What's a good rpm to cruise at in a 408w?

[lalojamesliz 56]

I'm calling on Tues to order my rear diff. I have a 4r70w with a .78 as a final and my rear tires will be 26.65 

I was looking at the currie calculator and if I put a 3.70, at 70mph my rpms will be at 2289

A 3.5 ratio will get me 2186rpm @ 70mph

Does that sound too high for a carbed 408w?

I still plan on getting something much lower like a 3.0 for trips to visit family 110 miles away and I'll be at 1856. He'll I'll probably just end up buying a trailer and tow it. It'll be almost the same price!

Obviously I've never done this and I need to get it right the first time. 

I don't remember the cam specs now but its a mild cam from Chris Staub (I think 250's lift and 1.6 rockers)  air gap intake, a 850cfm carb with vacuum secondaries, I'm getting the giant msd coil and probably a 6 series box, afr 205 heads, a built 4r70w (I'll find out) , Bauman controller

[jmlay 80]

4r70w is a 0.70:1, assume that is a typographical error. It really depends on the tq available at the cruise rpm, the converter and the lockup schedule. My lightning cruises at 2200 rpm at 70 mile an hour, lots tq available. I believe I would have a discussion with to the cam designer about this. However, my guess the 3.70 or 3.50 will be fine. 3.00 may be fine as well but may require a different shift schedule specifically for it.

[lalojamesliz 56]

6 hours ago, jmlay said:

4r70w is a 0.70:1, assume that is a typographical error. It really depends on the tq available at the cruise rpm, the converter and the lockup schedule. My lightning cruises at 2200 rpm at 70 mile an hour, lots tq available. I believe I would have a discussion with to the cam designer about this. However, my guess the 3.70 or 3.50 will be fine. 3.00 may be fine as well but may require a different shift schedule specifically for it.

 

Thanks for the response,  the 4r70w I have is supposed to be built and the gear ratios are slightly different.  That's what I was told the final gear is. 

I believe I'll go with the 3.70.  With that I'll be around 6100 @ 130 mph in 3rd if i took it to the 1/4 

I'll ask Shaun from SOT if that would also work on a autocross track since I plan to take it to willow springs and Buttonwillow raceway. 

 

In the near future I'll install a manual trans.  

[jmlay 80]

You should have the builder of the trans validate. I know of no .78 option for an aod/4r70w/4r75e. As well the math does not support .78.

[lalojamesliz 56]

1 hour ago, jmlay said:

You should have the builder of the trans validate. I know of no .78 option for an aod/4r70w/4r75e. As well the math does not support .78.

I bought it years ago. It wasn't the smartest decision but it's already done.  I would need to find someone who can verify what it has but I don't know anyone or any place I can take it. 

I've considered just buying a new one with the controller but that's not a cheap option.  I wish I would have just bought a new one to begin with. 

This one is supposed to be built. I just wish I could verify it before I install it. It came with a ptc converter and b&m oil pan

[Vicfreg 771]

Make sure that your highway cruising RPM is above the stall speed for your torque converter.  

[EastYorkStang 63]

What did you go with ?

[lalojamesliz 56]

 

16 hours ago, Vicfreg said:

Make sure that your highway cruising RPM is above the stall speed for your torque converter.  

Ah what? I guess I'll need a different converter.  The one I have is a 3500 I believe....I need to verify now. No way in hell I'll be cruising at 3500rpm

If I need a new converter and still need to buy a baumann controller...... I might just buy a manual trans. A converter is like 500+ right?  The baumann is about 1k. Damn

I'm supposed to finally install my coilover system/3-link  today or tomorrow since I'm off and the next step was to remove the coilover/3-link then apply the undercoating then reinstall and install/plumb my brakes. I'm not ready to install the engine trans anytime soon so I have some time to decide and figure out the trans now. 

Only reason I was doing the 4r70w is so my dad and wife can drive it for a bit then later I was converting to manual for my pleasure 

[lalojamesliz 56]

11 hours ago, EastYorkStang said:

What did you go with ?

3.70 truetrac

[Vicfreg 771]

If you’re running the 4R transmission, you should be able to turn on and turn off the lock up torque converter, so should be no issues with your set up you’re proposing

 

[lalojamesliz 56]

1 hour ago, Vicfreg said:

If you’re running the 4R transmission, you should be able to turn on and turn off the lock up torque converter, so should be no issues with your set up you’re proposing

 

I have so much to learn......thanks for that Rollercoaster of emotions ha ha ha

[Vicfreg 771]

Here is a graph from an excel file I used to calculate the RPM vs Speed at various rear end ratio values.  The three lines are for a 3:25, 3:50, and 3:70 rear end ratio.

It is based on the following:

 - 1:1 final drive ratio (I have a C-6 Automatic, so no overdrive)

 - 245/40-18 Rear Tires - 25.7" height - 80.7" Circumference, 785 revolutions per mile.

Yes, the tire size matters.   

if you are using an overdrive transmission with say, a 0.8 final drive ratio, then take my RPM number from the chart, multiply by 0.8  

Example: for a 3:25 rear at 60 mph, the RPM Value is 2500 RPM. 

If you have a 3:25 rear at 60 mph with a 0.8 overdrive ratio, then it is 0.8 x 2500 = 2000 RPM

 

Speed vs RPM Graph.pdf

[lalojamesliz 56]

I used something like that to determine what ratio I wanted for my for my 3rd member.  

I don't remember what the rpm's were exactly but I was happy with them. 

Thanks for sharing that Vicfreg

[EastYorkStang 63]

Wise choice.  Used that same combo from Strange in mine as well. 

[Mach1 Driver 560]

On 9/13/2021 at 7:05 PM, Vicfreg said:

Here is a graph from an excel file I used to calculate the RPM vs Speed at various rear end ratio values. 

Vic, that sounds useful, can you attach the excel file here?

[Vicfreg 771]

Done.  Added some notes to help interpret the way I built it.

 

 

Speed vs RPM Chart.xlsx

[skidmarky 50]

I put a T5 in my car a few years ago. 3.25 rear. At 75 I turn 2100 rpms. Hard to drive it between 55-60, in and out of 4th/5th alot. I can cruise at 85 at 2300 rpms though. You need to be turning a min of 2k rpms for driveability, car will lug any less than that.

[lalojamesliz 56]

8 hours ago, skidmarky said:

I put a T5 in my car a few years ago. 3.25 rear. At 75 I turn 2100 rpms. Hard to drive it between 55-60, in and out of 4th/5th alot. I can cruise at 85 at 2300 rpms though. You need to be turning a min of 2k rpms for driveability, car will lug any less than that.

What engine are you running?

[skidmarky 50]

14 hours ago, lalojamesliz said:

What engine are you running?

351w