Picking a cam and torque converter

[jgchemie]

I'm building a 390 (4" stoker kit) from my 318 LA engine to power my A108 van (adding aluminum heads and probably EFI). The purpose of this van will be to cruise on the highway but I want good low-end torque. I'm planning to keep the compression ratio low to run off pump gas. I'd like to retain my stock 727 transmission.

My question is what information do I need to gather and then how do I select the best torque converter and matching cam? The sales rep from Summit asked me about my rear diff, wheel size, cruising RPM, etc. How is all that information used to select a cam and torque convert?

I'd love to see some equations!

He also wanted to know if I was planning to "upgrade" to a four-speed transmission, which sounded great but also another project that I'm not sure I need to tackle right now...

Thanks

 

[moparmat2000]

Use a 46RH out of a ram pickup or van.

 

[jgchemie]

Use a 46RH out of a ram pickup or van.

Are those electronically controlled?
If I were to do that eventually, how do I pick my cam now to give me the best low end torque and low cruising RPMs?

 

[moparmat2000]

1991-1995 46RH series can be wired to be controlled with a toggle switch for lockup and for O/D. When those are turned off it functions as a regular 3 speed. Uses a TF727 sliding yoke

 

[rigger3006]

call a cam company, like bullet racing, they know more than the summit guy, their website has a cam recommendation form, from there you can figure what torque converter ypu need

 

[jgchemie]

1991-1995 46RH series can be wired to be controlled with a toggle switch for lockup and for O/D. When those are turned off it functions as a regular 3 speed

Do you replace the standard dash gear select? I've seen a part from Compushift- is that what you recommend?

 

[moparmat2000]

Use a standard 318 flexplate.

 

[moparmat2000]

Nope. It has 3 forward gear detents reuse stock shifter linkage and arm off your original trans. Shifter arm same spot and same diameter as the old torqueflight one. Your original shifter arm off your original trans will bolt right on. Use a cable operated kickdown such as lokar or jegs for TF727.

 

[moparmat2000]

What year van?

 

[moparmat2000]

The lockup and O/D work off solenoids. 46RH Trans is small block torqueflight A727 based. Even the pan and filter fit from A727. The 42RH is based off A904, and a transmission length chart. Pin out on the lockup/OD plug is same for 46RH

 

[moparmat2000]

Not sure on the cam, but you can run more gear with one of these and cruise at 75 mph at a lower RPM.

 

[jgchemie]

It's a 1969 A108 with the heavy duty axles. The van is actually at the paint shop so I don't know the rear differential ratio. These older vans hang from the cross beam on an eyelid in the tail. I'm assuming I'd have to swap the tail from the 46RH.

The reao

call a cam company, like bullet racing, they know more than the summit guy, their website has a cam recommendation form, from there you can figure what torque converter ypu need

How sensitive is cam selection? In other words, if I pick a cam that works for my 727 transmission and an appropriate torque converter today to get me an 70 mph cruising RPM of 2,000, will I need a totally different cam if I switch to a four speed 46RH in a year or two?

 

[moparmat2000]

Cant swap tailshaft housings. The tail on the 42RH and 46RH is the O/D unit. You would have to get creative making a mount. Theres a pair of drilled ears on the tail housing. You could get creative there even though its farther back. Maybe move the mount crossmember back, and make a support mount with an upside down U bolt going through those 2 holes in the tailshaft housing. Heres a pic of 42RH that will be going in my kids car. Note the mechanical speedometer drive in the tailshaft housing. Your 69 cable should thread right in. If it cant make the bend since the speedometer drive is further back, you can use a longer cable, or a 90° right angle adaptor.

 

[moparmat2000]

Heres a closeup pic showing the 69 up 3 prong neutral safety reverse light switch, and the 3 wire plug with wires attached for the lockup and O/D. If it has a connector with more than 3 wires for the lockup and O/D then it's too new and is a 42RE or 46RE. These require a computer to work. There is a place online that sells a premade kit using fluid pressure switches, solenoids, and relays to make the lockup and O/D seamless and automatic in engagement and disingagement, while providing a single switch allowing you to turn it off instead of you having to constantly remember to turn it off below 55mph.

Also in pic #2 it shows a standard 1961-1990 V8 flexplate which will bolt right up. Not sure if all magnum engines which is what this trans came out of are externally balanced using drill holes in the stock magnum flexplate, but the flexplate on the 3.9 V6 had additional drill holes on one side probably for external balancing. The trans I have did not have a counterweight on the torque converter. I am using this with a 318. A standard ATP flexplate is what I will use.

 

[jgchemie]

This is great information. Thanks! I will add the transmission to the list of upgrades I might do after I get my engine installed with the stock 727. Right now the van is at the paint shop, engine block at the machinist, gas tank in the drive way... lots of time to plan and put things back together!