View Full Version : Cadillac 425 Performance Question

08-10-10, 02:47 PM
Hi all,

So I acquired a 9k original mile 1978 Eldo Biarritz Custom Classic last year and finally got around to tinkering with it. When I bought the car it was a complete dog. If I floored it, I couldn't feel any difference at all. This was even after I changed the plugs/wires/dist cap/rotor and rebuilt the carb. The previous owner already had the catalytic converter removed. The car ran impeccably, and was fine if I didn't push it, but had zero guts if I floored it.

I've owned several 1976 Eldos with the 500, and they really scooted once I removed the catalytic converter and advanced the ignition timing from the stock 6 to about 11 degrees base advance (drove one car 40k miles this way--ran premium fuel and never spark knocked).

Anyway, I started advancing the ignition timing on my 78 Eldorado to see if it would improve performance. Low and behold, it did significantly. I know the stock tag says to set the advance at something like 26 degrees @ 1600 rpm, which is pretty much where it was when I got the car. Anyway, at that specification, the base timing at idle was about 6 degrees. Now, I've advanced it so that it's about 13 degrees base timing at idle.

The car runs soooo much better. Way peppier, no spark knock, no bearing noise, and better gas mileage. I'm a bit worried that 13 degrees is too much, but the car seems to run significantly better than if I had it set to 10-11 degrees. I can actually feel the extra power as the secondary barrels open up. Has anyone tried this configuration out? Seems like these 425s have very conservative ignition advance, a different distributor from the 500s, and are a total dog in stock form.

08-11-10, 01:42 AM
I run a 425 in a 77 deville at 12 degrees base with no problems. I think I have some carbon issues in the carb but even with that its no dog. You may have some timing chain wear that needs the advance to compensate or the vacuum avance is inop or leaking. Well with 9k its probably not the timing chain more likely vacuum issues or carbon buildup.

08-12-10, 12:03 AM
Thanks. It's decently peppy now running 13 degrees advance. Remember, the Eldorado is much heavier than your Deville...I'd guess about 800 lbs or so, so the engine is burdened with a lot of extra weight.

I think I remember reading that the 78 Eldorado took 13 seconds to hit 60 mph from rest in stock form, which is a rather leisurely pace. By my wristwatch, I'd guess I can hit 60 now in about 10.5 seconds, which is a substantial improvement.

My 76 Eldorado, with the converter removed and running 12 degrees advance would hit 60 in about 9.0 but it had the 500 ci V8.

08-30-10, 02:17 AM
I had forgotten about the weight difference, the Eldo would be a bit heavier. The only problem I have had with too much timing advance is preignition/detonation or hard starting. If you do not have those issues you should be OK.

08-30-10, 09:15 PM
It's possible the harmonic balancer outer ring has slipped and set your readings off. It seems improbable for a low milage vehicle to have this happen, but the rubber may have dried from age and a lack of use.

09-08-10, 09:37 PM
You should take off the distrubtor cap and check the weights and springs and make sure they are free and not frozen....and check the vacume advance is working.....I'm running a fuel injected 472 and my timing, after it starts, is never under 28 degrees... I don't think your going to hurt anything advancing your timing like you did...