1981 Corvette ECM vs engine mods

[Don61]

Can anyone explain to me exactly what the computer (ECM) on my 81 vette does? How will it limit or effect engine modifications such as heads, cam, intake & carb? I was planning on changing heads to some 64cc aluminum ones, changing to a slightly larger hydraulic cam, changing headers & changing to a performer type intake & carb. But I have no experience working with computer equipped cars & don't know how these mods will affect the computer or vis-versa.

 

[Peer81]

The only thing you need to look out for is the cam selection. A to bigga cam will lower the vacuum signal to the ECM at idle and higher. At that time the ECM is thinking your accelerating and signals the carb to give a richer mixture. So the engine will be running richer all the time. If I remember correctly Gerry has a XE268 cam in this engine so that size will be save. And as with any Q-jet change heads, cam, compression etc etc will change the setup for the carb and internal passage dimensions.

The only thing the ECM does is checking the CO mixture and adjusting the carb mixture to compensate for a rich or lean condition. As the system is a 10Hz system (max changing 10x a second, this equals the system speed of a crossfire or TPI at 600 rpm idle. At 3000 rpm that is 50 rounds per second. At that rpm the system only changes the mixture every 5 revolutions....


Greetings Peter

 

[TimAT]

Peter is right on with his description. Talk to your cam company and tell them this is going into an ECM controlled car. Other option is to pick the cam, get it installed, take vacuum readings, then talk to someone like Hypertech that does custom PROM programming. They can burn the PROM to compensate for non stock vacuum, mixture, and timing requirements.

 

[Don61]

Thanks for the answers guys! It eases my mind to know that it's not as big a deal as I thought. When I get ready to do the mods I'll be sure to check with knowledgable people about the combination before I install it.

 

[GRAND SPORT #12]

This may not be correct but i am sure someone has input on this thought. If you have a 81 and want more power and a lot of states do not require a check engine light function for emissions inspection on such an older car could you not bypass the engine ecu with a carb and distributer change and then do what ever performance mod you wanted to do.

 

[Wajulia]

This may not be correct but i am sure someone has input on this thought. If you have a 81 and want more power and a lot of states do not require a check engine light function for emissions inspection on such an older car could you not bypass the engine ecu with a carb and distributer change and then do what ever performance mod you wanted to do.

Yes, the only thing you have to do to get rid of the ECM in an 81 is to go to a non computer controlled carb and a vacuum advanced distributor The only thing left is the torque converter will no longer lock up at speed. Slight hit on gas mileage. Many have done it.

 

[Don61]

Yes, the only thing you have to do to get rid of the ECM in an 81 is to go to a non computer controlled carb and a vacuum advanced distributor The only thing left is the torque converter will no longer lock up at speed. Slight hit on gas mileage. Many have done it.

Would eliminating the converter lockup hurt the converter or trans? How bad would the gas mileage be affected?

 

[Wajulia]

Would eliminating the converter lockup hurt the converter or trans? How bad would the gas mileage be affected?

There is no adverse effect on the transmission, I think 1981 was the first year for the lockup torque converter, all earlier years didn't lock up normally

You will only see mileage difference if you do a lot of highway driving ( only time it really locks up) and it should only be a couple of mpg

 

[Hib Halverson]

Before you begin your mod problem, if you think you're going to go the aftermarket calibration route and are expecting to find someone to do a custom-programmed, "chip", I'd make sure you can find someone to do it. My guess is aftermarket cal guys who can do PROMs for 81s might be hard to find.

As for converting the engine to run without a computer, you, first, want to make sure your State does not do emissions tests on 81 models which involve a test of the engine controls. As for the TTC lock-up, you can always as a switch and control that manually as sort of a "poor man's overdrive".

Lastly as for mods which are computer friendly...the advice given earlier about engine vacuum is well taken. Unless you can find an aftermarket calibration, you will not be able to use a camshaft which changes the manifold vacuum the computer expects to see at a given throttle opening. If you do alter that relationship without a new cal, the car will run poorly and or get poor gas mileage in closed loop.

