Timing for Rebuilt L48

[1978L48jafo]

As you can see from my signature list my L48 engine has had a rebuild and is no longer the 8.5:1 smogger. I have kept the factory timing setting since the rebuild and I am wondering if this is a mistake based upon some posts I was reading through.



My compression is 9.5:1 due to flattop pistons, steel shim gaskets, and machining done to the heads. I am also running a mild street cam (Performer Energizer) Performer intake with the stock Quadra jet. The distributor was overhauled with the proform performance kit and I choose the middle of the road centrifugal advance springs. Exhaust is chambered under the car system from Corvette America.



I believe the factory timing for a 78 L48 is about 6 BTDC. Is this ok for my current setup? Please understand the car runs excellent I just wondered if it could be better....



John

 

[SG4206]

Every engine requires a different total ignition advance. Base timing,try 6,8,10,12 and maybe even 14 degrees on your combination and distributor.

Engines are like women ; they all have the same equipment ; but need to have special treatment.

 

[ernbuild]

timing re old dizzy

i have just replaced my old hei stock disturber with a new MSD one . the old one seemed to be working ok, but the engine did not seem just right.
after replacing i put it over a dyno and found it produced an extra 8 hp,or so at the top end,
and had a much smoother power curve.
i am sure with a little more twiking it will produce a bit more.
for myself it seemed worthwhile for the dollars especially since i want to do some more changes yet. regards ernie

 

[JohnZ]

The distributor was overhauled with the proform performance kit and I choose the middle of the road centrifugal advance springs. Exhaust is chambered under the car system from Corvette America.



I believe the factory timing for a 78 L48 is about 6 BTDC. Is this ok for my current setup? Please understand the car runs excellent I just wondered if it could be better....



John

Initial (base) timing doesn't mean much - what matters is the centrifugal advance curve, when it starts, how much it adds, and at what rpm max advance is reached. Until you have it set up on a distributor machine (or "map" it yourself with a dial-back timing light and experiment with weights and springs), you don't know what you have.

Most small-blocks like total timing (initial plus centrifugal, vacuum advance disconnected for the check) of 34-36 degrees, all in by 2500-3000 rpm. With vacuum advance connected to full manifold vacuum (NOT "ported" vacuum) and an advance can that's fully deployed at idle, you should see a difference and will have a baseline to work from.

 

[1978L48jafo]

I upgraded the old HEI unit with new chip, advance and such. I don't realy have access to a dyno, I suppose I was hoping for some thumb rules based upon the compression and cam change.
Thanks,
John

i have just replaced my old hei stock disturber with a new MSD one . the old one seemed to be working ok, but the engine did not seem just right.
after replacing i put it over a dyno and found it produced an extra 8 hp,or so at the top end,
and had a much smoother power curve.
i am sure with a little more twiking it will produce a bit more.
for myself it seemed worthwhile for the dollars especially since i want to do some more changes yet. regards ernie

 

[1978L48jafo]

Hi JohnZ,
I choose middle of the road springs from the kit during the rebuild, I don't have a dial back timing light but I would imagion one of my local speed shops will have a distributor machine.

I believe my advance is currently plugged into ported vacuum from the quadrajet. I have not tried to run it on manifold vacuum yet.
John

Initial (base) timing doesn't mean much - what matters is the centrifugal advance curve, when it starts, how much it adds, and at what rpm max advance is reached. Until you have it set up on a distributor machine (or "map" it yourself with a dial-back timing light and experiment with weights and springs), you don't know what you have.

Most small-blocks like total timing (initial plus centrifugal, vacuum advance disconnected for the check) of 34-36 degrees, all in by 2500-3000 rpm. With vacuum advance connected to full manifold vacuum (NOT "ported" vacuum) and an advance can that's fully deployed at idle, you should see a difference and will have a baseline to work from.

 

[curtis]

I don't have a high dollar timing light either, so this is what I did. unhook vacuum advance and plug line. set timing to 10 btdc. drove car and it ran better and more responsive to throttle. set to 12 btdc ...... still better feel running good. set to 14btdc even better yet...... set to 16 btdc oh crap pinging when you tromp it. Backed it up to 13 and left it there, reset idle speed and hooked up vacuum advance. Since then a buddy checked it and I am at 36 total. strong runner , no pinging on 89octane works for me!!!!

