4.12 or 3.88 irs is there much difference

stimey · Post by **stimey** » Sun Nov 13, 2005 9:26 am

I am needing to rebuild my 71 AH IRS and have been wondering if I should go with a 3.88 R&P instead. ( should be a 3.88 in a GHIA any way)I understand there may be a issue with the nosecone but that is no worry. My concern is, is it worth the hassle and will my gas mileage increase or decrease. I figure I will lose a couple hundred RPM around 70 (maybe save gas) but with the 3.88 will I have to hold it in gear longer to get to 70 and will I have to hold the pedal down more to keep it at 70?? If that makes any sense.
Why did VW change over to a 3.88 in 73? Was is due to F.I. and maybe more low end throttle reponse.??
Thanks
Lee

arlo · Post by **arlo** » Sun Nov 13, 2005 11:02 am

This will give you a 6.5% drop in RPM at a given speed in each gear. Thats not much but you would notice it if you pay attention. For example 3500
rpm with the 4.125 would be 3288 at the same speed and gear with the 3.875. Another way to look at it if you were going 60 with the 4.125 at a given rpm you would go 64 with the 3.88 at the same rpm. This is not a big differance but if you have plenty of torque in 4th to drive how and where you do and you are considering replacing the R&P anyway then I would go with the 3.875 If you are doing this yourself and are not experienced with the setup process it will double the complexity and odds of making a mistake if you do the swap.

Yes, I think the FI gave a little better low end torque to allow this small gear change.

Plastermaster · Post by **Plastermaster** » Sun Nov 13, 2005 12:49 pm

I think you are on the right track with the the throttle position compensating for any MPG drop you get with the 3.88. That is not to say the MPG would be the same or worse, but that it is not soley rated to RPM.

The nice thing about the RPM drop is the reduction in engine drone (noise)
Also, What engine are you running? If it is a stocker, I would stay with the 4.12 to keep the torque, but that is a matter of taste.

Ron

Guest · Post by **Guest** » Sun Nov 13, 2005 1:00 pm

Thanks Guys!!!!

stimey · Post by **stimey** » Sun Nov 13, 2005 1:02 pm

Thanks Guys ..I am running a stock 1600.

Bruce2 · Post by **Bruce2** » Sun Nov 13, 2005 1:23 pm

arlo wrote: Another way to look at it if you were going 60 with the 4.125 at a given rpm you would go 64 with the 3.88 at the same rpm.

Yes, I think the FI gave a little better low end torque to allow this small gear change.

Not true in 4th gear. VW made the 4th gear in a 3.88 gearbox lower to compensate for the taller R&P ratio. The difference between the final drive ratio of a 3.88 and a 4.12 gearbox is only 1%. So with your example, if you are going 60mph with a 4.12 gearbox, you will be going 60.6mph with the 3.88. Not measureable by anyone in any car.

FI on a Beetle didn't appear until 2 years after the switch to a 3.88 R&P, so FI has nothing to do with the switch.

IMO you are far better off sticking with your 4.12 R&P. It gives noticably better acceleration through the first 3 gears and will give you the exact same mileage cruising on the freeway.

arlo · Post by **arlo** » Sun Nov 13, 2005 8:39 pm

I was assuming he would not change out 4th but only the R&P so he would see the final ratio change in 4th. I agree though that it probably wouldn't be worth it if the original gears were in good shape.

I had a Ghia with the same engine and I believe the same ratios but I am not certaian. Mine was a '72 with a DP 1600 and it could definatly have used a taller 4th out here in the flat lands. These cars seem to have much less wind drag at higher speeds than the bugs.

Bruce2 · Post by **Bruce2** » Mon Nov 14, 2005 1:44 am

arlo wrote: I had a Ghia ..... a '72 with a DP 1600 and it could definatly have used a taller 4th out here in the flat lands. These cars seem to have much less wind drag at higher speeds than the bugs.

I agree with this. Not only is a Ghia more slippery, it has less frontal area. This means it takes less hp to push it through the air. This is where I would recommend a 4.12 x .82 or a 3.88 x .89

Post by **Marc** » Mon Nov 14, 2005 9:42 am

Late 'Ghias had a 3.875 R&P but retained the .88 4th that was used in the last of the 4.125 transmissions, so there's a ~6% difference in overall ratio. Evidently it was felt that the taller gearing was appropriate with the better aerodynamics of the 'Ghia.
Beetles got the .93 4th at the same time as the 3.875 R&P, which kept the difference down to .7% in 4th...the overall ratios in 1st, 2nd, and 3rd are all taller by the full ~6%, but there's only about 20RPM difference at 60MPH in high gear.

stimey · Post by **stimey** » Mon Nov 14, 2005 12:06 pm

So if I understand it right only the GHIA box gives you the 6 % rpm drop. Which would mean I would need a AN box to get the advantage. Now it makes it more difficult. WHat are the chances if you get a AN box that it will still have the .88 4th or are they even interchangeable. Sounds like the 4.12 box is the better bet?
Thanks
Lee
PS I am not trying to do the rebuild myself. Just wondering if I should get a ghia box for the rebuild or rebuild my current 4.12. I drive about 70 miles a day (highway speed) and was hoping for a little improvement in gas mileage. I will keep my eye out for a ghia box.

Post by **Marc** » Mon Nov 14, 2005 1:26 pm

You could count turns on an assembled trans to determine what 4th is in it. Lock down one output flange to eliminate any error due to differential movement, put it in 4th and see how many turns of the input shaft it takes to get four turns of the free output flange. If it's under 7 (~6.8) you have a 3.88x.88; over 7 (~7.2) it's a 3.88x.93 or 4.12x.88.
In second gear a 3.88 will need ~8 turns of the input shaft for two of the free output flange and a 4.12 will need 7½. [edit - that should be 8½]

If your AH is rebuildable, slipping in a .82 4th from a bus will approximate the "AN" final in 4th but the gap between 3rd & 4th will widen (with the stock AH if you shift to 4th at 4000, your RPM drops to ~2800; with the 4.125x.82 it'd drop to ~2600). This could be objectionable if you regularly drive on long inclines where you're just pulling 4th without lugging now.

Plastermaster · Post by **Plastermaster** » Mon Nov 14, 2005 2:32 pm

Reguardless of what they are "suposed" to be, If you look inside a tranny that was made the year of a ratio change, there is no telling what you will find. VW used their parts until they were all gone. I have a consnsus on this from different tranny shops.

Ron

Bruce2 · Post by **Bruce2** » Mon Nov 14, 2005 11:06 pm

Correction: The 4.12 gearbox will take 8½ turns.

Marc wrote:
In second gear a 3.88 will need ~8 turns of the input shaft for two of the free output flange and a 4.12 will need 7½.

Post by **Marc** » Wed Nov 16, 2005 8:37 am

Indeed, the lower R&P will obviously need more, not fewer (a 4.375 takes ~9).
Since I'm not likely to ever start typing any better it's good to know that someone is proofreading my posts - thanks.

Bruce2 · Post by **Bruce2** » Wed Nov 16, 2005 9:19 am

You only using 2 fingers to type, Marc?