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Discussion Starter #1
I have a 350 with a TH350 backing it in my 84 Elco. Issue is my Daily died so I am dailying the Elco like I used to. But my Hwy MPG is terrible due to no OD. I was looking at an OD unit but I might as well sell my TH350 (With a shift kit) and buy a new trans instead of dropping 2.6K on a unit.
What would be the best trans to put behind my 350 for MPG and performance? In the future (couple years) I plan to upgrade the engine to an LSX or LS1 pushing 400-600HP, so I want something that will give me the MPG now and the durability later.
I am willing to add a computer if needed for the trans as i'll need one in the future for my impending engine swap.
 

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You would also want to figure cost for changing it versus gas mileage and see how long (miles) pay back/break even is.
 

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I'm in the middle of a tranny swap right now.
I wanted a 2004r, the slightly higher first gear and the slightly larger OD (.67) and MUCH easier installation process. (until......)
I got a 700R4 for free, rebuild on it was 1600 (9 clutches, different servos etc..)
The issue I am having with this swap, crossmember doesn't work, changing all the speedo stuff, driveshaft getting cut and a different yoke.
Also, the 700 has a steeper first gear (1st to 2nd is quite a jump) and the OD is .70, I will be adding a 3.42 gear to the mix (versus the 2.78) so revs will be slightly lower, much better pull on the low end though).
When completed I can give more feedback on gas mileage and real world daily driving. (I should note I am running 16 wheels (25.7 height)

Rev calculators come up with this information;
Existing
2.78 rear, 25.7 tire height, 3-speed auto = 2726rpm @ 75
If I replaced rear set only (3.42) = 3353rpm @ 75

New (700R4)
3.42 rear, 25.7 tire height, (700R4) 0.70 = 2347rpm @75
2004R
3.42 rear, 25.7 tire height, (2004R) 0.67 = 2247rpm @75
If I leave the 2.78 rear set @ 2.78 I get these numbers with the 4-speed AOD
700R4 = 1908rpm @ 75
2004R = 1826 @ 75
 

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Discussion Starter #5
http://www.gearvendors.com/index.html
or
https://en.wikipedia.org/wiki/List_of_GM_transmissions


mpg is not just OD, but 30% OD helps a lot..


carb/carb tune, timing advance curve, cam/cam timing, motor build/cid, type of fuel,
trans/#speeds/od, rear gears/tire size, arrow dynamics, weight, maintenance/lube, tuneup.....
and of course the speedo and tach need to be right for accurate measurements..


:texas:
I know a lot more goes into it, but transmission is where I am planning to start. I plan on trans, new rear diff, then engine.
 

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I know a lot more goes into it, but transmission is where I am planning to start. I plan on trans, new rear diff, then engine.
Very smart most people start up front.:alc:
 

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My tired old 454 with 2.42 gears and a Turbo350 tranny got 11mpg. My rebuilt 440+hp 454 with a built 700r4 with post 3.55 gears get 17+mpg. Yes the drive shaft will need to be shortened (approx 3") and the cross member modified. However, I like the results with mine. Your mileage may vary.
:texas
 

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Final drive calculation is important to figure out. For example; if you have a 2.42 rear end gear ratio and a turbo 350 trans which has a 1.00 3rd gear you have a 2.42 final drive.
If you are using a 700r4 with a .70 4th gear and a 3.55 you have a 2.48 final drive.
Other things to consider are whether you have a lock up torque converter which on a car that is driven daily is a nice feature. It drops ultimate engine rpm another 150 +/- rpm and keeps transmissions temperatures down.
Fuel economy is best with lean mixture and high ignition advance during steady highway cruise.
 

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Discussion Starter #9
Very smart most people start up front.:alc:
The engine I am looking to save to put up front is the LS 427 from blueprint. (But that's going to be a long while from now)
Currently I was looking at an 8L90e, but would require a ECM when I am carbureted with no computer (long run spending a extra couple grand I don't have). So that pushes me back to going manual with a T56. I want slightly better MPG now that is futureproof to work great with what I have later. I know my engine will bottleneck the efficiency of whatever trans I put in, but it will be better than a TH350.
 

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While I haven’t done the swap on the el Camino yet, it’s going to happen. I have a 94 Buick RoadMaster Estate wagon that has the LT1 Vette motor and 4L60e od. This big wagon will run my poor el Camino off the road 0-60 . The el Camino has. 300/500 and T 350 and 3.42 gears while the Buick has the 265/350 and 4 spd od and 2.92 gears.

This 4L60e is the best auto I’ve ever driven of the classic cars. The low and 2nd gears give a huge performance kick and adding a 3.42 to your elky will make it even better. The trans shifts perfectly at all speeds and loads. Yeah it takes a controller and some speedo trickery but it’s well worth the extra trouble.

There are a number of streetrod guys I cruise with that have 200 r and 700r that have had all kinds of problems with them plus every one that has a carb has drivability quirks. The electric od allows you to tune these out. It’s even better if you use a multi port EFI. A couple guys have this combination and these cars run great with the electric od. It takes a little tuning but it’s easier with the hand held controller than messing with mechanical stuff.

Not all guys get great mpg but it has to do with driving habits. I used to get 14-15 with my 6-71 blown sbc Willys T 350 while guys with 300/350 and t 350 got 10-12. On the same cruises.

I’ve researched these swaps extensively lately especially the 4th gen but they are about the same regardless of gen with a few specifics.
You need crossmember mods that you can do your self or just purchase. Also the drive shaft needs to be altered or just have a new one made. Some shifters you can get new gates others you simply.......costly need to replace. You may need to alter the trans tunnel a bit. As fare as building for the future, you need to look at the rebuilders lists. The electric trans can be tuned to cover most any reasonable motor until you get to the bigger race stuff. Personally I’d just get what you need for the present and budget for a rebuild at the “ race” motor time. It may not happen so why spend now.

The biggest factor is ultimate cost. 4L60e I more costly than other od’s especially when you add $600+ for the controller. In my mind this is hard to swallow until it comes to doing the work. Getting linkages to work correctly can be time consuming and more than likely will cost a trans rebuild if you make a bigger mistake.

I’m just starting to gather parts for the EFI install then I’ll go after the trans swap. I simple can’t have the elky being manhandled by the Big Buick.

The pay back from mpg just isn’t in the picture. These are Streetrods and as such are discretionary income funded not 401k funds. As we used to say in the race world go only as fast as you can afford to maintain.

Byron
 

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If you're planning ahead to an LS 427, then the Magnum 5 speed would be the toughest choice for both efficiency & durability. Has larger spline input & output than basic t-56.
Proper working clutch linkage is very important. Sicspeedmonte has a setup with custom add in pedal, mounts & tilton clutch that has the usual issues worked out.
While a manual gives more free RWHP an auto can give better & more consistent drag times if that's you're main interest. The new a-10 10 speed auto is the most awesome slushbox ever, as seen in the new ZL1 Camaro's. Not a budget item though.
 
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