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Compression vs Octane

6K views 49 replies 13 participants last post by  Mario Torchio 
#1 · (Edited)
After about 4 months of searching and research I finally found a compression vs Octane chart that gives you an easy answer as to the optimum Octane rating for your specific bike. All you have to know is your compression ratio. I made the adjustment using an octane booster to the correct rating, and wow what a difference - clear crisp throttle response and more power. Here it is:

Comp Octane Brake Thermal Efficiency
Ratio Requirement ( Full Throttle )
5:1 72 -
6:1 81 25 %
7:1 87 28 %
8:1 92 30 %
9:1 96 32 %
10:1 100 33 %
11:1 104 34 %
12:1 108 35 %
 
#2 ·
I finally found a compression vs Octane chart that gives you an easy answer as to the optimum Octane rating for your specific bike. All you have to know is your compression ratio.


Comp Octane Brake Thermal Efficiency
Ratio Requirement ( Full Throttle )
5:1 72 -
6:1 81 25 %
7:1 87 28 %
8:1 92 30 %
9:1 96 32 %
10:1 100 33 %
11:1 104 34 %
12:1 108 35 %
Could I correctly guess that those octane #'s match up with the RON measurement and not the (R+M)/2 method?
 
#4 ·
After about 4 months of searching and research I finally found a compression vs Octane chart that gives you an easy answer as to the optimum Octane rating for your specific bike. All you have to know is your compression ratio. I made the adjustment using an octane booster to the correct rating, and wow what a difference - clear crisp throttle response and more power. Here it is:

Comp Octane Brake Thermal Efficiency
Ratio Requirement ( Full Throttle )
5:1 72 -
6:1 81 25 %
7:1 87 28 %
8:1 92 30 %
9:1 96 32 %
10:1 100 33 %
11:1 104 34 %
12:1 108 35 %

That would mean my Suzuki Intruders (805cc) need 100 octane gas, which is not carried by any stations. I think your figures are for fuels sold in Europe which use the RON method.
 
#6 · (Edited)
That chart might look a little better with a fixed font and an some HTML tags.

I'm pretty sure those percentage #'s are thermal efficiency (controlled explosion in the combustion chamber) at wide open throttle listed for each compression ratio but it doesn't take into consideration the air/fuel ratio.

I don't ride around at wide open throttle...like...never...so that chart doesn't apply to me.

This rule of thumb would be a simpler predictor of necessary octane:
up to 170PSI - 87 Octane
170 - 185 - 90 Octane

According to the OP's copied and pasted chart one of my bikes needs 102 octane to run and operate most efficiently. One of the problems with that is that the front wheel would never touch the ground through at least the first 3 1/2 gears at wide open throttle :grin2: .

One last point to consider (or not)...the octane booster in a bottle that you can buy off the shelf anywhere...when it reads out to say it increases octane 3 or 4 points it actually means that it raises .3 or .4 tenths of 1 octane.
 
#7 ·
One oz per gallon of turbo 108 will raise it by 30 total points or "3 consumer octane points". I believe this is the (R+M)/2 commonly used in the USA. The Turbo 108 Racing octane booster has a chart on the back of the bottle which says how many oz per gallon to raise it more than that. I'll follow up with those specific details. It has nothing to do with Shadows being "low compression engines" which is not correct anyway - it's about the science of getting the octane rating to best match the compression ratio(whatever it is). Each series of Shadows had different compression ratios. The first two years are high compression high performance engines with 9.0 to 1 ratios. Then from 87 - ? It went to 8.0 to 1, i believe. and after 93ish? it went down again... So you have to do your own homework here. Just google for it, specifically the year and size of your shadow to find the compression ratio. Then go to the gas station and see what octane the high test is. Probably 91 or 93 in the USA. Then using math, the size of your gas tank, and how much gas you need to fill the bike - you will be able to determine how many oz of octane boost to put in. Probably 1 to 2 oz per gallon. You will be pleasantly surprised, at the very least, to feel the way the manufacturer actually designed your bike to run!
 
#8 ·
My '99 750 ACE has 9.0:1 compression and Honda calls for "Any gasoline with a pump octane number of 86 or higher"
IIRC when the VT750s went to single carb the bumped up the compression to 9.5:1 and I think that was held over for the injected models too. I believe they call for 87 octane.

I did at one time have a bike with 12.5:1 compression that called for 100 octane leaded gasoline, back when 96 was the best I could get locally and I ran it on that with 15% toluene and it was happy enough.

Don't believe everything that is written on a can.
 
#11 ·
Somewhat

"somewhat" being the key word. While it's correct that the ecm adjusts so the bike can use a lower octane fuel such as 86, you would not be getting it's optimum efficiency. You would need to adjust the octane to about 102 in this case. The difference would be crisper throttle response and more power.
 
#16 · (Edited)
How's about actually get out and do it instead of telling people who have done it that it must be BS?

