A518 Overdrive Questions

[Jacksboys]

What year is your 500? '92 they did a pump/gear change(slots or flats on the hub)

Thanks for the info.
I am not sure on the year of my 500.  I looked down the shaft and I could see two small tabs (picture example below) Does that mean I have the slots type?

[tactransman]

Yes,you have the '92 and earlier type (slots)

[Jacksboys]

Yes,you have the '92 and earlier type (slots)

Thanks for the help   

[PlumCrazyChris]

ROB, YOU ARE THE MAN!!!!!

That is a sweet setup.  Just think, you are the first person to prove that it can be done.

Not exactly...

"I've heard that you can angle the original shaft and just mock up a lower pivot point for the shaft to get it away from the trans body and it will still work fine.   This is from 1WildRT out in Cali, that did it to Ransom's 70 vert. "

....But I'm sure glad to see its just that easy though.   I'm really ready to do this soon.  My tranny and 90 TBI 318 motor have been ready, sitting in the shop for almost a year now.   I'll just have to section out the trans mount to allow for the speedo/computer hookup to connect.   I'll use the stock computer for now, this is the Variable Speed Sensor I will use.

I'm hoping with this mild combo to avg about 20 mpg. and use it as my daily driver.   

[PlumCrazyChris]

Anyone get their OD on the road?   My friends shop where I was working, is not going to work out.  I can only work there while he's open for business, which are pretty much the same hours I work at my day job.   I've been making room above my garage for storage and cleaning it out for a couple of weeks. 

When its ready, I'll start working on it in my garage.  I'm going to be putting it in the paint shop soon too, but want the trans in and working first.  Then I'll strip it and take to the shop.  Then I hope to have everything ready to go back in when it gets back.   Its going to be a challenge I think.   I hope my wife will be understanding.....

[ShelbyDogg]

Sorry, but I didn't go with the overdrive. I'm going with the 4-speed setup as discussed in my restoration thread. It didn't take much thought, since the manual e-bodies are so much better than autos. (but those are fighting words to some people here)

I thought that I this is all moot since I got it hooked up and shifting?

Rob

[PlumCrazyChris]

Ya a manual would be nice and I may go that way at some point, but since I have this, I'm going to go with it and see how it goes. 

Did you put in a an 833 or a kiesler?    or perhaps a Passon OD 4spd? 

Chris

[MyMopar]

I've been looking into doing an auto od swap for this witner when I restore the Challenger.  After reading this thread I guess for me a lock-up would do the job.  3.23 gears and a 408 stroker.
I can get a non-lockup but I don't know if it will live with the 3.23 gear.  Can always swap them out I guess for a 3.73 or 3.91.

[tactransman]

I would definitely go with 3.73 or 3.91 gears with the OD installed. I will have 4.30's with mine but that is with 30.5 inch tall rear tires also.

[PlumCrazyChris]

I have to agree, I don't think I'd go any lower then 3.55, which is a pretty good gear, it would put you right at 2000 RPMs at 70mph,  3.23s come in at about 1850 RPMs and give you an effective final drive of 2.20, 3.55s about 2.40

In a great thread in moparts on gas mileage and motor efficiency, it was suggested that for a 350ish size motor, varying very little with other sizes, the most efficient RPM is ~2000 or about 1200 fps piston speed.  So if you average crusing speed is about 76 instead of 70, I like 73 myself, you'd be doing great.  I think the 3.23 is kind of low, but if you have a real torque monster it would probably work just fine, but I'd go with a A500 to get a little more gear down low and help you get off quicker from a stop.

I'm sure you'd be happy with any gear north of 2.76. 

Let us know how it turns out.

[71chally416]

I'm interested in how much of an ET hit you take when you switch from a 904 to a 518 tranny. I've heard it can be well over a second. Has anybody actually ran a car with both trannys and no other changes? The lower cruising RPM helps a bunch with gas mileage, but at what cost to performance?   

