Jebby - thanks. You’ve given me something something to think about. I think I like that idea. And it preps the car for an engine swap later. What’s a new tranny cost installed? Any idea?

 

Do your homework….but it will be in the $2500-3000 range….you will be able to pick a converter too that will add even more torque multiplication.
You will need a new driveshaft….
I like the 2004r because it is light and small….but the 700r4 is ultimately stronger and can be built to take more abuse….I say generally.
You need to shop for a trans builder that can help you with the complete package…..you will know him when you hear him….ask here where others bought theirs….
Installation is a crap shoot….you would have to find a guy out near you….if you had all of the stuff, I would say $1000 would swap it.
The cam can wait really….but add the headers for later. See how you like it….it will be much faster overall.
And yes…..it would set an awesome platform for future upgrades….

Jebby

 

This was built for under 4k look at the TQ tell me I can't run a 308 gear and have fun. With out OD I would not run 373s they suck on the highway

 

The engine itself was just over $5K (all new components including block). But then I added fuel injection (Edelbrock Pro Flo 4 XT) and a GM serpentine belt kit which added roughly another $3K. The transmission was just under $4k. I put in a new fuel tank setup for EFI and ran new fuel lines. I installed everything myself. I picked up the transmission kit from Silver Sport Transmissions. I looked at buying everything separately but their kit includes everything you need and the price was about the same. For example, their kit comes with a new driveshaft, a new cross member, shifter conversion kit, transmission fluid, Compushift Sport controller, etc. There was nothing I had to buy separate from their kit. Plus, they offered transmissions built to different power levels so I could match one with my new engine. You could certainly spend less on a transmission swap but I think SST is great option if the budget allows.

Speaking of which, if your budget allows, I think a new crate engine with an SST transmission kit is a great way to go. But, you won't go wrong following Jebby's recommendations. He's fairly close to you also so it might not hurt to pick his brain quite a bit on this venture. You might even get him to build your engine for you.

Disclaimer: I ordered the engine, tranny and most of the parts just before Covid hit so prices today might vary quite a bit.

DC

 

I would agree that the right transmission, torque convertor, and gears will do more for the seat of the pants feel than just dropping a 450HP crate engine in it. I just dropped in a mid 400's hp 383, but matched the transmission, torque convertor, and gears for the cam and power band of the engine. Most performance cams come with a recommended gear ratio and stall speed as do most performance crate engines. I would probably start with transmission and gears if you have a good motor and maybe a little stall speed.

I know bow tie overdrives sells transmissions, crossmembers, and shifter detent for the 200r4/700r4 conversion. You will probably need a local machine shop to shorten the driveline. I probably had ~2k into my conversion using a local builder for the transmission. I pulled my rear differential and had it rebuilt and new gears installed by vtec, I think it was ~$1100 plus a couple hundred UPS shipping.

 

If all I do is drive around town - no highway, and no desire - wouldn't it be ok to just keep the TH400?

 

Keep the 400! My 1979 came with the th350 and 3.55 rear end and 3.4 miles on the OD. I started hopping it up and long tube headers with big true dual exhaust. I had installed a manual/auto valve body, 3000 stall 9.5 inch TC........ I probably put 50,000 hard miles on the tranny and on long trips it averaged about 20 mpg over 4500 miles in two weeks.

It finally blew chunks of metal in the pan and it was time to retire! Then I went to a 700R4 racing tranny. It actually lost time in 1/4 mile and I tried to figure out why. Gearing ratios was kinda wrong, but the real problem was that it sucked about 25 more HP than a built TH350. It also lost MPG because my motor wasn't turning enough rpm to be in it's cam efficiency range. So I install a 4.11 rear end to prop up the 700R4. The 1/4 mile kinda returned to previous normal, but the MPG dropped more than a MPG on the same long trip that I did a couple of times a year.

OD 4 speed trannies are not all they are cracked up to be. Now if you could jump up to a modern many speed auto it would be a different story.

