70 LT1 new tuning results

[Dennis D.]

Set total timing to 38 degrees.(vacuum plugged) max at 3k approx. Checked initial. 14 degrees.

Hooked up vacuum. Timing 38 degrees@ 900rpm. 13" vac.
B28 can and black springs. Seem to be getting a lot of added mechanical at Idle.

[Duke W.]

Re: 70 LT1 new tuning results

Did you verify that all centrifugal is in at less than 3000? ... blip the throttle to at least 5000 to verify no more advance? Try finding the start point of the centrifugal. It could be 700 or less and might be tough to get the engine to idle stably that low or lower to find the start point.

Check max cruise timing (VAC connected) at an engine speed above where centrifugal is all in. It should be 38 +16 = 54.

High overlap cams like low thirties total idle advance, so 38 is above this range, but is it causing drivability to detonation problems?

Duke

[Dennis D.]

Re: 70 LT1 new tuning results

Went out for a hyway cruise, (60mph) , noticed slight pinging.
Going to recheck total again. Also try to get start point of mechanical. I think there is some at idle which is throwing off initial.
Dont know if this makes sense, but used the heavy springs and checked initial. Then switched to black springs and checked initial, and found 4 degree difference.
Btw all conditions the same with vacuum plugged.

[Michael L.]

Re: 70 LT1 new tuning results

Pinging can be from a variety of things, not just the timing. But you can try plugging the VA completely and checking out how it drives. If there's no loss of performance and it also doesn't ping you can keep it plugged.

[Dennis D.]

Re: 70 LT1 new tuning results

Rechecked mechanical. Stops advancing at 2850 rpm.(vac plugged)
Then set dial back timing light to 38 degrees, rev'd to 2850prm. Adjusted distributor until it read "0" on the balancer.
Rechecked timing. Ends up at 17 degrees.
Hard to get a reading as to when the advance stop advancing, because engine stalls. Best I can tell it is 600-700. Will try again.

[Duke W.]

Re: 70 LT1 new tuning results

If the centrifugal advance is all in at 2850 then you should set total WOT advance a few hundred revs above that, like 3200-3500. Once set blip the throttle hard to at least 5000 to verify there is no more advance.

Are you saying the initial timing is 1700? You can only measure it if you can get the engine to idle long enough to check it at below where the centrifugal starts, and this can be difficult if not impossible on high overlap cam engines, but so what?

When I set up a high overlap cam engine with a quick centrifugal advance curve that starts below the speed that the engine will idle stably, I set total WOT advance (VAC disconnected and plugged) as high in the 36-40 range as the engine will tolerate without detonation a few hundred revs above the point of maximum advance that I have previously measured. Then I connect the VAC and check total idle advance. Then I rev it a few hundred revs above the point of max centrifugal to check maximum cruise advance.

What about "initial timing"? I don't care... makes no difference. As long as the total WOT advance is in the 36-40 range and total idle advance and total cruise advance are in the range I expect, I'm done. It takes more time to set up/take down the equipment than actually do the tests and adjust the distributor.

Duke

[Dennis D.]

Re: 70 LT1 new tuning results

After I set the WOT to 38 degrees at 2850, I then checked the initial at idle and it was 17 degrees. (Not 1700).
If I do the math, 38-17=21. So mechanical advance max is 21 degrees? Is that correct?
Also, if I set WOT to 3200-3500. Won't that bring initial over 20 degrees?

[Larry M.]

Re: 70 LT1 new tuning results

Dennis:

It is a strong possibility that your 17 degrees at "idle" was both initial + some mechanical advance. Initial should be set below 700 RPM or so. You can put a rubber band around the weights to hold then tight and not advance, and then try it again. I bet your initial is really around 10-12 degrees.

Centrifugal advance is the difference from initial to full distributor advance, with the vacuum can disconnected and plugged. Once you get a true initial advance, I think your centrifugal advance will be a bit more than 21 degrees.

WOT advance is without the vacuum can in service and includes both initial and full centrifugal advance. It is the maximum mechanical advance (initial and centrifugal) that can be obtained from your distributor. It does not vary with RPM..........but is achieved when your RPM no longer changes the distributor advance number. It is all there is.

A lot of folks get into trouble setting WOT advance at say 3000 RPM not realizing that if they go higher in RPM the advance will continue to change. At WOT advance there is no more change.

Larry

[Dennis D.]

Re: 70 LT1 new tuning results

All the tuning was done with vacuum plugged.
The 2850 rpm was the the max advance without really knowing initial.

Once I established that mechanical was no longer advancing, I set my dialback timing light to 38 degrees. Then it was just a matter of reving to 2850rpm and adjusting the distributor until the timing light read zero.

Just to check what initial was, I adjusted the timing light and ended up at 17 degrees at idle.
I figured there was some mechanical in that.
Its difficult to get the idle low enough to see when mechanical starts.

So if I understand what you're saying, restricting the weights should give me a lower advance at 2850. Correct? Guess I wonder if that matters.

[Larry M.]

Re: 70 LT1 new tuning results

No. Restricting the weights temporarily with a rubber band will show you what your true "initial advance" is, since the weights will not be able to add any centrifugal advance.

