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lower revs at 100kph

Started by _johndefender_ , Jun 30 2006 10:57 PM

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16 replies to this topic

#1 _johndefender_

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Posted 30 June 2006 - 10:57 PM

I've got a 77LX with a six and the engine rev's really hard at 100kph. At present the car doesn't have a taco so I can't quote numbers, but it's working it's guts off. I would guess somewhere around 4,000-5,000rpm easy. Is there a diff centre that will lower the rev rate at higher speeds. I'm guessing it will effect the car at low speed, but that isn't a problem. The other thought would be is there a three speed auto with overdrive (four speed auto)(yes, it has to be an auto :banghead:

) that I can replace what I have at the moment? I don't know the diff ratio, not sure where to look etc, I can't believe that GMH would put a transmission/diff compo that reved that hard at highway/freeway speed. Any and all suggestions are welcome, I'm still learning as I go. :huh:

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#2 _devilsadvocate_

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Posted 30 June 2006 - 11:14 PM

John, 1st thing to do would be to check the rear axle code on the build plate.
Either gu4,7,8 or gv4. These are uc codes, possibly lx has different codes being a banjo....but possibly the same just to identify the ratios.
It might have started off as a four and kept the original diff.
If it is a 173 possibly has the gv4 3.36:1, or a 202 then a gu4 3.08:1, if its a gu8 that would be your prob.
What you are observing is a fairly typical complaint. The cars were undergeared for highway use.
Cheapest fix is possibly to change the diff ratio, replace the small 13in tyres with 14in,dont know of any overdrive conversions for the trimatic or other autos that will fit.....that would be your most expensive solution.
If none of those diff codes sounds right, post back here and Ill give you a way of working out the diff ratio manually.
ps: its tacHo

Edited by devilsadvocate, 30 June 2006 - 11:16 PM.

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#3 surfmaster

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Posted 30 June 2006 - 11:27 PM

If the car has a trimatic it would rev less at speed with a 2.78:1 ratio diff.

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#4 TerrA LX

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Posted 30 June 2006 - 11:29 PM

yeah sounds like u got 4cyl gears poss 3.9. you need 6cyl gears poss 3.36 medium revs or 3.08 lower revs.

if its doing 5000rpm at 100 kph it might be in second. :tease:

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#5 rodomo

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Posted 01 July 2006 - 01:39 AM

Was it a Sunbird in a previous life?

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#6 _uglybob_

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Posted 01 July 2006 - 03:19 PM

if you want it to stay auto and your looking at doing a conversion you can put a th700 whcih is an auto out of a vn or vp commodre, they have a fairly big overdrive. the conversion cost me about $900. its a bit expensive but its really nice if you do alot of high way driving becuase it alows you to keep your acceleration but not rev so high on the highway as well. or you could wak in a $90 2.78 centre which would make it a slug.

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#7 _johndefender_

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Posted 01 July 2006 - 05:59 PM

Okay, yes to the best of my knowledge it was a sunbird. At the moment (or just about any moment) I don't have the cash to do a $1,000 changeover. There are plans on the drawing board to put 14 inch rims on, I'll check the rear axle code on the build plate, by the sounds of things a 3.36 medium revs or 3.08 might be the solution and yes, you eat a taco, and watch the rev's rise on a tacho! I truley apprectiate the replies and thank you for the advice.
JohnB
Oh, nearly forgot, It's not in second at 100kph If it where, I think we would have pistons shooting thru the bonnet :clap:

Edited by johndefender, 01 July 2006 - 06:05 PM.

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#8 _devilsadvocate_

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Posted 01 July 2006 - 06:20 PM

It would seem strange that someone would go to the trouble of putting in a six and not changing the diff......
Id be looking for a 2.78:1 GU7, especially if you do a lot of travel at 100kmh...reduce noise and fuel use.

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#9 surfmaster

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Posted 01 July 2006 - 08:20 PM

If you do change your diff ratio don't forget to change the speedo gear, you don't want any greeting cards from the boys in blue.

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#10 _johndefender_

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Posted 01 July 2006 - 08:46 PM

Firstly thank you for the replies. The speedo's out by 8km's at the moment anyway. Those funny little yellow speed detection trailers come in very handy at times.
john

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#11 makka

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Posted 01 July 2006 - 10:00 PM

a 2.78 diff will solve your problems at speed.
to give you an Idea, I am running a trimatic, 3.08 diff gears and 225/60/15 tyres (heaps taller than standard) and I sit on 2500 at 100km/h and can get 100km/h out of first if I feel like it

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#12 makka

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Posted 01 July 2006 - 10:19 PM

just for a comparison, my tyres are 651mm in diametre. standard LH/X are 603mm, it makes a fair difference!

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#13 Heath

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Posted 01 July 2006 - 11:05 PM

John mate, I tried ringing you the other week a few times with no responce. Hope you're in good health.

I can almost guaruntee that you're running a 3.9:1 centre, having heard the impression you got from the previous owner multiple times I wouldn't be suprised if they didn't change it at all.

You won't have any trouble getting a diff centre around here anyway

That's a cheap easy fix, I can discuss other options with you if you have some time to chat. Give me a ring in the next few days.

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#14 _johndefender_

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Posted 02 July 2006 - 09:08 AM

ok, I know alot more now, than I did 24 hours ago! It seems that 3.36 or 3.08 are fairly cheap diff centres going by a quick look at eBay last night. There was one 2.78 LSD but it's asking price was around the $150 mark. Another couple of questions. They mention a fine, or course spline. I would find out the thickess of the gears that are in there at the moment? Also, can I guess that I have a Banjo, not a Salisbury?
Thanks again.
John

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#15 rodomo

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Posted 02 July 2006 - 12:05 PM

More than likely fine spline but I would pull an axle out to be sure. The salisbury has a plate the bolts on to the back of the diff, you should see this looking under the back of the car.

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#16 rodomo

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Posted 02 July 2006 - 12:10 PM

More than likely fine spline but I would pull an axle out to be sure. The salisbury has a plate the bolts on to the back of the diff, you should see this looking under the back of the car.

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#17 _Loki_

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Posted 02 July 2006 - 03:01 PM

if you want it to stay auto and your looking at doing a conversion you can put a th700 whcih is an auto out of a vn or vp commodre, they have a fairly big overdrive. the conversion cost me about $900. its a bit expensive but its really nice if you do alot of high way driving becuase it alows you to keep your acceleration but not rev so high on the highway as well. or you could wak in a $90 2.78 centre which would make it a slug.

In my experience in a number of VN's from ones thrashed to ones babied, the TH700 is a piece of shit waiting to break. :blink: