Highway flyer trans, question!

NextGen · Post by **NextGen** » Tue Jan 25, 2005 2:13 pm

I have been involved in a heated dispute with Bruce over at Samba over the following.

I have found the 74 and later trans to make a real difference in lowing RPM's and noise levels. I guess he never tried going from an early trans to a later then 74 trans. He keeps on stating the math that the difference is same. Guess he never tried one. Real chip on this guys shoulder. I refuse to go back to that post but was wondering if any of you guys see the difference as I do.

Joe

Plastermaster · Post by **Plastermaster** » Tue Jan 25, 2005 2:32 pm

I have not experienced both trannies, but the math works. (providing you take into consideration tire size. The rpm drop can not be other than what it mathmatically works out to. As far as noise, a few hundred rpm can make a difference between quiet and loud depending on the engine. I am actually quieter at 75MPH than I am at 65. except for wind noise.

Ron

Post by **Marc** » Tue Jan 25, 2005 2:44 pm

It all depends on how early a trans you're replacing. If it's a 4.125 ring & pinion and a .89 4th, that's a final of 3.67; if it's a .88 4th, it's 3.63.
The `73-up trans has a 3.875 R&P and a .93 4th for a final of 3.60. The difference is not earth-shattering in either of these cases, only .19 or .07%, respectively....Bruce is correct, there's only about 25-50RPM (not 500) difference at 60MPH.
Now, if the late trans happens to be from a `Ghia and still has the .88 4th with the 3.875 R&P (or if it's custom-built with those ratios) you're talking about a 6 or 7.66% difference, which starts to be significant. And if the early trans had a 4.375 R&P and .89 4th, that'd be a 14.2% change.
I run a stock late trans (3.875x.93) in my `69 and it's not even enough taller to make up for the slightly shorter-than-stock 195/60-15 tires I have on the back, compared to the factory 4.125x.89 ratios.

NextGen · Post by **NextGen** » Tue Jan 25, 2005 2:45 pm

I figure at least 500 rpm drop, have a tach!! Like I said the car sounds and feels at 70 as it did at 60.
Joe

Post by **Marc** » Tue Jan 25, 2005 3:22 pm

If you replaced a 4.375x.89 trans with a stock latemodel 3.875x.93 box, you'd be ~8% taller - not even 5MPH difference, let alone 10, at 60.
The combination that most builders market as the "freeway flyer" is a 4.125 R&P with a .82 4th from a bus - that makes an 8½% change compared to an early IRS 4.125 box, less than 7½% compared to a late DSC unit in 4th and leaves the lower gears unchanged. Personally I dislike the wider gap that creates between 3rd & 4th, but it's tolerable most of the time.

aircooledtechguy · Post by **aircooledtechguy** » Tue Jan 25, 2005 3:26 pm

I know that when I traded my original bus tranny (4.12 R&P w/ stock .82 4th and 1.25 redux boxes) to a 3.875 R&P trans (everything else the same), there was a noticable decrease in engine noise. The interior was WAY quieter @ 65mph than before. It probably didn't drop my RPMs much more than a couple hundred at best (I didn't have a tach installed at the time), but at the top end RPMs, noise increases exponentially.

Mathmatically, the decrease seems slight, but in reality, it does make a noticable difference in noise reduction. Stick to your guns brother!!

Post by **Marc** » Tue Jan 25, 2005 3:39 pm

nate@nateswaterart wrote:I know that when I traded my original bus tranny (4.12 R&P w/ stock .82 4th and 1.25 redux boxes) to a 3.875 R&P trans (everything else the same), there was a noticable decrease in engine noise....
Mathmatically, the decrease seems slight, but in reality, it does make a noticable difference in noise reduction. Stick to your guns brother!!

If you kept the same 4th gear ratio, that's a 6% change - far more than you get by replacing a 4.125x.89 or .88 with a 3.875x.93. But it's still only about 250RPM on the highway, one can't help but wonder how much of the "engine noise" was coming from the inside of the old transmission.

NextGen · Post by **NextGen** » Tue Jan 25, 2005 5:45 pm

Guys, seriously. My commute is 70 miles every day at a steady 75mph, the noise dropped, to like I said about what it was at 60 to 65mph. Also the engine feels the way it did at 60 to 65. When you drive your car trying to keep up with traffic every day, then you make the change and find out you can criuse as the same speed as the traffic and the engine is not buzzing like crazy. There is a big difference. and my tach shows it. I totally under stand you guy are going by the numbers, but this is the way it feels and what my tach shows. My tires are 195 15 if that makes any difference.

