compression ratio vs octane

[adn406]

first, what is the highest compression ration you would run 93 octane in if it is an aluminum headed motor?

second, is a 1/3 race gas to 2/3 93 octane adequate for an aluminum headed 360 sbc with a 520 lift cam (350 .040 over) and 11.5:1 compression?

[406shark]

first, what is the highest compression ration you would run 93 octane in if it is an aluminum headed motor?

second, is a 1/3 race gas to 2/3 93 octane adequate for an aluminum headed 360 sbc with a 520 lift cam (350 .040 over) and 11.5:1 compression?

There are too many variables to give an accurate answer.

Quench..... Camshaft overlap..... Piston shape..... Total timing..... are just a few of the variables off the top of my head.

Most car magazines will tell you that you can run 10.0:1 compression on 92-93 octane fuel with aluminum heads. Sometimes it works out, sometimes it doesn't.
I have personally seen guys running 10.96:1 compression on pump gas... but the engine was specifically built to run on pump gas.

Sorry I can't be more helpful...


Jeff

[xten]

Al, mine's 10.4:1 and it runs fine on the street on 93. I put race gas in it when I go to the track just for general principals.
Did you get your motor back from Jake? How'd it do on the dyno?

[adn406]

i got it back and raced it last week for the first time. best of 11.05 at 122mph. i attached the dyno sheets showing before and after.....

[xten]

Wow Al! Nice numbers! Ten's are just around the corner!

[wrktrkfromhell]

My 72 Nova was 10.3 with 041 iron heads & Edb RPM cam and a 125dose on 93 .
Alot of variables

[Diabolical]

Great numbers..
Getting her done!

[Tweeder]

12:1 on 93 with cast heads if that helps.

11:1 on 87, ofcourse all gas contains atleast 10% corn juice now, tho so the numbers might not be as acuarate

what ive ran without detonation

[adn406]

Wow Al! Nice numbers! Ten's are just around the corner!

i haven't shifted above 6200 yet and launching at 2200. i can buzz it up to 6800 and launch at 4000. i think there will be a 10 second pass this year. I need a cage now;-(

i need to replace my crank pulley, i am still throwing belts. i'll update more later.:cool4:

[adn406]

12:1 on 93 with cast heads if that helps.

11:1 on 87, ofcourse all gas contains atleast 10% corn juice now, tho so the numbers might not be as acuarate

what ive ran without detonation

sweet. i am glad to see that.

[1fast10]

from what i hear on a old school sbc 10.5 is the majic number that is gona work no matter what. but a worked head ,intake, piston design and of course heads can bring that up .i know the ls motors are around 11.1 pump gass but the head plays a huge part in that motor

[83blazerV8]

running 11.1 on 92 at the track with 34 total timing on aluminium heads cast is suppossed to be one point lower. but it all depends on valve overlap which can affect static compression its all in the camshaft and hotspots in the compression chamber

[blazaboy86]

Mine is 11.28:1 and all we have here is 91. Looks like a 110/91 mix is in order for me.

[bbcc]

I'm 11.4:1, 32 degrees timing, low low 10's for dynamic compression IIRC. We only get 91 swill here as well. A big key, as stated before is the shape of the chamber and the quench area. I set my quench at .038". Basically the tighter, the better (can be said for a lot of things haha). Also, polish the chambers to reduce the amount of carbon buildup. Coated pistons help also. Since you're mill is already put together, I guess those points are inconsequential. Nice numbers BTW.


Cheers,

Ryer

[83blazerV8]

.040 should be your min when you run tighter you run the chance of detonation unless your running racegas which has anti detonation properties added to it if your running pump go no further than 11.0-11.3 to 1 91 to 93 octane if your polishing the chamber and sanding any sharp edges on the piston top top you can get 11.5 to 1 but it takes time and the plug gap plays a part in that to

[sbctols1]

I'm 9.4:1 and run 87 octane. Im at 32 degrees of timing. Its a carbed 6.0 liter. I buzz is past 7k. I could probably get away with more timing.

[bbcc]

83 blazer, here's my thoughts on quench:

Minimum quench is dictated by bearing clearances, elastic elongation of the connecting rod, bending deflection of the wrist pin, all taken into account at a given engine speed and a known thermal expansion of the block/piston/rod.

Having said that, tightest is the best. The tighter you get to the head, the more turbulent this quench area gets giving you better homogenizing of the air/fuel mixture. This turbulence also prevents "stagnant" hot areas from self igniting. In essence the turbulence "quenches" any areas that has the potential to self ignite....that's where the name comes from. It's the same idea as eliminating sharp edges. Corners will always be hotter than the surrounding metal because they have the largest contact with the flame front, and the smallest heat conduction path into the surrounding metal. Radii will have less surface area. Heat control within the chamber is key to making any engine, regardless of fuel type, run at it's potential.

I agree, .040 is what most should aim for with stock components and a tall compression height. But this is to avoid piston to head contact. While the engine is running you want the quench area to be as near zero as possible. This reduced quench will obviously effect the CR, so alterations should be made to the chamber or piston to get your desired CR.

The big thing to running huge CR on pump swill is dynamic compression. Pump gas can only withstand a certain BMEP (break mean effective temperature) before it auto ignites. A lot of valve overlap will bleed cylinder pressure at lower rpms, making even 12:1 livable.

[Mailman]

I had a aluminum headed 355 with 11.9 to 1 compression that would run on 93 on the street but didnt like it over 6k rpm. I would run 110 at the track....may help...may not....

[83blazerV8]

83 blazer, here's my thoughts on quench:

Minimum quench is dictated by bearing clearances, elastic elongation of the connecting rod, bending deflection of the wrist pin, all taken into account at a given engine speed and a known thermal expansion of the block/piston/rod.

Having said that, tightest is the best. The tighter you get to the head, the more turbulent this quench area gets giving you better homogenizing of the air/fuel mixture. This turbulence also prevents "stagnant" hot areas from self igniting. In essence the turbulence "quenches" any areas that has the potential to self ignite....that's where the name comes from. It's the same idea as eliminating sharp edges. Corners will always be hotter than the surrounding metal because they have the largest contact with the flame front, and the smallest heat conduction path into the surrounding metal. Radii will have less surface area. Heat control within the chamber is key to making any engine, regardless of fuel type, run at it's potential.

I agree, .040 is what most should aim for with stock components and a tall compression height. But this is to avoid piston to head contact. While the engine is running you want the quench area to be as near zero as possible. This reduced quench will obviously effect the CR, so alterations should be made to the chamber or piston to get your desired CR.

The big thing to running huge CR on pump swill is dynamic compression. Pump gas can only withstand a certain BMEP (break mean effective temperature) before it auto ignites. A lot of valve overlap will bleed cylinder pressure at lower rpms, making even 12:1 livable.

x2 but most people dont build strickly race engines they are street cars/trucks that they take to the track most people dont like running huge cams on street engines(like i do or my family C.A.M.S C's Automotive Machine Shop) WOO WOO
and yes i like the saying is if its not kissing its not close enough but for something somebody drives around the street and occasionally takes to the track it better to be safe then sorry