Some thoughts on the 2M, 3.91 diff, 5M dizzy & fuel economy

[jbautist2]

As we all know, the M series engines are quite of gas guzzlers, and so I'd like to share the things I did in an attempt to decrease the fuel consumption of the 2M in my daily-driven ’69 MS55 3-speed manual without doing too much modifications – I wanted to keep my car as close to stock as possible.

First thing I did was to overhaul the carburetor, which stopped the spark plug fouling and greatly improved my fuel mileage, jumping from 4.7 to 8.5 mpg. Most of my trips are stop-and-go, low-speed city driving (15-20 mph) and I use my air-conditioner 95% of the time (can’t drive without A/C). Highway mileage also jumped from 12 mpg to 15 mpg.

Idle speed is also a significant factor, especially in stop-and-go driving. Increasing the idle speed from 550 to 750-900 increased my fuel consumption by about 15-20%, and caused difficulties in gear-shifting. I also found that my 2M does not like to be set to anything other than stock specs. Well it will appear to run properly, but it won’t be long before it starts to have problems. The engine will run smoothly with the mixture screw set to lean best idle or 50 RPM lean drop (engine fully warmed up of course), but driving on a hot day in heavy traffic, the engine would idle rough, very prone to stalling. This is because when the engine gets hot on a summer’s day in heavy traffic, the hot-idle compensator (mounted next to the carb fuel inlet) opens to lean out the supposedly over-rich mixture caused by elevated underhood temps, to prevent the engine from stalling. However, with the idle mixture already set to the leanest side of smooth idle, there is little to no more leeway to go leaner. Thus, when the hot-idle compensator opens up, the mixture becomes too lean, causing the engine to idle rough and eventually stall.

Leaning out the carb main jet does not help fuel economy, in fact it can cause drivability problems. A 1.06 main jet was spec’d for the 2M in 1966, 1.08 in 1967 and 1.10mm in 1970. The biggest main jet size that can be run on the stock 2M carb is 1.15, but it is actually slightly too rich, slowing down the acceleration and increasing the fuel consumption by about 5-10%. 1.20 is already way too rich, causing a very annoying off-idle stumble/bog. I personally use a 1.10mm on my 2M carb to be slightly on the rich side, and to compensate for today’s ethanol-laced fuel. 1.05 can be used without any problems (but you may have to retard the timing a bit) while 1.00 is already too lean (the engine will backfire from the carb at mid to high RPM!)

It was also easier to detect engine problems (especially in the fuel system) when everything is set to factory specs. So all carb adjustments in my 2M are set to values called for by the 2M Engine Repair Manual – idle speed at 550 RPM, idle mixture screw set to give maximum idle vacuum, choke thermostat set to fully close the choke valve at 77 deg. F ambient temperature, and fast idle set to 2500 RPM. If the engine fails to run properly at the factory settings, it is for sure that it has a problem. Even the engine's inability to idle properly within 1/2 turn from the base setting of the mixture screw (2 complete turns out from full in) indicates a problem.

Second thing was the conversion to a 5M distributor. My original dizzy was already worn out, its vacuum advance was dead. Having a functioning vacuum advance helps fuel economy by advancing the ignition timing further at cruise/light-load conditions. A replacement for my 2M was no longer available, so I decided to convert to a 5M dizzy. I used a brand-new 5M points-type dizzy, and set the ignition timing to 13 degrees BTDC. The engine became more responsive and only required a minimal amount of throttle to maintain highway speeds. However, it was pinging on every uphill grade at 40-45 mph, and pinged very loud at 65-70 mph on the flat, so I pulled it back to 10 degrees BTDC (1966 spec for the 2M). The pinging was gone, and after a hundred miles, my city driving went up from 8.5 to 11 MPG, while my highway mileage went up from 15 to 18 mpg.

Third thing I fiddled with was the gearing. I love the column-shift and I do not want to deal with anything more than 3 forward gears so I kept the 3-speed manual transmission in place. With that out of the equation, my only option to get the revs down (and hopefully get even higher fuel mileage) was to replace the original 4.88 diff with a higher-geared (numerically lower) differential. I found out that the diff centre of the ’77-’80 Cressidas are of the same length and bolt pattern as the ’68 to ’71 Crowns. I eventually found one, 3.91 ratio, pulled from a ’79 RX30 Cressida. It was bolt-on swap, finished in about 4 hours. The only modification necessary was to replace the original driveshaft flange with that from the Cressida (the bolt patterns of the flanges are different). To compensate for the resulting speedo error caused by the diff ratio change, I installed a speedo correction adapter (1.2844 ratio).

