Use of stock CVs with big(ish) 6

[bumble]

Questions for the transmission gurus:

I've just been testing my almost complete teener at the track. Car has a 2.7L six with about 180HP at the wheels. After several progressively faster sessions, shift from 3rd to 4th, slight 'clunk', no drive - damn! Load on trailer and take home. Imagine all sorts of $$$ internal gearbox problems. Turned out to be the bearing cage in one of the (new) inner CVs has broken - much relief all 'round .

So:

1. What's the general opinion on using the stock 914 CVs with this sort of power?

2. Given the diff is a Quaiffe, would you expect to get drive to one wheel? It didn't which concerns me somewhat

Cheers,

Mark
914/6

[Jim Smolka]

There are couple of previous post on CV options.

Some folks go to the 911 CVs and stub axles.

Here is my configurations. From the transaxle:
901/911 drive flanges (rare as hen's teeth)
Spacer
Early 911 CVs
Early 911 Stub axles
Carrera Hubs
Carrera rotors and brakes

It is possible to use 901/914 drive flanges and mate them to the 911 CV's, but you will need an adapter (I have these)

[bumble]

Thanks Jim for the reply - that's an interesting setup.

Before posting I did a search for CV related stuff. Problem is there is a lot of info on what's been done with various setups, but not why it's been done.

I guess I'm looking for a definitive opinion based on the collective experience of the group whether or not the stock CVs are strong enough to last maybe 1000 miles of fairly hard driving. The car is entered in a Tarmac Rally in four weeks but I have no time until then to begin a major re-engineering job.

I was hoping that I simply had a CV with a fault or something and that the stock CVs, while maybe not ideal, would go the distance. After that, I have no proble tossing out the existing setup and replacing it with something more robust.

Cheers,

Mark

[Jim Smolka]

I trashed a 914 CV in my 3.2 IMHO, if you are going to Rally the car and are near 200 HP or better, then the 914 CV's are not going to hold up to the rigors of a race.

However, you may be lucky and get by with a new set of 914 CV's

[Dave at Pelican Parts]

The Torsen-type differentials will not spin one wheel when the other is up in the air or disconnected. It's just how they work.

I have heard a lot of conflicting information about CVs. One thing I have heard is that most of the time it isn't power that breaks the CV, but rather being pushed hard at the wrong angle or in the wrong direction. Supposedly a good fix for this is to modify the axle shaft to let the CV slide further down the shaft. This keeps the angles from getting so extreme, and (according the the several people I have heard this from) cures most or all of the breaking CV problems.

I think the modification involved re-locating the groove for one of the C-clips that holds the CV joint in place.

--DD

[Will98D]

What kind of torque does your engine produce? You can figure the max input like this: engine x gear x differential x 0.5 (as the max torque to either wheel is half the total unless you have a locking diff or no diff like in a circle track car). I'm going to assume that the stock (GKN) vw/porsche 914 cross-groove (CV) joint in question has a rating of about 2,100 N-m, or 1,550 lbs-ft torque. I've assumed a 4.43:1 standard ring gear and the standard 914-4 gear sets for the calculation. This would mean the max engine torque input per gear could be (1) 226 lbs-ft, (2) 370, (3) 554, (4) 756, and (5) 986. Now obviously if you put 986 lbs-ft torque into the trans something else would blow before the CV joint. However, you can see that in 1st gear, at 226 lbs-ft, you could likely exceed that with your 2.7, but it's unlikely you'd ever get close to the 370 of 2nd gear. It also might explain why a lot of the V8 guys have better luck with skipping 1st. Plus, my 2,100 N-m was just a guess... it could as easily be 1,900 or so, getting the 1st gear low end down near 200 (271 N-m). No guarantees, but I think if you went with a standard joint and stayed out of 1st gear until you had a chance to upsize things then you should be OK. I know you said it broke in 3/4, but I suspect the damage was done in 1st gear with a hard launch; I'd change them both. One other thing: these are for joints close to a zero degree angle (~0 - 4 degrees). Once the angle rises, the load capacity drops, so if for some strange reason your wheels are not at a normal ride height this will add to your problems. Lastly, do not disassemble the joints without really figuring out how they go together first. They have 3 pairs of two grooves in a V (hence 'cross-groove'), and if you flip the inner race over accidentally when reassembling you may get it together, but it will lock in the mid-stroke position and soon break the cage. It is best to always mark them before disassembly so the parts go back in the same relative position to one another (the only thing you don't have to worry about is getting the 6 balls in the same grooves).

wrt Dave's discussion about moving the snap ring groove: a line between the joint centers will determine the angle of the axle bar. The outer races are fixed by the wheel bearing and trans flange, so you can only move the inner race further into the outer race. This will change (improve) the angle very slightly, but moving the ring groove you will have the same load over a shorter spline which will lead to an earlier fracture of the bar or inner race. Plus, then you'd also have to worry about the joint bottoming out inside the outer race.

[bumble]

Thanks to y'all for the replies.

Given my lack of time, I'll replace the broken CV, check the others and race it as is with crossed fingers. Then, regardless of whether or not it breaks, I'll install a much stronger setup.

FYI, the tranny (901 side shift) has a much higher 1st than standard (15:36) and I have been very careful not to do any hard starts in 1st.

Ride height is lower than normal but this has been obtained by shortening the front struts (Carrera 3.2) by about 50mm and shortening the rear shocks (Koni adjustable) by a similar amount. Drive shafts are more or less straight at rest and given the short suspension travel, I find it hard to see that excessive angle is the cause.

The day before, I was doing some shakedown runs on local back roads. Found a stretch with a deepish water race (empty - we're still in drought here) so took a run at it at 140kmh (about 80mph). Car got some air but nothing too dramatic. This may have contributed. Regardless, the car must be able to withstand this sort of treatment.

Regards,

Mark

914/6
928 S4

[vwzorm58]

I know this is a slightly different animal, but it may be pertinent. I've got a 73 VW Westfalia Camper with a stock 914 4cyl 2.0 motor and a 901 5-sp transmission. When I initially did the conversion I used 914 flanges, some 9/16" spacers (the 901 is narrower than the bus tranny), and 914 CV joints (a perfect fit on the bus axles).

My experience was that I was eating inner CV's every 6000-8000 miles. Pretty bad considering nearly $100 each for CV's! I attribute the early failures mostly to the much exagerated travel of the rear suspension on a bus vs the very slight amount on a 914. Failure may also be due to the 4500 pound weight of the bus vs the 2300 pounds on the 914. I don't know how to figure out how much force that would exert, but it's gotta be a LOT more than the 914 CV's were designed for!

Then one day, much to my excitement, I found that Pelicanparts was selling a kit to allow use of BUS CV's on a 914! Their kit ($600) includes 4 adapters, 4 CV's, 2 axles, and all hardware to bolt it all together. Since I only needed 2 adapters, I sent them an email and they kindly agreed to sell me just the pieces that I needed for my application at a reduced price. I can tell you, the quality of the adapters was 100% awesome. Whoever is doing the machine work on these should be commended.

Anyway, the point is, if you're eating CV's due either to too much travel or too much power, perhaps you should consider this "upgrade". If it solves your problem for $600 for the complete kit, it'll save you money in the long run.

YMMV

Mike g.

[bumble]

Thanks for the tip Mike - I shall have speaks with PP about this

Cheers,

Mark