hey all anyone got a fair idea what the most suitable compression ratio is for 98 octane fuel.??
caompression ratio
Started by
_75LH_
, Apr 23 2006 05:29 PM
12 replies to this topic
#2
LS1LX
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Posted 23 April 2006 - 05:37 PM
ever engine is different, every batch of fuel is different.
ive seen cars make power N/A with over 12:1 on 98 octane, but personally wouldnt go over 11:1 on a street motor, the days of high comp street engines are over, people make respectable power with respectable comp these days.
ive seen cars make power N/A with over 12:1 on 98 octane, but personally wouldnt go over 11:1 on a street motor, the days of high comp street engines are over, people make respectable power with respectable comp these days.
#3
_Keithy's_UC_
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Posted 23 April 2006 - 06:08 PM
Damn straight. I'm running just over 10.0:1 in mine on 98 Ultimate... No pinging at all...
Edited by Keithy's_UC, 23 April 2006 - 06:12 PM.
#4
_CHOPPER_
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Posted 23 April 2006 - 10:46 PM
High compression HQ engines were designed for Super, which was at the time 98 octane. So 9.5 - 9.7 should be safe if you're building a stocker.
#5
_1QUICK LJ_
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Posted 23 April 2006 - 11:26 PM
it depends what size cam you run because thats what dictates what cylinder pressure you end up with.
if using std cam up to 260 adv duration use 9.0-9.5 comp
270-280 adv duration 9.5-10.1
290-300 adv duration 10.1-11.5
300+ adv duration 11.5- 11.8
this is only a rough guide of what ive found seems to work well with different combos ive built for people,dont be tempted to run comp that is too high or you end up loosing any real gains by having to reduce ignition timing to suit.
if using std cam up to 260 adv duration use 9.0-9.5 comp
270-280 adv duration 9.5-10.1
290-300 adv duration 10.1-11.5
300+ adv duration 11.5- 11.8
this is only a rough guide of what ive found seems to work well with different combos ive built for people,dont be tempted to run comp that is too high or you end up loosing any real gains by having to reduce ignition timing to suit.
#6
_Keithy's_UC_
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Posted 24 April 2006 - 02:34 AM
^^^ They are talkin about 98 octane fuel dude (wants to know what compression ratio is ok to use 98 octane)!!
#7
_[BOTTLEDUP]_
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Posted 24 April 2006 - 02:41 AM
Has a lot to do with combustion chamber shape, cam timing events and of course ignition timing. Cast Iron heads aren't able to run as high a comp ratio as alloy either.
#8
antelopeslr5000
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Posted 24 April 2006 - 02:55 AM
I think 75LH is referring to static compression ratio and not dynamic. 
Cams have nothing to do with determining static compression ratios.
I agree with Chopper, about 9.5 to 10 would be about right.
Cams have nothing to do with determining static compression ratios.I agree with Chopper, about 9.5 to 10 would be about right.
#9
_75LH_
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Posted 24 April 2006 - 12:07 PM
cheers guys,mines currently ~10.5 on 98 but just wanted some opinions for future rebuilds and part selection.gotta plan these things.
#10
_1QUICK LJ_
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Posted 24 April 2006 - 02:26 PM
that is with 98 fuel. static compression is only part of the deal like i said cam choice is the factor that decides if that comp will work.as well as chamber design if you have pretty basic chamber design then go for slightly lower comp,
antelopeslr5000: dynamic compression ratio is far more important because thats what you actually end up with when the engine is running, what i quoted was static comps and what size cams suit them quite simple really.
you cant say that its ok to run 10.5 comp in every combo because one motor might have say a 270 duration cam(25/65 timing) and will rattle its head off,but a motor with a 290 duration cam(35/75 timing) will be no probs because it bleeds off cylinder pressure, then if you put in a 320 duration cam(50/90 timing) it will still run fine on the fuel but be a guttless pig due to very low dynamic comp.so the cam does matter when it comes to a running engine,which means a cam does come into deciding a static compression ratio.as does chamber and piston crown design, if you build engines with just the static comp in mind and no other factors such as cam, chamber design ,and volumetric efficency then how can you tell what comp you really need? its the old saying its all about the combo
antelopeslr5000: dynamic compression ratio is far more important because thats what you actually end up with when the engine is running, what i quoted was static comps and what size cams suit them quite simple really.
you cant say that its ok to run 10.5 comp in every combo because one motor might have say a 270 duration cam(25/65 timing) and will rattle its head off,but a motor with a 290 duration cam(35/75 timing) will be no probs because it bleeds off cylinder pressure, then if you put in a 320 duration cam(50/90 timing) it will still run fine on the fuel but be a guttless pig due to very low dynamic comp.so the cam does matter when it comes to a running engine,which means a cam does come into deciding a static compression ratio.as does chamber and piston crown design, if you build engines with just the static comp in mind and no other factors such as cam, chamber design ,and volumetric efficency then how can you tell what comp you really need? its the old saying its all about the combo
#11
myss427
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Posted 24 April 2006 - 04:57 PM
I totaly agree with 1QUICKLJ, I had a strong solid cam in a couple of years ago with 11.7 to 1 and would ping its head off even with Avgas. Changed to a roller cam with 310 advertised duration and now can run on 98. Runs best on 100 or higher but will tollerate 98.
#12
antelopeslr5000
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Posted 24 April 2006 - 06:52 PM
I agree too 1QUICK LJ. Dynamic compression is more important when considering engine build/design. 
Cams don't determine your static compression ratio, as I stated eariler, but they do play a major roll in deciding what static compression ratio to aim for.
Cams don't determine your static compression ratio, as I stated eariler, but they do play a major roll in deciding what static compression ratio to aim for.
#13
_CHOPPER_
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Posted 24 April 2006 - 07:18 PM
More cam overlap = more compression bled off. Therefore, more static compression is generally required to compensate for the losses. This is of course a guide only and doesn't mean that the dribble somebody else posts is incorrect.
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