Reminds me very much of the whole "Dark Side" stuff - those who say it cannot be done go to great lengths to ignore those who've found it works well for them :)


I also have no clue and do not claim that my vehicles have gained torque or HP. I do claim I've gained mileage (and have done tests for over a year in some cases) and smoother running in most circumstances. For example I'd often get pinking coming up to my place, reduced with a change of fuel (also reduced by kicking her down another cog at the last intersection).

EDIT to add : The AA, MTA and other respected groups have tested and proven that yes, a higher octane can result in less fuel used, however it is not always the case and not always cheaper. Sometimes it can be based more on additives in the fuel than the octane itself. But to tell if it does or doesn't work for your vehicle, test it for yourself and see. Don't take the word of some random person on the net - check what the motoring organisations say (who're paid to properly test products and have no brand loyalty) and test it for yourself with your vehicle under your normal conditions. If you get a good enough improvement keep it up, if not then stop.
 
#17 ·
I find no ethanol premium 91 octane here gives me consistently better fuel mileage than 10% Ethanol 87 octane, but only a couple of percent better and it costs about 20% more. So the financial returns from the switch are not to be had here in Ontario, Canada, at least not with my particular Honda Shadow.

Ethanol when it burns produces less energy than typical gasoline and some octane boosting additives like my previously mentioned toluene actually produce more energy when burned. So even to this former racer and kitchen table amateur scientist it does not strike me as necessarily entirely BS, but I'd be interested in how they blend gasoline in NZ and what the chemical break down is of the referenced octane booster in a can. (-:
 
#18 ·
For a late model Honda Shadow, then it is correct that you wouldn't need an octane booster because the compression ratio is such that the "high test", which generally has an octane rating of 91-93, is actually ideal. The difference between the regular and high test fuel, which is there, may not be as noticeable on a bike with this compression ratio.

To kiwirider, and for example, your cbx and 700 had higher compression ratios then your 750 did, so that's why the difference was more noticeable on the older bikes.

But for those of us who have bikes with an approximate 8.0 to 1 or higher compression ratio then you will need an octane booster, mixed at the appropriate ratio, to get the most out of your engine because even high test isn't high enough octane for your bike. For example My 1986 Honda Shadow 1100, according to the owners manual, requires a minimum of 91 octane. That's the minimum! Based on the compression ratio, 9 to 1, my ideal octane is 96. When I mix the octane booster correctly and get the gasoline up to 96 octane then I feel a noticeable difference! This is the way Honda designed the bike to run based on the compression ratio.

As far as the MMT causing harmful deposits goes, it's likely not an issue if you use it with high test gasoline(which you would do anyway) which has better detergents. All this being said, MMT is toxic. So you don't want to get it on your skin, eyes, or breath it in. If this isn't your "cup of tea" then that's fine... But some people might like it!
 
#28 ·
I found the NASA dyno test interesting. Thanks.

The BMW e30 is a pretty old tech from around the same general design period as the Shadow engines.
I'm thinking the sample gasolines with the exception E15 probably had the same 10% Ethanol content, hence similar power output.
I understand that due to the corn lobby, most generally available US gasoline is 10% Ethanol, with 15% introduced for "flex fuel" vehicles.
It is noted in the dyno test that the E15 would probably require larger injectors to match the power which are not permitted in this "Spec" race engine, this really points out the reduced energy of alcohol blended fuels.

This would be different from my area in Ontario, Canada, where 87 octane typically has up to 10% Ethanol, 89 has up to 5% and 91 has none, this is normally the grade breakdown at most name brand stations.

This probably accounts for my observations of differing fuel fuel mileage.

I have little doubt that there would be a power increase on the dyno with appropriate jetting to take advantage of the increased energy density of the higher rated fuel. Note that I don't attribute this possible increase to the anti-knock function of higher octane rating, but to the varying constituents of the particular fuel blend.

Perhaps NZ fuel grade formulas are similar and I have no idea what fuel looks like out in BC, but note that all your references of "no advantage" are US ones.
 
#30 ·
...

This would be different from my area in Ontario, Canada, where 87 octane typically has up to 10% Ethanol, 89 has up to 5% and 91 has none, this is normally the grade breakdown at most name brand stations.

This probably accounts for my observations of differing fuel fuel mileage
Ethanol (E10) use in the US has dropped with only 7 states mandating its use. In Canada the mandate is federal with most provinces mandating minimum E5 in regular gas but in actuality station use about E7. Ontario has proposed a minimum E10 for regular gas starting 2020. If your recommended fuel is 86 AKI, as mine is, then using an 87 with ethanol will meet the requirement. Note that as our engines age, or by using fuel with low additives you may experience some pinging and if that occurs you should use a higher octane fuel.

