How to convert RM octane number to R+M/2

Well, Ive always heard that when all else fails -you should read the manual, so that's what I've done, and here's what I've found. The fuel my IH tractor (gas) is supposed to use is 93 octane rating (Research Method).

For years I've used regular and have not had any problems, except for one stuck valve after I switched from leaded to un-leaded fuel. The motor shop changed the valve seats to a type suitable for un-leaded fuel to correct for this, and I use Sta-Bil to treat the gas if it's not going to be used immediately.

All of the gas pumps at the filling stations have the octane number shown as by the R+M/2
method. This means that the octane number shown on the pumps is the average of the octane number as measured by the Research Method and the Octane number as measured by the Motor Method.

My options are octane ratings of 87, 89 and 92.

Which of these conforms to 93 octane (Research Method)?

Maybe I should leave well enough alone, but now that I've opened this can of worms I would like to know which fuel conforms with the 93 octane etc.

Your advice is needed.

If fuel test at 93 reserch method it will test about 8 to 10 points lower for using the Motor Method.
Since the Anti-Knock Index R+M/2 is a average of the two it would be
93 - 9 = 84
84 + 93 = 177
177 / 2 = 88.5
So 89 would be about right at sea level.
Now as you move above sea level you can use a lower octane so 87 may work for you.

Or you could use what works with no knocking.

(quoted from post at 20:28:45 10/07/12) Well, Ive always heard that when all else fails -you should read the manual, so that's what I've done, and here's what I've found. The fuel my IH tractor (gas) is supposed to use is 93 octane rating (Research Method).

For years I've used regular and have not had any problems, except for one stuck valve after I switched from leaded to un-leaded fuel. The motor shop changed the valve seats to a type suitable for un-leaded fuel to correct for this, and I use Sta-Bil to treat the gas if it's not going to be used immediately.

All of the gas pumps at the filling stations have the octane number shown as by the R+M/2
method. This means that the octane number shown on the pumps is the average of the octane number as measured by the Research Method and the Octane number as measured by the Motor Method.

My options are octane ratings of 87, 89 and 92.

Which of these conforms to 93 octane (Research Method)?

Maybe I should leave well enough alone, but now that I've opened this can of worms I would like to know which fuel conforms with the 93 octane etc.

Your advice is needed.

Click to expand...

I have been running octane engine since 1981. The spread between RON and MON varies and depends on the crude composition and the cut the refinery makes to create the gasolines. Back in the day they rated octane by reaserch only because that is the only engine they had. It strikes me as strange that soemeone would be recommending a 93 RON. Propbably just needs an editorial change. I don't see why an 87 pump octane wouldn't fit the bill. Now a days the darn low heat of combustion(BTU) from the ethanol, messes with engines, but they don't care because they get cheap octane. Actually the whole octane measurement system needs to be revised to allow for the down grading that we get from etahnol.

In the US a 91-92 RON is equal to a 82-83 MON, the average of which is 87 RON+MON/2 - what nearly all pumps display.


If your manual is recomonding 93 RON you should probably be using a 89 octane rating. However - if you have been using "regular" for years (probably 30 years) why the sudden concern?

What IH tractor recommends 93 RON? I thought most recommend 91 MON. Most of the issues I've seen with knock in these low compression engines has much more to do with improper timing, excess carbon build up and incorrect carb settings (too lean) than detonation due to low octane rating.

Two ASSUMPTIONS are being made here:

1. Motor method is ALWAYS lower than Research method. Is it?

2. Motor method is ALWAYS 10 points lower than Research method. Is it?

I could see a situation where your M octane is 87 and your R octane is 91, giving you a pump rating of 89.

I can also see a situation where your M octane is 90 and your R octane is 88, giving you a pump rating of 89.

In either situation, your "89 octane" gas is not suitable for the engine that specifies 93 octane by the Research method.

Also, the 10% ethanol used in most fuel these days gives a "false" high octane reading. There's more octane but fewer BTUs which means your air-fuel mixture runs lean.

John, help me with this please.

I took a trip several years ago out west and went through NM. and AZ. I noticed the octane was lower than the 87 I purchase here at 600 ft. I know the air is thinner at altitude, but not up on the relationship to thin air and a drop in octane. I would think that you need more fuel to get a given bang out of an internal combustion engine with the air being less dense and hence lighter.

On the trip, I remember a mountain I went over, I think around Alamogordo, NM. that was 8500 ft as I recall...just a big blob of rock out in the middle of nowhere. I was running a 4.7L Dodge truck and I mean that thing turned to pure unadulterated puke crossing that thing. I guess the computer eventually would have figured things out, but the crossing didn't take but about 30 minutes and I guess it didn't have time to get it's thinking done. I was coming back east so the fuel I had was the lower octane I bought out west.