Marc if you don't like this gap, you would really hate the 914 gap.

Joe

Plastermaster · Post by **Plastermaster** » Tue Jan 25, 2005 6:25 pm

If you are running the same size tires before and after the change and you are experienceing a 500 RPM drop... well, what can I say?

Rpm aside I still hold that a small rpm change can make a very noticable difference in noise level that is NOT directly proportional to rpm, due to harmonics, ballance issues etc.

It would be nice to have a big get together, where we could all try out each others rides.

Ron

MASSIVE TYPE IV · Post by **MASSIVE TYPE IV** » Tue Jan 25, 2005 6:47 pm

Did you change engine/trans mounts??

BUT, lower RPM can drastically make for a quieter car. It uses the MassIVe torque of the TIV engine and multiplies it into solid speed.

Class 11 streeter · Post by **Class 11 streeter** » Tue Jan 25, 2005 6:49 pm

This thread might be of interest in the transaxle forum, but didn't VW change the gear tooth count from the early IRS trans to the late trans for quieter running when the AH (73?) case trans was issued?

raygreenwood · Post by **raygreenwood** » Tue Jan 25, 2005 10:53 pm

There can be a BIG difference...math not withstanding...from small changes in ring and pinion ratio. What the math does not take into account...because it cannot......easily...is that the torque at take-off in first, the peak torque and ignition advance levels of any particular engine configuration can make different trannys...ever so similar mathematically, drive differently. A case in point, the 411/412...with the 3.79:1 final drive is drastically different off the line and through 3rd gear than that with the 3.91:1 final drive. Its only a small amount of rpm....and its no discernable difference at highway speed, but its easily felt off the line. And it really depends on what your ratios are in 1-3....not just final and fourth. The 411/412 is a 1:1 fourth. Ray

Post by **Marc** » Wed Jan 26, 2005 6:28 am

I have a tach too. I've also had a variety of transmissions over the years and half a dozen different tire sizes on my car in the last year or so (my son has been playing around with different combinations between 4 bugs and a 912E).
The question was, as I recall, if anyone had any experience to back up your argument against Bruce.
I do not. Actually I cannot, since you have not specified exactly what transmission you are comparing to the late one so it's impossible for me to know if you truly have a significant ratio change, or if you had a particularly noisy transmission before, or if it's all just wishful thinking.
All I can tell you is that the difference between a stock, bug, SSC trans and a stock, bug, 4.125 trans is barely discernible. The fine-tooth gears came on the scene in about 1970 as I recall; personally I've never owned a car in which the ambient noise level was low enough to make it possible to tell any difference between them and the coarse-tooth gears - If you ask me, there's more variation in mainshaft bearing noise from transmission to transmission. If there was a "real difference" it would be between a pre`70 and a `70-up, or a pre`73 and a `73-up, not `74 - the only thing different between a `73 and a `74 is the Reverse linkage.
The numbers don't lie. Guess you'll have to try to find a third forum if you can't accept someone disagreeing with you. If you do, may I suggest that at some point you let on what transmission you are comparing the late one to?

NextGen · Post by **NextGen** » Wed Jan 26, 2005 7:15 am

Ok I went back into a hidden away part of my brain and pulled up some data that may make this a bit more understandable. Actually less understanable because the data is not complete.

My 74 was an automatic that I made stick using a 69 trans. I asked Mike, the late Joe of Oregon Performance to make me a 74 trans. ( By the way working with that guy was a mistake). Anyway, here is what he said he was going to do, now this was 14 years ago, so lets see what I remember.

74 trans, welded 3rd and 4th gears.

Also said he was going to use some early gears because they had less teeth and were stronger. What and where they were I don't remember.

He may have made some weird ratio ( but I like it ).

Also my tires were 160 and I went to 190 15's

I may have a weird set up because am doing 2500 rpm at 60, not 3000 as before.

BTY he sent my tranny by UPS it got damaged. So I paid him $350 and he also got $350 from UPS. Plus it took 8 weeks to get my trans.
Joe

Plastermaster · Post by **Plastermaster** » Wed Jan 26, 2005 7:31 am

You could possibly have a 3.88 rp with a .82 4th. What is the aspect ratio on those 190 tires? We'll get to the bottom of this!

Ron