[PlumCrazyChris]

It shouldn't make any difference.  They are the same except for the overdrive, which is not used while racing.  There may be some internal loss, but its hard to believe that there is a 1 sec loss in the quarter.   

I have no personal experience on that though.  Let us know what you find out.

Chris

[tactransman]

He is going by the SMR guys.http://smrtrans.tripod.com/smrtransmissionsintro/id21.html It will effect it alittle but stock to stock comparison I don't think would be a 100 H.P. difference. Going to one of their $6000 low drag trannies from a 518 might. 

[71chally416]

I know my own experience. I picked up almost 4mph and 3-4 tenths switching from a 727 with a loose 10" convertor to a homebuilt 904 with the low 1st gear, a Turbo Action manual VB and a stock 904 convertor in my 318 Cuda. Part of that was the inefficency of the 10" convertor (I think it was a B&M Super Holeshot?) but part of it was the lighter rotating mass of the internals, the lower 1st gear ratio and lighter overall tranny weight. I suppose the motors low compression and lack of tq had something to do with it too because it really liked the change. I could immediately feel the difference. It would have ET'd much better with a good convertor because it only did 12.70/80's @ 109 with 9" slicks hooking up with consistent 60 ft times. It should have did low 12's.

Why do you suppose so many GM racers use the Powerglide instead of the much stronger TH350 or TH400? Not because it only has 2 gears and is inherently weak. I remember it being a joke back in the 70's because they commonly failed, even behind 6 cylinder engines. We called them "Powerslides". I never would have guessed they would one day be the predominant automatic racing transmission in bracket racing. It simply uses the least amount of hp of any automatic. Each additional gear in an automatic adds more parasitic rotating mass going down the track. Rotating mass hurts performance much worse than stationary mass does, like added vehicle weight. Carry a car rim for 50ft and then carry the same rim while you also spin it and see which expends more of your energy. The same applies to pistons and valves and rods and and water pumps and anything else that's moving going down the road. All that stuff saps your engine hp.

I'd be very surprised if anybody has installed a 518 where a good 904 was and not lost a bunch of MPH and ET. I'd guess at least 5mph if everything else remains the same, but I've never seen an actual A-B comparison. 

[ntstlgl1970]

I'm interested in how much of an ET hit you take when you switch from a 904 to a 518 tranny. I've heard it can be well over a second. Has anybody actually ran a car with both trannys and no other changes? The lower cruising RPM helps a bunch with gas mileage, but at what cost to performance?   

I think the main thing is that no one is going to use a 518 just for racing, for me I'll take the performance hit if it means I can comfortably drive the car longer distances. To drive from Sacramento CA to Reno NV (Summit Store)takes you over highway 80/Donner Pass (7000 feet). With my current setup (727, 28" tire, 3.91's) I turn between 3100-3400 on the freeway to keep up with traffic (around 68-70 mph) it gets really old really fast.

 
The extra overdrive rotating mass is probably close to 40 or 50 lbs, but I have no idea how much of a difference that makes in the 1/4 (is there a rule of thumb for drivetrain rotating mass?). Going from a 904 to a 518, between the added weight in the converter (thinking lockup here) and the overdrive assembly and the overall weight of the unit itself I could see there being a loss in ET. I think a full second is BS though. 1st through 3rd operate exactly the same as a 727/904 except there is the extra rotating weight to overcome. Just for speculation (assuming same converter stall speed and gearing between the two trans) lets say the extra rotating mass is 50 lbs and the extra total weight is 50 lbs. So the 60ft would suffer cause you have to get the extra inertia weight overcome, and the extra total weight would be .05 in the quarter (using the 100 lbs = .10 rule of thumb calculation). I could see 4 to 5 tenths due to the 60 foot maybe but a full second?? Feel free to correct me if I am wrong...

Tough to do a back to back comparison but maybe one of the drag calculators would help.