spark advance

[cngodfather]

Hi guys!

I have a 91 5.7 with a howards cam
110235-12

it is about 9.5 - 1 compression.

I am doing some experimental stuff with my fuel and ignition. I am running a 600 cfm carburetor at the moment. Due to my current situation, I will be running with no ability to have any variable spark advance. Therefore, I am limited to a single set vacuum advance. What do you all think it should be set at to run decent for the meantime? Once again, this is only temporary.

Thanks

 

[cngodfather]

I mistakingly posted vacuum advance. there is no vacuum advance, nor electronic. Just fixed.

 

[cngodfather]

anybody?

 

[Erik the Awful]

If it's absolutely locked, I'd probably start with 30* and tune from there.

 

[JWOK]

You are losing a lot of power and driveability with no advance. You will most likely have to set initial timing low enough that it starts and doesn’t detonate at low rpm acceleration which loses a lot of power at cruise and higher rpm. If you are forced to run this way, you will have to set it at the highest advance that will still allow you to start the engine and avoid detonation. I have seen lazy engines run 20 degrees initial just fine, but you will just have to experiment to see what works. Detonation is going to be your enemy by running a high base timing.

 

[Pinger]

I think that if the truck is expected to cruise at any kind of speed then as per post #5 as high as you can and still start but be oh so gentle on the throttle accelerating.
Or, set it low if you need acceleration and have hills to climb and keep the rpm low and watch coolant temps. Set low will give the exhaust valves a hard time. For that, you might consider going rich on the mixture - to avoid detonation if you use sharper timing also.

Keep your ears on hyper alert for detonation whatever you set it at and jump out of the throttle pronto if you hear it.

 

[cngodfather]

ok thanks. I know it was a wierd question. I am doing some experimental things to my fuel injection. The timing will be computer controlled in the future, but until that is set up it has to be locked in a fixed position. It has been a slow process getting the parts that I want. The truck rarely is driven, I just needed to get into the ball park of where it might be happiest at the moment. The only time I would need to drive it is if I needed to pick something up at home depot or something like that.

 

[Pinger]

A large amount of spark advance is taken up just establishing a viable flame.
Info I was looking at recently had the burn well underway at 10deg BTDC, 50% of the charge burned at 10deg ATDC and the combustion complete by 20deg ATDC. But there's no way timing was set at 10deg BTDC for that!
Probably around 25-30 CAD is required between spark and established flame. And that will vary with temp, AFR, cylinder pressure and rpm. Thereafter, in actual time, burn rate should be consistent (maybe faster with rising rpm if the combustion chamber promotes turbulence) but juggling between it keeping up with rpm and not so early as to cause detonation is the game.

 

[cngodfather]

Posting an update for someone who may be in a similar situation. My engine may be different from others. I have set the timing to fixed 20 degrees advanced. engine runs great through most of the powerband. When the engine comes to temp it struggles at idle. I am purchasing a bored out throttle body to get it back to efi. My new efi harness is in the engine bay with the wires extra long. It would be nice to install the computer to at least control the spark advance, but I am waiting on getting the throttle body installed. Once it is in there, I can route the wires better and some sensors must share the same ground. I think that in the short term, I will retard the timing a bit so that it idles more favorably.