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Octane rating vs CR

General engine tech -- Drag Racing to Circle Track

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David Redszus
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Re: Octane rating vs CR

Post by David Redszus »

Static compression ratios have little relevance to the ability to predict compression pressure or compression temperature.

Pump gas (or any other gas) octane index values (93-94) must be discounted by about 5 points or MON values used.

An engine with a SCR of 12.0:1 would normally see a DCR of 9.5:1, with compression pressure of 110 psi, and compression temperature of 530F at a firing point of 32 deg BTC. This could be marginal for even high octane pump gas, except for
short bursts of full power and controlled inlet air temperatures.

If in doubt, run 100 octane race gas.
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Re: Octane rating vs CR

Post by rfoll »

This may be a shock to some, but "max power" is not always the correct answer, especially if you have to drive it every day.
So much to do, so little time...
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Re: Octane rating vs CR

Post by Steve.k »

F-BIRD'88 wrote: Tue Jun 15, 2021 12:57 pm I don't have a hate against high compression nor premimun pump gas... I do have an issue with ignorance and incompanence reguarding combining excessive high CR with fuel that lacks the octane performance to protect that expensive engine from knock damage..

Running around with retarded timing is a crutch..

I had a 350 with shaved ported 305 heads CR was 12.65:1.
Ran fine on 94 unleaded as long as timing did not exceed 27 deg BTDC... But when you gave it the correct fuel for the cr, allowed optimum spark timing (36deg btdc) oh what a difference in power and performance..

I don't have a problem with cr nore pump gas.. Just incompitance and those that advise others that this engine works (with ECM spark timing control) , or (clearly retarded timing well short of optimum as a tuning crutch) thus your engine will too.
Yes plus new. Ecms have the ability to adjust cam timing so another big help.
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Re: Octane rating vs CR

Post by frnkeore »

It is sounding to me, that no one has tuned a 87 oct engine, in a old school wedge headed engine or a more modern heart chamber?

I had assumed that it would be understood that timing would be set in the 36 to 42 deg total range, with mech and vacuum adv no vac or electronic retard or knock sensor. Imagine your tuning this in 1975.

If I must specify a particular engine, I'm a Ford guy so, make it either a SBF or FE. Those are what I mess with and what this project would be.
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Re: Octane rating vs CR

Post by mopardave »

I'm at about 500'. I use sunoco 93 mixed with dragon 110 leaded. awesome mix, the cars never ran this good on anything else i put in it and i have tried a few. I have a loose vert, but its very streetable. I run this with a 6 heat range plug as well. Doing the things most will tell you ,you cant do. Its possible, just have to have the right combo and keep an eye on things. I think most over octane their combo's. Easy to do as i have done during my learning curve. With another combo, I once ran low on pump gas at the track and filled back up with track 110 torco. I saw 3 tenths added to my time slip. That stuff really slowed me down.
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Re: Octane rating vs CR

Post by skinny z »

frnkeore wrote: Tue Jun 15, 2021 10:02 am Is the conscious that no one can build a high performance engine, that will run on the the 87 octane gas, at the pump, closest to where they live?

Or does it mean that dynamic compression ratio means nothing?

Or does it mean that DCR is only useful if certain other criteria are met and if so, what are they, in the order of how important they are, to retard detention?

It's pretty, universally thought, that ~8/1 DCR is good for 91-93 oct in a good street application but, I've never heard what it might be for 87 oct.

I've specified squish, chamber, air temp, eng temp and altitude but, I will add a carb and 12.5/1 A/F and unequal length, short headers.
In case you missed it...
skinny z wrote: Mon Jun 14, 2021 3:19 pm Well, if you believe what's printed in the car magazines of old, about 20 years ago Super Chevy put together a Vortec headed 355.
8.75:1 SCR and used Comp's XE268H cam (I believe on a 108 ICL).
Quench at .044".
87 octane fuel (from who's pump I can't say other than being California based) and netted peak TQ of 427 at 4000 and peak HP of 390 at 6000 on the magazine dyno.
DCR works out to 7.2:1.
Make of it what you will.
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Re: Octane rating vs CR

Post by RW TECH »

You cannot make an accurate enough guess for octane tolerance based on a theoretical dynamic compression ratio.

As a real-life example, a COPO 427 engine running at 150°F water temp with 13.3-ish mechanical compression ratio and 44° ABDC @ .050" intake closing (at lobe-factory build) will definitely tolerate 98 octane unleaded gas with zero incidences of knock and spark set to produce max TQ in a sweep range of 4000-8000 RPM @ 200 RPM/sec.

A completely different setup, like older generation engines with either aluminum or cast iron heads, but other original parameters the same, may not like 98 octane fuel at all.

Your best overall bet is to play it safe. You can do more (tuning-wise and safeguards) with an EFI system than you can with a carb & distributor, so if you are a carb guy play it extra safe.
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Re: Octane rating vs CR

Post by panic »

Ecms have the ability to adjust cam timing
?
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Re: Octane rating vs CR

Post by F-BIRD'88 »

panic wrote: Wed Jun 16, 2021 1:47 pm Ecms have the ability to adjust cam timing
?
tNot in response to engine knock detection..
The ECM only retards the spark timing to stop knock.
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Re: Octane rating vs CR

Post by RW TECH »

panic wrote: Wed Jun 16, 2021 1:47 pm Ecms have the ability to adjust cam timing
?
Sometimes, and to some extent.