Open loop is another story. If you go to an aftermarket cam, the carburetor calibration at high part throttle and wide open throttle will probably have to be adjusted. That will involve oldskool Quadrajet changes in jets and rods.

 

[6880 Mike]

...I think 1981 was the first year for the lockup torque converter...

The 80 models had lock up torque converters.

 

[Peer81]

I think to much people have not a really clear idea what the ECM is doing and in reality how little control it has or how little input it has to control the carb. Most of the input info are a general things like engine temp (don't think you want to cool it down below 160F or above 240F. Barometric pressure is the same for every engine etc etc. The ECM doesn't know if it's a 305 or 454ci and you can take advantage of the things it doesn't know or see. Besides the coolant temp I think the biggest problem with the stock ECM is the spark control, there is no way you can change the stock spark table by hacking into the prom. The only thing you can do is change to an aftermarket PROM with a different spark table diagram.

Greetings Peter

 

[Yoda]

I think to much people have not a really clear idea what the ECM is doing and in reality how little control it has or how little input it has to control the carb. Most of the input info are a general things like engine temp (don't think you want to cool it down below 160F or above 240F. Barometric pressure is the same for every engine etc etc. The ECM doesn't know if it's a 305 or 454ci and you can take advantage of the things it doesn't know or see. Besides the coolant temp I think the biggest problem with the stock ECM is the spark control, there is no way you can change the stock spark table by hacking into the prom. The only thing you can do is change to an aftermarket PROM with a different spark table diagram.

Greetings Peter

Peter

Years ago I tried a "Super Duper Chip" I bought off Ebay... I installed it and expected smoking tires, power through out the entire RPM and better gas milage... I didn't get any of the forementioned, just out whatever the $$ was for the Chip.. I ended up sending the chip FREE to a 1981 owner in Texas, with the promise of a fair critique... that was over 12 years ago and Nothing from him so I am guessing he didn't use it, sold it or didn't bother to let me know his results ...

I found getting the old QuadraJet rebuilt by an old Quarderjet Gearhead here in Amarillo that understands the carb and it's intricacies to provide more seat of the pants Umph and better gas mileage too...

Bud

 

[Don61]

Thanks for all of the info guys, gives me a lot to think about. You all are very knowledgeable about corvettes & very generous to help one not so knowledgeable.


So it sounds to me like it would be best to just bypass the ecm, install the cam, heads, carb & distributor that I want & then install a manual lockup switch for the converter. Does this sound reasonable? The only thing I would have a problem with is how to do the manual lockup switch.

Btw - there are no emission tests in Ohio for my 1981 so that will not be a problem.

I really don't want anything too radical, just a hundred or so more hp. I want to keep everything so that I can return it to stock whenever I want. I've actually thought about just pulling the engine & replacing it with a crate motor. Money $$$ will be the deciding factor!

 

[Wajulia]

I really don't want anything too radical, just a hundred or so more hp. I want to keep everything so that I can return it to stock whenever I want. I've actually thought about just pulling the engine & replacing it with a crate motor. Money $$$ will be the deciding factor!

My thoughts on HP for a 1981 -- ranked by cost (HP numbers are just guesses -- would love to hear other peoples opinions)

The 1981 has about 190 HP stock, and the 1/4 mile times of low 15 seconds bears that number out. Improvements go like this
1) true duals and high flow or no catalytic converter -- 210-220 HP
2) New high flowing heads -- 230-250 HP -- ECM stilll should work with this modification, no need to change the carb or dist
3) Bigger cam -- 250-280 hp ECM will begin to get confused -- ECM, Carb and dist may have to go at this time to increase any more
4) With the ecm gone it is a jsut any other small block at this point HP = Money -- how fast do you want to go.
5) A new crate motor -- HP=Money again.

