I Need Help With Compression Ratios

[copasetic808]

sorry for the caps......i have a 97 ex vtec...and i want to change my compression ratios....im not sure what my stock ratio is anyone know/????i got on hondaswap.com and it said that it was 9.8:1....is that right..my auto teacher....who really knows his sht..said that anything over 9 is starting to get a little unstable..like he said muscle cars ran like 11:1...and you will be pre-igniting...anyone can help me...also if i change the compression...(by probably changing piston heads to make them flat) how much extra hp am i talking????like 5-10...if im lucky...thanks guys....

[jcrx]

Stop listening to your musclecar friendly auto instructor dude. I've known people running 12.3:1 on pump gas in Honda engine. It's all in the tuning, and yes the Hondaswap stats are right, well that one is. You don't change piston "heads", you change pistons, mill the head, or use a thinner head gasket. High compression pistons aren't flat they are convex.

[crxlvr]

your auto teacher really doesnt know his shit. im running my b16 with about 11.5:1 ratio on 94 octane with no problems. with proper tuning and more hardcore internals i could prolly push that to around 12-12.5:1 i just gotta keep upgrading the octane.

[PunkAlex]

Dont listen to your shop teach when talking about jap/compact motors...hell today i argued with mine about double over head cams-he said that a DOHC motor is a v-8 with one cam on each side...

[copasetic808]

Originally Posted by crxlvr

your auto teacher really doesnt know his shit. im running my b16 with about 11.5:1 ratio on 94 octane with no problems. with proper tuning and more hardcore internals i could prolly push that to around 12-12.5:1 i just gotta keep upgrading the octane.

94 octane???i think that like premium is 91 right??i think that unleaded is like 89 or something..i dont know..proper tuning meaning?????you and the guy above all said with the right tuning???this is my functional car that i take around to school and stuff so it has to be able for me to use....you can tell a differernce in your horsepower from raising it up???

[YOUNGSTER]

one of my friends hads b20vtec set up its built he runs 13 5 0 wel its a lil lower cause he has turbo valves but hes driven it around with (91)premium but he didnt punch it but he siad thats pushing it and i agree lol

[crxlvr]

in cali the best you guys get is 91, over here in NY, we dont have such restrictive laws, so 93 is our premium, then 89, and 87. however, Sunoco carries 94, and a few places carry 113 race fuel, so its limitless over here.

[bbyzfr6]

GREAT TOPIC!!! Listen to the muscle car / gear heads!!! Ok lets find you a calculator. (http://www.kb-silvolite.com/temp/calc.php?action=comp) This will work for your car too it's just math not domestic specific. I believe it is about 4% increase in power with 1 point increase in CR.

Now the mfg. will claim certain compression ratio, but they are usually wishful numbers. Be sure and measure you deck height and head gasket along with the gaskets diam. People leave these out a lot when calculating compression ratios. You'll be suprised how a little number like .010 can drop you CR half a point.

Also your block probably has a chamfer at the top of they cylinders from the factory to make piston installation easier. This also increases volume driving CR even lower. Super Comp drag racers do not put the chamfer in...they use a funell, bore specific, ring installer.

If you port or rework your heads that will increase their cc which need to be accounted for in your calculation.

Cam design can also play a big role in your total compression. Over lap and big durations bleed off cylider pressure and alow for more static CR. Go to compcams.com and read about this. The cam will come in when calculation dynamic compression. I still need to do this on my motor, but I haven't begun to selected the cam grind yet...

It's not just CR that causes pinging it's QUENCH height that is important as well. If you go swapping heads, changing pistons, stroke, ect and get the quench greater than .060 you will ping with even a 10:1 motor.

Now I'd really really like to see a TRUE 12.5:1 run on pump gas. I'm sure it is possible with a lot of tunning, but don't wait too long or you piston will look like you took a cutting torch to the top of it. I'd stay lower 11's:1 or maybe high 10's with everything set up correctly on a STREET motor.

Just my opinion and for those that dissagree I want hard evidence on the motors you've seen over 12:1. That means the dial indicator on top of the block showing me the numbers. Along with the oil level for the cc or you heads and pistons. Not my friend had one and my dad can beat up your dad stuff.

[copasetic808]

hey i really appreciate it....i mean its nice to hear from someone who hasnt just passed on something they havent actually tried...or maybe just learned from another thread and recycled it..