The Stovebolt Site Forums The Shop Area Engine Shop Minimum RPM question?

Last weekend I entered my 41 Chevy GI in a "slow speed race" designed primarily for antique tractors. Unfortunately I got beat by a 1956 JD 420 that the owner was able to get to idle at an incredibly low RPM. Being very "green" to this type of race, I showed up unprepared as my engine was screaming in comparison. With a crawl ratio of 93.608 I think I still have a chance. So here is the question...if I can get my 54 235 engine to idle real low, will there be any ramifications? Will the oil pump move enough oil, or will I harm the bearings? The duration of the "race" is very brief and there is little load on the drivetrain. Any tricks to "tune" an engine for this type of match? I need your all's help with this as I have already told this years winner (who is a real good guy BTW) that I will be back for a rematch next year. The trash talk has already been flung, and I don't like the taste of crow...help me out guys!

Here is a video of the race:


Paul

put on really small tires!

The course looks to be down hill which will help keep an engine running at a very low RPM more so than climbing that same slight hill.

I wouldn't worry about the engine idling along without any real load and for such a short time.

Back out the idle speed screw on the throttle linkage to lower idle speed. You may need to provide more fuel with the idle mixture screw. Seems backwards but with the lower air velocity through the carb it has a harder time picking up that fuel, so opening the mixture screw helps get the necessary fuel in there.

After that smaller tires as mentioned and or some different gears are about all you have to work with. Idle speed then being the only practical method.

Good luck,

I spoiled my chances on winning a slowest truck race when I changed my 1954 Suburban's Ring&Pinion gears to 3.55 (from 3.90). The next best "standard" equipment 1/2 ton truck combination would have been the SM420 in an earlier 4.11 R&P.

It had been almost unbeatable with its SM420 transmission and the very slow/smooth idling engine. It moved slower than Grannie.

Paul has the extra advantage of a transfer case and the gear reduction for low range.

hmmm, maybe my COE would be a good contender (in the 2-ton class)?

Paul's ratio of 93.6:1 is each transmission low ratio multiplied together X the axle ratio.

I was thinking I might have a chance with main and auxiliary transmission in low low at 12.84 X axle ratio, but Paul would be about twice as slow with same tire size and engine speed.

I don't think your COE will be very competitive either.

What kind of engine RPM are we talking about?

I'm guessing you can get down to just a couple hundred, I know my GMC will idle real slow, have to check and see just how slow

At some point of throttle opening, rpm, gear ratio and load the engine will not rotate smoothly throughout the 720 degree cycle, it will tend to stop @ TDC.
Avoid this.

I'm wondering if you adjust the clutch so it slips slighty at full engagement? That would allow you to reduce the engine's "effective" RPM to the drive train. A slipping clutch for that short of distance and with no load would not hurt it much.

SimS

There was a slow competition at the reunion in Kansas City. Lightholders Dad finished 2nd in his COE.

Gentlemen,

Thanks for the ideas, I will try to incorporated this collective knowledge in next years rematch. More brains on this is better than one so the help is much appreciated. Here is my synthesis.

1)Smaller tires/ wheels: this one is out as it is too expensive and it would kill my already low top end speed
2)Lowering idle, adjusting fuel mixture: I was going to investigate just how low the engine would idle. I figure it should be able to go as low as 200 rpm? Fuel mixture is something I had not considered.
3)Engine oiling: good to hear that you guys think the engine would still be adequately oiled at 200 RPM. Jerry, Dave, do you engine gurus concur, as this is my greatest concern?
4) Adjusting the clutch: that is a good one, and not too difficult to do on "race day". Part of the strategy is to engage the clutch without increasing RPM.

On this 75 year old gal, all of the drivetrain is bone stock just as she rolled off the Atlanta assembly line in February 1941. I am one of those purist guys so no alterations/modifications are on the table...at least not on this truck. Here are some stats. Crawl ratio- 7.23:1 (first gear) x 1.94:1 (transfercase low range) x 6.67:1 (differential ratio)= 93.61.

Ah yes, fun with trucks...life is good.

Paul

Paul. I've got an idea. Refill your crankcase with some of that 0-20 synthetic oil that goes into our Crosstreks. Should help that engine spin real slow. Yes ?
Or maybe something even lighter.

We had a nearly identical looking army truck with a 5000 gallon water tank for watering the pull truck/tractor track that my friend owned. We put a second 2:1 transfer case in the drive line so it could creep slow enough to soak the track in one pass and thus avoid making tire ruts while allowing the rpm to be high enough to handle the 40,000 pound load. Worked good and empty with minimum rpm it would really creep slow.

200 rpm + 0W20 oil = seized crank

1954 235....full pressure oiling, correct?

This might be a rare instance where the earlier, dipper-style, stovebolt would hold an advantage.

Not terribly helpful to the conversation at hand, it just made me think.