Note I have been using E10 in motorcycles and cars since 1985 (is it really 34 years?) and have had no engine or fuel delivery problems. My 1978 Goldwing has 380k km and my 1995 VT1100 has 80k km. The spark plugs in my VT1100 always look new when scheduled to replace. I use Top Tier 87 AKI exclusively.

G.
 
#34 ·
Thank you all for participating. Please note that when the manufacturer recommends an octane level - that is the minimum! I basically laid out the science in the last post that I made, but here it is again for those who want the take-a-way:

For a late model Honda Shadow, then it is correct that you wouldn't need an octane booster because the compression ratio is such that the "high test", which generally has an octane rating of 91-93, is actually ideal. The difference between the regular and high test fuel, which is there, may not be as noticeable on a bike with this compression ratio(comparatively low).

But for those of us who have bikes with an approximate 8.0 to 1 or higher compression ratio then you will need an octane booster, mixed at the appropriate ratio, to get the most out of your engine because even high test isn't high enough octane for your bike. For example My 1986 Honda Shadow 1100, according to the owners manual, requires a minimum of 91 octane. That's the minimum! Based on the compression ratio, 9 to 1, my ideal octane is 96. When I mix the octane booster correctly and get the gasoline up to 96 octane then I feel a noticeable difference! This is the way Honda designed the bike to run based on the compression ratio - For those of you who doubt this then I challenge you to try it for yourselves and post the results here.

The fine print. All this being said, MMT(octane booster) is toxic. So you don't want to get it on your skin, eyes, or breath it in. If this isn't your "cup of tea" then that's fine... But some people might like it!
 
#36 ·
No, you are still misinformed. An 8 to 1 compression ratio is no where near high compression. If your engine is not pinging, you do not need an octane booster. your seat of the pants tester needs calibration. I had an 82 750 Magna that ran fine on 87 octane methanol and didn't run any better on higher octane gas(which does not add any horsepower). You are wasting your money on octane boosters, why don't you just send your extra cash to me.
 
#35 ·
Also if you are having trouble figuring out how much octane booster to put in your bike then reply here with the year, make, model and size of your bike along with the octane level of the high test that you can get in your area. I will do the math and tell you how much booster to put in!
 
#37 ·
8:1 high compression? Ummmm...... most muscle cars ran compression ratios of 9:1 or even 10:1 on regular pump gas and mazdas SKYACTIV-G engine runs and astounding 14:1 compression ratio on pump gas. I highly doubt you v twin needs octane booster but if it makes you feel better keep on running it. Its not like it'll hurt anything except your wallet
 
#38 ·
Like the old lady said in the commercial "Where's the beef?", I won't be convinced until I see the dyno slips showing how higher octane gas will make any of my vehicles produce more hp and torque.

Seat of the pants 'feelings' are no indicators of anything except what a person wants to feel. How many times have we seen guys hacksaw their exhaust systems, run shorty straight pipes, then claim how their bike is so much quicker now? What they hear is a hell of a lot of noise, and somehow figure that it just must be quicker, so they swear they can actually feel how much faster it is.
 
#39 ·
Maybe I missed something, but which octane method was used in the compression ratio/octane table in the original post… RON or (R+M)/2?
The octane equivalence table I use matches 90REC {(R+M)/2} with 93.5 RON, and 87 {(R+M)/2} with 91 RON. I find myself in agreement with @Chuck_Michigan that the only really scientific test of performance with any given bike is the dyno, and will be specific to that bike and atmospheric density altitude when the test is run.
 
#43 ·
As a gasoline blender for a major oil company, you are correct JPR1968. High Octane is for high cylinder pressures to prevent pre-ignition. If you run high octane in a low compression engine it’s possible you may actually lose power, it would be like retarding your ignition timing. My 03 ACE runs great on good ole 87 and that’s with the 4 deg timing mod.
 
#46 ·
#47 ·
@Chuck_Michigan, that's funny about the magnets. People were putting cow magnets on their fuel line to help the gas molecules line up and they would burn better for mileage.

Crazy thing is you can still buy that crap. =

"High octane performance with lower octane fuel".

"And it can increase the life of Engine".

Those ads remind me of my teenage years, when we'd get some $50 car, bring it home, scrub and wax it from top to bottom, inside and out, slather brake fluid all over the tires, jam as many spacers into the front coil springs as we could to lift the front of the car as high as possible, and then finishing it off by removing the air cleaner filter element (which gave us untold amounts of extra horsepower), and then cruising up and down the main road in town. If money was no object, we'd buy a K&N air filter and a set of chrome rocker covers, chrome oil cap, and chrome air cleaner cover which would immediately put us into the 'elite' crowd of kids with the hottest cars.

Ah, the good old days... :-D
 
#48 ·
Exactly!!!
And some of the cool guys would advance their distributor timing so the engine would lope at idle and sound like a bad bad hot rod in the Dairy Queen Friday night.
 
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