Advice needed -Retarding the timing to run pump gas

[69L88clone]

Hello All,
I have a 69 Vette which the PO tells me was built to L88 standards & runs on 110 octane fuel.
With gas being so expensive I must now, regretably, retard the timing so I can run pump gas (Sunoco 93) (this is what I have been told needs to be done to run pump gas)..
I'm not a mechanic. I will ask my mechanic to do this for me. I'm embarassed to ask but what exactly is involved in "retarding" the timing?
Thanks for any/all replies.
Jim

 

[goingballistic]

Retarding ( or advancing ) the timing determines where in the piston stroke the plug fires. Example before TDC, After TDC etc.

Retarding the timing will change where the spark happens on the stroke ie later rather than sooner. The thing you need to be careful of.......IF that engine is designed to run 110 octane ( which is very high ) the compression ratio must be way up there, ie higher than 11:1. With the compression ratio that high, you run lower octane and retard the timing, you could do something called "predet" which is predetonation of the plug in regards to the stroke ( happens way too soon) if this happens you can damage the engine if it continues.
I would talk to your mechanic and ask him to check the CR for you and then determine what you can do to get back to pump gas, you might find that the CR is OK and you can run pump gas anyway but talk to someone who can decide based on testing not speculation.

Good Luck, let us know how it turns out.

 

[69L88clone]

Thanks GB, excellent advice! I'll follow it.
Regards,
Jim

 

[goingballistic]

Thanks GB, excellent advice! I'll follow it.
Regards,
Jim

Just so you know, retarding the timing is as simple as loosening the distributor and rotating it to set the timing, not rocket science but if you don't know the internal workings or CR or anything else about the motor, I'd check with someone who does, might save you some $$$$ in the long run.

Cheers.

 

[srs244]

most common pre detonation problem is blowing a hole in the top of the piston (yeah, it's that serious). as already stated, check everything first with a qualified mechanic if you are unsure. i agree with the above statement, however that the requirement to run 110 octane fuel is pretty severe and would mean that someone really worked that engine over pretty good to get the compression ratio way up there. while i wouldn't recommend it (and i assume from your original question you drive the car a lot (like a daily driver type??), you can always run avgas or something similar to get the required octane, but that's really not a great solution to your problem as that is pretty expensive on it's own. it might be helpful to this discussion if you tell us what you know about the engine specs and any rebuilding that took place.

 

[JohnZ]

There's no way to determine the actual compression ratio (or the dynamic compression ratio, which is more important in terms of sensitivity to detonation) without pulling a head and taking accurate measurements to determine the static ratio and understanding the installed cam profile to develop the dynamic ratio. If it has to have 110 octane to run, it's gotta be pretty nasty - another street engine "mistake". Do you have the buildup specs from the previous owner or engine builder?

 

[AKRAY4PLAY]

does the motor have aluminum or iron heads?? that will greatly effect the required octane to avoid detonation. i agree more info is needed about the engine combo before getting to serious about direction of re-tuning. the good thing about an L-88 is the cam overlap. your dynamic compression is low at most speeds, except high rpm where the engine will like to run. it should be possible to run 93 octane with it, unless it is a seriously nasty lopey poppy engine. besides just retarding your timing, you will most likely have to recurve the distributor advance curve. also are you running vacuum advance??

 

[lonestarvette79]

Be careful if you do retard the timing... as the others have stated, you will get detonation. Remember to, another side effect to retarding it will be overheating at slower speeds. If this engine is near L88 specs then it should be somewhere in the neighborhood of 12 to 12.5/1 compression. I don't know If I would risk running less than 100 octane. But the choice is yours.