Welcome to the Corvette Forums at the Corvette Action Center!

Engine Timing

minifridge1138

Well-known member
Joined
Feb 2, 2007
Messages
908
Location
USA
Corvette
1982 Black Fastback
Does anyone know what would be the correct engine timing on a 1982?
Instead of the crossfire, i have a holley 4-barrel carb, if that makes a difference. The camshaft is stock. I've heard values ranging from 6-12 degrees before top dead center. Anyone know for sure?

Thanks
 

minifridge1138

Well-known member
Joined
Feb 2, 2007
Messages
908
Location
USA
Corvette
1982 Black Fastback
Under the hood

ok, thanks for the tip. I've looked under the hood and the only plate i have seen was describing the emissions and how to arrange the fan belts on the pulleys. I'll look again when i get back to my garage. Just in case the plate was removed by the previous owner, does anyone have a suggestion? i'm at 12 degrees btdc at the moment. The exhaust gets louder and throatier as i get closer to 6 btdc.
 
Joined
Apr 29, 2001
Messages
2,141
Location
Rio Rancho, NM
Corvette
1981 HD Suspension; ZN1 Option
fridge,

If you have the TBI disconnected and have the Holley -does that mean that you're still using the old distributor?

If you're still using the stock distributor with the Holley, then perhaps the timing advance selected by the computer won't be worth any increase in performance. The computer will either attempt a guess based on available parameters, or it'll switch to an open circuit mode, but I doubt that it would be any good to extract some improvement. You may want to consider using a federal HEI distributor from a 1980 Vette or older.

Can anyone tell me if I am in the ballpark or totally out by the parking lot entrance? (I Can't wait for ML Baseball to begin :L ) ;shrug ;help

Also, the timing of 6 deg BTDC is perhaps a best-emission-reduction setting. For improved performance you'll have to dial a little more (like 12 deg BTDC for starters). Case in point; on my Dyno run last Saturday I got an improvement of 20 HP by increasing the initial timing on the L81 from 8 deg BTDC to 13 Deg BTDC.

The L81 has other issues that I have to correct before I report on the horsepower achieved at the rear wheels.
 

minifridge1138

Well-known member
Joined
Feb 2, 2007
Messages
908
Location
USA
Corvette
1982 Black Fastback
Distributor

Yeah, i replaced the stock distributor with an aftermarket HEI unit with vacuum advance. Right now i believe i'm running 12 btdc and it runs fine, but doesn't have much pep. And for a car with Flowmasters, it sounds very quiet. The last time i was playing with the timing, it had a throaty-flowmaster kind of sound the closer to 6 btdc. I may just play around this weekend and see what happens. If i advance it too far, it should be very easy to tell.

Thanks for the tips. The only way to learn is to ask.
 
Joined
Jan 1, 2002
Messages
7,246
Location
Washington, Michigan
Corvette
'67 Marina Blue Convertible
Initial timing (with the vacuum advance disconnected and plugged) is just another reference point; do you know exactly what your timing/advance map looks like? How much centrifugal advance is the distributor adding and at what rpm is it "all in"? What your "total timing" is (initial + centrifugal, with vacuum advance disconnected and plugged) and at what rpm it's achieved? How much the vacuum advance is adding to initial at idle (connected to full manifold vacuum, not to "ported" vacuum)?

You can check this out in a few minutes if you have a dial-back timing light. A good baseline to start from is 10*-12* initial, with 24* of centrifugal in the distributor, all in by 2800-3000 rpm (which will give 34*-36* total timing), and the vacuum advance should add about 15* to the initial at idle, for 25*-27* idle timing.

:beer
 

minifridge1138

Well-known member
Joined
Feb 2, 2007
Messages
908
Location
USA
Corvette
1982 Black Fastback
more timing

Wow, that is more about engine timing that I had ever thought of. I guess i still have a lot to work with. We had a cold front move in, so i haven't had time to play with my car, but it is starting to get warmer so I will give it a shot this weekend.

What exactly is a dial-back timing light?
 
Joined
Jan 1, 2002
Messages
7,246
Location
Washington, Michigan
Corvette
'67 Marina Blue Convertible
Wow, that is more about engine timing that I had ever thought of. I guess i still have a lot to work with. We had a cold front move in, so i haven't had time to play with my car, but it is starting to get warmer so I will give it a shot this weekend.

