C4+3=/=C7

Sorry for making what seems like a redundant post topic, but I have a few questions relating to transmissions for my '85 MH5 and am interested to hear your opinions on this situation.

First of all, I've been having some transmission/clutch issues (I made a thread here) and found out recently that AC Delco will replace the clutch for free. I will hopefully have the transmission out of the car soon, but I have a few concerns that I discussed in greater detail in the other thread but will repeat: I've been hurting the synchros by driving around with a partially-releasing clutch, there is significant gear whine in direct drive, the car sometimes will buck easily in lower gears, and reverse grinds badly. The first point has obvious consequences (bad synchros), while the bucking would seemingly have to do with first and second gears and the whining might have something to do with the overdrive's annular bearing.

In any event, even though it kills me to put thousands more into an otherwise-ailing car to fix one problem, I am determined to keep improving the car and have been reviewing options for a transmission rebuild or swap. I looked seriously at rebuilding the 4+3, swapping a ZF6, or installing a TKO or Richmond 6 speed. I eventually narrowed the decision down to ROD or 4+3; I really like the Doug Nash/Super T10 combo, but the direct swap to the ROD seemed really appealing as well. Both are around the same weight (heavier than Tremec, lighter than ZF) but with favorable gearing for mixed use while being robust enough to handle as much power as I'll want to throw at the car.

Well, I called SK Speed today and talked to someone at the trans shop who was also a fan of the 4+3 and was very friendly and helpful. He said they were on their 1,029th 4+3 rebuild and that the cost of overhauling the entire transmission was around $2,500... a lot of money, but still less expensive than other options. He told me that he'd go with a 4+3 rebuild from an experienced rebuilder and with revised parts rather than a 5 speed swap and also informed me that Richmond Gear was bought out and no longer makes the 6 speed, which he thought was a good transmission. That explains its absence from their site... However, in terms of rebuilding the current transmission, I am pretty confident that SK Speed would be a great option. SK bought Medatronics' 4+3 tooling and business and apparently has been working on the 4+3 from the beginning. Medatronics has been working on 4+3s since the beginning as well, having upgraded some OD parts for GM, modifying them for Callaway B2Ks, and doing work on Challenge cars, so I think it's safe to say that the combination of Medatronics' furthered engineering and SK's experience is reassuring.

It seems that the ROD is out of the question, but at least my decision is easier now. A 4+3 with all revised components but still with on-demand overdrive sounds pretty great, and there's always the "4+4" mod and the very remote chance that a short shift kit is still out there for sale. However, this is a big decision that will probably sideline the C4 for several months and I'd appreciate some input.

-Is it possible to change gearsets among different Super T10 ratios? If so, recommendations?
-Would it be possible to get a taller overdrive gear than my non-Z51's 0.60? Worthwhile?
-Does anyone else have experience with loud transmission whine? Is it possible that it isn't a symptom of a bad OD annular thrust bearing?
-Does anyone have any experience with ZF6, TKO (swapping or driving)? Any opinions between those?
-Does anyone here have experience with SK Speed?

Thanks a lot guys. I'd really like to hear any thoughts you have on the matter.

1985 Corvette

Mine started off a 700R4, then was slated to be a 4+3 car. Somewhere along the way I got frustrated with trying to comprehend the 4+3 and I shelved it for a ZF, which I swapped instead. The shifting feel of the ZF is superior but for a street car, I feel the 4+3 is plenty and you're right, the gear splitter on back of the 4 speed is a neat trick for its day. When clutched in and OD is activated in any gear or as a "fifth" gear, the transition is seamless in a properly function 4+3.

Heard nothing but good things about SK speed. Sounds like this car isn't a daily driver but more of a toy. So my question is why not give the rebuild a shot yourself? The OD rebuild guide is usually floating around on ebay all the time for $20 and you also have the FSM to fall back on. You're right, $2500 is a lot of money. If all the gears check out, a rebuild kit for the 4 speed is about $150 and the OD rebuild kit is about $300 to your door. Why not take some time to learn how to do it yourself, and be able to say you went through the box and refreshed it yourself?

Try Cobra Transmission's website, as they usually have all the stuff you would need. If I was going to spend $2500, I'd save up an extra grand and go for a TKO 5 speed. Not that I don't love the 4+3 but it's basically a T10 with a gear splitter bolted on back....nothing out of this world complex. You'll just need a few special tools and some research.

But if you're not the type that likes to take stuff apart, disregard.

C4+3=/=C7

I've actually read a few threads started by you some years ago when trying to figure out the whole ZF and 4+3 swap specifics while doing some research. Good to hear about SK Speed. I have very little ability or work space and few of the tools needed for the job, so I don't really trust myself to do it myself. That will be something to consider, though, especially considering the difference in cost. I might go ahead and drive a 6 speed C4 just to make my mind up better, but I really would find it hard to leave the Doug Nash camp. The car actually is a daily driver (although that may have to change quite soon) but will eventually be used for track days and I'm planning on one autocross; however, I can justify the 4+3 for such use because the ratios are nice while the weight and torque capacity are... competitive with ZF6, TKO 500, etc. Maybe my affinity for the overdrive is just a result of having to defend it outside the Corvette community so often--I'll certainly have to weigh my options ("test drive" the overpriced 6 speed LT1 at the Lexus dealership perhaps ) and think about the DIY route if I decide on a rebuild. Thanks for the suggestions!

ChimpWithWrench

Wait how did he break down $2500? I just called myself and talked to Brian. I thought he said it was $1300 for a rebuild, unless I missed something. Maybe shipping? I am a town over from SK so maybe thats it?

C4+3=/=C7

Sorry for the late reply. He was saying it usually runs about 1,250 for the manual trans portion and about the same for the OD. I don't even want to think about shipping (being hundreds of miles away). Maybe you were inquiring about the overdrive only. Anyways, if it's really $1,300 for the entire transmission, I'll be ecstatic as it looks like the Corvette will be going away for quite a while otherwise. Just had the suspension rebuilt to make the car liveable as an almost-everyday driver, too.

Also, a question about the carrier bearing for anyone who might know: if it fails, will it cause the transmission to be badly damaged? I read somewhere that a failed carrier bearing can result in the planetary gearset moving out of place and, while my OD works, the gear whine in direct drive definitely seems like a symptom. If I'm not risking anything other than the carrier bearing itself, I'll be able to continue to drive the car. The noise has been present for at least 5k miles of mostly city driving and some spirited driving and there hasn't been any overdrive problem yet. Thanks again.

c4cruiser

You can read thru these pages from S-K Speed concerning the various components of the OD. The carrier bearing info is the first tab.

C4+3=/=C7

That's actually the page that got me worried about additional related damage caused by a bad bearing, but I can't find whether or not it will potentially cost me more in the long run to continue driving as is. In other words, am I damaging other components by driving with the whining bearing (as it describes the planetary gears coming out of place as a result)? Thank you for the information.