Rpm at 65?

[Dave _S]

The tach on my rig says the engine is turing 3200 rpm at 65 mph. I have a TH400 with a 3.54 axle ratio. According to the following formula "RPM= [MPH x gear ratio x 336]/[tire diameter], I should be turning about 2600 rpm at 65. The formula assumes 1:1 lockup which of course a TH400 doesn't have. Anyway, for purposes of evaluating a possible trans swap, I am trying to get a feel for how much slippage there is in a TH400. I have the feeling my tach is inaccurate, so I need some comparison data.

So if anyone with a TH400 and 3.54 gears happens to know what their tach reads at 65 mph, would you please post the number? If you have an overdrive part-time kit installed please so state as that changes the gear ratio.

Thanks,
Dave

[RudyC]

HEHE, you have too much spare time on your hands Dave http://www.ifsja.org/ubb/biggrin.gif

Torque converter slippage from wind, rolling resistance. There is no way the TC will spin both side evenly without a lock up device or a clutch. It might work if you put the Chero on jack stands and run it in gear.


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79 WAGONEER (SOLD IT, BUT WILL FIND A LESS RUSTY ONE LATER)
79 DODGE STEPSIDE, 318,HOLLEY STREET DOMINATOR INTAKE, 4BBL HOLLEY,TF727/275X16 GOODYEAR EAGLE GTS
pictures at:
http://albums.photopoint.com/j/AlbumIndex?u=1500380&a=11321052

[Dave _S]

Perhaps I should clarify. I am not referring to traction. I am referring to engine rpm at cruizing speed. Specifically, if I swap in a four speed manual, I will have true 1:1 lockup, and should have an rpm of about 2600 at a cruizing speed of 65 with my current 3.54 gears. What I want to get a handle on is what rpm am I currently turning at 65 with the TH400? My tach says 3200, I think the tach is wrong. Of greater interest, what is the difference in rpm between a four speed manual and the TH400. Both are 1:1 final drives. But the TH400 has slippage through the torque converter. What I want to quantify is how much. If my actual cruizing speed with the TH400 is 2800 rpm versus the 2600 rpm I would get with the 4 speed mtx, a mere 200 rpm reduction, well then I am not going to bother with a four speed manual swap. If that's the case, I would only consider the hassles of a drivetrain swap if I could get overdrive. On the other hand, if my engine really is turning 3200 rpm at 65 and I can drop 600 rpm by going to a 4-speed, well then maybe its worth the hassle.

Anyway, I am a gearhead and these things interest me.

Dave

<BLOCKQUOTE>quote:</font><HR>Originally posted by RudyC:
HEHE, you have too much spare time on your hands Dave http://www.ifsja.org/ubb/biggrin.gif

Torque converter slippage from wind, rolling resistance. There is no way the TC will spin both side evenly without a lock up device or a clutch. It might work if you put the Chero on jack stands and run it in gear.


<HR></BLOCKQUOTE>



[This message has been edited by Dave _S (edited May 29, 2001).]

[Stuka]

According to that calculation at 80mph in 4th gear (1:1) with a T18a tranny and 3.54:1 final drive, i am running 3281rpm. Now, I have never gone 80mph in my rig. I have gone 75 once or twice, and it sounds like I am at alot higher rpm then that. You sure that calculation is correct?

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<UL TYPE=SQUARE>
1975 Cherokee
360 w/ Edelbrock Performer Intake and Carb
T18a
Spicer 20 T-Case
Dana44 w/3.54 Gears
[/list]

[Mike Z]

In my jeep, at 65 she is turning about, 2800
rpm. I have no idea what gears i have, so it may be different for me, since i have the tow pakage? l8tr
Mikex

[243]

GM Automatic Transmision Gear Ratios

Transmission Type

1st Gear 2nd Gear 3rd Gear4th Gear
Power Glide 1.76 or 1.82 1.00
TH350 2.52 1.52 1.00
TH400 2.48 1.48 1.00
700R4 3.06 1.63 1.00 0.70
200R4 2.74 1.57 1.00 0.67
4L60E* 3.06 1.63 1.00 0.70
4L80E 2.48 1.48 1.00 0.75

What makes you think the Jeep TH400 does not have a 1:1 third gear?


