Pump Gas - Octane Rating and Compression Ratio

[Muad'Dib]

What compression ratio range allows the use of pump gas .. whether it be regular or premium...?

[gradon]

I'd say shoot for a max of 9.5:1 SCR, 8.25:1 DCR, and a quench of .050". On my build, 2.5K-3.0Krpm is ping territory(@60+%tp accelerating, while still in closed loop, but right before open loop) using 93 octane w/ ~10:1/8.5+, low .060s" quench(dunno if -.007" w/ the new mp gasket is worth the trouble, but might give it a shot), and stock obd2 programming w/lt1 25.5#injectors and map voltage set to 6v to make sure 3K+/OL is in the 12s:1 AFR.

[Muad'Dib]

My particular build comes out to:

9.68 SCR
8.47 DCR
.06 Quench

Which at this point im still not sure is right or wrong...

Thats what made me wonder about it. It would be good to have like a particular range of Compression Ratios and what Octane rating will work with each particular range. Or maybe its just not that simple...

[ajmorell]

I don't think it is that simple (I asked the same question). I agree about shooting for a CR of about 9.5:1 but from what I've seen quench that low (0.050") is hard to do unless you want to drop tons of money (read custom pistons & decking of block). I think that 0.070" quench is a more realistic number personally.

[RAPTORFAN85]

Something that nobody ever mentions about runnung pump gas is that they mix gas different depending on were in the country you live. Out in the woods you get dirtier gas that burns better (less ping) and in the more populated areas you get the cleaner mixes that seem to be ping and preignition prone. I think this makes more of a difference them people think...

[1bolt]

so whats the theory behind cities getting cleaner gas versus out in the country?

On the topic, a lot of performance builds on V8 can get up into the mid 10:1 compression intended to run on pump gas. The difference besides often running aftermarket heads with current technology combustion chambers. Is usually dyno tuning the fuel and timing curve. I would say that's the biggest single difference between those street/strip builds and a Budget Stroker. Most of us are going to try and get away with the factory fuel/spark.

There was a bit of discussion about this on the Email list side the other day but it kinda died, I think because the old timers have all argued about it extensively and are tired of it.

My opinion is that you might get more performance and reliability, AND in the long run cost yourself LESS money by spending a little more short term on custom pistons.

The budget build ends up with compromises (cam shaft) loose quench (.080 to .060) and a compression ratio that is basically an accident of where the budget pistons end up in the hole, and how much machining you pay for.

Because budget strokers tend to commonly have ping issues, the "long term" costs are higher when you start trying to diagnose and cure the Ping.

(or when the ping softens up the thin piston crown from deep dishing an OEM replacement cast piston; or beats a hole in a ring land).

[RAPTORFAN85]

so whats the theory behind cities getting cleaner gas versus out in the country?

Cities have a higher density of cars so if they burn cleaner gas then the smog will be less noticable, but in making the gas burn cleaner you reduce the quality of it. Out in the country we are more spread out so you can get away with dirtier burning gas and not notice it.

Thats also why gas tends to be more expensive in larger cities.

[RAPTORFAN85]

This study says that in 2004 they found AT LEAST 45 different blends in the US alone...

http://www.gao.gov/highlights/d05421high.pdf

[1bolt]

Interesting stuff, I know there are seasonal blend changes but I didn't know they tried to get away with less refined fuel in rural area's.

[yuppiexj]

Interesting stuff, I know there are seasonal blend changes but I didn't know they tried to get away with less refined fuel in rural area's.

I wouldn't say less refined, I would say more gasoline stock with less crap in it (MTBE, ethanol, MMT, etc)

I know when I buy fuel in Northern VA the Heep runs like crap and drops 1-2 mpg. (that and I pay 20-30 cents more per gallon)
South of the Rappahannock River it runs much better.

the Renix really likes the 10% ethanol blend though (just not the NOVA pollution zone)

There's a whole other can of worms. How is it polluting less to burn 10% more fuel with a bunch of new combustion byproducts?

