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post #1 of 11 Old 11-15-2011, 08:50 PM Thread Starter
rdsk8ter
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another 258 power question

Here's what I have
on the highway my jeep runs great in 4th gear street driving its a monster 5th gear not so much it cant keep enough power in 5th and I'm trying to figure out why
258 bored 30 over very small cam (cant remember what it is)
nuttered (kinda, mine is an 82 but still had some stuff that could be removed)
TFI upgrade with 8mm wires and new plugs, HEI upgrade (in the dura spark box),mc2150, nv4500(Chevy), Dana 300, 4.56 gears, 35 12.5s, 3" snorkel all of this made great differences but it still just doesn't like 5th gear.
One question is would the exhaust make a huge difference and allow me to hit 5th would a plugged cat cause it to bog down after running for a while at speed? |It will hold 4th no problem 3000rpm and still pulling but 5th gear hates me. Any ideas? I know the carb is built right and timing seems to be great idles like a dream. My father is a mechanic of 30 years and he is giving up and saying its just the way it is I would love for once in my life to prove him wrong.

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post #2 of 11 Old 11-15-2011, 09:13 PM
pouge
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Not sure what the answer is but my jeep is the same. I just don't use 5 th. Curious to see what the answer is.
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post #3 of 11 Old 11-15-2011, 09:22 PM
james04si
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Im pretty sure its a combination of gearing tires and the final drive ratio of the transmission when in 5th but ill let the number crunchers do the math for you.
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post #4 of 11 Old 11-15-2011, 11:16 PM Thread Starter
rdsk8ter
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Originally Posted by james04si View Post
Im pretty sure its a combination of gearing tires and the final drive ratio of the transmission when in 5th but ill let the number crunchers do the math for you.
That's what my dads saying but again i just really wanted to prove him wrong just once lol
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post #5 of 11 Old 11-16-2011, 02:06 AM
BagusJeep
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You have 35" tyres, they came with 29", you have 4.56 gears and an overdrive 5th gear which could be the 0.73 ratio in a NP4500, guess what??

The table below is for 4th gear at 55mph, which is 1:1 and your 35" tyres work OK on this ratio. If you have an 0.73 5th gear you will be pulling 27% less revs and you will struggle if you are pulling 1757rpm at 55mph.

If you had the 0.86 overdrive ratio in a T5 box you would have been pulling 2070 rpm and that is also really sluggish but more manageable.

Sorry, it is your gearing in overdrive top. I would be tempted to steer clear of it unless going downhill as it will bog and use more fuel than 4th which is nicely geared for 35" tyres.

P.S. Dads are always right.
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post #6 of 11 Old 11-16-2011, 07:26 AM Thread Starter
rdsk8ter
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yeah it just darn lol hmm maybe time for the darn v-8 lol now question how does this chart work is it when you shift into the gear or? fourth gear at about 55-60 is really high rpms like 3500 rpm range
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post #7 of 11 Old 11-16-2011, 11:58 PM
BagusJeep
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Quote:
Originally Posted by rdsk8ter View Post
yeah it just darn lol hmm maybe time for the darn v-8 lol now question how does this chart work is it when you shift into the gear or? fourth gear at about 55-60 is really high rpms like 3500 rpm range
If you work the maths these are the engine speeds at a constant 55 mph when in direct top gear 1:1 for a range of tyre sizes and diff ratios.

If you are showing 3500rpm at 60mph in 4th you most likely have not changed the speedo drive and EITHER the speedo is pessimistic OR you are really doing 80 mph.

You may also have a different diff ratio to what you believe.

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post #8 of 11 Old 11-17-2011, 12:14 AM Thread Starter
rdsk8ter
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if im doing 80 then the pissed off semi drivers need to slow down lol it could be closer to 65 ill check later this weekend.
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post #9 of 11 Old 11-17-2011, 02:22 AM
John Strenk
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Originally Posted by rdsk8ter View Post
That's what my dads saying but again i just really wanted to prove him wrong just once lol
Dad-1 Son-0

Just tell your Dad he was right and make him smile.....
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post #10 of 11 Old 11-17-2011, 02:24 AM
BagusJeep
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Even if he was wrong tell your Dad he was right, he deserves it!!!!!!

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1981 CJ7 258ci - Bagusjeep
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post #11 of 11 Old 11-17-2011, 08:37 AM Thread Starter
rdsk8ter
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yeah i already did lol any one know if i could change 5th gear to a better ratio?
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