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  #16 (permalink)  
Old 05-25-2005, 08:57 PM
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compression ratios

I grant that there is always someone that gets away with running higher compression etc, and a lot of that has to do with cam profile.
But doesnt it make you wonder, that if gasoline for high compression engines was readily available, the auto manufacturers would be making a lot more engines with 10.5 to 11.5:1 compressions just as they did in the 60's.
Since Im a poor boy, and cant afford to experiment and take chances, Ill settle for some nominal losses in hp and know that my engines will survive.

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Old 05-25-2005, 09:05 PM
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If you have access to Av gas ( 115/145 ) , you can go up to 13:1 with retracted tip spark plugs and have a normally aspiraterd, carbuerated engine and use a distributor to fire them.
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Old 05-25-2005, 09:10 PM
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Av Gas 105

Our local airport used to be pretty nice about selling us Av Gas as long as it was pumped into a barrel, however, like many of the airports now, they wont sell you gas unless its going into an airplane.
Running Av Gas will require richening up your jets 2-3 sizes due to the lighter specific gravity of Av Gas Vs pump gas.
Av Gas weighs 6 lbs per gallon where pump gas weighs 6 1/2.
BTW, used to run 12.5:1 comp. with conventional spark plugs in my stock car engines, running 101/105 Av Gas.
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Old 05-25-2005, 09:21 PM
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You don't need AV fuel to go 13:1, what you do need is a thorough understanding as to what causes detonation and how to keep your combustion temperatures from "hot spotting" on the piston.

I run almost 13:1 on 93 octane with no detonation. Operating compression is what you need to gauge your set-up on. Don't fall into the trap of using only the popular manufactured components...design your own, it's not that difficult but it does take time and understanding. The bad part about designing your own parts is that if you are unable to actually make them yourself it will cost you an arm and a leg (and your first born).
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Old 05-25-2005, 09:23 PM
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That's right Max. It's better to err on the safe side. That said they do have a couple of high compression fuel injected aluminum headed V8s out there.... and even some 12.5:1 motorcycle engines(ie 250 Ninja)....

But there are some many variables in this equation like vehicle weight, gears, stall, transmission, fuel, piston & combustion chamber shape & smoothness, cylinder bore, plug temp, intake design, Cam Profile, temperture, load, humidity, alltitude, driving style....{the list goes on and on} that there is no clear cut answer.

As was stated 8.5 can ping and i've personally rode in a 13:1 BBC that ran on pump gas. ( it had a 700"+ solid roller cam & aluminum heads). Personally I would have NEVER drove it like that but it was to prove a point. When we pulled it apart it showed signs of leakage around the head gasket surely from detonation.

And to back up ll77 I've seen it before, 220PSI in a 400 sbc (Lunati "412 stroker" kit with their 515" lift cam and double hump heads) ran great on pump gas. And i've seen dish top 406's ping like there was no tomorrow.

Be real, play safe. Keep it {well}under 10.5:1 with iron and 11.2:1 with aluminum unless you are a pro.
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Old 05-25-2005, 09:33 PM
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This has been asked a million times, there is no right answer...too many variables.

Stay under 11:1 and you will be fine.
“She may not look like much, but she's got it where it counts, kid. I've made a lot of special modifications myself.”

— Han Solo
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Old 05-25-2005, 10:27 PM
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Thanks guys for the many responses

Big thanks to everyone that answered. This was a discussion at my job with some guys that were not really aware of the disadvantages of running a high compression (11.5) engine on pump gas. Which leads to the (IMO) crazy combination of using chevy 305 heads on a 350 block along with a peformance cam shaft of some kind in a late 80's chevy suburban of all things. IMO I didn't think the combination would be successful for hauling groceries, bass boats, etc. I'm not certain though because the engine rebuild shop that has agreed to do the build has not objected to the build and have said that they have the neccessary components to get the job done. Thanks again
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Old 05-26-2005, 07:47 AM
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Whatever you do. Get a good set of aftermarket or vortec heads. Factory heads suck (except for the vortex heads). Don't waste your time with 305 heads. You will loss 50hp with stock heads!

Dart Iron Eagle's or Trick Flows are pretty cheap. AFR's are the best. Vortecs are the best deal but require some machining and a vortec intake, vortec valve covers, and vortec rockers.
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Old 05-27-2005, 02:54 PM
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The Duratec engine in the Lincoln LS is 10.5 from the factory, and although they recommend 91 octane I get away with 89 usually.

The variable cam timing seems to allow a little more compression.... the non VCT motors stop at 10.0 C/R
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