Cadillac Owners Forum banner

1 - 3 of 3 Posts

·
Registered
Joined
·
446 Posts
Discussion Starter #1
I don't know if I'll ever do this, but I've been thinking about it again lately.

I have a 1979 federal emissions Sedan Deville, but I live in California. The car was originally sold in Washington I think. So, no smog pump!

But, I do still have to smog it, which, I don't mind really because I care enough about the environment to keep it legal, but I guess not enough to dump it and drive an EV or hybrid.

Anyway, so, I have a factory EFI setup, but I don't think I'll use it, because I would have to make too many mods to tune it to support whatever else I do, and then I'm pretty sure it would fail the smog test visually.

So, I'll just rebuild the quadrajet and make some invisible improvements to it.

Then, I have an intake off of a 500, which I think I could get away with using since it would still look stock, but what I would rather do is port the crap out of the aluminum 368 carb intake that I have and use that. I thought I would cut out the whole plenum area, and then match and blend the ports. If it doesn't work out, then I can always just use the 500 dual plane instead.

I would obviously port and do large valves on the heads, and get a slightly more aggressive roller cam.

Hopefully I could figure out a good way to boost compression to about 10:1.

The exhaust is pretty much set. I'm already running manifolds (with the EFE valve) from a 500, and I swapped the stock catalytic converter for a 3" magnaflow, which then goes into dual 2.5 the rest of the way back. I can't do true duals, but I think the single 3" to dual 2.5" is fine. I could probably upgrade the manifold to converter y-pipe in the future and that might help a little bit too.

My gut tells me that I would still pass tailpipe by keeping the converter and other smog equipment in place. My concern I guess is that I would need to run more timing than stock, and when I smog it, ignition timing has to be set to stock or they fail you. I don't think I'll mind changing it every two years, but I would just hope that it didn't run so bad that it caused suspicion.
 

·
Registered
Joined
·
13 Posts
10:1 compression is probably overkill unless you plan to run unleaded race gas. True 9.0-to-9.5-ish would be adequate. Better to have full ignition advance, and a point less compression, than to have half-a-point too much compression and have to retard the timing. Get the quench/squish distance correct. NOT MORE than .045, and .035--.040 would be better. Which pretty-much means zero-decking the block 'cause I bet all the common head gaskets are right around .040.

I can't help with the intake manifold. What are the ports like in the 425 head? I thought they were small. Might have to hire a genuine expert to get flow from them.

The Q-Jet, properly calibrated, should work very well. Same deal with optimizing the distributor--find a stock one in good condition for emissions testing, and one that's been recurved for actual use in between tests. Swap distributors for the test, swap back to make it run right.
 

·
Registered
Joined
·
2 Posts
If your Caddy is required to pass Calif. emissions for that same year, I believe you will need an EGR intake manifold and EGR heads with smog pump as EGR was what was required to sell that car in California. You could use a caddy 500 EGR manifold, with the factory EGR valve on your 425. Do your 425 heads have the smog rail and ports for EGR? I think only the California sold 425 motors had the smog rails and intake with EGR. These items should be available in California bone yards. I have a California emissioned 425 and it has the 425 EGR intake manifold, EGR valve, the giant mult-port vacuum switching valve on block near the water pump (to run the EGR) and smog ports and steel tube cross over manifold on the front of the 425 heads. It came with a Quadrajet with a California specific number, probably with different jetting for the EGR but I don't know the difference or metering changes.

On your build,
I would zero deck the block (if you are doing any machining, and also take 0.020 off the heads). You are going to be pretty limited on piston selection and compression ratio unless you go full custom piston. The 425 heads have the same size intake and exhaust ports as a 500 heads, but the intake port has a pretty bad short 90 degree turn right under the valve seat. Larger valves aren't the cure unless you can match it up with some pro level porting and re-radius the short turn. You can definitely help the factory intake manifold with some internal cleanup, radiusing, 1" carb spacer, and port matching. An Eldebrock is pretty, but won't perform much better.

In the heads, you can clean up the bowls, unshroud the valves, blend the bowls, narrow the guides, smooth the rough castings, etc., new 3 angle valve & seat grind. I would try not to remove much material in the combustion chamber as that will lower the compression ratio. If you want to go big hp, start with a 472 or 500. The 425 crank is a super lightweight version of the 472 crank and the horsepower limit to about 400 hp, before there are main bearing issues and cap walk. Haven't been there, but read about it.

The factory rocker assemblies and rods will need to be upgraded well before you get to 400 hp safely. All the money spent on this 425, could also be done to a 472 using the same parts with bigger results. The 425 is a great motor stock, or with mild mods, but if you want much over 300 horsepower on a 425 the bang for the buck is pretty low. The bang for the buck on a 472 is pretty good up to about 400 hp.

If it were me, I would find a complete caddy 472 or 500 with working EGR, EGR heads, HEI distrib, and working smog pump from an earlier caddy 1971 - 1976 caddy. Then swap on your 425 pan and oil pickup to match your chassis and swap it all in your '79. It would look almost identical, especially if you kept a 425 manifold on it. This is a pretty common swap and would look 100% factory. I doubt anybody at the emissions place could tell the difference, and it should pass '79 level emissions with a working cat and rebuilt carb. A 425 and 500 look identical on the outside, except the 500 has a dual plane manifold and a 425 has a single plane manifold.
 
1 - 3 of 3 Posts
Top