I have a 1991 454 Mag engine. Just replaced the 33 yr old Thunderbolt IV ignition with a DUI distributor. Since the purple coil wire (+) on my motor isn’t 12 ga., I added a Mercruiser starter relay so I could run a 12 ga. wire to the distributor. However my fear that a starter relay may not be designed to be on for so long was realized, as it only lasted about 25 minutes of running before the relay failed. I can run the distributor on just the existing purple wire, but I’ve read you need a 12 ga. wire to get the best performance.

Any recommendations on a good reliable relay? Preferably a marine relay, and I don’t want to bother with any cheap Chinese knockoffs on Amazon.

And for anyone considering the upgrade, the distributor is nice upgrade on this 33 yr old BBC. So far, faster starts, motor is smoother at all RPMs. Certainly feels like more power, but I can’t provide a true comparison, as I also just replaced my prop with newly retuned one, before the Thunderbolt ignition started failing.

The factory wire is 14 AWG - should be fine. There's no need to make it complicated. I would leave it as is.

The instructions do say it should be no smaller than 12 ga. Measure the voltage at the distributor while the engine is running. As long as it is at battery voltage it should be ok. If it is significantly less it will be a problem.

The instructions also say that if you have the factory Pink wire that it is ok. I just looked in the wiring diagram for my Wife's car and the Pink wire to the HEI is a 3mm wire which is just under 12 ga. 14 ga is more like 2mm so its a tough call

If you do want to stick with a relay I would call DUI and ask them how much current the distributor draws and go from there.

That purple wire originates from the ignition switch and has several, to many, things tied into it. It ends at the coil. Quite a journey it has !

If you do a search here, several peeps on oso did a relay to their dui and picked up some good rpm.

Oh, your typical 30amp Bosch relay will be fine.

Here’s one of the peeps:

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Thanks everyone for the feedback. I think it will be interesting to check voltage or voltage drop as distributor is pulling power. I’m on the purple wire for now, but if I can get a little more power with a relay, I’ll likely go that route, as it would help in my case. Realized, after posting this, that a simple relay that may be used for headlights, like the Bosch mentioned should work fine. I have found a potted relay for marine use at a marine distributor. Think I’ll just use the push in spade terminal type, and then make an emergency jumper, so if a relay fails, I can still get it home. And I’ll carry a spare.

It’ll be few weeks before I get back to boat, I’ll report back with my voltage test, preferably under load at various RPM.

And just a reminder to others doing the switch, I’d question the use of a Merc starter relay, even though I’ve seen it recommended somewhere. Didn’t last long at all for me, and I used a Mercruiser part. I’m thinking it would the same for a trim relay as they are not typically on, all the time. Maybe something was up with the one I received, not sure, but it didn’t last for 30 min.