Changing the 5th gear ratio on gearbox

[gotoy]

I know theres kit on the market, but they cost more than the gearbox I have.

What I am after is a better ratio on the 5th gear of a R151 gearbox for cruising on the highway.

Would greatly appreciate any advise as to how to do this.

[ozrunner]

Fit the overdrive gears from a Dyna Truck R452 trans and you will go from your current 17% overdrive to a 30% overdrive.

Check the Dyna trans to ensure it has the correct OD gear count (23:49).

JD

PS. Mudbasher2 has just screwed his Dyna box so he might sell these gears

[gotoy]

Thanks for the reply.

Is that a common box and easily available?

[ozrunner]

I've seen a few on ebay and I would suspect truck wreckers would have them.

[gotoy]

Are the parts available of the shelf?

Made a few calls today and all I get is ask me buy the whole gearbox.

[ozrunner]

Probably but knowing Toyota they will cost more than buying a whole used trans and be ex Japan.

Buying a complete trans is the easiest but that is the penalty so it becomes a matter of how bad do you want a 30% overdrive .

You could maybe sell the remaining gears to guys that do comp rock crawling etc and don't require an overdrive as 1st gear is 5.14.

I've seen boxes on eBay for $200 and recently.

This shop seems to have some but you would need to confirm the gear teeth count.

http://www.japparts.com.au/products/gearboxes.html

[RUFF]

Or buy the R452 and install all the internals into the R151 and get the best of both worlds with the low first gear and the tall over drive.

[ozrunner]

Nope as you would have a normal 17% overdrive

[bj on roids]

pls explain?

[ozrunner]

OK I’ve done this before but I’ll try

When I was contemplating my 5 speed swap I wanted to retain a good overdrive similar to the existing auto’s 30%. After a lot of research I decided on using the Turbo Supra R154 as its stock overdrive was 25% and it’s 1st gear ratio was more suited for my purposes (3.25).

Part of this research was to also examine the internals of the various R series to determine if any overdrive gears could be swapped to alter ratios as they are the only gears that are removeable and it worked .

Overall gear ratios are determined by the amount of teeth on each gear . Thus the R Series that we know of, ie R150/151/154 and the Dyna R452 all share parts to determine their different overall gearing etc.

All gearboxes have a set of “cluster gears” ie fixed gears on the input shaft and on the lay shaft that are used to alter the input shaft rotation speed. This combined with the actual gear ratios give each trans its own final overall gear ratio.

The R150 and R151 have the same cluster gear ratio, ie they both have cluster gears with 43:29 teeth counts so 43 divided by 29 gives an overall ratio of 1.482 meaning the input shaft speed is then multiplied by 1.482 at the layshaft.

This is how they get the different overall gear ratios in these two R boxes, ie they both have the same cluster gear ratio but the R150 1st gear teeth counts are 31:12 = 2.583 x 1.482 = 3.828 (3.83 1st) whereas the R151 1st gear teeth counts are 32:11 = 2.909 x 1.482 = 4.31 1st .

By having a different cluster ratio but using the same gear teeth ratios you again get different overall ratios such as in the Dyna R452 and R154. The R154 and Dyna both also use the R151 1st gears, 2.909 but multiplied by their different cluster you different 1st gears, ie R154 = 2.909 x 1.117 = 3.25 . The Dyna R452 = 2.909 x 1.769 (46:26) = 5.14 1st.

Thus the same applies for the R150/151 overdrive gears which have tooth counts of 26:46 giving a 0.565 ratio so 1.482 x 0.565 = 0.83 which gives you your !7% overdrive in the R150/151

But the Dyna has overdrive gears with a teeth count of 23:49 which equals 0.469 so multiplied by 1.482 = 0.695 thereby giving an overdrive of 30.5%

I used the R154 and I found that with its cluster gear ratio of 1.117 if a set of R150/151 overdrive gears were fitted, ie 1.117 x 0.565 = 0.631 = 37% overdrive perfect for my application

Conversely, if you put R150/151 overdrive gears in a Dyna it would be 0.565 x 1.769 = 0.99 meaning you now have two top gears

So it’s all to do with the different cluster ratios

For information of you comp guys that want a low 1st and what pees me off with shops selling this Dyna trans as a "heavy duty replacement" for 4Runners/Hilux’s, when it is nothing more than a normal R series with different ratio’s, ie it uses exactly the same 1st gears as a R151 and R154. The only HD aspect is it has a cast housing and it uses a sloppy outside shift selector mechanism.

If you want a 5.14 1st rather than pay the exhorbitant cost for this so called “HD trans” go find a R452 in a truck wreckers for $2-300, strip it and fit it’s gears to a R150/151 housing using the R150/151 selector shafts etc and you now have the lower gears but also with the normal 4Runner/Hilux trans with internal shifter and a lot cheaper than $3000 plus