Compression ratios

[400hp_76]

When i was redoing the rotating assembly i wanted to change the compression, so i installed 10.25:1 compression pistons with 72cc combustion chambers. i wanted 11:1 but the guy over the phone said that it was too much comp. without buying high octane gas. i can buy 93 octane gas here which im sure most people can. i was just wondering if this guy really knew what he was talking about, because in magazines i have seen where crate motors run 10.8-11.0:1 compression on 92 and 93 octane.

 

[inferno-vette]

I agree with you,I don't think he knew what he was talking about. 11.5:1 is the limit for premium gas. But the safe zone for not having detonation 10.5:1/11:1. It's very common to see a 11:1 on the street. (just look at those hondas and acuras with 11:1) there is only one thing, youjust have to retard you ignition a little.

 

[JohnZ]

It depends a lot on cam overlap, but as a rule of thumb, 9.5 - 10:1 is about the practical limit for pump gas with iron heads, and 10.5:1 with aluminum heads. Anything beyond that and you'll be into backing out timing to stop the detonation and/or having to blend race gas with pump premium, which is a PITA for a street-driven car. Don't be fooled by the magazine buildup articles - a dyno run only lasts 5 seconds, but you drive your car all day long.

 

[Rowdy1]

94 Sunoco Does It For Me

I'm using 12.5 to 1 with 64cc aluminum heads, .030 over on my 327ci with a complete MSD ignition and I have NO problem with 94 octane Sunoco. My timing is 34 degrees total advance, no detonation. Just power, LOTS of it!!! Oh yeah, very lumpy cam also

 

[400hp_76]

thanks

thats exactly what i have been thinking, i mean i knew that unless fuel injection just made a huge difference that you could run more compression. thanks for the comments. T

 

[Bullitt]

Do a "search" for compression ratios. Anthony, with his 427 aluminum heads, was asking about this awhile back. I think the thread was titled "Jet Fuel."

--Bullitt

 

[cmegga]

which gas ??

Im having the edelbrock rpm heads put in this week. They are 64cc and say they will run at 9:5:1. I also have their manifold and 750 cfm carb. A crane hot street cam and headers. All else is pretty much stock. What do you think as far as gas for this set up. Should I be running the 93 or something middle grade ??

At the moment it seams perfers the cheap stuff, with the stock heads.

 

[rpounds]

You can run higher compression ratios. Someone mentioned that all you have to do is retard the timing a bit in order to get rid of detonation.

Well . . . . what's the point? So you build a little bit of HSP by upping the compression ratio. But then you retard the timing in order to eliminate detonation, which in turn REDUCES power. Why not, as JohnZ says, stick with compression numbers that are safe . . . 9.5:1 with cast iron and 10.5:1 with aluminum and be able to run an optimum timing advance. In so doing, you will increase your low end, rather than sacrificing it by retarding the timing.

Remember that, on a street driven engine, you spend more time in the lower RPM range than you do at the upper limits. So, torque is your best friend . . . retain as much as you can. Keep a safe compression ratio limit and crank the timing to 34-36 degrees total timing . . . makes your vette happy, happy, happy . . .

Ron

 

[400hp_76]

Good point,

Retarding the timing would sorta defeat the purpose. You can make good hp and tq with 10.5 comp.

 

[Stingray72]

One thing to consider is that octane consistency from pump gas is not something to rely upon. For safety's sake, 10.5:1 is a reasonable limit.

 

[jsimpson]

I'm with JohnZ on this one. Determining factors are cam overlap, head material, sharp edges in compustion chamber, ignition timing. You can always back your ignition timing down enough to handle preignition, but you lose power and tend to overheat. 10.25 is a good compression ratio for high performance on street gas. Now if you work at an airport.......

 

[400hp_76]

I could see running a higher compression ratio than 10.5 if you never really drove your car but say on the weekends and you did a good bit of drag racing. So cooling would not be a big problem because you only drove it to the track or to shows. Get to the track fill her up with race gas change the timing back and then let her rip. I could see that. T

 

[rpounds]

That's a good point T. I thought about that after I posted the other day. However, for a daily driver, best to keep it in an area that it'll be happy . . .

 

[jsimpson]

Mine's 10.5 with iron heads, so I'm gonna use thick head gaskets to help out after I port it and put new valve springs in. The porting will help just a little, as will the valve job.

 

[Stingray74CC]

Sunoco 94

Like Rowdy, I use the Sunoco Gas. My engine is 11:1 compression with Aluminum heads and I'm not hearing any preignition. I do carry a bottle of Octane Booster in the back compartment incase I need gas and have to stop at a station that doesn't offer 94 octane. I've used that bottle more than once...A VERY noticeable difference in performance!!! I wish I could afford 100 octane all the time.

I'd agree with these guys, if you want to stay safe go with 10.5 with Aluminum if you can and work on power from other sources.

Good Luck

 

[resto75]

Mine is 10.5 with double hump iron heads and runs fine on 93 oct fuel

 

[jsimpson]

11 would be way too much for iron. 10 is about max for iron.

 

[sscam69]

keep in mind

I am not saying what you guys are suggesting is wrong but, it also has to do with how much overlap you are running as well.

As I have learned here you have static and dynamic compression. I am assuming you guys are all talking about static which is just measured by calculating the volume with the heads, gasket, and cylinders.

there is dynamic which can be checked by putting a pressure gauge on the spark plug hole and cranking the engine without firing and reading the compression that way.

If you have a lot of overlap your compression is actually less.

Are you guys with high compression, 11 and above, running a lot of overlap?

Frank

just my .02 cents

 

[jsimpson]

actually, dynamic is more related to cylinder pressure and comes into play when the engine is running, rather than at cranking speed. And yes, more overlap allows a useably higher static compression ratio at the expense of a smooth idle and useable vacuum.