Engine RPM Seems High

[mikekaidan]

Sent: 1/31/2005 2:01 PM

Hello group!

I've been a member now for about a month and a half and have found an increadible amount of information from this group... I thank everybody, as I'm a first time RV owner... a 1980 Brave that we've appropriately have named 'Rink-E-Dink'  After a month of figuring everything out, (for the most part) some minor repair work on the engine etc I have one lingering question.... My brave has a Dodge 360-3 (was told that it was 440 when I bought it, didnt' know any better at the time, but I'm fine with the 360... so far so good) I've noticed while on the highway at a speed of about 55-60, my RPM's are running around 4K. Maybe I'm used to the little cars these days with the higher gear ratio, but to me that seems high.... just some feedback on this would be great!

Thanx again, hope to see ya all on the road!,

Mike/ Rink-E-Dink

[denisondc]

Sent: 1/31/2005 5:00 PM

4000 rpm seems too high. Hopefully the tachometer is "off". If you can figure out your rear axle ratio, then based on your tire diameter we can calculate what your driveshaft rpm would be at 55 or 60 mph.

The usual rear axle ratio on Winnies in the early 70s was 4.56 or 4.88. The 4.10 ratio was only on the shortest/lightest chassis, and a 5.86 ratio came with some of the longest/heaviest chassis. I would presume yours would be the 4.56 or 4.88.
To learn that ratio you can either try to read the two numbers off a tag that is supposed to be on the front of the differential, numbers like 41/9, for the 4.56 or 39/8 for the 4.88. They are the numbers of teeth on the bevel gear and pinion gear.
OR, if you cant find that tag; you can jack up one side of the axle off the ground, and turn the driveshaft, counting its turns. Block the other wheels so you don’t get run over.

You make a mark on the tire, and turn the driveshaft until that tire mark has gone around TWICE. (Since the other half of the axle isnt turning.) The number of turns the driveshaft made will be the rear axle ratio.

With my tires being 33 inches tall my rear axle turns 611 times/mile. With the 4.56 rear axle ration, that means the driveshaft turns 2786 times in a mile. At 55 mph that is 2554 rpm. Allowing 300 rpm for torque converter loss on a level road, I would expect my engine to be spinning about 2850 rpm at 55 mph, or about 3050 at 60 mph. The amount of torque converter loss is going to be less at higher driveshaft rpm, but will be higher if you are towing anything. This agrees with what I see on my tachometer â€" JCWhitneys finest.
Since my 727 transmission has a 2nd gear ratio of 1.45 to 1, if my tranny was staying in 2nd, then at 55 mph the engine rpm would be 2554 x 1.45 = 3703 rpm, plus mabye 200 for TQ loss, giving 3900 rpm. If your winny had the 4.88 rear axle ratio and tires similar in diameter to mine, 55 mph could be 2730 driveshaft rpm, plus about 300 for TQ loss, or 3030 rpm in 3rd gear, or over engine 4000 rpm if it were staying in 2nd gear.

My experience is that with the tranny shifter cable very slightly out of adjustment, my tranny will stay in 2nd gear when I move the lever from 2nd up to Drive. But if I put it into Drive from Neutral, then it goes into 3rd gear properly.

[HeavyHaulTrucker]

Sent: 1/31/2005 8:47 PM

denison, another possibility could be the kickdown linkage not being adjusted properly; I found that out on mine.  A dirty tranny filter could also be the culprit.

Since I have never seen a description of how to adjust the A727 mechanical kickdown lever (also called a Throttle Linkage), and having received "I don't have a clue" looks from most mechanics when asked, I will detail what I have found out by "trial and error" below.

This procedure should be performed only after you have properly adjusted your carburetor (mixture & idle screw settings).

The transmission throttle linkage (kickdown lever) consists of a long linkage lever, a pivot link mounted on the valve cover bolts, and a shorter lever with a slotted end that connects to your carburetor throttle cable attachment point.  The first step is to loosen the 7/16 bolt that secures the slotted end of the long lever to the pivot point.  Adjust the pivot point's "C" linkage so that it is exactly vertical.  Now, holding the pivot link so that it does not move forward or backward, pull the long linkage lever forward lightly until you can just begin to feel resistance; hold the linkage lever and pivot link immobile with one hand while tighting the 7/16" bolt.  This adjustment is your "starting point".  You can move the slotted end of the long linkage lever forward or backward 1/16" to 1/8" at a time to obtain the proper shift points -- remember, "Longer = Later, Shorter = Sooner".  Do not be concerned if the shorter lever does not fully "bottom out" against the carburetor throttle lever pivot stud -- the spring connecting the carb throttle lever with the pivot point for the kickdown linkage will keep the proper geometric relationship between the throttle position and the kickdown lever travel.

There is an alternative method -- holding both the carb throttle and the kickdown lever full open while tightening the bolt -- but IMHO it doesn't work nearly as well as the method above.  And is not nearly as accurate -- I adjusted mine right on the money the first time with the method detailed above.

John

[mikekaidan]

Sent: 2/4/2005 12:27 PM

Well guys, thanx for the imput. I haven't manually checked the ratio's as described yet... but thats great information. I'm guessing it's my tach thats off based on the fact that I know the transmission is shifting twice (I can feel and hear it)... into a 3rd gear. Engine sounds OK, but I see that 4K RPM and I get concerned. It's not running hot or anything. I just wish it one more gear!

Thanx again

Mike
Rink-E-Dink

[DaveVA78Chieftain]

Sent: 2/4/2005 2:35 PM

Not sure about other versions of the chassis manual, but mine (73 plus 74-75 supplement) has 3 different descriptions, depending on which version you have, of how to adjust the the transmission throttle rod.  They are in the Transmision section (group 21), Maintenance and Adjustments, under the title Throttle Rod Adjustments.  There are different procedures for a 3 section linkage, single section linkage, or a 413 linkage.

Dave

[denisondc]

Sent: 2/4/2005 2:46 PM

I thought of another thing to check. If someone replaced the original fan clutch with a solid fan hub, one that didnt allow the fan to slush along at lower than engine rpm; that might cause excess fan roar, and sound like an engine turning too fast.

The fan clutch could have been replaced because the old one had gotten feeble, resulting in overheating on hills.. You radiator fan should turn with a thumb and forefinger on it. And above about 2000 rpm engine speed, the fan clutch should let the fan "lose" rpms, not keeping up with the water pump hub rpm.

[Cooneytoones]

Sent: 2/5/2005 8:15 PM

Mike......I said the same thing the first time I drove mine....."It feels like it needs another gear".....The 3 speed auto does need another gear.....No tach in mine, but engine seems (to my ears) to be near or somewhere a little below 3 grand...

Timmy