Otherwise the ECM offers more fuel tuning latitude to equalize distribution vs a carburetor situation where part throttle/load and transients are, at some point, "just what they are".

Ditto with spark, so at the end of the day the ECU is safer than a carburetor, assuming both are "in the right hands".
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Re: Octane rating vs CR

Post by BLSTIC »

I recall reading an article where the commenter (a well known Australian engine builder, not sure on the name) that when unleaded came out here it was 91 RON only (basically cats piss rather than fuel). They would limit street EFI 304 builds to 9:1 until 95 octane was a thing.

So I'd say given those were a somewhat modern head design and came with multi-port injection and a knock sensor (i think) you wouldn't able to safely go much higher
Last edited by BLSTIC on Wed Jun 16, 2021 3:53 pm, edited 1 time in total.
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Re: Octane rating vs CR

Post by dannobee »

panic wrote: Wed Jun 16, 2021 1:47 pm Ecms have the ability to adjust cam timing
?
Yes, through the cam phaser, but not for knock detection per se, but for egr, which will definitely reduce the detonation threshold.
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Re: Octane rating vs CR

Post by NewbVetteGuy »

F-BIRD'88 wrote: Tue Jun 15, 2021 12:57 pm I don't have a hate against high compression nor premimun pump gas... I do have an issue with ignorance and incompanence reguarding combining excessive high CR with fuel that lacks the octane performance to protect that expensive engine from knock damage..

Running around with retarded timing is a crutch..
Real question, mostly because I increasingly am starting to LIKE the idea of an EFI engine pushing the edge of detonation and running higher compression with retarded timing in the risky areas of the map, but maybe I'm thinking about this wrong:

-Doesn't running on the edge to the point that you have to pull out timing in parts of the map and leverage knock sensors to pull timing also improve performance at part throttle and highway cruising and improve cruise MPG?

Higher compression == more efficiency at converting fuel into work; more power per unit of fuel everywhere seems good to me. My thinking is higher compression is more better and just chase after it like you chase after weight savings -little bits EVERYWHERE add up: maximize squish area, maximize quench action, get the coldest air possible, cool engine coolant and reasonable oil temp, pull heat out of the piston with oil squirters if you have to to stay in the power longer without over heating, and pull timing where / when necessary.

Compression just seems beneficial everywhere you're not pinging to me right now: WOT, part throttle, torque at low RPM, more hp at high RPM, fuel economy. -Push it and pull the timing to stay just below the threshold of detonation on your octane and temps and have some safety net in the tune and well-selected and calibrated knock sensors.

I like the idea of having more "dynamic range" in an engine: more efficiency and cheaper cost-to-operate when you want it, and more power when you want it and add more octane. -Electronic thermostats are another example of pushing the "dynamic range" of an engine: run hotter at highway cruise to improve fuel economy and then start cooling off that engine to make more power when you want to.

-This just seem to me like where you end up with good engineering.

It's definitely a hallmark of the best designed and most expensive computer and device processors: a processor that can throttle down and use the minimum amount of watts to prolong battery power when you're not using it and that can then throttle up and produce the most power when you need / request it: the best "dynamic range" requires the most serious engineering and use of the newest technologies.


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Re: Octane rating vs CR

Post by hoodeng »

Many years ago an article was written in a motorcycle magazine that extolled the virtue of not having to spend what providers of performance products and services were charging….Now that got a lot of people’s attention, especially those that want a higher performance level at the least amount of money for their V2 engine.

So the formula went:
Don’t pay for head porting and performance valve train parts or even high comp forged pistons etc, all you need to do is install brand X cam which will bolt in without any clearancing/setup required but has the longest duration, overlap and lift you can get into a stock layout engine,also adjustable pushrods, the next step was to leave the stock cast flat top pistons and instead deck the heads ·140” to go from 8.5:1 to 10:1 comp,cut the cylinder base to get ·025” to ·030” squish, this combo could make a change from 60hp to 85hp!! worked a treat for a while,,, scroll forward a little and the failures started rolling in, beat up top ends, broken springs, piston failures,destroyed lifters, lunched engines. They all had one thing in common, an article in a magazine.

The race to find just how much you can squeeze from the lowest octane fuel on the market is a race to what?? I don’t get it, what is the quest? We don’t live in a country, or countries where fuel is not much better than kerosene, [even in those countries, the cars look like the fuel they use] what do we save? A few cents per litre?

The difference [variances not factored] in m/joules/litre in commercial pump fuels from lowest octane to highest as far as energy goes is roughly 31mj/l for std, 34mj/l for premium, the difference in engine knock protection is huge. The need of engine protection as far as fuels goes is pretty much run what your engine demands, and if you are not sure go up a level, just for the insurance.

There is a cohort in some vintage bike ownership's that suggest premium fuel makes more soot or something and fouls plugs, to hear this rubbish extolled as informed comment grinds me,, blaming a fuel for a crap tune needs better explanation to me.

Cheers.
Last edited by hoodeng on Wed Jun 16, 2021 10:30 pm, edited 1 time in total.
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Re: Octane rating vs CR

Post by hoodeng »

Don't know how that line got through the later part of that post??
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