 

[1978L48jafo]

Thanks Curtis,
This seems like a very practical approach, I just happen to be off today so maybe I can give this a try.

Follow up question...I read somewhere that the HEI distributer needed to run off of ported vacuum. for the members still using their HEI unit are you running your advance from ported or manifild vacuum?
John

I don't have a high dollar timing light either, so this is what I did. unhook vacuum advance and plug line. set timing to 10 btdc. drove car and it ran better and more responsive to throttle. set to 12 btdc ...... still better feel running good. set to 14btdc even better yet...... set to 16 btdc oh crap pinging when you tromp it. Backed it up to 13 and left it there, reset idle speed and hooked up vacuum advance. Since then a buddy checked it and I am at 36 total. strong runner , no pinging on 89octane works for me!!!!

 

[curtis]

You should run your vacuum advance off full manifold vacuum. the reason is when you tip in the throttle it lessens your vacuum advance to avoid preignition (pinging). Your mechanical should be all in a 3000RPM. At highway cruising the manifold againe has vacuum and gives you more advance for better mileage. Mine is running 48 degrees at 75MPH cruising. again when you step onthe throttle it backs off vacuum advance to avoid preignition. At wide open throttle there should be 0 vacuum advance. also the vacuum advance provides a much smoother idle. the original purpose for port vacuum advance is unknown to me now (AHLZEIMERS MAYBE ) BUT i remember an article in Super Chevy magazine recommended manifold vacuum.

 

[1978L48jafo]

Thanks Curtis, I will move the vacuum advance this evening to a manifold source. I already set the timing to about 12 BTDC and it seems to be running fine. Don't notice a huge difference in power.

I expected big MPG gains from the installation of the six speed. first tank was arround 18 MPG but I had several hours of working on the AC with the car ideling in the garage. Timing and advance changes should help with this.

Having trouble getting the quadrajet to idle smooth I don't think it knows how close it is to being replaced with something less long in the tooth.....;-) I saw a tune up procedure someplace I will give that a try first.
John

You should run your vacuum advance off full manifold vacuum. the reason is when you tip in the throttle it lessens your vacuum advance to avoid preignition (pinging). Your mechanical should be all in a 3000RPM. At highway cruising the manifold againe has vacuum and gives you more advance for better mileage. Mine is running 48 degrees at 75MPH cruising. again when you step onthe throttle it backs off vacuum advance to avoid preignition. At wide open throttle there should be 0 vacuum advance. also the vacuum advance provides a much smoother idle. the original purpose for port vacuum advance is unknown to me now (AHLZEIMERS MAYBE ) BUT i remember an article in Super Chevy magazine recommended manifold vacuum.

 

[curtis]

1978L48, you can check for performance increases in a rather crude way with a stop watch. Use second gear and time how long it takes you to accelerate from say 1500 RPM to 5500. do it a couple times to get a good average. then make your tuning changes and check it again for a average. If the average is shorter time you are headed in the right directions. Just watch out for detonation. If it happens when you hit the gas, you better back off the timing.

 

[1978L48jafo]

I will give this a try, noticed detonation on the way home from work today. Was not an issue during the morning ride in. Moved the vacuum advance to a manifold port maybe the will be enough. Running rough now may have fouled a plug.....
John

1978L48, you can check for performance increases in a rather crude way with a stop watch. Use second gear and time how long it takes you to accelerate from say 1500 RPM to 5500. do it a couple times to get a good average. then make your tuning changes and check it again for a average. If the average is shorter time you are headed in the right directions. Just watch out for detonation. If it happens when you hit the gas, you better back off the timing.

 

[curtis]

causes of detonation other than timing can be low fuel octane rating, carbon deposits in combustion chamber , high engine temperature, high intake air temperature, lean mixture, stuck egr valve dumping exhaust gas into manifold. do your tuning for afternoon temperatures outdoors. this gives you best chance to eliminate detonation because of advanced timing for most all driving situations.

