A fellow club member has asked me to help him detune the engine in his Chevelle. It is a 402 in a '69 and I've heard it run and it must be fairly wild inside. I think it was a semi-serious drag car before he got.

Neither he nor the previous owner know what is inside the engine. The first task is to check out the heads (close/open, square/oval) and carb. It has a Torker II and I'm guessing square port heads. He says it runs poorly on pump gas so I'm guessing it has fairly high compression. The engine needs to be pulled to repair some broken starter bolts and re-gasket the thing. He'd like to avoid going through the whole motor, or at least the short block to get it to be more streetable but is willing to do a full rebuild if necessary. First we need to know what's inside to make informed decisions about what path to follow though.

A lot will be learned when we determine what cam is in the thing, but assuming it has fairly high compression, are there any kinds of street cams that could be used to lessen the effective compression ratio while maintaining the pistons and heads he has? If this is not possible or a bad idea I would suggest changing heads alone to lower the compression ratio to avoid getting into a short block rebuild.

It runs fine and doesn't blow smoke, it's just too wild for the street. I'm hoping a carb/intake and cam change alone can get him back into the realm of the streetable regardless of the heads and pistons he's got in there.

You can not make good decisions on what parts to use by simply assuming.

It doesnt cost much to freshen a motor, and will save you alot of hair pulling in the future.
At that time, it would be wise to swap pistons/cam/intake, ect.

A smaller cam will build more cylinder pressure, and require higher Octane, if not too much cylinder pressure and blow out alot of head gaskets.

Originally posted by Motor Martyr:
You can not make good decisions on what parts to use by simply assuming.

It doesnt cost much to freshen a motor, and will save you alot of hair pulling in the future.
At that time, it would be wise to swap pistons/cam/intake, ect.

A smaller cam will build more cylinder pressure, and require higher Octane, if not too much cylinder pressure and blow out alot of head gaskets. The "assuming" is temporary, just until we get into the engine and determine what's in there. Hopefully, if it has high compression it is created by the heads more than the pistons. If the short block is sound with reasonable pistons, the detune could be obtained without getting into the short block other than the cam. Of course if the block or bearings have a lot of wear the best route is a full rebuild. He wants a fairly mild cruiser so another option might be to simply swap to a less radical engine altogether. Depending on what we can locate, a swap might be more cost effective too.

Overlap on the cam would determine how much cylinder pressure would be built but I think a cam with perhaps less lift and duration but more overlap would still not be all that happy on the street. If the engine has pistons with large pop ups and closed chamber heads he's going to have to look at some fairly meaningful changes all around.

If it has closed chamber heads, he's in luck. Swap them for open chamber 781's (or something similar), a milder cam, and a dual plane intake. You can do it all withthe engine in the car. If it has domed pistons, things get stickier. I am told that domed pistons for closed chamber heads often will not work with open chamber heads...the piston could contact the edge of the chamber. In this case, you're looking at pulling the motor, swapping pistons, re-balancing, etc. Might as well go the whole 9-yards and freshen the rings and bearings too at that point.

Most closed chamber pistons will work with open chamber heads,I have closed chamber pistons with my open chamber heads,it's just like everything else you have to do your checks to be sure!

Anything short of opening the motor up and finding out what is has is pure speculation.Get it apart,and go from there..it probably some mismatched combo,or one that isn't tuned correctly,but thats speculation too ;) :D

The engine has to come out regardless. The pan gasket is leaking quite a bit and the starter bolts are broken off in the block. There is no way to easily get at those starter bolts with the engine in the car so out it comes for a look-see.

Considering it appears to be a barely streetable race build up I'm expecting some serious pistons and heads in there. The good side to that is that it has a better than average chance of having a really good crank and connecting rods which can certainly be reused in any event barring any serious problems.

He's going to get the head casting numbers to me yet this week so we can begin to know more what we are looking at. I've only seen the car once and if I remember correctly the carb was a full race version without a choke set up so that too will need to go. It also just occurred to me that it might have really big headers too so this could get more expensive quickly.

Once I have a few more specifics like the head, carb and cam info I'll start another thread so we can figure out what the best and most cost effective route may be. It's possible it's simply an overcammed steet motor at heart.

He's also going to try to contact the previous owner (two owners ago) who built up the engine to see if he can shed any light as to what's inside before we crack her open.

Don't be suprised if you find .180 domed pistons with open chamber heads (about 8.5 to 1 if you're lucky) and too big of a cam..

That'd be a stroke of luck, one street-friendly cam and you're done! Rich, other option is the quick swapo as Mike points out, or you might have to stick an old-time, ala GM, wide seat duration 114-115 lsa cam in it to drain pressure and gain idle/low speed driveability. At at least relative to where he is at now.

The path will become clear once you get some more info. He might want ( I would) to do a hot/cold compression test now ( unless the starter isn't working), to determine sealing in case he gets to a decision node down the road where overall sealing quality is something he'd want to know.

Leakdown would be best test. And oil filter "spec per pleat" count to get a handle on internal wear. To know how far into the thing he needs to go.

Prior owner info only valid if the guy/gal was careful about documentation. He starts getting, "Well, I think it was a Comp XXX, or was it a YYY ", then assuming NOTHING is more efficient than getting led down the wrong path by a well-meaning, but forgetful, owner.

Do what I would do, just put all the same stuff you were thinking of running in your own engine design, in his, to see how it works before you pay up! smile.gif

I'd love to do a compression and/or leak down test but the starter is broken off from the engine and the bolts are broken off in the block. There is no quick and easy way to repair this in the car so it just makes sense to pull it, fix that and use that opportunity to see what we have to work with. I'm trying my best to prepare him that things can get expensive quickly once we have it out of the car and start the evaluation. :D

If a cam swap can solve most of our problems, I think we can stir up a little more business for LunatiHarold! ;)

Well, the first tidbit of info we have is that the heads are: 1971 oval open 113cc chamber 255/114 ports.

Not a bad head for the street. Of course a lot yet will hinge on what kind of pistons and cam we've got to deal with. Maybe we'll luck out and it will be a "if I just bolt on the biggest stuff I can find" engine that can be easily resurected by taming down some of the bolt on equipment. And he'll be able to fund part of the process by selling his old parts since they are all in fine shape.

The way the thing ran it has to be WAY overcammed and maybe over carb-ed as well.

The carb turned out to be an Edelbrock 750cfm square bore unit. We might be able to make use of that in the end. It has an electric choke but I don't think it is hooked up.

I'm beginning to get the feeling it is simply and overcammed street engine. We'll know for sure when we crack it open in a couple of weeks.

Rich,what is the head the casting # ?

Originally posted by mr 4 speed:
Rich,what is the head the casting # ? Sorry, I thought I had posted that with the head specs. They are 3993820 heads.

Ok, so far so good. A decent head I hear. Had a set on on old 396.

Got DOME!? ;)

Originally posted by 427L88:

Got DOME!? ;) We won't know for a couple of weeks when we pull the engine.