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Fuel grades for 3.9 engine

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MrToad Chris Coote
Brixham, Devon, UK   GBR
What grade of unleaded fuel have people found best on this relatively low stressed engine? all it says on the car is "unleaded only". Some engine management systems have better anti knock arrangements than others. Do these engines go better on super unleaded (97 octane)?

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davewhite Dave White
ARCATA, CA, USA   USA
The owners manual for my 2003 Plus 8 recommends 95 to 98 octane. I have always run premium.

Dave

GR08MOG Geoff Roberts
Lindfield, West Sussex, UK   GBR
In 22 years with a Plus 8 I used both. No knocking but premium 'felt' better, and with a relatively small mileage the cost difference was insignificant.

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Morgan13 Timothy Barlow
Vancouver, WA, USA   USA
I have a 4.0 in the +8 and I always use the best premium gas I can find in both my Morgans. Why would you want go cheap?

2013 M3W
Morgan stage ll motor kit

1998 +8
SS sport tuned exhaust

GoMoG Avatar
GoMoG Lorne Goldman
Cuenca, Azuay, Ecuador   ECU
1984 Morgan Plus 8 (+8) "The Phoenix"
1990 Morgan Plus 8 (+8) "Humpty Dumpty"
US pre-1996 Morgan Plus 8s have no knock sensors and have distributors rather than computerized ignition systems, so fuel quality and the timing it allows is VERY important. Though you can get away with kaka petrol on most crs since the early 1990s, don't try it with pre-1996 US +8s (or pre-2000 +8s elsewhere) unless you are into castrating your sports car. I would also examine your timing. People unavoidably and unknowingly adjust to a poorly performing car they own...it happens so incrementally.

Always use the best fuel you can find. I learned that when I opened up a US propane Plus 8. Propane is around 118 Octane. The inside of the engine looked better than new!!! Absolutely clean.

There are different things about octane levels,

1. UK uses the the RON rating system to rate Octane levels, So when the Morgan Manual says 95 to 98 Octane that means 91 to 93 at a US gas pump.

2. USA used the MON rating system to rate Octane levels, it is the average quality of the fuel being pumped

3. I believe Canada uses the MON system but the rating indicates the minimum quality of the fuel being pumped.

But these things change fast..especially with this weird fascination governments seem to have with ethanol in the last decade.

SJM1 Jan Morgan
Thousand Oaks, CA, USA   USA
I have been using 91 from my local California stations (USA, Shell, Arco, depending on price and convenience). No knock or ping with standard timing, 10.5:1 compression carburetor equipped 3.5 liter.
There is nothing to be gained by running more octane than this engine needs. If the fuel burns slowly enough to prevent knock or ping, that's enough.

However, if your engine has an octane sensor, running higher octane may result in the engine using more advance, making more power, if a higher knock threshold is detected by the engine. You may be able to increase power by as much as 5%.

My Morgan's engine has a distributor equipped with a magnetic trigger, so the timing remains stable.

What is just as important as octane rating is that the car is always parked with a full fuel tank. Half full (or less) fuel tanks are water traps for condensation, and high compression engines don't run well on water. Keeping the fuel tanks full ensures good performance, even when the car has been parked for a month or two. This also saves the fuel system and the carburetor (or fuel injectors and fuel pumps) from corrosion.

Another consideration is that modern pump gas doesn't have a long half life and can deteriorate over time, especially with water contamination. Don't forget to drive the car, and burn that old gas. Then fill the tank before you park it.

Propane cars run CLEAN, and when opened up, have virtually no deposits at all. That's not due to octane, but due to the propane fuel itself, and its clean burn characteristics.

GoMoG Avatar
GoMoG Lorne Goldman
Cuenca, Azuay, Ecuador   ECU
1984 Morgan Plus 8 (+8) "The Phoenix"
1990 Morgan Plus 8 (+8) "Humpty Dumpty"
Let me try and help you guys... it will be very easy to understand until I get to 1998.

OCTANE: Europeans and Americans measure octane differently...by a different calculation method. RON vs MON
GoMoG has a conversion table under the fueling index. In a nutshell, all Morgan manuals use British measures not American. So Dave's 95 to 98 means 87-90 in the USA..of course, that is merely a minimum fuel grade suggested.

PRIOR TO 1998

In a nutshell, the better the quality of the fuel, the cleaner and more potent the combustion it produces. Ergo, assuming the ignition is correctly adjusted to match the fuel quality, more power will be produced. You will be able to advance the ignition timing without pinging...aka knocking.

