im fixing to build a 396 and i want to run a 150 shot on it and was just wondering how many of you do that with high compression and how it affects the motor? i was gonna build my 396 with about 9.5:1 cc but thats gonna be harder than i thought so i may just go with a good flat top. anyways opinions would be nice here. thanks guys

 

nitrous isn't like turbo or S/C
you dont need to lower compression for it
it doesn't increase cyl pressure like the other 2 forms of FI

 

are there truly an advantage to going with higher compression? i doubt i'll ever run a supercharger , just nitrous so would you suggest say 11:1 over 9:1? just wondering

 

Your's is really a NA motor and that will need higher compression to develop more power and be more effecient. You'll have to consider a bigger cam (which goes without saying) to bleed out cylinder pressure at low rpm, i.e. to reduce the dynamic compression to get you further from detonation.

Good luuck

 

Higher compression works well with nitrous. To clarify what Trey said, nitrous certainly does increase cylinder pressures but most of the increased pressure (and therefore power) comes after the combustion event is initiated. Forced induction elevates the compression on the compression stroke more than nitrous. The nitrous also does a wonderful job at cooling the intake charge which reduces the possibility of preignition, whereas forced induction elevates intake temps by compressing the air.

11:1 should be plenty safe. Many people run ~12:1 with larger cams and good gas on their 396's.

The higher the static compression ratio the more favorable dynamic compression ratio you will have at cruise, this makes the engine more effecient. The higher compression also yields a bit more peak power.

 

You can go 11:1 and still use pump gas without a problem. If you go with a big cam like stated it'll bleed of some cylinder pressure cause of the extra duration more so its the extra overlap of the cam. High compression=good for nitrous, low=less efficient. Nitrous give off oxygen once its heated to a high temperature dont remember specifically, but more compression=more heat so to a certain extent a higher compressing will make you more efficient on the bottle.

 

you should be safe w/ a 150 shot, shouldnt have anything to worry about!!! good luck!!

 

Another way to look at it is to consider the limits of the engine, and the cylinder pressure it can take before it lifts the head off the block. Start with lower CR (and I'm thinking of 10.5-11.0 as "lower") and add more juice, and you can reach the same cylinder pressure limit and roughly the same HP. I run a 300-shot on a 10.8:1 setup.

With "big" nitrous on a 396, I would be more concerned with rod length, wrist pin placement and ring pack location. The longer stroke of the 396 starts to push the pin too high and makes rod/piston/ring selection very critical.

But there is no reason to drop the CR down into the 9's.

 

i bought a cc306, should i consider selling it and going to a bigger cam? im going to have a VERY nice P&P job on my heads w/ 2.0 1.57 valves. if so what cam specs would you suggest? or should i just call someone and let them figure it out? thanks a lot guys for all the responses.
Rob

 

I agree with what Fred says.

I assume you will be running a forged rotating assembly since you building a 396 and planning on using N2O. Just pick a rod piston set up that gives you plenty of material in the piston dome and you can run 11:1 with shots approaching 300 HP. In my 383 I run 11:1 on 93 octaine with no problems. When I begin spraying this year I plan on using 100 octaine VP unleaded in the tank and back down the timing according to the size shot.

 

the 306 has a VERY long exhuast lope
you'll be fine

people run 200 shots on stock LT1s with 10.5:1 compresson

 

Have the person who is porting your heads call and figure out what cam will work best with your combo. There are a lot of factors that come in to play with the cam you pick.