Steps 1 and 2 are really the only thing you can do with a stock 81. Some people smarter than me have successfully put a bigger cam in but you need to modify the inputs to the ECM

 

[Don61]

My thoughts on HP for a 1981 -- ranked by cost (HP numbers are just guesses -- would love to hear other peoples opinions)

The 1981 has about 190 HP stock, and the 1/4 mile times of low 15 seconds bears that number out. Improvements go like this
1) true duals and high flow or no catalytic converter -- 210-220 HP
2) New high flowing heads -- 230-250 HP -- ECM stilll should work with this modification, no need to change the carb or dist
3) Bigger cam -- 250-280 hp ECM will begin to get confused -- ECM, Carb and dist may have to go at this time to increase any more
4) With the ecm gone it is a jsut any other small block at this point HP = Money -- how fast do you want to go.
5) A new crate motor -- HP=Money again.

Steps 1 and 2 are really the only thing you can do with a stock 81. Some people smarter than me have successfully put a bigger cam in but you need to modify the inputs to the ECM

I have built numerous smallblocks in my lifetime, just never one with an ecm/computer. I think your numbers are pretty close but I think (hope) that going from the crappy stock 76cc heads to a set of 64cc aluminum ones would give a little more hp jump than your prediction.

Thanks for adding your thoughts!

 

[xfirez51]

Can anyone explain to me exactly what the computer (ECM) on my 81 vette does? How will it limit or effect engine modifications such as heads, cam, intake & carb? I was planning on changing heads to some 64cc aluminum ones, changing to a slightly larger hydraulic cam, changing headers & changing to a performer type intake & carb. But I have no experience working with computer equipped cars & don't know how these mods will affect the computer or vis-versa.

Don,

in order to optimize the calibration for your mods, you or someone will need to modify fuel and timing parameters along with some other secondary controls. I'm not certain what the Service Number is the '81 ECM, but I doubt that it has been "hacked" to the point of allowing the necessary changes to the calibration. I faced a similar situation when I modded my 84 Xfire along the same lines you are considering. Ultimately I swapped the stock ECM w one from DYNAMIC EFI
DynamicEFI. This would allow you to consider a TBI or TPI induction system.
Just another option to investigate.

 

[Don61]

Don,

in order to optimize the calibration for your mods, you or someone will need to modify fuel and timing parameters along with some other secondary controls. I'm not certain what the Service Number is the '81 ECM, but I doubt that it has been "hacked" to the point of allowing the necessary changes to the calibration. I faced a similar situation when I modded my 84 Xfire along the same lines you are considering. Ultimately I swapped the stock ECM w one from DYNAMIC EFI
DynamicEFI. This would allow you to consider a TBI or TPI induction system.
Just another option to investigate.

Thanks! Some interesting ideas.

 

[WHEELS 81]

Upgrades

i have built numerous smallblocks in my lifetime, just never one with an ecm/computer. I think your numbers are pretty close but i think (hope) that going from the crappy stock 76cc heads to a set of 64cc aluminum ones would give a little more hp jump than your prediction.

Thanks for adding your thoughts!

everyone ideas on the cam upgrades are good. But before you do that look into making the vehicle a fulie. Edelbrock and several other companies have several kits on this which include the fuel pump , lines, pressure regulator,injectors, manifold, chip and thermo-stat. Its a 350 pretty universal engine, you can do a lot with it.

 

[Don61]

everyone ideas on the cam upgrades are good. But before you do that look into making the vehicle a fulie. Edelbrock and several other companies have several kits on this which include the fuel pump , lines, pressure regulator,injectors, manifold, chip and thermo-stat. Its a 350 pretty universal engine, you can do a lot with it.

Thanks for your thoughts! It will probably be next year before I actually start any of the improvements. There are a lot of ways to get where I want to go. I appreciate everyone's input on various things to do & look for. I think that money $$$ will be the biggest deciding factor for what actually gets done. The car needs a tune-up but runs fine other than that, it just doesn't have enough hp for me.