With the 3.91 diff... at 65 mph, the engine is merely doing 3200 RPM, as opposed to 4000 with the 4.88 diff. My highway mileage went up from 18 mpg at 45-55 mph to 21 mpg at 60-65 mph! My city driving mileage, however, went down from 11 mpg to 9.5 mpg. That is because, with the 3.91 diff, more revs and clutch slip are needed to get the car moving from a dead stop, more so with the A/C on. Gear-shift points also moved up; with the 4.88 diff, my 1st to 2nd shift was at 10-12 mph, and the 2nd to 3rd shift was at 20-25 mph. With the 3.91 diff, 1st to 2nd shift is at 15-20 mph, 2nd to 3rd shift is at 30-35 mph. My maximum cruising speed went up from 70 to 80 mph, without making the engine scream.

Slight pinging was present at 55-60 mph and 80 mph on the flat with the 3.91 diff. In an attempt to compensate for this, I retarded the timing to 5 BTDC. It was no longer pinging on the flat, but on an uphill, the engine would still ping at 55-60 mph. I retarded the ignition timing to TDC (0 deg.) but it was still slightly pinging on uphills even at that setting, and would backfire from the carb if I try to restart it with the engine hot after short parking (about 10 to 20 minutes). The only time it won’t ping is when the timing is retarded to 5 ATDC (AFTER top dead center), but the carb backfire became worse and acceleration became very dismal. However, with each 5-degree retard in timing that I made, I found it easier to get the car moving, with just little revs and minimal clutch slip. I eventually discovered that the 5M vacuum advance is too sensitive, thus it is incompatible with the 2M. It was not readily apparent with the 4.88 diff since with the very low gearing, the engine was able to tolerate a lot more advance.

It was not the case with the 3.91 diff. When I move off from a dead stop, I open the throttle slightly, sending a ported vacuum signal to the vacuum advance, which then pulls the timing by about 15 degrees. However, when I let the clutch out, engine vacuum drops, releasing the vacuum advance and losing all the 15 degrees that was added to the initial timing, which causes the revs to drop big-time. More revs is therefore needed to prevent the engine from stalling and, eventually, get the car moving. That wouldn’t be the case if I had a 3.34 ratio 1st gear, or better yet, the 3.58 or 3.67 ratio 1st gear of the 4-speed gearboxes.

However, I want to keep the 3-speed box and no longer wanted to switch back to the 4.88 diff… so as a solution to my above problem, I permanently disconnected the vacuum advance and plugged the vacuum port on the carb. I then advanced the initial ignition timing to 15 degrees BTDC, which brings the mechanical advance curve very close to that of the stock 2M distributor when set to 13 deg. BTDC. With no vacuum to influence the timing, it now takes little revs and minimal clutch slip to get the car moving. The now close-to-stock mechanical advance curve resulted in more torque at low RPMs and faster acceleration. Pinging has been completely eliminated in all speed ranges, even when the pedal is floored on uphills.

The elimination of vacuum advance required more pedal to maintain the same speed and further dropped my fuel mileage from 9.5 mpg to 8.3 mpg city, and 21 to 17 mpg highway… but the higher initial timing made up for the quicker acceleration and more low-end torque. Setting the spark plug gap to the 1970 spec of 0.032" (instead of the 1966 spec of 0.028") also contributed to better acceleration.

My advice is, if you want to run a higher-geared (numerically lower) diff behind the 2M, the minimum torque multiplication in low gear for decent performance and fuel economy in city driving is 13.38, especially if you plan to run A/C. With an early 3-speed manual (3.059 first gear), that is equal to a 4.375 ratio diff. With a 4-speed manual, you could go as high-geared as 3.73 ratio diff. If you don’t plan to run A/C, you might be able to get by with a 4.11 behind the 3-speed, or a 3.55 behind the 4-speed, while still maintaining decent off-the-line performance.

-Jonathan Bautista
Manila, Philippines

[jbautist2]

Here is a graph that I made, showing the mechanical advance curves of the 2M, 4M and 5M distributors. "2M 1967" shows the advance curve for the stock 2M distributor when set to the 1967 spec of 10 degrees BTDC; "2M 1970" shows the curve for the stock dizzy when set to the 1970 spec of 13 degrees BTDC; "2M SMOG" shows the curve for the emission-controlled 2M when set to the 1970 spec of TDC (0 deg.); while the "4M/5M A" and "4M/5M B" show the advance curves of the 5M distributor when set to 13 and 15 degrees BTDC, respectively.