With our V8s, David Harcastle, estimated 7 extra bhp per degree of advance. As an example, when in Quebec, I live near a famous race circuit. Two adjacent petrol stations sell petrol at a minimum octane level of 96 MON aka 104 RON in England or 4 points better than the 5 Star Petrol made famous in the days of Morgan winning LeMans. With that fuel, I can set my timing at 11-12 degrees (yes 11-12!) BTDC.

However, when traveling in the USA, the best I can easily find is an average of 93 (MON) and worse on the backroads I prefer. So I have to retard the ignition to 6-8 BTDC..sometimes worse until I can rid of a bad tank of fuel By retarding the timing and using USA fuel, the same dyno shows me losing 26bhp.

So compared to NA, the fuel quality in Europe is decidedly wimpy. The fuel quality in England is very reliable if not super potent, though I DID find a little place selling 5 Star near Oulton Park! On the continent, off the autoroutes, it is much worse. It was so bad, that I swapped in a new British Army 4.6 Rover V8 engine with a low compression ratio (8.75) after third trip there. It was made to run on ..er.... anything. winking smiley

So there you have it, all pre-1998s should run on the best octane they can find. They should mark their timing wheel (white paint) for quick timing adjustments changes for when they are home, and detune for traveling. With the marks, it takes a second at any old garage with a timing light.

POST 1998

In 1998 Morgan starting shipping GEMS cars overseas (aka the USA and Downunder) These systems are modern and came standard, with LRs with computerized ignition coupled with knock sensors... So like all cars fitted similarly, (all modern cars) there was no possibility for easy timing changes as the cars do it automatically. In other words, they adjust to whatever you feed them. As well the US 1998-2000 GEMS Plus 8s have a very good bespoke fuel map..

After 2000, (with some codicils) all Plus 8s, UK and USA became stock GEMS. Ergo, no distributor you can adjust whihc, as explained, should have been no problem because of the sophisticated ignition system and knock sensors.

BUT Morgan in their wisdom, did not fit the knock sensors to any of their GEMS from 1998-2004. Of course, many have followed the emog advice and converted the cars by installing the missing knock sensors and using a Mark Evans bespoke chip/fuel map. Once sorted, this makes those cars reliable and potent.

What fuel to use? Assuming what? Without knock sensors, who knows? Even the LR guys who programmed the later cars couldn't tell you. (I asked them) And in America, there are two different fuel maps! So each post 1998 GEMS Plus 8 must be advised separately. Sadly, I am most familiar with 3.5 and 3.9 EFIs 1984-2000 (UK). The best GEMS fellow is/was David Poole and he, like so many of us old timers, has gone underground.

So for GEMS, I would simply use the best petrol quality to be found (with 98 in the Europe) or 93 in the USA and hope...or more fully describe your present system and mods.

BTW, in Ecuador the cost of a US gallon (aka 3.73 L) is $1.20 smiling bouncing smiley

Lorne

P.S. Just noticed your name Geoff. So good to "see" you!



Edited 1 time(s). Last edit at 2018-03-08 07:31 PM by GoMoG.

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Britmog Bruce M
Evergreen, CO, USA   USA
1964 Morgan 4/4 "Megan"
1994 Morgan Plus 8 (+8) "Maurice"
2013 Morgan 3 Wheeler "Olga"
Lorne,

Thank you for the details on Octane, most interesting. I have always used the highest octane available for my 1994 3.9 +8 as well as try to use the big brands. Do you use an additive such as ZDDP in pre 1998 +8s?

Bruce

GoMoG Avatar
GoMoG Lorne Goldman
Cuenca, Azuay, Ecuador   ECU
1984 Morgan Plus 8 (+8) "The Phoenix"
1990 Morgan Plus 8 (+8) "Humpty Dumpty"
In reply to # 12132 by Britmog Lorne,

Thank you for the details on Octane, most interesting. I have always used the highest octane available for my 1994 3.9 +8 as well as try to use the big brands. Do you use an additive such as ZDDP in pre 1998 +8s?

Bruce

I am going to answer you in a separate thread under that heading.

L.

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Britmog Bruce M
Evergreen, CO, USA   USA
1964 Morgan 4/4 "Megan"
1994 Morgan Plus 8 (+8) "Maurice"
2013 Morgan 3 Wheeler "Olga"
Lorne,

Saw your post of 4 March under Engine Oil Viscosity that answers my question.

Thanks,

Bruce

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