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Larger tires and less power

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jhg View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote jhg Quote  Post ReplyReply Direct Link To This Post Topic: Larger tires and less power
    Posted: 09 Aug. 2022 at 10:25am
I want to confirm an observation/warning that our taskmaster Bruce pointed out in another (or other) threads about going up in tire circumference from original and running the Go Devil powered jeeps anywhere high altitude. High altitude being above 3000' for purposes of this thread.

You will see a difference in power (less) and in crawl speed(faster).

Now I love my 6.50 16 STA Super Traxions. But I see how they would be better complimented by a T90C version of the transmission over the T90A.

I can live with it, but the crawl speed increase also makes wheeling harder in many circumstances. The loss of power is noticeable on steep grades when starting from stop, Add a passenger, some gear and full fuel tank, a spare, that tool kit including extra distributors, axles, leaf springs and kitchen sinks, well, you will see a lack of available power.

In my case its all good. I may build a "C" version trans one day. Or not. For now I can live with it and still have a blast on forest service roads and etc.


Edited by jhg - 09 Aug. 2022 at 10:38am
1948 cj2a. Rebuilt L-head, steering, T90, WO 636, steering, brake lines. So far.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Spinnas Quote  Post ReplyReply Direct Link To This Post Posted: 09 Aug. 2022 at 10:33am
On flat highway at Idaho Vintage Jeep Rally 2 Go Devil powered 2As on 7.50-16s both hit over 60mph and were cruising consistently at 55mph at 4500-5000ft elevation. Tire size and elevation with affect anything to some extent. Making sure your equipment is dialed and setup correctly is a bigger factor in power and speed than elevation and tire size.


Edited by Spinnas - 09 Aug. 2022 at 10:34am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote jhg Quote  Post ReplyReply Direct Link To This Post Posted: 09 Aug. 2022 at 10:41am
I think hitting mph on flats is a lot different than pulling a 12% rocky grade from stop, fully loaded at elevation. They were not hitting 55 pulling hills of any significance. 
I agree with you about maximizing what is there but going up in tire size will affect power in the negative.


Edited by jhg - 09 Aug. 2022 at 10:42am
1948 cj2a. Rebuilt L-head, steering, T90, WO 636, steering, brake lines. So far.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Spinnas Quote  Post ReplyReply Direct Link To This Post Posted: 09 Aug. 2022 at 10:51am
Well they have both have T90C, OD, and Teralow, so fast or slow is not an issue. Along with 1 having a Hickey head and the other a Supersonic. But also 7.50-16s.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Spinnas Quote  Post ReplyReply Direct Link To This Post Posted: 09 Aug. 2022 at 10:56am
Originally posted by jhg jhg wrote:

In my case its all good. I may build a "C" version trans one day. Or not. For now I can live with it and still have a blast on forest service roads and etc.


Do you have an OD? Overall unless strictly crawling crawling, the combination of T90C and OD is the best upgrade you can make IMO in terms of drivability on pavement or dirt.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote jhg Quote  Post ReplyReply Direct Link To This Post Posted: 09 Aug. 2022 at 12:10pm
Don't disagree regarding gearing. My initial comments were directed at those with stock jeeps going to larger tires and the performance cost of doing that.  Changing gearing, adding an over drive and/or transfer case gearing changes are significant mods from original, and beyond my experience.
1948 cj2a. Rebuilt L-head, steering, T90, WO 636, steering, brake lines. So far.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote storm_factory Quote  Post ReplyReply Direct Link To This Post Posted: 09 Aug. 2022 at 4:19pm
I live at 6512'

I have a L134 in who knows what condition with a weber 32/36 and an NP435 running 34 super swampers on 5.38's. Using 2-4th as my 1-3 (basically the same as a t90A) around town 40-50mph on flats is no problem but any hill of significance it's 25-30mph in "second". If I had an OD and could split 2/3 it could do a little better. 

Before I had 750-16s and it was basically a racecar. You'll be fine if your brakes, suspension, and steering are up to it. The hum of the go devil makes me smile every time i drive it and people always wave as they blow my doors off in the air conditioned space ships with the stereo blasting

Edited by storm_factory - 09 Aug. 2022 at 4:33pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Mark W. Quote  Post ReplyReply Direct Link To This Post Posted: 09 Aug. 2022 at 9:37pm
This is why I am running 235/75R-15's 28.6" tall 5.38's T90A D18 ATV 30% OD. 2655rpm at 60mph in OD according to my dyno test my Max torque is at 2410rpm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote bight Quote  Post ReplyReply Direct Link To This Post Posted: 10 Aug. 2022 at 7:53am
had 31x9.50R15LT Super Swampers (31 inch diameter) on aluminum slots and changed to McCreary 6.0-16LT NDT's (27.5 diameter) on stock 16x4.5 steel rims and it is a very different jeep. it now can go up long paved hills in 2nd gear. had to be in 1st with the larger tires.