 

[Bob Chadwick]

I will give this a try, noticed detonation on the way home from work today. Was not an issue during the morning ride in. Moved the vacuum advance to a manifold port maybe the will be enough. Running rough now may have fouled a plug.....
John

John,

Do you really know what the timing is on your car at this point. As JohnZ pointed out you need approximately 34- 36 degrees of mechanical advance around 3000 rpm. If you are above this it will cause the detonation. Buy yourself a dial back timing light from Sears and see what you have. Otherwise I suspect whatever you do is just conjecture.

Bob

 

[1978L48jafo]

Bob is this the actual name dial back timing light or a nick name? Can you point me to a procedure to do this? I will search the site shortly. Unfortunately I know a little about many things and a master of none so timing has usually been a matter of placing back into factory specifications. I don't remember the spring color I used in the mechanical advance but I am guessing this timing light allows this to be checked.

I am also fighting a quadrajet that seems to leak vacuum around the base of the secondaries (sometimes):confused .

Will do on the timing light it may need to wait until the weekend
Thanks
John

John,

Do you really know what the timing is on your car at this point. As JohnZ pointed out you need approximately 34- 36 degrees of mechanical advance around 3000 rpm. If you are above this it will cause the detonation. Buy yourself a dial back timing light from Sears and see what you have. Otherwise I suspect whatever you do is just conjecture.

Bob

 

[1978L48jafo]

Curtis, I guessed that the ambient temperature was probably the issue. The day I changed the timing was very cool and the test drive had no pinging. I hated finding this out so far from home (40 mile commute). Bob has suggested I can't accomplish this correctly without the dial back timing light, can either of you elaborate on what it allows you to do in comparison to a plain timing light.



Sorry to be such a dah on engine timing.......



Thanks,



John

causes of detonation other than timing can be low fuel octane rating, carbon deposits in combustion chamber , high engine temperature, high intake air temperature, lean mixture, stuck egr valve dumping exhaust gas into manifold. do your tuning for afternoon temperatures outdoors. this gives you best chance to eliminate detonation because of advanced timing for most all driving situations.

 

[Bob Chadwick]

John,

A dial back timing light allows you to set the timing back so that you can see exactly where your timing is. I bought the Inductive Advance Craftsman Timing Light from Sears and have not had a chance to play with it yet. The way I understand things is that by dialing in the advance, it will compensate for the advance put in place by the distributor and show the correct advance on the timing marks on the front of the engine. The timing marks on the front of the engine don't go this high and without the dial back unit, you would have to measure and mark the balancer to see whether or not the timing is correct at higher rpms as the mechanical advance engages.

I have been running a similiar post on changing out my distributor with an MSD unit. Take a look at it, particularly the reference to a paper by Lars on timing. I hope by setting the initial timing at 8 degrees as indicated on the spec plate under the hood and limiting the mechanical advance to an additional 28 degrees to get 36 degrees of mechanical advance by about 3,200 rpm. The timing on when the advance is allowed is controlled by the springs that I put in the distributor.

Too much advance will cause the pre-ignition as will allowing the advance to come in too soon. Once you have the timing down, you can look at some of the other causes.

Bob

 

[MrGibbly]

I used the timing light that bobchad mentions in the post above to set my timing after I replaced a spun harmonic balancer. It made it really easy because I just set the gun to the initial timing I wanted (12 degrees in my case) and then fiddled with the timing until the mark on the balancer showed TDC in the indicator on the block. Then I installed my Moroso recurve kit in the factory HEI and checked full mechanical advance. At 3,000 rpm I just kept turning the dial on the back of the timing light gun until the mark on the balancer ligned up TDC. The dial on the light read 34 degrees. Piece of cake! I was pleased by how straightforward this light made things.

 

[1978L48jafo]

Bought the timing light last night (thanks Mr Gribbly, Bob & Curtis) and after playing with the centrifugal advance, I settled for 8 BTDC. Ran great this morning will see what the PM ride home is like. I am getting in all of my advance by 3600 RPM so I guess I need a set of weaker springs.





For vet owners with higher than stock compressions at what point are you running higher octane gas or additives? I am wondering if I better start feeding it the good stuff.....;-)

Thanks
John

 

[Bob Chadwick]

John,

Did you measure with the vac advance disconnected and what was your total advance?

Bob