MK4 Tremec 600 BP347 pinion angle question

[vtullomk4]

138038Greetings. Been a while since I posted. Build is coming along. Actually got engine in yesterday. Thus my question.

I have a MK4 with Blueprint 347, Tremec TKO600 and IRS.

When I installed engine, the tail shaft of trans was sitting on the cross tube. Based on forum posts, I ordered a transmission mount spacer kit from Mike Forte (Thanks Mike!) and installed 3 spacers, each 1/4". All went fine.

Motor mounts are installed correctly, and seated properly.

But looking at the forum posts, I see lots of discussion on pinion angle, so I did some measurements with a digital angle meter.

I set the meter at "0" sitting on the trans u-joint fork. The angle to the drive shaft is measured at 6.5 degrees up. The angle of the drive shaft to the diff U-joint is measured at 4.7 degrees down. (Sketch should show 4.7, not 5.7) thus the pinion angle is 1.8 degrees - difference from the trans shaft to the diff shaft.

If I add another 1/4" spacer the pinion angle becomes .9 degrees. The trans will be above the tube about 1 inch or so.

My questions are - did I calculate this correctly, and are those angles all within spec? 4 spacers seems more than usual. Any suggestions?

Thanks for everyones help.

Vince

[vtullomk4]

I ordered a 4th space from Mike. Any help/suggestions are appreciated.

[MSumners]

Interested to hear others as well. I have the same setup, Dart block 347 from mike forte with tko600. I made my own spacer as i needed slightly more than the 3 spacers included. I would have to measure again but I suspect I was very close to the equivalent of 4 spacers


https://thefactoryfiveforum.com/attachment.php?attachmentid=117185&d=1573010893

[GoDadGo]

You just need to get the angle of the dangle right so don't be afraid to do a little transmission shimming.

https://youtu.be/Idk3BVDVHq4

We've all been there before so I know you'll get through it too.

[vtullomk4]

Very helpful. Thank you. Based on other threads, the pinion angle needs to be less than 1 degree. And the operating angles less than 3 degrees. I think 4 spacers will get the correct pinion angle, but with IRS, not sure I can get to 3 degrees of operating angle. Is this ok?

[GoDadGo]

Very helpful. Thank you. Based on other threads, the pinion angle needs to be less than 1 degree. And the operating angles less than 3 degrees. I think 4 spacers will get the correct pinion angle, but with IRS, not sure I can get to 3 degrees of operating angle. Is this ok?

Though my car is not an IRS, the principal is the same.
You just need to get the yoke in the same plane as the pumpkin like you illustrated in your drawing in the original post.
In addition, you may also want to look at Edwardb's 20th anniversary MK-4 or his Type-65 build.
His documentation really supplements the heck out of the manuals with great photos and detailed explanations.

[Hoooper]

Unless I am reading your diagram wrong, adding more spacers to the trans will make your angles worse by making the angle from trans to diff further off but improving your u-joint angles. Ideally in a driveshaft with single u-joint on each end you want the trans and the diff to be parallel, u-joint to trans and u-joint to diff angles under 1 degree difference, and operating angle under 3 degrees. In the real world you want the diff pointing below the trans ever so slightly so that under power they are lined up.

https://www.youtube.com/watch?v=gmV4qwLfOMY&ab_channel=JCG30200

[NAZ]

Unless I am reading your diagram wrong, adding more spacers to the trans will make your angles worse by making the angle from trans to diff further off but improving your u-joint angles. Ideally in a driveshaft with single u-joint on each end you want the trans and the diff to be parallel, u-joint to trans and u-joint to diff angles under 1 degree difference, and operating angle under 3 degrees. In the real world you want the diff pointing below the trans ever so slightly so that under power they are lined up.

https://www.youtube.com/watch?v=gmV4qwLfOMY&ab_channel=JCG30200

And this is the best answer.

The key phrase here is UNDER LOAD that's why the dif will many times be pointing down X number of degrees from parallel with the trans output if the suspension or in the case of IRS, the dif is mounted in elastomer bushings as during hard acceleration these soft bushings will allow the dif to rotate and the pinion end will elevate. For solid mounted axles like mine, there is no offset to account for rotation. A rule of thumb is to have ~1-degree minimum operating angle to ensure the u-joint needle bearings get a minimum of grease smear and don't brinell. And 3-degree max operating angle is a compromise as the farther away from 2-degrees operating angle and the shaft critical speed is reduced exponentially.

If this is a high speed/low drag race car that will turn the driveshaft at high RPM these are adjustments you should be paying close attention to. I've run operating angles approaching 30-degrees on off-road vehicles that saw street duty with only minor annoyance from vibration and significantly reduced life. But these driveshaft turned less than 4,000 RPM on the street. My race car can turn the driveshaft at 7,700 RPM so I pay very close attention to operating angles and mine is set to 2-degrees max under full suspension travel. My engine is set at 2-degrees down in the front where most OEM cars have the front of the engine elevated 2-3-degrees. The attitude of the engine is not nearly as important as the angles of the driveshaft.

[vtullomk4]

Thanks to all. Now that I look at it, my sketch is not quite accurate. The diff does point down, so adding a shim to the trans makes it more parallel to the diff. I added a shim today and the pinion angle - difference between trans and diff - is now less than 1 degree. I'll check operating angles again, but my guess is around 4. Pinion angle less than 1 seems more important?

[Hoooper]

You can go a little over 1 degree if it helps your operating angle, even better if it's because the diff points below the trans. At your current angles you're within the allowances for u joints but could potentially cause some vibration. At some point if you're within acceptable range but not ideal you may want to just run it and see. From the sketch it seems like your next step would be to start shimming the engine and trans up together to keep the parallelism with the diff but decrease the operating angle

[vtullomk4]

Thanks. I’ll try it first. A ways before first start. Still have cooling and wiring to do. By Shimming engine, do you mean putting spacers between engine mounts and frame, basically lifting engine up?

[vtullomk4]

I ended up putting 4 of Mike Forte spacers between trans and mount. About 1 inch in total. Pinion angle now measures .8 and the operating angles are 2.3. So it looks like I am good to go. Thanks for all your help.

Vince

[CraigS]

... Ideally in a driveshaft with single u-joint on each end you want the trans and the diff to be parallel, u-joint to trans and u-joint to diff angles under 1 degree difference, and operating angle under 3 degrees. In the real world you want the diff pointing below the trans ever so slightly so that under power they are lined up.

I agree. And w/ the FFR IRS, I'd capitalize the phrase EVER SO SLIGHTLY because the diff mount bushings are quite firm so I can't imagine the diff changing angle that we could measure.

[NAZ]

Of course the IRS rear DOES rotate if mounted in elastomer bushings, similar to a solid axle that has elastomer bushings in the control arms. The key is knowing how much rotation under acceleration so you can offset the pinion by that amount. If you look close at the differential in this video you'll notice rotation and you can see that the OEM rubber bushings allow significant movement. https://www.youtube.com/watch?v=dA0kzu4Ukzk