Highway RPM, 700r4, doubler

[Peteasauras]

I'm at the point of putting the trans in my 70 k5 and am wondering about highway cruzing. The plan is 700r4\203\205. It's got a well build small block with around 350HP. I have 33" tires and 4.10 gears. I want to take long trips in the truck and wheel to the limits of the 33's. So I figured I should try to get as low and as high of gear choices as possible. I bought the truck not running so I've never got to drive it yet to get a feel. I want it to be able to cruse at 90mph comfortably. This combo should spin about 2600-2700 at 90. So my questions are, is running a dubbler with 33" tires stupid, and does anybody think this truck will "cruse at 90 comfortably" or will the old truck feel like it's comming apart? (It's got new poly suspension and body mounts.)

Thanks for all the great info, I've been building this truck based upon the posts here.

 

[Greg72]

90MPH in a lifted K5?

Your interpretation of comfortable must be different from mine.....that sounds terrifying to me.

Beyond that, the Doubler has nothing to do with highway RPMs. I must be missing the meaning of your question...

 

[pauly383]

If its solid ( body mounts , motor mounts , all bolts tight , good bearings , steering , tires ) highway cruising is no big deal , although your speed should stay under 75 for safety sake ( big brick flying )

Doubler with any size tire is your personal preference , I would prefer at least a 35 for starters though . A 350 would move it with 4.10's fine in street/highway driving ( doubler not in gear , using just the 1:1 with the trannies OD ) for now ...... I say for now because if you wheel it , your gonna want lower and lower eventually

 

[Peteasauras]

Thanks for the comments. I'm going thought the entire blazer, basically all new, rubber, bearings, seals, brakes, anything that wears. I just drove up to Colorado and was amazed that many were driving 80-90 on the highway, I even got passed at 90 by a new 2500 chevy pulling a rock crawler! What I want is a truck that I can drive across country or disconnect the swaybar and wheel the snot out of. The 7004r was for highway gearing and the doubler for the rocks. Just wondering with 33's if the doubler is ridiculous overkill gearing with such small tires. I figure that I may need to go up to 35's anyway to run more trails which is more reason for the doubler. The other option is to skip the doubler if there is already plenty of gearing or maybe run a lowmax. Thanks.

 

[bp71k5]

I'm running a doubler with 33's and it's just fine on the highway. I need to get better gears as my RPM's are too low on the freeway at 65mph. I guess if I was going 90mph, it might be ok, but I'd be scared to drive a truck like that so fast. I've got it to about 75mph and that's about all I'm confortable doing.

For OH use, I can't say how 33's are...yet.

 

[vtblazer]

Aside from the drive train combo, your going to want lockers in both axles if you hope to get anywhere off road.
A whole different subject sure but one that will need to be addressed eventually.

IMO, you should forget about the need for 90mph and start off with a gear chart.
A 700r has a really decent 6.12:1 first gear for starters.
Decide on the tires you wanna run and work backwards from there designing your set up.
Get a comfortable cruise RPM @ 65/70mph, cause from there 90mph is only a few hundred RPM's away.
I'd try to hold off the "gotta keep up" thing too, drive safe and enjoy the ride for ALL of our sake.

 

[Greg72]

Thanks for the comments. I'm going thought the entire blazer, basically all new, rubber, bearings, seals, brakes, anything that wears. I just drove up to Colorado and was amazed that many were driving 80-90 on the highway, I even got passed at 90 by a new 2500 chevy pulling a rock crawler! What I want is a truck that I can drive across country or disconnect the swaybar and wheel the snot out of. The 7004r was for highway gearing and the doubler for the rocks. Just wondering with 33's if the doubler is ridiculous overkill gearing with such small tires. I figure that I may need to go up to 35's anyway to run more trails which is more reason for the doubler. The other option is to skip the doubler if there is already plenty of gearing or maybe run a lowmax. Thanks.

Here you go, I ran some CrawlCAD numbers for you for both 33" and 35" tires (33" are in the upper window, 35" numbers are in the lower window):

There's certainly nothing wrong with your crawl ratios vs. tire diameter with the setup you are considering. 33" tire has a 103" circumference and with 4.10s your crawl ratio would be at 99:1. That's just about 1 in of tire travel per engine RPM....considered by most to be a good target value for low-speed crawling.

 

[Yukon Jack]

90MPH in a lifted K5?

Your interpretation of comfortable must be different from mine.....that sounds terrifying to me.

I gotta agree with Greg, even when my 35"s were brand new I wouldn't have dared go 90 mph in my K20 (course with it's 4.56 gearing and 1:1 ratio in 4th I wouldn't push my truck to go that fast anyway). You better have those 33"s balanced really well or you are going to shake real bad at 90 mph. Maybe a stupid question, but do tire that large even have a speed rating in that range?

 

[Peteasauras]

I agree with everyone. 90 is really fast.I figure if I build it to run 90 if will run great at lower speeds. BFG muds, even the new KM2 are Q speed rated (99 MPH). I'm going to try to get everything balanced well including the tires and driveshafts. I have the factory double-cardon front shaft that will need lengthend. And in the back I have a huge 3/4 ton shaft from some 70's longbed. I didn't measure but it's probably 4". It's got the 1350 joints to match the 14 bolt, and I converted the 205 output to 1350. I hope with only 4" lift this shaft will work without vibrating. I figured the big shaft was strong and it had the joints I wanted. Anybody have any experiance with these big shafts? I could also move the drivetrain forward a bit to get a longer rear shaft and more clearance for the HEI if recommended. Greg72 where did the 6.12 number for the 700r4 come from? I thought the low gear was 3.06? I must be missing something.( Ratios TH-700R4 3.06 1.63 1.00 .70)

Thanks again.

 

[Greg72]

6.12 is the "real" ratio of 1st gear below stall speed (torque converter)....

The torque converter is not in a pure 1:1 relationship with the engine until it reaches it's stall speed, so the effective gear is actually about 2X larger than indicated (3.06). This is relevant for low-speed crawling since you almost never go fast enough for the converter to stall. The calculators will always have some sort of correction factor like this for the automatic trannies.

 

[AJs72K5]

Actually...building a truck that's going to be stable at 90mph isn't necessarily going to have the off road manners you may be looking for. At 90, you're gonna want a much stiffer suspension with almost zero body roll or sway, otherwise that 6000lb rocket you're driving will be on it's lid after taking the first curve, or even quick lane change.

My trucks pretty solid, but at speeds above 75 or 80, things start to get a little sketchy. And all bets are off if you pick up a death wobble at that speed. Just my 2 cents.