rpm

[brian wafer]

hi ,does anybody know what rpm my 6.2 is turning, with 700r4 in drive 3.42 ratio, 245 75 r16 tires.at 55 miles an hour!this would be my cruise speed when towing my small travel trailer.

[80Sierra]

After factoring in some torque converter slippage, and assuming that a 245/75R16 is approx 30" in diameter, your engine should be turning approx 1500 - 1600 rpm at 55 mph

[brian wafer]

thanks sierra 80 are you using a chart,would be nice too know how too figure it out myself! how are you making out with your motor!

[opto]

http://www.ringpinion.com/Calc_RPM.aspx

[DmaxMaverick]

1475 is what it should be in OD, with torque converter locked. Are you asking about 3rd gear (direct)? TC locked or unlocked? RPM's unlocked will depend on loading. Should be about 2075 if locked in direct.

[Robyn]

The actual RPM is about 1600

The easy way to figure this is to rolla tire(with load on it 1 revolution and measure the distance it travels in inches.

To do it with the the formula is (Pi X D) Or 3.1416 X Diameter in inches

This equals the circumference or distance traveled in one revolution.

1 MPH = 88 Feet per minute
1 mile = 5280 feet

5280 X 12 will give you inches in the mile and make the calcs easy.
1 mile = 63360 inches

A 30 inch tire will roll approx 94 inches per rev

88 X 12 = 1056 / 94 = 11.23 RPM of the wheel

11.23 X 3.42 (Gear ratio of the axle) = 38.40 RPM of the drive shaft

Now with the OD engaged you have a .75 ratio which means the input turns .75 RPM and the output turns 1 RPM.

** Now we cant have 1 MPH in OD but these numbers make it easy to understand and you can simply multiply the outcome by whatever speed you want to know the numbers for and your there.****

38.4 RPM X the .25 ratio difference will = 9.6

subtract the 9.6 from the 38.4 to get RPM in OD = 28.8 RPM

Now mutiply this by the desired speed and you have the engine RPM for any speed you choose (as long as the converter is locked and the tranny is in OD) (wont work in Hydraulic drive due to converter slippage)

With this you will be running 1584 RPM

This will be very close but subject to slight variations due to the fact that we cant get an absolute on the rolling distance on the tire with all the frictional factors plus as the tire speed increases the diameter may increase slightly due to centrifical force giving a slight decrease in the actual value as the rolling distance becomes more and the axle speed will be less for any given speed.

Hope this helps some. :eek::eek:

Actually it fairly simple and if you follow the numbers its easy.

Best

Robyn

[DmaxMaverick]

Robyn is almost right. So was I. The 245/75/16 tire is 30.5" dia. Actual tire size will vary slightly from brand to brand, style to style, and according to wear. I calculated it with a .71 OD at first, which is the wrong OD ratio for a 700R4 (cranial flatulence). Robyn's math is right, the tire size was wrong.

In OD, it will be about 1554.1 RPM's. In direct (3rd gear), it will be about 2072.2 RPM's.

THIS SITE (http://www.4lo.com/calc/gearratio.htm) should help. Makes it a lot easier.

[JohnC]

No, now you're both wrong. ;)

The 700 has a higher OD ratio (.71 IIRC) than the 4L80 (.75).

I think....

[DmaxMaverick]

OK already. The 4L80E does have a .75 OD. Bowtie Overdrives shows the 700R4 has a .70 OD (originally, I thought .71, too). So, in OD, it will be about 1450.5 RPM's at 55 MPH.

[80Sierra]

Truth is, even with the converter locked up there is still a bit of slippage there regardless (at least in my experience!)

I calculated approx 1450 rpm using a 30" tire diameter, and guesstimated 1 - 200 rpm worth of slippage at 55 mph :)

My engine is coming along slowly, just got the word that the second set of heads I bought are garbage. Fella who sold them to me is a member on the other side, and is refusing to return PMs / emails etc, so its lookin like I've wound up with a 400 dollar set of paper weights.

Next set of heads I buy will be brand new, just haven't decided which yet...

[DmaxMaverick]

If you are slipping ~200 RPM's, either the TC isn't locked, or the tranny is toast. My guess is the TC isn't locked. 200 RPM's is about the magic number for slippage with an unlocked TC.

[brian wafer]

thanks on the response,i was hoping to install a lock up switch on my transmission i have a 4 plug connector with only 2 wires a blue with black strip an a green wire .i was told too get a 12volt switch too the green wire for locking up converter.should i cut the green wire or splice in to it to actived loc up.how should i use it only when towing will it lock in every gear,what can i do or not do with torque lock!

[DmaxMaverick]

What year is it? If you click on the "Truck Stuff Pics" in my sig, you can view and print a complete electrical diagram for 1985. Just so happens the TCC circuit is highlighted (been there, done that).

[brian wafer]

thanks its an 1984! will check out diagram

[brian wafer]

for some reason i can,t see your diagram,maybe me i am not too good with computers ! will try the green wire this afternoon.

[Robyn]

Hmmmm, Robyn making a mistake?????????/ OOOOOOOOPPPPSS

yes the .70 is right. Too many numbers in my little head. Must be having a Blonde Moment, :eek:

Any way the math is good and you can figure any ratio or tire size you choose.

As far as the converter goes, when it is locked up it better not be having any slippage or the clutch will soon "NOT BE"

The major fault with the lockups is the size of the clutch and the methodology of applying them.

We can discuss that tech in another thread.

Anyway, you can use these tools to figure out what your little beast is doing when its churning down the Boulevard.

Just thinking here

My Humvee (Kit truck) has 35 inch tires with 3.08 gears and a TH 400 so there is no overdrive.

The RPM at 55 is gonna be low even in 3rd gear. 1641 by the math.

Hmmmmm Imagonna need some gears me thinks.

Little 6.2 gonna stuggle with those 3.08's and them 35 inchers.

Maybe it will get good mileage. :D


Have fun with the numbers and be safe

Robyn

[brian wafer]

thanks robyn ,apreachyate reading you! (i know its wrong spelling but i tried)