2M Ignition Curves.jpg

You can see that the 5M distributor is under-avanced (when compared to the 2M dizzy) at the lower speed ranges (possibly to lower the NOx emissions at eliminate pinging at low RPM), but its curve runs parallel with the 2M dizzy from 2500 to 4400 RPM. The 2M dizzy reaches it maximum advance at 4400 RPM, while the 5M dizzy continues to advance further by 3 degrees until it reaches its maximum advance at 6000 RPM. Even though their curves are different, the maximum amount of advance for both distributors are equal, being at 20 crankshaft degrees.

I personally used this spreadsheet heavily in my analysis of the different advance curves at different initial timing settings, to determine the RPM points at which my 2M is pinging; reducing the amount of guesswork and road tests needed to perfect the timing setting. Personally I used 15 degrees BTDC for the 5M dizzy in my 2M since it is very close to the 1970 spec curve. I do not worry about the possible ping that might occur above 4400 RPM, since with the 3.91 diff I never hit anything above 4000 RPM (which is equivalent to 80 miles per hour). On the lower gears I rarely go above 2500 RPM.

["Z" UTE]

Mate you certainly have documented this fuel consumption issue very well.

I had a new MS55 in 1971, I serviced it every 3,000 miles, oil change and filter as well as grease all the suspension nipples. Every 6 months I would warm the engine and adjust the valved clearances 8 and 6 thou, still remember those numbers. I would regularly cover 750 miles on weekends and the Crown would return 30 to 32mpg on the highway @ 60-70mph. Never bothered checking the city consumption, but it was not horrible.

Have you done a compression test lately on the old 2M, excessive blowby will result in higher fuel consumption. The quality of fuel has a huge impact on knocking/pinging. A water methanol injection system helped us up the hills on very hot days. That was also a blast on really cold nights! felt like the engine had just hit the mega like a 2 stroke bike.


cheers Chuck.

[jbautist2]

Mate you certainly have documented this fuel consumption issue very well.

I had a new MS55 in 1971, I serviced it every 3,000 miles, oil change and filter as well as grease all the suspension nipples. Every 6 months I would warm the engine and adjust the valved clearances 8 and 6 thou, still remember those numbers. I would regularly cover 750 miles on weekends and the Crown would return 30 to 32mpg on the highway @ 60-70mph. Never bothered checking the city consumption, but it was not horrible.

Have you done a compression test lately on the old 2M, excessive blowby will result in higher fuel consumption. The quality of fuel has a huge impact on knocking/pinging. A water methanol injection system helped us up the hills on very hot days. That was also a blast on really cold nights! felt like the engine had just hit the mega like a 2 stroke bike.


cheers Chuck.

Recently my 2M has began blowing moderate amounts of blue smoke out of the exhaust as well as the road draft tube, so I'm sure that the compression is already low. It would even blow large amounts when on the highway, but after about 10 miles of driving at 65 mph, the blue smoke is completely gone. It will begin to show up again after 10 miles of low-speed city driving. I'll get that sorted as the engine is already scheduled to undergo a complete overhaul next year. But even before it began blowing out those blue clouds, the fuel consumption was pretty much the same.

My A/C compressor is a Denso 10PA17C which is quite a heavy load on the 2M (I should have went for a 10PA15C instead) so that adds significantly to the consumption. Without A/C, my city driving was 14 mpg, highway 22 mpg. The figures dropped by 4-5 mpg when I added A/C.

By the way, all of my MPG figures are in U.S. measure so they would be slightly higher when in Imperial measure. My 22 mpg highway is equivalent to 26 mpg Imperial. Not bad considering I have a 4.88 diff back then and no overdrive, so the engine was doing 4000 RPM at 65 mph!

95 RON fuel is what's commonly available here in our area so I had to tune the engine for that octane rating. 97 and 100 RON fuel are available but I still have to drive several miles to the nearest station that has them. I could probably advance the initial timing by 5 degrees more if I used those fuels. That would lower my consumption even further.

Having a functioning vacuum advance was really a big help in lowering my fuel consumption. Too bad I could not use it with the 3.91 diff so my figures are back to as if I still had the 4.88 in the rear.