Low compression ignition timing considerations...

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blackhand

blackhand

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Just curious, would a low compression smog motor need more mechanical timing to maximize power? I know that 38* is the best baseline for most big block mopars, but what about on late model 400's or 440's that have about 8:1 compression?

Seems to me that the flame front would travel slower with the lower compression (or would it travel faster??) and 40* or more degrees may be better. Also, would low octane fuel be better for these engines since it burns faster??

Gonna be a while til I can afford to stroke my 400 out to 470 so for now I'm just trying to turn my lemon into lemonade. Thanks!
 
Lower compression can handle more timing,40-45 degrees depending on cylinder condition,and lower fuel grades would help,to a point.As long as the motor is tuned and fueled correctly,it should run great on regular fuel.Each car is different,and tuning is a big key to making it run 100%.Running 93 octane fuel will cause a very slow burn,as a lower grade fuel causes a faster burn.The secret is what is right for your car,only you can tell by trying each level of fuel and tuning it,mrmopartech
 
i think you're suppose to have peak cylinder pressure a few degrees after tdc. so like said with a low compression motor you should add timing until the thing doesn't go any faster. you'll also find out that lc motors with nos you don't have to retard the timing either. it's that peak pressure thing.
 
I've had two engines liked as much as 50 degrees!! One was a 440 that ran 12's and the other is a 318 that runs a 13.50 and yes, they are both low compression....really low compression lol.
 
When one is over cammed and under compressioned is when it can require a LOT of timing. You can still make them run pretty good like that though. Especially up in the higher RPM ranges.
 
Yup, the teen didn't turn on until 3000 rpm but it also was saddled with a stock converter. It did however have a 4.88 in the rear. Talk about a totally mismatched bunch of parts but what we did was just throw it together with parts that were laying around. It surprised many including us.
 
With a good converter, it would have surprised you.
 
blackhand said:
Just curious, would a low compression smog motor need more mechanical timing to maximize power? I know that 38* is the best baseline for most big block mopars, but what about on late model 400's or 440's that have about 8:1 compression?

Seems to me that the flame front would travel slower with the lower compression (or would it travel faster??) and 40* or more degrees may be better. Also, would low octane fuel be better for these engines since it burns faster??

Gonna be a while til I can afford to stroke my 400 out to 470 so for now I'm just trying to turn my lemon into lemonade. Thanks!
Click to expand...

You've already got a handle on it.
And more octane than required would burn slower/resist ignition with too low compression.

On any motor u want to raise the total timing up till it doesnt mph any faster at the end of the track/1/4...so to get the highest mph.
 
StrokerScamp said:
With a good converter, it would have surprised you.
Click to expand...
A 13.50 @ 102 was a surprise based on the mismatch of parts.....but with the right mix of parts, the car may have dipped into the 12's? Who knows. A 102 points that the car has a low 13 in it. The stock converter is holding it back by at least 3 tenths. Anyways, it's just sitting rotting it the corner of the shop now. The guy that owns the car isn't interested in doing anything with it at the moment but I wouldn't mind having my 4.88's back and a few other parts that were mine....
 
Well that's too bad. Sounds like it was a fun car. I call them old school builds. When somebody basically throws a cam and bolt ons on a stock motor. Those can be some of the most reliable because you're not lookin for every single 1/10 out of them. Usually very consistant cause they ain't as "high strung" as somethin with a lot of compression.
 
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