IMO, Put a shift kit in your th400 and have fun! The day it blows go to plan "B"

 

Based on what I'm hearing here are my recommendations:

1) before doing anything else, find out what your rear-end ratio is. The quick and dirty way to do this. Jack up the back of the car so the wheels are off the ground. Put the trans in Neutral. Put a piece of tape on the wheel to mark a spot, same with the main driveshaft. Spin the driveshaft by hand and count how many times the wheels go around for each time the driveshaft does. If it's 3 1/2, but have approx 3.50 rear gears, if it's 3 3/4 you have ~3.73 rear gears, etc. If you are running something below a 3.42 rear end, definitely consider a rear gear swap. If the rear is original the factory limited-slip is probably worn out anyways.

2) I agree with Stormin Norman, a modern transmission is probably the way to go here. The TH7004R/4L60E is a popular choice as it has both a very steep 1st gear, and an OD gear. You can run a 3.73 or 4.11 rear gear set and still have a very livable cruizer.

3) Headers will make more peak power, but there are tradeoffs with fitment, ground clearance, etc. You will lose all of your ignition shielding and bracketry stuff in the swap. Not saying it's a bad idea, but just know this going in.
Expect to spend at least $600 on a set of headers for them to be any good. I really like Sanderson for streetcars.

4) Once you get the drivetrain where you want it to be, seriously GO GET DYNO TUNED. You'll probably spend $1000 for a full tune, and you'll be absolutely shocked at the results

 

66 Impala - I'm just curious, did you read my post, #9?

 

Hi BK. Yes, that is what I was originally planning on doing. New cam and headers. Regarding gear ratios, I guess a good tranny shop can do that? As I mentioned, I am mostly interested in off the line and passing power. Just riding around town, zipping from light to light. Probably about 100 mls per week at best. I prefer my new RAM truck for longer drives. Car just seems heavy and mechaniic said 383 would make noticeable difference. From some of the posts here, it sounds like it would. I mentioned the tranny swap to him, and he said if I am just riding around town, he would stick with. the TH400. And that the street tires on it are fine for want I am trying to do. Its a 1972,with 73K miles. 6K on the crate engine. PS, PB, AC. All stock except for engine (and accessories like waterpump, starter, etc), and mufflers. Has the original AC compressor which still works.

 

I applaud your decision to choose all brand new GM crate. Nuthin's perfect, but me thinks the Chev Perf/GM Powertrain choice is best.

ya do know GM's 383 has longer arm than most? It's 3.800" v. others' 3.750"

 

You aren't changing the gears in the transmission but in the rear differential. These are generally specialized shops
Zip Corvette sells rebuilt rear diffs
https://www.zip-corvette.com/68-82-c...erentials.html

You will need a trans shop to properly recalibrate your speedo after the rear ratio is swapped, but that's minor

 

If it is a stock unmolested 72 L48, it came with 3.36 gears if no air, and 3.08 if it has air.
Since you are in TX I will assume A/C.
You really need to check the rear gear ratio.
So if all you really do is drive around town, I would drop a set of 373s in it.
Your 50 mph rpm only goes from 1900 to 2300, so it will still cruise back roads fine.
But you will have 21% more TQ in every gear until then.
Your 380 TQ will now feel like 456.
If you also put the 383 in it, and got another 20% TQ, it won't hook up under 70 and you'll need bigger tires.
Go have fun!
Your (likely) 308 rear does not give you much gear multiplication, only 7.6, IE: really lazy, first gear good to 60+ mph.
Changing that to a 3.73 will make a huge difference, you'll have a 9.3 in 1st gear, which is just about right! 1st gear will now be redlined at 50mph.
Play with this gear spreadhsheet if you like. A TH400 has a 2.48 1st gear.

The gear change is basically the same kind of TQ change as the 383. The Best part is the rear gear and tranny is 3-4K not 10k!

 

Leigh - Wow! That's good stuff. Im going to have to re-read your message when the sun is going up instead of going down. I wasn't expecting to get so much feedback from everyone regarding gear ratios and the tranny. I was always thinking the solution was cam and headers. I appreciate you all. Truly. Cheers!

 

Most transmission shops are not equipped to rebuild a rear and particularly to change the ring and pinion. The first time I had the rear rebuilt and the gears changed, I sent it to Bair's Corvette. They did a good job at a reasonable price.
BTW I have a 383 in my 77, with a TH350 trans. You would certainly benefit from the extra torque over a similarly built 350, but I think your money would be better spent on cam upgrade, headers, converter and rear gears based on what you do with the car.

