compression

[96z28]

I want to rebuild the z cause it has a lot of miles on it. When I say rebuild i mean full. Like piston, crank, rods, heads, and cam. My question is what is the highest compression i can have and still run 93 octane.

[Joe 1320]

Well..... it's not quite that simple. Alot depends on the combustion chamber shape and condition, piston shape, camshaft, fueling and tuning. Generally though, about 11:1 is about as high as you'll want to go on 93 octane and you better have a dead on tune. I've seen guys run higher and were successful, but if fuel quality or delivery is for whatever reason a little off, something is going to give. I usually try to stay no higher than 10.5:1.

[fastTa]

Essentially there are two types of compresion ratios used. First there is the most commonly talked about compression ratio, STATIC. Basically, your static compression ratio is the relationship between the volume above the piston at TDC and BDC.

Your DYNAMIC compression ratio combines both the values used to obtain static compression ratio and then also factors in the timing of your intake valve in relation to to crankshaft degrees (specifically the number of degrees ABDC). This is important because the a/f in the cylinder does begin to compress until the intake valve is closed. Since 99.999% of all internal combustion motors will close the intake valve after BDC (ABDC) and thus effectively shortening the stroke in relation to the beginning of actual compression, the DCR will always be lower than the SCR. Given this info, it is much more beneficial to base the ability of your motor to effectively run pump gas on DCR.

In an LT1 with 54-55 cc combustion chambers a 12:1 SCR is about the limit. A 11.5 SCR or less will be much more friendly. Generally speaking, the bigger the cam the later the intake valve will usually close. So given that and again generally speaking, the bigger the cam you choose the higher you will want to start with your SCR.

But like I mentioned above, once you choose a camshaft for your build you will know the IVC (intake valve closing point). From there you will be able to calculate your DCR and adjust your SCR accordingly.

Here is a good online calculator for both SCR and DCR:

http://kb-silvolite.com/calc.php?action=comp

And like Joe mentioned above, pump gas quality and local octane available will also affect your build goals.

[fastTa]

What type of bottom end components do you plan on using? (Crank, rods, pistons, bearings, wrist pins)?

[96z28]

I was just going to get a 383 stroker kit, I think it comes with all the stuff I need

[fastTa]

I was just going to get a 383 stroker kit, I think it comes with all the stuff I need

I don't mean to sound like I am prying, but who were you planning on buying the kit from and which kit?

[96z28]

i have no clue at the monent, i've been looking around but im in no hurry cause i dont have the money anyhow. I have to w8 for my after school wages to pick up. I dont make much so it takes me a while to save up that much and they only let you work from 4-9 on school nights so thats only 25 hours a week. Do you have any suggestions of what i should buy and how much it would cost

[fastTa]

Do you have any suggestions of what i should buy and how much it would cost

That really would depend on alot of things. How much power do you plan on making? Naturally aspirated, boost, or n20? How much money do you want too spend? How many cubes?

[96z28]

in a perfect world i would like to run boost, but on a 16 year olds wages i'll keep it naturally asparated. I may make it bottle fed later but i think that would be hard to find a place to hide it from my rents

[Mr Nice Guy]

If you are going to go with a power adder sometime in the future I would recomend against upping the CR. Boost or the Bottle, neither likes high compression.

[fastTa]

On the street with boost it is a good idea to run a slightly lower compression ratio, but with n20 it really doesn't matter much because there is nothing being forced into the cylinder. The a/f and n20 mixture is still being drawn into the cylinder by vacuum. How big you jet your n20 setup will just depend on your final DCR.

Strokerkits.com seem to be very knowledgeable and supposedly have the latest state of the art CNC equipment. They are also very reasonably priced for their basic all forged rotating assemblies or complete shortblock packages. Also try Speed Inc. (ls1speed.com), or AP Engineering. They also have competitively priced shortblock packages.

[Injuneer]

Agreed.... nothing wrong with high compression on a nitrous motor. But you better look at something closer to 9.0:1 for a blower setup.

The other thing to look at is what you hope to gain. I did a simulation with my engine, and by going from 10.8:1 to 11.8:1 I would have gained 18HP, or about 3.7%. That's on a fairly mild street cam (selected to pass NJ rolling emissions) with a 114LSA for nitrous, that reduces overlap even more and increases DCR, 94 octane fuel, and a lot of engine dyno time to optimize everything. We even tried tuning the engine on 100-octane fuel, and couldn't find any more power. Going to the higher compression ratio doesn't gain all that much, and puts you at risk of the variable quality in the pump gas that is available.

[fastTa]

Another advantage to running n20 is that it is basically ignorant to the outside ambient air temps. Even if it is 110°F outside, the decompression of the n20 will significantly cool your induction charge and provide a much more dense charge of ambient air to compliment the o2 that is released from the n20 during combustion.

Ambient air temps can wreak havoc on supercharged cars.

Boost and n20 both have very distinct and unique ways of making power, but the one important thing they have in common is that they both get more oxygen into the cylinder and thus allowing you to therefore add more fuel. Superchargers get more oxygen into the cylinder mechanically by compressing the intake charge, and n20 gets more oxygen into the cylinder by means of a simple chemical reaction.

Here are some very informative links on nitrous, turbos, and superchargers:

Here's one for how nitrous oxide works:

http://auto.howstuffworks.com/question259.htm

Here's one for how turbos work:

http://auto.howstuffworks.com/turbo.htm

Here's one that compares superchargers to turbos:

http://auto.howstuffworks.com/question122.htm