[IH 392]

General rule of thumb is to run your compression ratio no higher than the octain rating of the fuel in a cast iron cylinder head and you want your quench as tight as you can get it without the piston hitting the head allowing for a forged piston in 4" bore with .007" clearance you should allow for .040" quench, in our engines with a smaller bore and cast or hyperutectic pistons you could reduce it some, I personally would shoot for 0 deck and use the thin head gasket.

[1bolt]

Interesting stuff, I know there are seasonal blend changes but I didn't know they tried to get away with less refined fuel in rural area's.

I wouldn't say less refined, I would say more gasoline stock with less crap in it (MTBE, ethanol, MMT, etc)

I know when I buy fuel in Northern VA the Heep runs like crap and drops 1-2 mpg. (that and I pay 20-30 cents more per gallon)
South of the Rappahannock River it runs much better.

the Renix really likes the 10% ethanol blend though (just not the NOVA pollution zone)

There's a whole other can of worms. How is it polluting less to burn 10% more fuel with a bunch of new combustion byproducts?

Yeah I get what you're saying but some of those things actually improve octane. If City gasoline has higher effective octane then it's better gas from a performance standpoint; and worse gas from a commuter standpoint. Yeah you may very well lose mileage from "better" gas even at the same exact pump... Higher octane tends to have less specific energy content.

I think in this case better is in the eye of the beholder, but I certainly wont be going out of my way to buy gas around the beltway

Doesn't everything have 10% ethanol for the last couple years? I can't remember seeing a pump without 10% ethanol in recent memory. Not like I've driving around looking at pumps though.

[gradon]

Been in the beltway most my life, save for 5 college years in Lancaster and 1 year in Costa Rica.

[PolloLoco]

Doesn't everything have 10% ethanol for the last couple years? I can't remember seeing a pump without 10% ethanol in recent memory. Not like I've driving around looking at pumps though.

Not true. In Michigan I'm used to seeing everything with 10% ethonal. Didn't pay much attention to it when I moved to the Tacoma, WA area. While driving back to Michigan from Ft. Lewis, WA I got gas in Gillette, WY and had the choice to buy 100% gasoline or save several cents per gallon and buy the 10% blend. Knowing my Pontiac Vibe ran fine in Michigan I saved myself myself enough to buy another coffee.

BTW - It's easily possible to drive 2,340 miles in 56 hours. This was March of 2008. That's recent memory for me because it was my last whole month in the States.

[PolloLoco]

So back to the topic here.

From what I've read altitude can affect ping. The closer you are to sea level the greater the risk of ping. I am in the military and currently stationed at Ft. Lewis, WA but in Iraq right now. I will be running my Jeep at sea level. I may also get stationed at Ft. Carson, CO some day, be a mile up and be able to run low octane fuel all day long. I will plan for sea level, that is where my Jeep is and where the work will be done.

Another variable is the camshaft. Too much to explain here, besides, I read most of what I know from the great FAQ's section on the forums here.

I myself want to purposely build my stroker with high compression. I just don't want to get carried away and be stuck with having to add some damn octane-enhancer from Autozone while at sea level. I've done a little homework and this is where I currently am for my future stroker build:

• Crankshaft 3214723 12CW 66lb 4.2L
  Connecting Rods 6.125" 4.0L
  Pistons KB945 +0.060" 11.38cc dish
  Cam CompCams 68-231-4
  Head Gasket Mopar/Victor Reinz 0.043"

I will bore out the cylinders 0.060" and mill the deck to 0.

This gives me the following results form the Stroker CR Calculator:
SCR: 11.09
DCR: 9.66
Quench: .043
The above assumes a combustion chamber volume of 56.7cc, not the defualt 58cc of the calculator. Looking around last night I found that 56.7 is more appropriate and inputting a smaller value here increases SCR & DCR so in reality it may be less.

I know those values are pretty high, but the quench is nice and low and the word is that the CompCams 68-231-4 is good for high CRs as well.

I could easily lower the CR by using the KB944 piston with a 21.73cc dish, SCR=9.89 and DCR=8.63, but at the loss of "balls." My target is a great-torque trail-rig that runs on premium and is used mostly off-road.

Am I being sane by planning on the KB945 pistons being able to run on premium?