Detune advice
Thread Starter
Racer
Joined: Jan 2020
Posts: 388
Likes: 117
From: Tacoma, Wa
Detune advice
Gas prices amirite. I drive my vette every day theres no rain. I want to run 87 instead of 91 for the near future. I put the stock distributor springs back in, backed the time back to 8* (6 is stock). Vacuum advance is now on the ported side and not manifold.
where I need advice is getting a good idle. Its been along time since she’s ran on 87, and the idle seems to wander more with the lower advance in the tune. With it set up to lars specs, itd wander 30-20 rpm. With the stock set up it wanders 40-60 rpm and just runs rougher overall. Ive tried everything I know to get it to settle down but it seems like this is just how she wants to run. Is this normal with a stock tune?
‘79 with stock bottom end and heads, upgraded cam. True duals 2.5 in. Edlebrock 1406.
where I need advice is getting a good idle. Its been along time since she’s ran on 87, and the idle seems to wander more with the lower advance in the tune. With it set up to lars specs, itd wander 30-20 rpm. With the stock set up it wanders 40-60 rpm and just runs rougher overall. Ive tried everything I know to get it to settle down but it seems like this is just how she wants to run. Is this normal with a stock tune?
‘79 with stock bottom end and heads, upgraded cam. True duals 2.5 in. Edlebrock 1406.
Team Owner
Joined: Jan 2007
Posts: 53,648
Likes: 878
From: Southern Cal Ca
St. Jude Donor '22
Uh, I think you are going backwards in ways that are not necessary.
Lars dist curve won't affect gas mileage or
gas octane needs.
A simple base timing retarded so there is no ping or knocking is the answer.
Full time vacuum will help mileage.
Idle circuit can need a little adjusting.
Funny thing about octane and timing, in the end lower octane could lower gas mileage.
All we can do to get better mileage is gearing, more air in tires, accelerate smoothly, and drive at lower speeds.
Lars dist curve won't affect gas mileage or
gas octane needs.
A simple base timing retarded so there is no ping or knocking is the answer.
Full time vacuum will help mileage.
Idle circuit can need a little adjusting.
Funny thing about octane and timing, in the end lower octane could lower gas mileage.
All we can do to get better mileage is gearing, more air in tires, accelerate smoothly, and drive at lower speeds.
Le Mans Master
Joined: Mar 2008
Posts: 5,948
Likes: 72
From: Oxford MA-----You just lost the game!!!!
Why did you switch the vacuum advance from where it's supposed to be to ported vacuum? Now you have full vacuum advance at cruise, but none at idle. If you hook the vacuum advance back up to manifold vacuum, it'll probably idle better. If stock timing is 6 degrees, it's probably an L-48, right? If it's all stock except for a bigger cam, set the base timing at 6 degrees (with the vacuum advance temporarily disconnected) and it'll probably run on kerosene.
Scotty
Scotty
Old Pro Solo Guy
Joined: Dec 2017
Posts: 8,094
Likes: 4,449
From: Marlton NJ
When I had my stock 11:1 (blue-printed CR) LT-1 it really liked 97 Octane with a good optimized Lars-like tune. 36 total, 12 initial, 10 advance. More Octane than that made no difference, but it would knock with less.
Retarding the timing 8 degrees overall let it run on 92 Octane pump gas.
It was just a quick initial timing change to 4 degrees.
It made a huge difference with the knocking, cost some 20+ HP at the top end, and barely affected the mileage.
Should work similarly on an engine with less CR to start with.
I switched it back n forth for 27 years, depending on what octane gas I wanted to run.
Retarding the timing 8 degrees overall let it run on 92 Octane pump gas.
It was just a quick initial timing change to 4 degrees.
It made a huge difference with the knocking, cost some 20+ HP at the top end, and barely affected the mileage.
Should work similarly on an engine with less CR to start with.
I switched it back n forth for 27 years, depending on what octane gas I wanted to run.
Race Director
Joined: Mar 2017
Posts: 11,953
Likes: 4,510
From: Virginia
Now that I've cleaned the carbon out of the cylinders, I can run 87 (non-ethanol) in my L48 and still have a proper Lars-style tune, with manifold vacuum advance. Mileage is what it is. If I move the vacuum advance to ported vacuum, she will barely idle.
Le Mans Master
Joined: May 2007
Posts: 5,028
Likes: 1,120
From: North tonawanda NY
Is this motor stock other than the distributor? what engine is it? Theres no 79 vette that actually needed anything more than 87 that I know of? even the L82 was less then 9 to 1 compression and actual testing found those numbers were optimistic with true numbers being like half a point lower than advertised. (which would make the l82 about 8.4:1)
87 octane fuel has more stored energy than 91. "Premium" fuel is not really "premium" its just marketing to get more people to feel they are missing out by not using it, even when theres no benefit. If you have a higher compression engine thats a different story.
87 octane fuel has more stored energy than 91. "Premium" fuel is not really "premium" its just marketing to get more people to feel they are missing out by not using it, even when theres no benefit. If you have a higher compression engine thats a different story.
Dr. Detroit
Joined: Mar 2012
Posts: 10,097
Likes: 4,027
From: New Braunfels Texas
Do you have this 1406 on a stock Q-Jet intake with adapter or an aftermarket aluminum intake......?
The only difference in ported vs. full vacuum is you have vacuum at idle with the throttle blades closed.....it will effect zero from a pinging standpoint.
Even with 87 fuel it should never ping even with the performance timing setting....simply not enough compression.
Jebby
The only difference in ported vs. full vacuum is you have vacuum at idle with the throttle blades closed.....it will effect zero from a pinging standpoint.
Even with 87 fuel it should never ping even with the performance timing setting....simply not enough compression.
Jebby
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Thread Starter
Racer
Joined: Jan 2020
Posts: 388
Likes: 117
From: Tacoma, Wa
Do you have this 1406 on a stock Q-Jet intake with adapter or an aftermarket aluminum intake......?
The only difference in ported vs. full vacuum is you have vacuum at idle with the throttle blades closed.....it will effect zero from a pinging standpoint.
Even with 87 fuel it should never ping even with the performance timing setting....simply not enough compression.
Jebby
The only difference in ported vs. full vacuum is you have vacuum at idle with the throttle blades closed.....it will effect zero from a pinging standpoint.
Even with 87 fuel it should never ping even with the performance timing setting....simply not enough compression.
Jebby
Old Pro Solo Guy
Joined: Dec 2017
Posts: 8,094
Likes: 4,449
From: Marlton NJ
Agreed. If you have zero reason to believe the pistons or compression have changed since new, there is no way you need more than 87 octane in a very low compression 79 engine.
It will still benefit from the performance timing "tune"
It will still benefit from the performance timing "tune"