I remember hearing about an easy modification one can do to the intake manifolds of these big blocks. You had to drill something out, I can't remember exactly, what does it do even... I can't remember. Anyways I'll be taking my carb off for a rebuild soon, so I might as well pull the intake manifold off, clean 'er up and mod it if it's a good idea. I remember Rick did the mod to his 425, if you're out there, "Help me Rick, you're my only hope!" :)

I remember hearing about an easy modification one can do to the intake manifolds of these big blocks. You had to drill something out, I can't remember exactly, what does it do even... I can't remember. Anyways I'll be taking my carb off for a rebuild soon, so I might as well pull the intake manifold off, clean 'er up and mod it if it's a good idea. I remember Rick did the mod to his 425, if you're out there, "Help me Rick, you're my only hope!" :)


cut out the plenum

Is this advisable though, I've only heard of a few people doing this, are there any unwanted side effects?

Is this advisable though, I've only heard of a few people doing this, are there any unwanted side effects?
no there will not be any unwanted side efects it will be like having the Edelbrock manifold u will get the same flow so just cut out the plenum and thats all u really can do to the intake manifold

Yup, cut out the center section. If you'd like, tomorrow I'll pull my intake manifold out from the closet and snap some pics... I *DID* take my intake manifold off my car... :).

Also, with the '68 intake, you loose EGR... some say its good, some say its bad.... I think, when working correctly, its good, but the valve was origanal as was everything else, and it helped to clean it up nice.

To properly gain all benifits to the modified intake, you need to add a 1" carb spacer... if one can't be bought, stack 2 1/2" thick gaskets on top of each other, with some hi-tack on the manifold and between the gaskests etc...

There is no downside... well, by cutting the center section out, you do slightly loose some low end torque... but.... its a freakin big block, and its like 5ft-lbs at the most... you don't even realize it, and the benifits more then outweigh the slight loss. It gains alot in the mid and upper RPM, and just flows really nice.

I'd say to rebuild, get the carb rebuilt, or get a new carb while its off... don't do this mod, then stick an origanal 25+ y/o carb on there.... and keep the stock Qudra-Jet! Unless you have a wild engine with over 500hp, the stock 800CFM Q-jet is the best you can get.

I should make a write up and post it in the big block archives... I haven't been keeping up with that like I wanted... but I have just had alot of stuff going on... I'd like to though, as its a great benifit.

If you can snag exhaust manifolds from a 500, get them, the flow alot better then the 425's manifolds... and upgrade the exhaust! a nice 2.25" true dual with H pipe, no cats and hi-flow mufflers would be sweet.... between the intake, and the exhaust... and a #5.... MAYBE #10 MTS cam (#10 is the very edge for the stock valve train), you would have really woken the 425 up... there is plenty of potential there.... also, dump the 2.28 gears.... thats limiting it too... but damn, my 425 was quite powerful by the time I got done with it.... and my plans were just starting too.

Of course the above is in reguards to the '79 chassis.... still tho, the intake mods are the same, the 1" carb spacer, and a 14" Edelbrock open element air filter would be great.

Check http://www.500caddy.us - click "manifold" on the left hand navigation bar. There are a couple pics there.

If you're going to change the exhaust manifolds, you can port the stock ones a pretty good amount if you have a die grinder (which incidentally is the best tool for doing the intake manifold mod.)

My intake is different then the one in the pic... I have coolant passages for the carb, tho they aren't used.

Yeah, a grinder would open the exhaust manifolds up, but I woudln't even bother with the 425 manifolds... get 500 manifolds and take the grinder to those... then you'll really be doing good.

We took my intake manifold for a family friend with a machine shop... dunno the name of the machine we used, but it was pretty sweet.... couple hours there, BS'ing and stuff, and it was good.

Yeah that's what I meant, open up the 500/472 manifolds.

There has been a bit of confusion regarding my vehicle, I have a 472 in a '70 Deville. Although the mod still works exactly the same and will still show the same benefits will it not? As far as duals go, is 2.25" true duals with H-Pipe about right for the 472 as well? What about headers? Also I'll be damned if I can find my carb number, I have looked everywhere on that thing but it is nowhere to be found, it's hard to get a rebuild kit for a carb with no number, but it must have one somewhere.

Thanks you guys I really appreciate your advice :)

yes, the intake mod will help out great in any 425/472/500.... not sure if it fits the 368, but if it does, then that too.

Unless you are doing a really wild build on the 472, then I think 2.25" duals with H pipe would be great. No cats or resonator, just a muffler of your choice on each side and you choice of tips.

Lots of headers are made for the Cads, you can even get BBChevy headers and have them welded to Cadillac flanges.... the main limiting factor is chassis clearance, I am not too familer with the ~'70 as far as limitations go... I focused more on the '77+ for the engine swap... Some headers IIRC will have clearance issues, but others will fit. Still, if you open up the 472/500 exahust manifolds a bit.... like before, unless you are going really wild, they should flow fine.