1996 GM 5.7L Vortec Carb'ed and Thunderbolt IV Ignition Setup

[76SeaRay]

Ready to wire up my new engine build for breaking it in on a run stand. I will wire it up to my instrument panel using the wire harnesses from the boat (boat is in full restoration so wire harnesses are on the bench). I have a Thunderbolt IV ignition. What is the recommended initial timing? Need to make sure the timing will be correct after initial timing so how do I tell what the timing curve is for the TB IV module I have or are they all the same and not changeable? The TB IV is from a donor boat so I don't know what it was run on before...

 

[Scott Danforth]

8 degrees BTDC

timing curve will be on decal on module.

 

[76SeaRay]

Well, unfortunately somebody painted over the label on the side of the module. I am attempting to get the paint scrapped off and have uncovered some information but haven't found anything yet that looks like a timing number. Will try again tonight to get the rest of the paint off....

 

[76SeaRay]

While I sort out which TB IV I have, a further question on initial timing. I put in the Comp Cams Xtreme Marine XM264HR cam (roller lifters, 7.2 inch push rods, and 1.52 to 1 roller tip rockers). I assume this is more lift than a stock 96 Mercruiser 5.7L. So, does the initial timing need to be more than 8 degrees? The machine shop said 14 degrees but not sure he was thinking marine engine when he gave me that, however, he knew what cam I am using. I know the initial timing is going to be dependent upon which TB IV I have but don't know if it has to be to different to satisfy both the new cam and the TB IV. I guess the follow on question is "does the TB IV care what the initial timing is set at and only adds to that for total advance?"...

 

[Scott Danforth]

8 degrees unless you go to dry exhaust

 

[Scott Danforth]

10 degrees if you feel lucky and watch for reversion

 

[76SeaRay]

So, still trying to get the paint off the label to see which module I have. In the meantime, what is the max timing at all-in to avoid reversion - 30 degrees or 32 degrees (V8-22 or V8-24 with 8 degrees initial)?

Thanks...

 

[achris]

The module doesn't know (or care) what you set the initial timing at. If the module is -22, it will advance the timing by 22 degrees by about 3000rpm. 24 degrees for the -24 module...

Chris...

 

[76SeaRay]

Finally got enough paint scrapped off the label to get these numbers (as near as I can tell). These are the only numbers on the label that is on the side. The top simply has the Mercruiser Thunderbolt IV id label but no other markings:

6F27
98307
390 7839 1

Can anyone confirm what I have?

Thanks....

 

[Scott Danforth]

So, still trying to get the paint off the label to see which module I have. In the meantime, what is the max timing at all-in to avoid reversion - 30 degrees or 32 degrees (V8-22 or V8-24 with 8 degrees initial)?

Thanks...

its base timing at idle that you have to be concerned with

too much timing at idle and you suck in water. to much cam overlap at idle and you suck in water.

total timing is a function of your boat weight &the hull shape (load), the combustion mix of the motor (wave front propagation and pre-ignition control) and the fuel you are running (resistance to detonation).

30 or 32 degrees all-in is going to be fine. the difference between 30 degrees all in or 32 degrees all in is about 5hp

put a timing lite on the motor and bring the RPM's up to 3000 and see what you have.

 

[76SeaRay]

I don't really care if I lose the additional 5hp. Based upon the pistons and heads (per my engine machine shop), I should end up with the ability to run 87 octane pump gas and just about 300hp (will be running an Alpha One and mostly cruising)... Maybe I should go with a different module than the one I have since I don't know anything about it. I did find a used one with a similar number as above on NLA and it said it should be a V8-24 but then it also said some were V8-22. Go figure...

So, wondering if I should go with the module I have or replace it. If I replace it, I am wondering if I should get a newer used TB IV with V8-22 ($225 or so) or go to a used TB V (haven't found a used one but they look expensive). I have a brand new Mercruiser TB IV distributor so want something compatible with that.

 

[Scott Danforth]

go with the module you have, install, verify total advance, go boating

 

[76SeaRay]

Since I can't work on fiberglass with Covid (can't breathe well enough with the respirator), I am doing homework on the engine. Bringing up my older thread with a new question. I know that my machine shop said I should be able to run 87 pump gas but I am concerned no matter what I run, that should I start getting detonation that I won't hear it with my bad hearing. So, I am wondering if I should go ahead and replace the TB IV with a TB V and install a knock sensor or is there some kit to add knock sensor to the TB IV in case I get detonation and can't hear it? (I have new dry joint exhaust and new full closed cooling)

Thanks...

 

[achris]

...or is there some kit to add knock sensor to the TB IV...

No. TB-IV doesn't have the internal circuitry to use a knock sensor... If you want knock sensors, you have to use TB-V, and the right knock sensors...

Chris...

 

[dhrmx5]

its base timing at idle that you have to be concerned with

too much timing at idle and you suck in water. to much cam overlap at idle and you suck in water.

total timing is a function of your boat weight &the hull shape (load), the combustion mix of the motor (wave front propagation and pre-ignition control) and the fuel you are running (resistance to detonation).

30 or 32 degrees all-in is going to be fine. the difference between 30 degrees all in or 32 degrees all in is about 5hp

put a timing lite on the motor and bring the RPM's up to 3000 and see what you have.

I am intrigued. Why does too much initial timing cause water to get ingested? I ask because I am running 10BTDC with TBIV and the boat goes like a raped ape. (apologies to all who identify as simian on this page) I am fortunate in that I have an endless supply of high octane fuel I get for free so I didn't think the extra couple degrees would hurt anything.

 

[76SeaRay]

Is there such a thing as an after market TB V direct replacement ignition system?

 

[Scott06]

Is there such a thing as an after market TB V direct replacement ignition system?

No closet thing is the Delco EST but that’s a kit with distributor and requires minor rewiring so it is not a plug and play replacement for t bolt

 

[alldodge]

Don't know the stock level but I'm finding new OEM TBV for $700 (bravo) at Merc vendors.

That said Delco EST would be my choice at $400 for dizzy, wires, coil and hook ups
https://www.amazon.com/Marine-Ignition-Mercruiser-Distributor-805222A1/dp/B07NGSM9XZ

 

[76SeaRay]

Does the Delco EST have a knock sensor input or is this a replacement for TB IV without it?

 

[alldodge]

Delco EST doesn't have a knock sensor function. They were brought in when ECM came in, so knock function went to the ECM