 

Thanks BK. Why would my mechanic say that I wouldn’t benefit from a cam upgrade, particularly with the transmission I have. Is it because he just doesn’t want to do the job? Or does he have a point. He said that if it was a stick, then he could see it. But with an automatic, I should go the 383 route to accomplish what I’m hoping for. More power.

 

66 - The trans really isn't affected by the cam, but the converter is. The hotter the cam, the more stall speed you need because the engine will be making less power at low rpms (with the hotter cam). For example I have 225/232 duration with my cam, in the 383. With a stock converter, my car would be dead off the line. My TH350 trans has the same ratios in its 3 speeds as yours, but 3k stall and 3.73 rear. My car has run 1.69 sec 60 ft times, on the track, with drag radials. That is really good low end response, if I do say so myself.

 

some guys do engines, some do trans, etc. fewer can visualize the whole combo/package!
Jebby is definately one of those
listen grasshopper....

 

I agree with Jebby that removing a GM 350 HO crate motor for a 383 is not worth the time, effort, or money when you can easily get MUCH more power out of the current motor. If you were to change motors, As Jebby states, going 406 is a MUCH better bet and value proposition, for what you are after.

I also agree that upgrading your existing platform with a much higher lift Cam (flat Tappet or roller conversion), 1 5/8 LT headers, and I would add, new heads, will easily get you 400-425 Gross HP with your current engine. Vortec heads are great inexpensive heads but do have the cam lift limitations without head rework mods. I would suggest a decent set of iron high flow aftermarket heads (relatively cheap), along with a much higher lift roller cam conversion, (Howards Roller Cam conversion in my L-82 355 of .525 Lift, Duration 219/225 for low to mid range power or smiliar) along with some small tube headers for bottom end punch. Add a thin head gasket (.015) along with some other tricks and the engine could make 9.2-9.3 compression and some much bigger power than it produces now, all for lot less money, than a new crate engine, installed.

I would consider the trans mods later after the motor upgrades, especially since you are talking a street driven car, not a drag racer, but the car does have an automatic, which is VERY different than shifting a manual, when dealing with lower numerical gearing.

FWIW- I have driven several cars over many years with 3.08/2.73 gears both auto/manual and the automatics are much more hindered by the tall gearing for many of the reasons stated above, from others. A higher Gross HP motor putting power to a 3.08 gears, on the other hand, will scoot very well with a manual transmission. On the street, not racing, a high TQ motor does not need the higher TQ multiplication nearly as much, to assist the weaker power from smaller displacement/lower power engines, especially if you are not at the drag strip.

GM 350 HO Specs:

​​​​​​

Tech Specs

• Part Number: 19355662
• Engine Type: Chevy Small-Block V-8
• Displacement (cu. in.): 350
• Bore x Stroke (in.): 4.000 x 3.480
• Block (P/N 10105123): Cast-iron with four-bolt main caps
• Crankshaft (P/N 14088526): Nodular iron
• Connecting Rods (P/N 10108688): Powdered-metal steel
• Pistons (P/N 12514101): Cast-aluminum
• Camshaft Type (P/N 24502476): Hydraulic flat tappet
• Camshaft Lift (in.): .435 intake / .460 exhaust
• Camshaft Duration (@.050 in.): 212° intake / 222° exhaust
• Cylinder Heads (P/N 12558060): Vortec iron; 64-cc chambers
• Valve Size (in.): 1.940 intake / 1.500 exhaust
• Compression Ratio: 9.0:1 nominal
• Rocker Arms (P/N 10089648): Stamped steel
• Rocker Arm Ratio: 1.5:1

 

FYI

19418602 aka "602" circle track "race" motor and aforementioned 350HO are virtual twins with exception of pan & valve covers; both have same flat tappet cam but both are ready for OE roller cam & OE lifters. Yes, there's power in a 602 pan, but running a left turn-only gated pan on street is a poor choice. And typical drag pans hang too low. Ideally, run a Road Race pan.