Edited by bight - 10 Aug. 2022 at 7:56am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Steelyard Blues Quote  Post ReplyReply Direct Link To This Post Posted: 10 Aug. 2022 at 2:00pm
When I got her, she had 235-75-15 on split rims. I went back to stock. Stock rims with 6.00 NDTs weighed 45 pounds each. The other tires were 70 pounds each. I was surprised how the extra 100 pounds of rotating mass made the Jeep both slow to accelerate and stop. 

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Post Options Post Options   Thanks (0) Thanks(0)   Quote oldtime Quote  Post ReplyReply Direct Link To This Post Posted: 10 Aug. 2022 at 3:32pm
I was cool back in 1977 so I went extreme with a set of 35” tall gumbo wide Monster Mudders on my nearly standard CJ-2A (lifted for tire clearance and no overdrive).
I could not get it to go past 40 mpg on interstate with a fresh rebuilt Go Devil. A couple weeks of that nonsense was all I could handle. Yeah I Lost my “TOO COOL” factor but easily gained 20 mph with a set of  29” tires.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Bill2A Quote  Post ReplyReply Direct Link To This Post Posted: 10 Aug. 2022 at 10:37pm
Yep, taller tires are great for adding ground clearance, but it takes power to turn them.
Everything is a trade off. 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Spinnas Quote  Post ReplyReply Direct Link To This Post Posted: 11 Aug. 2022 at 11:33am
Originally posted by oldtime oldtime wrote:

I was cool back in 1977 so I went extreme with a set of 35” tall gumbo wide Monster Mudders on my nearly standard CJ-2A (lifted for tire clearance and no overdrive).
I could not get it to go past 40 mpg on interstate with a fresh rebuilt Go Devil. A couple weeks of that nonsense was all I could handle. Yeah I Lost my “TOO COOL” factor but easily gained 20 mph with a set of  29” tires.


And 1/2 of the mph gain was cause the speedo was reading correctly again! LOL
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Post Options Post Options   Thanks (0) Thanks(0)   Quote oldtime Quote  Post ReplyReply Direct Link To This Post Posted: 11 Aug. 2022 at 12:08pm
That Jeep would easily do about 60 max.
I drove a lot of Interstate in those days and normally ran 55 mph without an OD because 55 was the National speed limit
I have never babied my engines due to their diminutive displacement yet still got decent to excellent service life from them. How does 80K sound running them long and hard between rebuilds ? That’s what I got on my F134 3B with OD typically running 62 mph on interstates. 29” tires or 6.50 x 16”.


Edited by oldtime - 11 Aug. 2022 at 12:14pm
Currently building my final F-134 powered 3B .
T98-A Rock Crawler using exclusive factory parts and Approved Special Equipment from the Willys Motors era (1953-1963)
Zero aftermarket parts

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Post Options Post Options   Thanks (0) Thanks(0)   Quote wfopete Quote  Post ReplyReply Direct Link To This Post Posted: 11 Aug. 2022 at 1:20pm


I'd say about 1 out of every 10 L-134 engines are in good enough tune to pull a taller tire. 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 43 Flat-Fender Quote  Post ReplyReply Direct Link To This Post Posted: 11 Aug. 2022 at 2:12pm
Hi All ..... Wondering .... for those of us lucky enough to be heading to CFFC FCT in a little over a month, what "Tuning" steps can be taken while there to "Tweak for High Altitude Performance"?

As a practical matter, it would seem "Advancing the Timing" is in order, regardless of tire size.  (I'm running stock 6.00x16 NDT and stock gearing @ around 28 to 1 crawl ratio)  

Bring RPM up to around 1500 RPM, Advance Timing to obtain Maximum Vacuum and then back-off a Skosh to aid in engine starting.

Does that sound like a plan that will work?

Obviously a "Feel and Ear" kind approach.

Thoughts?


Edited by 43 Flat-Fender - 11 Aug. 2022 at 5:54pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 3A Steve Quote  Post ReplyReply Direct Link To This Post Posted: 11 Aug. 2022 at 2:52pm
I live about 610' above sea level in the panhandle of WV. Haven't done anything other than adjust my float once when my carb was leaking. Works fine here and in Colorado. It may be less of a problem than you think. Might want to wait until you get there and take the get acquainted run before doing anything. 
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