two Wire ???

[rockdog57]

I'm planning on changing out the thunderbolt ignition on my 5.7 chevy motor. The new marine distributor comes with a two wire lead. It has the coil in the cap. One lead goes to 12 volt keyed and the other goes to the tac. If I do this, how would i wire the shirt interrupter? I'm assuming the distributor grounds thru the block. So there is no way to interrupt the charge going to the distributor from the coil. Hook the interrupt to the tac wire? Has anybody done this conversion before?

 

[achris]

Has the module of the Thunderbolt failed? If not, why are you changing it? If the TB system is still working, there is no point in changing it. It is still one of the best and most reliable systems out there.

To make the shift interrupt work it would be prudent to contact the manufacturer of the new unit. Some do it by interrupting the incoming 12V, but there are systems that do not respond well to that, and would need to be interrupted in a different way. Grounding the tacho wire is not be advisable (and may actually burn out the new module).

And don't take the distributor being bolted to the engine block as a good ground (don't ask how I know!). Run a ground wire!

Chris.....

EDIT: Just read up on the Davis units. It's a Mechanical advance!!! Why would you do that to yourself???? The Thunderbolt is fully electronic, not weights and springs, nothing to fail, corrode or seize up!

 

[rockdog57]

Chris, are all the thunderbolt modules the same? My boat originally had a 305 in it. I’ve replaced it with an l31 (5.7 vortec) I can’t get it to rev above 4000 rpm no matter what prop I run. It goes like a scalded dog. But something is holding it back. New Holley spread bore 600 cfm. I tried a rebuilt quadrajet, same thing. So I figured it must be ignition th

 

[Bt Doctur]

or the gear ratio in the outdrive

 

[79Invader]

Chris, are all the thunderbolt modules the same? My boat originally had a 305 in it. I’ve replaced it with an l31 (5.7 vortec) I can’t get it to rev above 4000 rpm no matter what prop I run. It goes like a scalded dog. But something is holding it back. New Holley spread bore 600 cfm. I tried a rebuilt quadrajet, same thing. So I figured it must be ignition th

Gear ratio in the outdrive could be your issue. What’s your top speed at 4000 rpm? What pitch props have you run? SS or aluminum? 3 or 4 blade? Also check to make you have a solid ground on the distributor pickup sensor, that could cause issues as well.

 

[Scott Danforth]

Chris, are all the thunderbolt modules the same? My boat originally had a 305 in it. I’ve replaced it with an l31 (5.7 vortec) I can’t get it to rev above 4000 rpm no matter what prop I run. It goes like a scalded dog. But something is holding it back. New Holley spread bore 600 cfm. I tried a rebuilt quadrajet, same thing. So I figured it must be ignition th

its not your ignition. the TB ignition is years ahead of the old HEI.

DUI is normally paired with a cone clutch drive for that reason. you could probably rig up a switch to momentarily kill the power

you realize an L31 is only about 215hp stock. the 305 you pulled out was closer to 250 at the flywheel
look at your cam, if you have the stock L31 cam
look at your base timing
look at your fuel system
600 CFM is not that much. my 1988 boat came with 715 from the factory. most V6's are running 600 CFM carbs

 

[rockdog57]

its not your ignition. the TB ignition is years ahead of the old HEI.

DUI is normally paired with a cone clutch drive for that reason. you could probably rig up a switch to momentarily kill the power

you realize an L31 is only about 215hp stock. the 305 you pulled out was closer to 250 at the flywheel
look at your cam, if you have the stock L31 cam
look at your base timing
look at your fuel system
600 CFM is not that much. my 1988 boat came with 715 from the factory. most V6's are running 600 CFM carbs

Actually the L31 is way more than 215 horse. I have the top gearing in the outdrive. Running a 21 pitch SS prop. Carb might be my problem. I'll chase that some more. I'll recheck base timing too. Not trying to be a smart *** about the L31. I just did a lot of research before going with that crate motor.

 

[achris]

I would need to know more about the previous 305 that was in the boat. Do you have the number from the side of the module? It's going to be 'V8-24', or 'V8-22'. And both of those modules were also used with 350 engines. Total advance is only 2 degrees different. Your problem is not the ignition advance curve. Save the money that you would have spent on the Davis unit and buy a proper carb. Holleys are about the worst carb you can put on a boat. Merc stopped using them back in the 70s!.

What do you mean by 'top gearing'? What is the ratio of the drive? (It'll be 1.5:1, 1.65:1, 1.84:1 or 1.98:1) If you don't know there are a few ways to find out. The easiest is to give us the serial number from the side of the drive shaft housing... https://forums.iboats.com/forum/engi...serial-numbers

Chris...