In about four weeks, I will be pulling the stock cam and smogger 882 heads from my LM1 motor and replacing them with the 268 cam, and mildly ported 083 heads. Also will have the edelbrock performer intake. I will still be running the stock cc q-jet. Will I have a problem with this modification and the ECM unit ??? Will the computer recognize and adjust accordingly ?? Can't take the computer out as I have to stay emissions legal. Please advise, or comment. Also, what kind of power gains can I expect if all goes well ??? Thanks.

 

Yank,

You shouldn't have any significant problems. The feedback system on that engine is very simple. The BARO sensor checks atmospheric pressure and slightly skews the fuel/spark trims accordingly. The ECM monitors the oxygen sensor and tries to cycle the mixture control solenoid in the carburetor at a rate which will trim fuel to keep the oxygen sensor in an acceptable range. The coolant temperature and air intake temperature calculate slightly into this trim, and teh spark timing is adjusted slightly depending on those inputs and engine RPM. It is really a very forgiving system, and since it doesn't care much about intake vacuum/pressure, you should have no real problems using it. You may have to adjust the carburtetor to maximize your setup, but you'd have to do that even without the ECM. You may need at least different secondary metering rods and the hanger/cam, and may have to adjust the secondary air valve spring rate. If you get really good head flows, you may even have to re-jet the carb, but the primary side jets are usually adequate for the primary flow rates at lots of RPM and flow range.

 

I agree. The combo you talk about is well within the adjustment range of the system you have. It may need a little manual work dialing it in after you put it together, but it's not at all out of the ballpark of what the system is capable of. Just don't think that you'll be able to slap it together and the computer will take care of it on its own; remember, the computer is expecting some "peanut" cam and alot less flow, so you'll need to do some tweaking. But again, that combo will "fit" within the tolerances of the mchanical sizes of things and the computer's adjustment range, after proper mechanical setup, but it won't be perfect right off the bat.

 

OK, Thanks guys ! I can rest eaiser now. One other question......Can I use the stock springs from the 083 heads with the 268 cam ??? Or will I have to get stiffer springs ??

 

Yank,

Don't do it. The lobe profile is beyond the capability of the stock springs to keep the lifter compliant with the lobe at much above 4,000 RPM, and is dangerously near the limit of the springs lift capability.

Another thing that I missed in your first post - the "268" is a flat tappet grind. Exactly which cam grind were you considering to mate with your roller lifters?

 

Vader, don't know the grind of the cam. A friend who is a mechanic is giving it to me. He had it in a Hotrod and went with a more radical cam. He said it would work in my motor. I have the stock lifters for the 77 350. Pretty sure they are not the roller type. Sounds like i'm going to run into spending more money than I thought !!!!

 

Yank,

Sorry I confused the matter by my laziness. I totally missed the "77 350" part. I read the "87" in your post and my train of thought went straight down he "roller cam" track. Looks like I got off at "Oblivious Junction"....

You'll definitely want to improve the valve springs, and should check the retainer/guide clearances. And those factory pressed rocker studs may not survive the extra force for very long. Pinning would be a minimum step, but screwed studs are the final answer. If you have to machine the guides for retainer clearance, it becomes almost an automatice response to install screwed studs.

 

OK...Thanks Vader !! Do you see anymore unforseen things that might need improving before I put the heads on ?? I just don't want to get everything put back together and have something go terribly wrong because of lack of proper components in the valvetrain area, from camshaft on up. If you can think of anything that should be done at a minimum, please advise. Thanks Again !!