Thanks,
Mark

We use the 93 in our 706 gassers or for that matter all of the gas tractors we have get the 93. This gas we get today is NOT the same as back then . And for you the machine shop got into your nickers on the valve seat as the valve seat were hardened from the factory .as for the lead in the gas Ammaco had lead free gas way back. The problem with running the 87 in the I H tractors is that the 87 burns way hotter at peak combustion , And it will plum melt pistons . Back in the day of these tractor were new gas was higher octane as reg. was 95 and hightest was 105+ . compression ratio's were in the 9.5 to 1 on a standard engine and on high performance engines or as we called them back them 4 bbl engines or dual quads or tri power the compression ratio's ranged from 10.5 and up . Pistons back then were forged not cast they would take heat better . Problems with gas tractors did not raise it's ugly head till in the late 80's . Back then there was only like 3-5 cents between reg . gas and hightest . Now we are talking 20 cents. Ok so it is 20 cents a gallon more and for us that is around 120-140 a year more for the 93 over the 87 . It takes three years of buying the 93 over the 87 to pay for one piston replacement not figuring in labor.The tractor will work all day long doing what it was designed to do running the 93 . The 93 burns cooler and longer then the flash burn on the 87 . Myself i have found that running the 93 in my S/MTA or Vernon's S/MTA we have less plug problems . Let me put it to you this way your not going to take a car or pick up with a factory STOCK engine of the day with a 9.5 to 1 compression ratio and run it today on 87 , you can squeak buy on the 93 IF you lose maybe 5 degrees of timing on the top end . I have played with engines now for over fifty years i know a little about them and what fuels they need. Back in the days of gas powered semi's like the 477-534 fords the 549 I H the big mack gas burners the white Mustangs they all ran the 105 hightest and they were low compression engines BUT working engines and they got HOT same as a tractor engine as it is not like a car engine that is just looping along . A tractor engine working is running at or darn close to full throttle or i should say the butterfly is over half open to wide open tiering to maintain the perset throttle setting . A gas engine gets hotter then a diesel . Go out on a cold morning and start your car then get out and open the hood and touch the exhaust manifold , Do that on a diesel . Put a pyrometer on a gasser and go make a round plowing and see how hot she gets then do the same with a diesel . all that heat has to some place . and it is suppose to go down thru the pistons and transfer into the cylinder walls , Today with what we get in the rebuild kits for pistons that heat is not carried away thru the cast piston fast enough like it did when the pistons were forged. Now we have to have gas that burns cleaner so now it burns hotter and faster. The old M's -450's were long stroke slow turn the new 4-6 cylinder engines are short stroke fast turn higher compression make more heat they can not transfer that heat fast enough to the cylinder walls . I have a collection of Clevite pistons out of a 706 due to 87 octane gas. Twice due to the fuel supplier bringing use the 87 gas when the 93 was ordered and once due to my friend's kid nit thinking and bought 87 at the gas station one time while we were at the away fields. I fought with one customer for a year over his 460 over gas and oil issues because he was so cheap that he would even spend two hours looking thru buckets of rust bolts and when i found one he would wire brush it and rethread it then wonder whay it took so long to make a repair to save 20 cents on a new bolt . He use the 460 gasser on a manure spreader twice a day it ran great started great Going down hill , but when he made the turn to come back up hill with half a load on she would start missing and start pulling down and start to seize up and quit . Then it would not turn over . let it set for fifteen twenty min and it would start up and it might make the end of the long field and maybe not . And about every month it would eat and exhaust valve . Because he would not buy the low ash oil . He was blaming everybody for his engine problems . Two I H dealers and two other guys , then he started on me . After the third time on the valve jobs two of which i ate and at that time i showed him the scored liners and burnt piston tops we put in 6 new pistons with new rings and were able to lightly hone off the aluim. piston deposits Did the valves over and i changed the oil to low ash and gave him a five gallon bucket to go . Put in a full tank of 93 and set him on his way . And i would go over each day and check the tractor and when it got low on fuel i made him go to the little gas station and fill it with the 93 and if it needed oil i made sure that he put in the low ash did this for three months and told him that if it went down after three months the next bill was on him. That was twelve years ago and it is still running each and every day . Oh and now he has a bolt bin with new bolts .

The computer would not have figgered it out. At that high of altitude it needs a different 'chipset' to adjust for less oxygen and fuel to air ratio.

I also ran the Green Machines for 20+ years. I had a great time with them. Did you have the cetane engine too?

(quoted from post at 06:50:00 10/08/12) Two ASSUMPTIONS are being made here:

1. Motor method is ALWAYS lower than Research method. Is it?

2. Motor method is ALWAYS 10 points lower than Research method. Is it?

I could see a situation where your M octane is 87 and your R octane is 91, giving you a pump rating of 89.

I can also see a situation where your M octane is 90 and your R octane is 88, giving you a pump rating of 89.

In either situation, your "89 octane" gas is not suitable for the engine that specifies 93 octane by the Research method.

Also, the 10% ethanol used in most fuel these days gives a "false" high octane reading. There's more octane but fewer BTUs which means your air-fuel mixture runs lean.

Click to expand...

Having run both RON & MON octanes for 30 years, I can honestly say that I have only ever encountered MON octanes that were higher than RON's about 3 times.

The higher the MON rating the better. MON is actually a better than RON as a measurement of octane rating for these tractor engines as it measured used high heat and greater engine lug - the exact situations a tractor faces with hour after our of lugging. The manufacturer would have used MON for its rating had it existed as the standard when these were built.

Texasmark1
Think about why we would buy higher octane fuel. It is to deter knock in higher compression engines.
The air out west is less dense so less air is pulled into the cylinder on each cycle. Less air means less fuel and the compression will be lower when the pistion is at TDC. This deters knock by it self so we can now cut the fuel octane.
This is why you feel your truck runs like crap out there. It is drawing in less air leading to less compression leading to reduced power.

The problem comes in when a carbureted car comes back east with west coast fuel. We are now putting in more air leading to higher compression with less octane fuel. This can cause knock and could damage the engine if run for long periods like that.

Another problem comes with turbocharged vehicles out west. They are craming air into the cylinders but even with their best gas the motor can never see full power because of the lower octane fuel.

Appreciate the response and makes sense.

I don't intend to argue the point with anyone, but I will "comment" that I sure don't see my 5.9:1 CR SMTA needing 93 octane gasoline! Everyone just use whatever works for you.