What exactly is a dial-back timing light?

A regular timing light only allows you to read advance within the limits of the markings on the timing tab, which isn't enough to determine anything other than advance at idle; a dial-back timing light has a dial control on it that allows you to read advance far beyond the limits of the timing tab, which is necessary when "mapping" or setting up an advance curve. Sears has them, so does Actron (www.actron.com), who makes them for Sears (same guts, different housings).

:beer
 
Joined
Oct 7, 2006
Messages
295
Location
South Dakota
Corvette
76 Coupe
Lots of good ideas here on timeing. However I must say I set mine differently. I adjusted my base timing up until I got detonation at moderate to heavy throttle around 2200 rpm while driving the car, then backed it off about 2 degrees. It ended up at 15 advance, but I am not sure if my damper hasn't slipped. i can tell you that it was a dog at 8 degrees and a monster where it is now. "For curiousity we put a dial back lite on it and ran it up to 4000 rpm. With vacuum hooked up I was at 51 degrees advanced. Your perfect timing may not be the same as anybody elses. it depends on many factors in your engine, like cam overlap, compression ratio, mixture, temperature, etc. All these figures you have been given are nice starting places. I would suggest you set the timing to avoid detonation on 88 octain fuel, instead of premium grade unless you have modified your engine enough that it won't safely run on anything other than premium.
 

minifridge1138

Well-known member
Joined
Feb 2, 2007
Messages
908
Location
USA
Corvette
1982 Black Fastback
Ok, well i distracted by some fiberglass issues, but i finally got around to timing.
I noticed that at 10*, that if i floored it, the car paused for a sec, rpm dropped, then the car accelerated.

It started at 10*. I advanced it to 6*. It drove alright, but when i floored it, the car stalled.
I put it back at 10*. It drove better, and it almost stalled when I floored it.
I put it at 12 *. It drove the same as 10.
I put it at 15*. Drove better than 12*.
At this point i was out of timing marks, but i would guess it is closer to 20*. It has a lot more pep, but the RPM's still drop just before it accelerates.

Does this sound "normal" ? I never had any pinging or spark knock, but i run 93 octane. (lower octane results in funny smelling exhaust)
 
Joined
Oct 7, 2006
Messages
295
Location
South Dakota
Corvette
76 Coupe
I am no expert, but it sounds like you got the timing pretty close now and you need to adjust your carb. You are describing what they call a hesitation or stumble on throttle tip in. It has to do with idle mixtures, jets, accelerator pump adjustment and maybe even secondaries adjustment on your carb. . I know nothing about a Holley. The bad smell you talk about is probably misadjusted idle mixture and is usually not due to the gas octane rating.. You need one of the Holley experts on here to help you further.
 
Joined
Jan 1, 2002
Messages
7,246
Location
Washington, Michigan
Corvette
'67 Marina Blue Convertible
It started at 10*. I advanced it to 6*. It drove alright, but when i floored it, the car stalled.
I put it back at 10*. It drove better, and it almost stalled when I floored it.
I put it at 12 *. It drove the same as 10.
I put it at 15*. Drove better than 12*.
At this point i was out of timing marks, but i would guess it is closer to 20*. It has a lot more pep, but the RPM's still drop just before it accelerates.

Were these readings taken with the vacuum advance disconnected and plugged?
 

minifridge1138

Well-known member
Joined
Feb 2, 2007
Messages
908
Location
USA
Corvette
1982 Black Fastback
Yes, the vacuum advance was disconnected and plugged.
I also found a few other issues:
1) I replaced my PCV valve and the hose going to the intake. But i used vacuum hose and apparently i have so much vacuum that it was sucking the hose closed. That was part of my problem.
2) I had lowered my idle mix. I incresed it, and now there is no more stalling or hesitation.

So now the timing tab says i'm about 20* btdc and I have more pep than i did before. I"m just afraid that i have the balancer may have slipped. 20* sounds like an awful lot, since stock was 6*.
 

Corvette Forums

Not a member of the Corvette Action Center?  Join now!  It's free!

Help support the Corvette Action Center!

Supporting Vendors

Dealers:

MacMulkin Chevrolet - The Second Largest Corvette Dealer in the Country!

Parts/Accessories:

Vetteskins

Advertise with the Corvette Action Center!

Double Your Chances!

Partners

Top Bottom