Hey guys,

My engine builder has explained to me that I can choose to go 11.3 or 11.7 to 1 compression with my 383 and 59cc Trick Flow heads. LE suggested 11.6 would be fine with my local 91 octane but sounds like I can't land that number exactly with a 6" rod. I'd like to go 11.7 if its not going to be a problem with 91 octane because for the $$$ I'm spending I don't want to leave any power on the table.

Your thoughts?

 

That depends on your cam, gears, and quench. Do you know what they are? 11.3 is the safer bet, but 11.7 could be possible depending on the combination you have.

I'm running 11.7 on my 355 LT1 with a 230/238 Lunati Cam through a 6 speed and 3.42 gears (too tall I need at least 3.73's for this combo, will be going 3.90 soon). But that cam is the absolute smallest I could go with that compression. Your extra displacement changes how the cam reacts. What did Lloyd recommend?

 

I think that would be a quench decision. If the 11.3 gives you less than optimal quench then it would be a no brainer to go 11.7. If I doesn't change then it would be a question of if you might ever purchase higher octane fuel to go to the track. 11.7 isn't much higher than 11.6, so I would venture to the higher side anyway, I can easily pull a degree of timing out.

 

OP

few "variables" to think about. The 91 octane (vs 93) is one critical factor. Lets say you go 11.7 (based on your calculations using a 6" rod). You may find while tuning, with 11.7, detonation so you back off some timing to prevent that. If you had "slightly" less compression you could get away with more timing. Obviously this becomes a balancing act of "do I make more power with higher compression but a degree or two less timing...or do I make more power with less compression and more timing".

I think between Lloyd and your tuner....those would be the ones to advise the correct path.

I do think 91 octane is going to be a primary factor in the correct decision.....especially with the "summer blend" of gas

 

I run 91 octane on 11.7:1, but it's a 355 with a decent cam for lower dynamic compression. He can run 11.7 depending on the rest of his set up. But 11.3 is the safer and easier bet. If I remember right that extra .4:1 compression is worth maybe what 5 hp? 10 tops? Personally I'd go with the low CR for drivability reasons.

91 at 11.7cr is fine depending on the cam. The cam will determine the gears. The CR and heads and CID will determine the cam, etc. It's all balancing act.

Still interested to see what Lloyd recommended. I am interested to see what difference in cam selection 28CID makes on an engine.

 

Here in Calif we can only get 91 octane

I have 11:5 in my 383....tuner (Ed Wright) backed out some timing to stay away from detonation. If I had 93 octane available, could have run slightly more timing

It's all a balancing act...

 

Thanks guys....seems like 11.7 should be okay assuming it is tuned properly

 

A bit off topic but I find it bizarre that California only get's 91 with racetracks everywhere and in NY you can get 93 (Sunoco used to have 94) with practically no racetracks.

 

sadly for the state where drag racing grew up....there are very few tracks and of those tracks even fewer "test & tune" aka "public" days.

91 octane....even sadder as the summer blend is so aerated it really isn't even 91 octane. You can buy 100 octane "race gas" at a few select stations. Unless your motor is set up for that it is of no value....and is over $8 gal anyway now.

76 used to carry it at some stations and on their site there was a chart showing how many gal of 100 to mix with "x" gal of 91 to increase octane to "93" or higher

low octane, tracks that on any given day they are actually open typically in hot summer months with a DA of 4000+' along with 700 cars and little if any track prep where maybe you get 3 runs all day after hours of tech line and hours waiting in staging lanes between runs....sure takes the fun out of it

 

[QUOTE=hrcslam;18184144] Personally I'd go with the low CR for drivability reasons.


?? Drive ability reasons, Please explain.

 

The lower compression allows for a smaller cam, this allows for taller gears, and lower RPM's while cruising and driving without lugging and bucking. With a 383 that should still equal tons of power, but drivability too on 3.42's in 6th at only 1600rpm at 70mph.

Right now I'm running 3.42's on a manual and in 6th I have to be doing over 75mph to stay away from a bucking engine. 5th at 75mph and below puts me at 2500+rpms. Not exactly comfortable when you are used to 1600rpms at the same speed. Not to mention the gas mileage hit.

3.90s are soon in my future. But that's only to stay away from under 2K rpms. If I had lower compression (not possible with my set up) I could've gone with a smaller cam and actually had a power band under 2Krpms.