what happens when you put a cam in a 302 efi that is not for a computer controlled engine

 

Is this for the '86?

The ECU can compensate for some cams, but not all. Depending on the cam specs, you may run into run-ability issues (or a no run issue). You can also have the ecu re-flashed, but that can run $3-500 for someone that knows what they are doing. Somebody must know what the limits are for that computer, but not me. I usually stick with carbs.

You may also have issues with lean conditions, where you may need to up the injectors too, and possibly the fuel pump.

Just an example, I helped do a motor for a Mustang that had a larger cam. Just for the heck of it, we tried to run it with the stock computer. Wouldn't run for more than a couple of seconds. The ecu was replaced with a Cobra mustang unit. It now runs great after "learning" the new engine, with out doing a re-flash.

 

i picked up one that is .471 lift 276 intake and 286 exaust with a lobe separation of 114 and i want to change to a 1.7 roller rocker. Just trying to figure out what problems i might run into or if i need to go ahead and order a weiand 8020 intake and throw my 750 holley up there what do you think

 

and thats a nice looking ride there 81choptop i got an 86 stepside

 

ive heard that the lump from some cams will trigger the knock sensor. it reads the lump as detonation and keeps the ign timing retarded

 

going to finish pulling the engine this weekend maybe so in a week or two i shall find out... might pick up a set of injectors incase she is running lean maybe 24 and 30 what you think. my knock sensor has already been deleted so that shouldnt be a problem

 

The problem is the computer is a speed density setup. The factory set the engine up with the stock components, and then ran it on a dyno and set the timing and the fuel curves for THAT setup. At a certain throttle opening and rpm, the engine has a certain vacuum signal to the MAP sensor and the computer looks up in a chart in the memory how much fuel to send to the engine. If the oxygen sensor says it's lean, it can add a little bit more fuel, but not much.

You are going to change the characteristics of the engine, and it will throw the air/fuel chart already in the computer way off. Your cam will make the vacuum lower that it would be with the stock cam, and all your other mods will be increasing the amount of air entering the engine. The computer has no way of knowing how much air is entering the engine, because it doesn't have a mass-air sensor.

 

That was a VERY informative explanation, Franklin. Thanks!

 

X

EDIT:

Here I go typing up a huge response for the OP then Franklin beats me to it in a few lines. LOL.

X


If the cam fits in the motor, that won't be a problem. What does matter is the duration on the camshaft as this will determine how the computer will react to the drop in vacuum.

When you select a cam, obviously you're going to be looking at higher lift, and most people will go for a higher duration cam. The duration is the amount of degrees in crankshaft rotation that the valves will stay closed. Duration is what affects idle quality. And, to compare cams, you don't look at advertised duration. You look at duration at 0.050'' of lift. A cam with 208 or 210 degrees of duration will idle and run in a motor very differently than a cam with 218 or 220 or more degrees of duration.

Generally, as duration goes up, vacuum in the motor will go down. And, because of the early EFI systems using a speed density setup that reads the vacuum of the motor, you can only go so high on duration before the computer starts tripping ***** and thinking something is wrong, when there really isn't.

I don't know much on reprogramming speed density EFI vs EFI with Mass Airflow Sensors, but I do know that it is far easier to cam a vehicle made after 1995/1996ish that has a MAF sensor than it is to cam a vehicle that runs with speed density fuel injection because of the difference in how the computer monitors the engine while it's running. One system monitors the vacuum in the intake while another monitors the amount of air flow, if I remember correctly.

You can stick a bigger cam in there, and you can stick one in there big enough to make the computer think it doesn't have a motor hooked up to it, but when you cross that threshold where the computer starts to whig out, you'll need to reprogram it or go with a different setup, and this can get expensive in a hurry with EFI.

Not to say that EFI is bad, but people will generally be more inclined to build a motor with a carburetor than EFI as EFI can be a very expensive system to put together.

Also, when selecting a cam, make sure that you pick a cam that doesn't have more duration that you need. If you do, your motor won't run to it's full potential. Too much duration (or too "big" or "lumpy") and you have to wind it way up to make power, too little duration and you might not get the performance out of it you want. Therefore, it's best to pick the cam out that you want to use in the motor and build the intake, heads, exhaust, and bottom end around that camshaft. For instance, a cam with 220 or higher degrees of duration will want a higher flow set of heads and intake and exhaust as well as a larger carburetor because it has to wind up before it makes power. Ever notice how drag cars idle? They sound like marbles in a tin can. They also regularly run above 4000 RPM, with many going way higher than that. Their power also starts coming in above 2000 RPM in a lot of cases due to the higher duration (low vacuum means it won't run well at idle because of poor fuel atomization). Whereas with a street vehicle, most of those don't go over 6000 RPM and you'll want power off-idle to around say 5500. You'll also want to stay away (in most cases for street and daily driven vehicles) from high rise intakes, hugely ported heads, way over-sized valves, etc. that you would typically find on a vehicle designed for racing.

It's very important how you pick a cam. I'll say it's the most important thing that determines how well a motor can run.

If anyone sees anything in there that's wrong, feel free to correct me. I think I got it all right.

 

Just because it hasn't been mentioned, when "finding" a cam make sure you know the firing order. There are two different ones and your ECU is going to care about that a whole lot. With a carb you can just re-jigger the wires at the distributor.

Good luck,
- Jeff

 

For the trucks, the computer really doesn't know the difference, since it's a batch fire computer system it fires all the injectors at once. So like the carb setup, you can just re-arrange the wires. For instance if you installed a HO cam in a regular 302 truck engine.