This knowledge is not really necessary as Duke and others have said, but if you want to know what the real "initial advance" is, this will do it. Then you remove the rubber band.

On my 67 327/350 HP car, my initial advance is 8 BTDC, but at around 900 RPM I show 17 degrees BTDC.........with vacuum advance disconnected and plugged. I have to bring my engine RPM down to 500 RPM to read the initial advance, since above this RPM the centrifugal advance is kicking in. You likely have the same thing occurring. I normally set my curb idle at around 700-750 RPM for daily use.

Although my engine does not like 500 RPM, it will tolerate it for half a minute or so. Your LT-1 engine is not inclined to do this, and likely just quits before you can go that low.

Larry

PS: To directly answer your last question, if you temporarily restricted the weights with the rubber band as I mentioned, and vacuum can was disconnected, your advance at 2850 RPM would only be your true initial advance.......probably around 10-12 degrees BTDC.

I personally think that your WOT full advance at 2850 RPM is too low for your car with vacuum advance and today's gasolines. I think 3500 RPM is a better RPM for this. The reason is that you are likely running very close to detonation with the vacuum can in service.....unless you are running leaded racing fuel blend. But that is your decision to make.

[Dennis D.]

Re: 70 LT1 new tuning results

[QUOTE
I personally think that your WOT full advance at 2850 RPM is too low for your car with vacuum advance and today's gasolines. I think 3500 RPM is a better RPM for this. The reason is that you are likely running very close to detonation with the vacuum can in service.....unless you are running leaded racing fuel blend. But that is your decision to make.[/QUOTE]

I dont understand this. If I raise WOT to 3500 rpm, isnt that more likely to detonate? Wouldn't the slot in the distributor need to be elongated to accomplish this?
Also, do you put the rubber band around the perimeter of the weights?
BTW, plan on running ultra 93 with 112 race blend.

[Dennis D.]

Re: 70 LT1 new tuning results

[QUOTE


I dont understand this. If I raise WOT to 3500 rpm, isnt that more likely to detonate? Wouldn't the slot in the distributor need to be elongated to accomplish this?
Also, do you put the rubber band around the perimeter of the weights?
BTW, plan on running ultra 93 with 112 race blend.

Nevermind about the first part. Wasn't thinking right about it. I get what you mean.

[Larry M.]

Re: 70 LT1 new tuning results

[QUOTE
I personally think that your WOT full advance at 2850 RPM is too low for your car with vacuum advance and today's gasolines. I think 3500 RPM is a better RPM for this. The reason is that you are likely running very close to detonation with the vacuum can in service.....unless you are running leaded racing fuel blend. But that is your decision to make.

I dont understand this. If I raise WOT to 3500 rpm, isnt that more likely to detonate? Wouldn't the slot in the distributor need to be elongated to accomplish this?
Also, do you put the rubber band around the perimeter of the weights?
BTW, plan on running ultra 93 with 112 race blend.[/QUOTE]

https://www.corvetteforum.com/forums...tor-issue.html See post #83 for picture of rubber band around weights, courtesy of JohnZ. You might also want to read the entire thread.

Running the 93 octane + leaded 112 octane race gas mix should just about prevent any detonation. But there may also be times when you just have to fill up with 93 octane gas only. It happens, and then you may have some issues with detonation.

If you raise WOT total timing "all in" to 3500 RPM, then at 2850 RPM you have less total advance......since it is still increasing up to the 3500 RPM maximum. So less advance means less chance for detonation to occur. Say at 3500 RPM WOT you have 38 degrees BTDC. Then at 2850 you should have less, maybe 30-34 degrees BTDC.

The B28 vacuum can wants to hold its vacuum advance of 16 degrees until your engine vacuum drops to below around 8-9 IN HG. During part throttle acceleration, you might have all 38 degrees of mechanical advance (if you are above 3500 RPM) and still have most or all of your 16 degrees vacuum advance still in. That is a lot if you are accelerating, and not just cruising.

Larry

[Michael L.]

Re: 70 LT1 new tuning results

What about "initial timing"? I don't care... makes no difference. As long as the total WOT advance is in the 36-40 range and total idle advance and total cruise advance are in the range I expect, I'm done. It takes more time to set up/take down the equipment than actually do the tests and adjust the distributor.

Duke

Agree with Duke here. I always set total timing with vac plugged and typically rev it up to something like 4000-4500 briefly and set the timing there. After that I don't really care what the idle timing is.

But the issue here, if I am correct, is after setting the total timing the OP found the car pinging on highway driving. I agree with a prior poster that setting it at 2850 rpm might be leaving out some mechanical advance, which could contribute to the pinging. Then when you add in the vacuum advance you could be seeing advance into the high 40's low 50's which, when coupled to some other factors, could easily result in pinging. What I would suggest is you dial back the total timing like Duke suggested to maybe 34 and make sure you rev it high enough to get every last bit of advance out of it, and then try driving it. If the drive characteristics are good and there is no more pinging you are good to go. Just because maybe the Chevy service manual says total timing at WOT should be 38 doesn't necessarily mean that works for every engine, especially when they get some miles on them.

Mike

[Dennis D.]

Re: 70 LT1 new tuning results

Will check out the thread. Thanks.
In order to get WOT in the 3500 rpm range, would changing to a heavier spring be the solution?