Really ? I'm curious about this. If one were to run a very light viscosity oil, would more damage result because the engine is running less rpm, or would higher rpm cause more damage ?

I ask because I see where 10W is called for use in temps less than 32F, and even 5W for temps less than Zero F.

Of course we are not talking about a slow speed race in Fairbanks, Alaska, but it's not like the engine will be subjected to a lengthy time of operation. And wouldn't the oil pump have an easier time moving a lighter fluid, and maybe more of it, while operating so slowly ?

Heavier flywheel. No idea if there is a stock replacement or about modifying yours but that can really help maintain low rpm.

How serious are you about this?
Just adjusting and using what you have for a little fun every now and then.
Or are you going to go all out and compete professionally in the slow races, building and modifying your truck to make the best of it's slowness? The prize money must be interesting, do they attempt to make it as small as possible, trimming up a penny with some snips for the payout?

Heavy flywheel: X2

200 rpm means almost no pump output and very low pressure. The actual local pressure at the journal is not the same as the pump but higher (if it's working) because it's developed by the surface speed of the journal in feet per second. All the pump does is refresh what is used at the journal. The local pressure varies directly with rpm - 200 is 10% of 2,000.
If it's really low the leakage rate is less than the pump output, and it seizes.

If idling with little to no load and so long as you have a few psi oil pressure, like 4 or 5psi, I wouldn't worry about it. My Detroit diesel manuals list under normal operating conditions a surprisingly low allowable oil pressure at idle, pretty much if it registers on the gauge it's OK. Above idle more pressure is needed because more load will/can be taken above an idle.

Wow, the grass on the side of the road was growing faster then you guy's were. Jeffrey

Since you're not pulling a load, just idling along with lots of gears between the engine and the road, just overfill the crankcase by a couple of quarts and let the engine splash-lubricate itself. The oil pump will move enough oil to supply the rods and mains, and the pistons, rings and cam will get enough splash to survive. Just drain a little out before you get back on the road.
Jerry

Gentlemen,

Thanks for the replies, this has been a fun discussion. As stated earlier, I have no intentions of permanently modifying the truck as she is a real cherry original and I intend keeping it that way. I am seeking methods to tweak performance in a minimally invasive (effort-wise as well), so as to slow the old girl down as you can see from the video that she was going way too fast. Yes fast is a relative term in this instance. The data placard on this truck states a maximum permissible speed of 3 MPH in first gear/low range.

As for prize money? You could shave pennies and still have spare change. The only "prize" I have to win here is the adoration, love, and respect from the community at large! There are no plans to go on the race circuit.

For the short distance of the "race" I didn't feel I would harm the drivetrain driving a short distance on hard pavement while in 4WD. I just wasn't so sure if there would be some issues with oiling the crank/cam bearings if I was able to get the full pressure 235 to idle at as low of an RPM as it could stand. I suppose the state of tune will dictate how low the engine will successfully operate.

Thanks again for the words of wisdom and creative ideas.

Paul

Engine design calls for increasing timing as RPM is increased.
Would retarding the timing at lower than 400 RPM increase idle smoothness, thus allowing you to lower the RPM even lower?

Competing with tractors is a lost cause- - - -they're geared for crawl speeds, and most of them have a flywheel that weighs a hundred pounds or more. The old 2-cylinder John Deere tractors have a huge bore and stroke, and they will idle down so slow you can almost count the revs. No way you're going to compete in that league!
Jerry

The video shows he was competitive. And if his idle speed could have been cut in half he'd be half that ground speed, would have easily won.

Timing is a good point, just adjust it by ear at your extra low idle speed for smoothest running.

Too true. I restored a couple of 801 Ford tractors
http://i99.photobucket.com/albums/l286/Hookalatch/Tractors/1772cb87.pbw
The gas powered one has a 2" thick addition behind the transmission made for roto-tilling (Howard Rotovator). In 1st gear at idle speeds you can hardly see the forward progress. Perhaps the rules prevent add-on gear reduction or the use of hydrostatic transmissions. It looks like its all for fun anyway.

Wow Jerry, I was going to ask you to be the captain of my cheer leading team, but with those words of despair I may have to reconsider! We will see how it goes next year.

I was involved in doing the machine work for a big John Deere that competed in antique tractor pulls. The machine shop where I ran the connecting rod department on a part-time basis built up a welded stroker crankshaft by welding material onto one side of the rod journal and regrinding it for a 5/8" increase in the already long stroke. To fit that crankshaft into the engine case, slots had to be cut in the dividing wall between the crankcase and the transmission section of the block so the rods would clear, and a modified oil system was used to lubricate engine and trans from a common sump. The guy won his class regularly with that rig!

Even a stock John Deere will pull at incredibly low speeds, just by using the inertia of those big flywheels and that long stroke.

If you could get away with using a much shorter-profile tire, and/or smaller diameter wheels, you might game the system enough to drop your wheel speed by a pretty fair amount.
Jerry