Opinions needed on upcoming CST Blazer Build

[Rust Buster]

So I need some opinions here. I have decided to change direction on my 68 GMC K20 build (in my signature), and put all of the parts I've bought for that into my 71 CST K5. The drivetrain will be a HT383 GM Crate Engine with a custom Mass-Flo EFI system mated to an NV4500. What I need help on is the rear end choice.

I have a new full floater 14 Bolt in the truck that has 3.42 gears. I also have a very used rear Dana 60 from a 72 GMC K20 with 4.10s and a locker. I'd most likely rebuild the Dana 60 with 3.73 or lower gears as the Blazer is not going to be a hard core offroader but rather a daily driver with lots of highway use. I also have a rebuilt 12 Bolt already in the blazer with 3.07s, but I am pretty sure that is my last option. Which would you go with and why? Is the 14 bolt overkill for the blazer?

[truckster]

In my opinion, for the type of use you're planning, the 14 bolt is more than you need. I've actually considered the same swap (I have 1t front and rear sitting out back) but I just don't use my Blazer that hard off-road, mostly dirt roads and trails.

I'd be interested to hear other opinions, because I'm still in the initial stages of my build.

[DeadheadNM]

What size tires are you planning to use?

[Rust Buster]

Originally Posted by DeadheadNM

What size tires are you planning to use?

Current plan is 32s. That's what I have on their now and I like the stance

[71swb4x4]

Why is the 12 bolt your last option? For a daily driver? The gears? If it was mine, and I was building a daily driver that will mainly be highway driven, I would go with the 12 bolt. If I needed to change gearing I would do so in the 12 bolt. Probably run 3.73's with that trans.

[67Steven]

I would think the 12 bolt would be your first choice. It can be built to be strong and light.

You're using 32" tires, not thrashing off-road and running highway miles, all perfect for the 12 bolt.

The 14 or 60 are heavy, strong, but power and fuel abusing axles, not what you need for your claimed use.

[turp mcspray]

I used to do a lot of rock crawling with my blazer, with 36 inch swampers, a locker, 4.10 gears, and aftermarket axles in the 12 bolt. It never gave me any grief, untill I was goofing around in 2 low, and bent a spring perch trying to to climb over about a 3 foot bank.
For what you intend to do, I wouldn't hesitate to use the 12 bolt!

[tjsblazer]

For highway, use the 12bolt. Probably 3.73 with the 32 tires. Go to grimjeeper dot com and use their calculator to figure out engine rpms at different speeds. Excellent website with easy dropdowns that include the nv4500 and np205. Just be sure once you figure your engine rpm's in 5th gear at you highway cruise speed, that the engine you have will cruise happy at that rpm based on cam usage range.

[Rust Buster]

Originally Posted by tjsblazer

For highway, use the 12bolt. Probably 3.73 with the 32 tires. Go to grimjeeper dot com and use their calculator to figure out engine rpms at different speeds. Excellent website with easy dropdowns that include the nv4500 and np205. Just be sure once you figure your engine rpm's in 5th gear at you highway cruise speed, that the engine you have will cruise happy at that rpm based on cam usage range.

Thanks! That website seems to be down, but I'll check it out!

[tjsblazer]

Originally Posted by Rust Buster

Thanks! That website seems to be down, but I'll check it out!

I just went to the website. I input NV4500/NP205 with 32 inch tires. The first picture is with 3.73 gears the second with 4.56 gears. Picked 50,60,70 miles/hour as different highway speeds. You can see the rpms in each gear. Cam specs on the HT383 look very short @196intake and 206 exhaust so I would assume it could run happily at a fairly low rpm.
I am posting all this as my opinion. PLEASE CONFIRM ALL NUMBERS YOURSELF.
website link:
http://www.grimmjeeper.com/gears.html
Cheers.
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[truckster]

Cool website!

I put in my numbers (with the current TH350 and planned 700R4) and with the 700R4 if I can get the engine up to 6000 RPM I should be doing 226 MPH!

What's the take-off speed for a 72 Blazer?

[Rust Buster]

Ah! You left out the extra "m" in grimm when you wrote the first time. Awesome tool and thanks for passing along, it will be getting bookmarked!

[tjsblazer]

Originally Posted by truckster

Cool website!

I put in my numbers (with the current TH350 and planned 700R4) and with the 700R4 if I can get the engine up to 6000 RPM I should be doing 226 MPH!
?

I think it would get a little scary about the 85-90mph threshold.

[tjsblazer]

Originally Posted by Rust Buster

Ah! You left out the extra "m" in grimm when you wrote the first time. Awesome tool and thanks for passing along, it will be getting bookmarked!

Yes. Input your 3.07 stock gears. I wonder if you would really use 5th gear much with that combo.

[Rust Buster]

Apparently not! man...decisions decisions...

[72chevynut]

I have the 12 bolt 3.07 rear in my 71 K5 with 33" tires now. It is a little slow on take off, but works great running with 70 mph traffic on the interstate. With your tire size preference and daily driving I would recommend the 12 bolt. I should also add that mine is 350 mild build V8 and 350 auto (3 speed) trans. Enjoy the build!

[tjsblazer]

Originally Posted by Rust Buster

Apparently not! man...decisions decisions...

I'm restoring my k5 right now and swapping to an NV4500 also. I'm doing driveline test fit on Monday. Ive decided to run 4.56 gears on 33" tires. My highway cruise will be around 1900-2000. Just high enough to get in to the range of my cam although it is quite a bit taller and longer than the cam in your set up.