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| Re: A good slush box decision for drift or skids :-D |
Subject: Re: A good slush box decision for drift or skids :-D
by jmac on 2012/5/17 8:42:36
fwiw I agree. I've rebuilt more bw35/40/55s and 904/727 torqueflites than I can accurately count (and a good couple of dozen of various others, trimatics, ford c4/10 boxes and the like).
They are fantastic where you have a setup which has so much power you can't even remotely launch at full throttle (or with nitrous activated or whatever), they are reliable and consistent (as far as performance goes) - you don't have to 'get' each shift right like on a manual (esp the 2-3 or 4-5 shift or the 1-2 and 3-4 on a dogleg) - just push one click forward and you're there.
They certainly cost power (but on a high boost turbo 6 or big v8 or the like it's hardly a huge concern) but being able to precisely load them up on the start line, and launch with just the right amount of throttle/boost to suit the traction conditions is worth more than a tenth or two with the average driver (I mean the average driver at any drag race meet, who are likely a good distance above the 'average driver' if we take into account all the drivers on the road out there).
The other big deal is esp with non std cams. you'll have no power off the line with a stock convertor. It'll lug away until you get into the cam's rpm sweet spot, and by then the show is over. SO you run a looser convertor, and can launch a bit harder. Makes it run hot all the time on the street, but even IF that isn't a big concern on a track only setup, well that looser convertor also means FAR more slippage at higher rpm, so you lose some power, and also end up having higher rpm for any given diff gears. So then you have to either risk engine damage, or run taller diff gears so you go through hte traps at more sensible rpm, but then you have less torque multiplication with those taller diff gears, so the car doesn't accelerate quite as well.
THe other big deal is the ratio spread. Most 5 speeds out there have what you might call an 'ok' ratio spread for a racer on a tight budget. Not ideal, but we can't all afford to go to Albins (who by the way produce gears that are pretty much second to none - they really are tops, but they aren't cheap). But with an auto - esp a 3speed, the gear spread is a compromise. If you look at the US dino motor oriented boxes, you'll be lucky to get more than 2.5:1 or so for first, then a big jump to around 1.5 or so for second (the early th700 4sp has a low first gear which is why vn v6 commodores were pretty snappy off the line for what was a stock/production car in their day).
Put another way - they are the sort of ratio spread that with a hi-po small capacity 4 cylinder, will have it needing to rev way past it's peak power rpm, then yanked down to below it's powerband with each gear. It makes for a VERY unsatisfactory combination, even with a motor that would in fact perform great guns in manual guise.
What I'm trying to say here is that the loss in performance will be far more than it would initially seem even IF you already take into account the fact the power robbing issues of a torque convertor fronted auto..
When you get to the point you are making 800ft lbs or more (and plenty of drag cars out there do, believe it or not) of torque, you have so much torque that you don't need the torque multiplication of (relatively) low first gear. You sort of don't need first gear at all. In fact you could (in this hypothetical) take off in second, and still have to launch with care as it will still try and fry the tyres. And in those cases, you'll naturally see more people looking at the 2 speed powerglide (which is basically 1.7x or 1.8x : 1 first gear and a 1:1 top) - They are a strong box (and can be modded to be way stronger than standard) but the point is they don't 'need' the extra gears in those situations.
But this sort of thing is absolutely at the other end of the spectrum, a small capacity engine with a need for more or slightly closer spaced gear ratios, and less than an overabundance of power.
TO be completely fair and play devil's advocate, if someone wanted to fund an a series made to run a big supercharger and nitromethane (so likely the biggest power output an a-series would ever make, for however long it lasted) - then I think it could be made with a wide enough powerband and if for no other reason than consistency and predictability, I'd give an auto a go. It wouldn't be a jatco though. Taking nothing away from their attributes, they aren't cheap, and I could likely cobble together a trimatic with a 4cyl gemini (or toyota corona) convertor (albeit opened up and stengthened with a few things, including a v8 sprag) to do the job. It still wouldn't be perfect, but you'd have some justification for it at least.
FWIW I seem to recall one of the tilley's running a valiant circuit racer using a fully manualised torqueflite. Even with a (relatively) high torque slant motor, you'd still have to select the convertor very carefully, so that it launched ok, but also so engine braking, and transition back to full power was smoother than with a convertor too loose.
One of the guys on PF was running a very high output 2JZ based motor (I forget what in for the moment) he ran a toyota auto, that was considerably modified, but still had dramas with the mph not being where he felt it should have been. Went to the stage of modding the convertor/setup so that it would actually engage the lockup clutch in the convertor at full throttle at a certain rpm or speed (I forget which). It certainly helped, but I wouldn't be taking bets on how long the friction linings in the lockup convertor would last with that sort of punishment.
That's the thing though, a lot of the cars you see in magazines and the like have this or that transmission, and they run good times. But they likely see only a couple of hundred kms (tops) between teardowns. An example would be the modified trimatics. I wouldn't have too much trouble with one behind an a-series, hypothetically, but between a few of the people I know/mix with, some of the big name race spec trimatics won't even hold up in a street driven car that runs in the mid 12s. Fullsize HQ sedans and coupes and the like with v8s, so obviously not lightweights, but still. Just could not get them to hold together, and the specialists who sold the boxes just wanted more money to keep repairing with no real options left to improve longevity (th350s and 400s on the other hand, had no trouble in the same cars, and they are all running them now)
It's still not a dirt cheap option, but I strongly feel a 71b box out of a series 3 ca powered bluebird is about the best combo of price, vs likelihood of not being thrashed/damaged vs strength to cope with a very high output a series.
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