highest compression and still on pump gas?

fiveoho · May 11, 2006

question for you guys-

how high can you get the compression on a N/A 306 motor and still be able to run 93 pump gas? i've heard if its too high i will have to run 110 , mix or use a additive. i want to be able to run straight pump gas.

how high is too high?

thanks

89white50 · May 11, 2006

with aluminum or cast iron heads? For cast iron i would say 10.5:1 ish and for aluminum you can go up to 12:1 i believe.

HISSIN50 · May 11, 2006

Are you using a T4 puter as it comes, or will the combo be tunable?

fiveoho · May 11, 2006

aluminum heads and the stock computer . may change to A9L ( was told that comp may work better with the tfs cam )

i can go all the way to 12:1 on strictly pump gas??

Grn92LX · May 11, 2006

fiveoho said:
i can go all the way to 12:1 on strictly pump gas??

Depends on the cam and other things. Some cams WILL require the engine to run race gas with 9:1 compresssion. "It depends"

I run 11.045:1 on pump gas with no issues and full timing advance.

fiveoho · May 11, 2006

Grn92LX said:
Depends on the cam and other things. Some cams WILL require the engine to run race gas with 9:1 compresssion. "It depends"

I run 11.045:1 on pump gas with no issues and full timing advance.

what all factors would you need to know to give me a good guestimation?

my sig has most of it...

thanks

Grn92LX · May 11, 2006

Looks like you have a tfs stg 1 cam. Im sure you'll be plenty fine running 11:1.

What piston are you running? You will have to play with a good compression calculator to determine what head cc and such you need to get the compression you want. Gasket thickness, block deck ect.

fiveoho · May 11, 2006

actually the tfs stage 2....

i know the pistons will have to be cut

Grn92LX · May 11, 2006

What pistons?

65ShelbyClone · May 11, 2006

Grn92LX said:
Looks like you have a tfs stg 1 cam. Im sure you'll be plenty fine running 11:1.

Does it really bleed off enough compression to use 11:1? Even with aluminum heads I'd be reluctant to exceed 10.5:1 with a streetabe cam. Just me, though.

fiveoho · May 11, 2006

pistons will be forged but my mechanic has them...

if they will have to be flycut anyway to allow for that cam and with the aluminum heads and a thick gasket......9.5:1 / 10.0:1 ?

should it still make good power if it gets in that range?

Grn92LX · May 11, 2006

65ShelbyClone said:
Does it really bleed off enough compression to use 11:1? Even with aluminum heads I'd be reluctant to exceed 10.5:1 with a streetabe cam. Just me, though.

My FTI cam is healthy but very streetable. No issues what so ever running pump gas with 11:1 compression in my little streeter.

Fiveoho, just get all your specs do all the calculations. Piston cc, block deck, gasket thickness ect to see where you'll be. If this is going to be a n/a pump gas car, just try and shoot for 10.5-11:1. With your combo i'd go for a custom cam and shoot for good power but thats me. Let those afr 185's do their thing

89white50 · May 11, 2006

i was just going by what i have seen people running, i personally wouldnt go over 10.5:1 either, mostly b/c i dont want to have to put premium in all the damn time.

fiveoho · May 11, 2006

preciate it man

hope it works out like im wanting.

so bottom line- 11:1 is possible on pump gas......safe to say 10:1 and under is a safe zone? how low could it go before my performance starts to suffer?

(premium dont bother me much ....weekend car mostly)

5spd GT · May 11, 2006

Stick with 10.5:1 or in that range...since I'm assuming this is your daily driver, you will see all kinds of different scenarios in your car. Uphill, a lot of heatsoak (especially in a hotter environment), etc. A lot of those things come in to play more and more, the more compression you try to shove into the combustion chambers of the head.

Do not worry about the cams that need "race fuel" for 9:1...no custom cam builder, nor the TFS 2 is going to make you anywhere near that. Static or dynamic compression will not be enough for that with those cams.

More compression leads to more chance of blow-by if you plan on racking up a lot of miles in the next coming years. Blower will pretty much be out of the option block and keeping it streetable.

Grn92Lx - I thought your builder showed you were running 10.9:1? with his calculator

Grn92LX · May 12, 2006

5spd GT said:
Grn92Lx - I thought your builder showed you were running 10.9:1? with his calculator

11.045:1 on my builders calculator. Its all on my computer with the rest of the specs including bearing clearances, ring gaps, rod ratio ect

Rick 91GT · May 12, 2006

lol @ Mike

11:1 is about as high as I will usually go for an alum head street motor, even when you take all items in consideration it can be borderline. With some hot air temps or really cool good air you have to be careful that you do not detonate, timing is crucial. You can get into Dynamic compression, bleedig cylinder pressures off,etc to help explain it all but that is over most guys heads...

Depending on your pistons, heads you may only be able to acheive 10.x:1 compression or lower. What are the specs on the parts you are using. I have a very good comp calculator....

giddyup306 · May 12, 2006

Iron heads with no quelch? You'd be lucky if you get 9:1 w/o pinging. Good set of aluminum heads? 12:1 is dooable.

The 347 is 11.5:1 and the SVT has 11.25:1.

Grn92LX · May 12, 2006

giddyup306 said:
The 347 is 11.5:1 and the SVT has 11.25:1.

whats your 347 combo? Any track or dyno numbers?

5spd GT · May 12, 2006

Grn92LX said:
11.045:1 on my builders calculator. Its all on my computer with the rest of the specs including bearing clearances, ring gaps, rod ratio ect

How about that, Keith did the same for me...piston, pin, locks, ring, rod weights (in grams), ring combination, gap at the top, second ring gap, main journal, bearing bore clearances, rod journal, endplay, side clearance, torque specs, deck height on each square, etc, etc...oh, and the better rod ratio is also wrote down

Oh and for those that have static and dynamic compression mixed up or doesn't know anything about it...this might help:

Static - http://e30m3performance.com/myths/more_myths1/comp_ratio/comp_ratio.htm

Dynamic - http://cochise.uia.net/pkelley2/DynamicCR.html

Simple terms...static compression is volume at bottom dead center/the top dead center's volume which is left in the combustion chamber, quench area under the head, head gasket, and the amount of piston cc (valve relief area)...

Dynamic compression starts when the intake valve goes to close, where "actual" compression starts...not at the bottom of the cylinders travel (BDC). When the intake valve goes towards the valve seat that is where the engine actually makes compression and goes back towards the top to make the "boom" Cam timing effects this...

Detonation is still more likely to occur in a hot zone or heat soaked engine...of course you can make an engine that will detonate with cold air...LOL. Just keep upping it up...bigger is better right...

highest compression and still on pump gas?

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