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243

Ol'e Blue
79 Wagoneer 401/T400/QT

http://www.0hassleisp.com/dwilk/79sig2.jpg

Red 75 Wagoneer (project)
Mean Green 76 Wagoneer (parts car)
99 Dodge Ram TD

cummins_isb@hotmail.com



[This message has been edited by 243 (edited May 29, 2001).]

[smithhl]

At 65 my engine rpm is about at WWAAAAAAAAAUUUUUUUUUUUHHHHH, is that what about yours is at?

(Man I need a tachometer.)

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81 Wagoneer Brougham
327 Vigilante T-18 NP-208

74 Volkswagen Sunbug
Superbeetle

95 Olds Achieva
4cyl 5-speed

[Dave _S]

It does have a 1:1 third gear and I did not mean to imply otherwise. However, the TH400 does not have a lockup torque converter. With a non-lockup torque converter you always get some slipage and you never actually get a true 1:1 final drive. The best non-lockup torque converters achieve about 96% lockup. Thus, the engine of a vehicle equipped a non-lockup automatic will always run at a higher rpm than with a vehicle equipped with a manual (assuming both have the same top gear) since a clutch provides 100% lockup. Thats why I am looking for some data points, I am curious to know how efficient the TH400 is. Comparing rpm at a given mph with the only variable being trans type (atx or mtx, both with 1:1 top gears) is one way of getting a feel for this.

<BLOCKQUOTE>quote:</font><HR>Originally posted by 243:
GM Automatic Transmision Gear Ratios

Transmission Type

1st Gear 2nd Gear 3rd Gear4th Gear
Power Glide 1.76 or 1.82 1.00
TH350 2.52 1.52 1.00
TH400 2.48 1.48 1.00
700R4 3.06 1.63 1.00 0.70
200R4 2.74 1.57 1.00 0.67
4L60E* 3.06 1.63 1.00 0.70
4L80E 2.48 1.48 1.00 0.75

What makes you think the Jeep TH400 does not have a 1:1 third gear?

<HR></BLOCKQUOTE>

[jeepbob]

Your calculations are correct, but your tach is in the ozone layer. I run about 3300 to 3500 at 65 to 70 mph with a 400, 4.10 gears and 33's and I think my tach is a little optamistic. Make sure your tach is not for or set on 6 cylinder operation. And yes, my motor does sound like it is tying to come out from under the hood.

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65 wag. 360/edelbrock rb4/t400/20 t-case/4:10 d27/d44 broken power lok/onboard air/onboard 110v power(no inverter)/1999 Lincoln t.c.leather buckets/Lincoln ctr console/winch/33x12.50/tunes/water proof door pads
soon to have new motor/d44frt/d60r(4:10)welded diff/custom bumpers
see ya in da mud

[porchpiggy]

I know exactly what your talking about. Yhe best way I know to "check your cruisin' slippage" is to: when your flying along at 65MPH at (your Tach) 3200, let off the gas and see how far it drops. You should be able to see a "range" of RPM (Mine is about 100-300 RPM). You did not say what size tires your running, but with the gears you have I would think your tach is wrong, or your tranny is slipping in places it aint suppose to.

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Porchpiggy
"To old and slow to run with the Bigpiggys"
88 GW and 79 "burban

[joe]

Sorry for the ditto post but I think you need to smack your tach gently but firmly...

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Regards,
-joe
63 2-dr Wag, 72 J-4000, 73 J-4000, 82 CJ-8

[JeepFreak]

Any way to use a lock up converter in a TH400??

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JeepFreak

[WillyPete]

would you have to switch torque converters to achieve lockup (is anything actually that simple? http://www.ifsja.org/ubb/wink.gif)?

the formulas are right, i have no idea how much slippage a 400 or a 727 (what i've got) would provide. i'm supposed to be hitting 2700 rpm @ 70 mph (3.31 gears, 235/75R15), it sounds more like 2800-2900. go figure.

good luck with figuring it out

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'86 Grand Wagoneer
Sugar (http://albums.photopoint.com/j/View?u=1666079&a=12770766&p=47642337&Sequence=0&res=high) Magnolia (http://albums.photopoint.com/j/View?u=1666079&a=12770766&p=47642335&Sequence=0&res=high)