I am in the process of milling my heads, and was wondering how much compression i could go with before running into problems with detonation and such on regular street gas(93 octane). I will be getting a dyno-tune, so what is the most realisitic compression i can run, and still realistically stay on pump gas? Any help would be appreciated!!

P.S.-I think i'm gonna shoot for around 11.5 compression. I am hoping this will help a more aggressive cam be more useable on power under the curve. Is this a valid thought? Am i going in the right direction?
Tim

 

I could see this turning into a war because not many of us have a dyno at our
disposal.

Volumetric efficiency and Dynamic Compression Ratio (trapping efficiency) of
the motor are what really determine how much static compression you can use.

Cams with an early closing intake and high mechanical compression are usually
limited to 11:1 before you get into issues.

Now open the flood gates for "what if"...

AL heads, spark timing, engine temperature, quench height, plug temp, etc.

All things considered, I would personally limit the static compression to 11:1
on pump gas with a close watch on knock counts.

 

I was really hoping i could get away with a little more compression out of the motor than that.....hm....i'l have to talk to FMS more it seems about what this cam needs...

 

Just curious to why you want to raise the compression for pump gas?

For comparison, I'm running 9.7:1 static compression and getting 300+ HP
with 12 second quarter miles.

What sort of valve timing are you planning to use?

 

Well, the cam i plan on running is pretty large, and at this point, i'm running 10.12 on stock head gaskets. I am afraid that i am gonna have to spin this motor to 7k to make good power with it. I was told by FMS I would need more compression to acheive full potential of the cam.

cam is in the 240 range duration, 111LSA, and .620 lift

I don't intend on running 12's. I was gunning more for 11's on motor, and 10's on nitrous.

 

it depends how much timing u wanna run too but I will be at 11:8:1 I dont think Id go higher than 12:5:1 but thats just me

 

I'm not a nitrous expert, but I think adding more compression is going to limit
your charge intake.

IOW, you can compress the **** out of small amounts of charge and get decent
power

or

you can compress a large amount of charge mildly and get more punch.

The power comes from the energy in the charge. More charge in the chamber =
more power.

You might want to ask FMS if increasing compression is a good thing with
nitrous use. I think not?

 

ok, thanks for the help....

 

actually higher compression is better for nitrous.... Lower compresssion for boost apps

 

i was thinking the same..

 

Why is that?

EDIT: Boost is boost. Nitrous adds charge volume. Please explain the difference.

 

Before I head out, I'd like to re-iterate my initial thought and make it solid:

High compression and nitrous don't mix, especially on 93 octane pump gas.

It doesn't make sense to increase compression with added charge.

 

N2O is not boost.
What you are doing while shooting is multiplying the oxygen content of that air charge therefore allowing more fuel> bigger bang>more power, now increase CR and that releases higher energy, so even more power.
Of course forged internals are in order.
The higher the CR the better.
On stock internals, you can run 11 > 11.2:1 and a 100 shot very safely with proper tune.

 

I have forged rods, pistons, and a stock crank. I also have the LS6 GMPP heads with the stock(light!!!) sodium filled valves with titanium retainers. The problem is i lost all my compression with the valve reliefs in the pistons. I am setting the bottom end up for 150+ spray. I was gonna try around 11.5 compression.

 

Im with this guy the greater the shot and the higher the compression the more power you make just build it and tune it right and give it the gas it needs....But just let me reiterate NITROUS LOVES COMPRESSION

 

I was told about 11;3 was about as much as you want on 93 octane.

 

Nitrous is not "boost", but it's a charge volume adder.

I highly doubt you guys are talking from experience here (Aside from NastySS).

When you add charge volume, you need to consider DCR and static CR.

You may shooting more oxygen into the chamber, but you are taking up
combustion volume. You can`t compress large charge volume with a volatile
fuel octane!

Have a read through these many links, then we can talk on a higher level.

http://www.starracing.com/nitrous_oxide_general.htm

"Generally, the higher the compression ratio, the more ignition retard, as well as higher octane fuel, is required. For more specific information talk to one of our technicians."

http://www.kb-silvolite.com/article....on=read&A_id=7

`The strongest engines have less compression ratio, less spark advance, and more nitrous.``

http://www.urbanracer.com/articles/a....asp?a=780&z=4
It's common to see nitrous oxide used to add power to drag race motors that exceed 14:1 compression. On such an application, the nitrous/fuel ratio would be even richer, like 6:1, to add additional race octane (100-plus) fuel to the combustion chamber to control the heat and temperature.

Needless to say, you don`t want to creep over 11:1 static CR with pump gas!

 

im sure you know my SCR is 12.02 and my DCR is 8.49. my IVC @ .050 is 51. though i havent run the motor much as im still working **** out i havent detected much detonation, im running 91 now but will throw in some 100 octain until i get everything sorted out.

 

I wasnt saying not use a higher octane gas for it...Youd be stupid not to... What I am saying is while the STRONGEST engine may have a low cr ratio and more nitrous I guarantee the faster engine has a higher CR and dont say that i dont know anything either because you have no idea what my car runs or what it has in it

 

Bull.

First off, let`s stay on topic. The original post states he wants to use
pump gas, therefore, the compression must remain moderate.

The strongest engine is the engine that packs the most energy in the charge
per power stroke.

Like I stated earlier, you can compress the **** out of NOTHING and get
nothing, or you can compress a high energy charge mildly and have power.

Static CR`s are used to pad large cams with late closing intakes.

I don`t know what your motor has, but I can assure everyone it`s not a high
compression pump gas nitrous motor.

If you want to debate the physics of combustion further, be my guest. I`d
love to get into this in depth.