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RPM's too low at 65 mph, what's up with that?

Desert Rat

Desert Rat

Fetch the comfy chair
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Ok, here is my setup. 76 Blazer with GM crate 350 with no mods other than Doug Thorley headers an MSD HEI distributor, and Edelbrock intake with Edelbrock Q-jet. I've swapped in a 700r4 transmission and I have a doubler transfer case setup with NP203 rangebox up front and NP205 in the back. The drivelines are HAD's w/ a D60 up front and Corp14ff in the rear w/ 35" MTR's and 4.56's. Now, here is my problem. Last wheeling trip over the Sonora Pass, the rig was really dogging it. In fact, I had to put the TC in 2:1 a couple of times on the really steep hills just to keep up the RPM's to generate enough power. Granted, I had a full load of gear.

Now, during this trip I couldn't accurately see what my RPM's were vs. the speeds because my speedo was way off. I've since obtained a ratio adapter and it is much closer. It still reads a bit slower than actual speed, but is within 3-4 mph at least. So, I take it out on the highway today after installing the ratio adapter and find that I'm turning about 1600 - 1800 RPM's at 65 mph. According to the chart, I should be about 2800 RPM give or take. One chart says an auto speeds it up a tad but an overdrive slows it down a tad so that should even it out. However, 1000 RPM below the chart? What gives? :dunno: Anybody else out there have a similar setup? The TC's should be a 1:1 and not a factor in the equation I would think. The 700r4 I had built didn't use any odd ratios. Could all the weight with the heavy axles, full roll cage, winch bumper in front w/ winch and heavy bumper in back with full size spare put too much strain on a stock 350 engine? You would think it wouldn't change the RPM's but maybe? I don't want to spend more bucks to go to 4.88's or 5:13's if I don't have to.
 
Something ain't reading accurately.

My F250 with 4.10s, 33s, and OD 5 speed reads 24-2500 at 65. I know this speedo is right.

Find someone whom you KNOW has an accurate speedo and check against theirs.
 
I'm thinking the speedo is pretty accurate, but perhaps the tach is off. However, given the lack of power in the steep hills, I suspect the RPM's are way too low for some reason. When I put it in 2 low, and the RPM's came up, the power was fine.
 
Does it shift out of OD, into third? At those rpms, it should if you are pushing the go pedal.

Also, in 2 low, your axle ratio would be around 8 to 1. I would think that 350 would be screaming around 5500 rpm.:confused:
 
Uhh, hate to say ... factory tach? They're famous for being ... well, notoriously unreliable. I gather some of the capacitors wear out after lo these many years ... they can be rebuilt, which is costly, or you can go aftermarket.

At the very least you should check it in the driveway against another tach... I have a tach/dwell I can swing down with sometime if you want.

-- A
 
OTOH ... I just did the math:

35" tire is 110" circumference (loosely, 'cuz the ACTUAL tire size is unknown.)

65 mph = 343,200 feet per hour = 5720 feet per minute = 68640 inches per minute / 110" per revolution = 624 revolutions per minute at the axle.

624 * 4.56 (axle) * .7 (overdrive ratio of a 700R4) is ~2000RPM at the engine.

Be about 2850 for 1:1, i.e. a non-overdrive tranny -- that's prolly what you saw on the charts.

I think your speedo is still off -- find somebody with a GPS to check it? -- and/or your tach is off.

-- A
 
dremu said:
OTOH ... I just did the math:

35" tire is 110" circumference (loosely, 'cuz the ACTUAL tire size is unknown.)

65 mph = 343,200 feet per hour = 5720 feet per minute = 68640 inches per minute / 110" per revolution = 624 revolutions per minute at the axle.

624 * 4.56 (axle) * .7 (overdrive ratio of a 700R4) is ~2000RPM at the engine.

Be about 2850 for 1:1, i.e. a non-overdrive tranny -- that's prolly what you saw on the charts.

I think your speedo is still off -- find somebody with a GPS to check it? -- and/or your tach is off.

-- A
Click to expand...


Pleh,

Forget all that stuff ....math is hard.

Just put in a big block. Plenty of power at lower RPMs so you won't have to recalibrate your speedo or verify the accuracy of the tach! :wink1:



:usaflag:
 
Greg72 said:
Pleh,

Forget all that stuff ....math is hard.

Just put in a big block. Plenty of power at lower RPMs so you won't have to recalibrate your speedo or verify the accuracy of the tach! :wink1:



:usaflag:
Click to expand...

Agreed, i run a big block and have an extra one in the back yard for that just in case moment when mine goes kablooee
 
Greg72 said:
Pleh,

Forget all that stuff ....math is hard.

Just put in a big block. Plenty of power at lower RPMs so you won't have to recalibrate your speedo or verify the accuracy of the tach! :wink1:



:usaflag:
Click to expand...

Or you could just fill out a form that does the math for you:

http://www.grimmjeeper.com/gears.html


Running the setup with the 700R4, 4.56's and 35's, you're going to be under 2K RPM at 65. That's really too low, especially hauling a heavy truck over passes. Get you some 5.13's in the axles and you'll be running just a hair over 2100. That's a better range overall and will give you a little more perceived power up and over the hills.
 
Greg72 said:
Pleh,

Forget all that stuff ....math is hard.

Just put in a big block. Plenty of power at lower RPMs so you won't have to recalibrate your speedo or verify the accuracy of the tach! :wink1:



:usaflag:
Click to expand...

Greg, you know better :shame: 'cuz he's in a *smog* state :doah: ... no big block for DR, without getting an exemption and getting the attention of the DMV.

-- A
 
That is accurate , the charts are always off a bit in real world use due to variations in tires , your torque converter slippage , when and how it goes from locked up to non locked up , aerodynamics , grade , etc . And your not 1:1 when in overdrive , the charts and calculators are based on non overdrive save for a select few calculators .

I spin that rpm @ 65 , and I won't be going to a lower gear in the axle until I get larger tires , when I hit a grade and have to downshift to 3rd from 4th , or 2nd from 3rd ( damn 305 ) it goes to 3000 rpm , and I will not sit there spinning 3000 rpm all day long :thumb:
 
pauly383 said:
I spin that rpm @ 65 , and I won't be going to a lower gear in the axle until I get larger tires , when I hit a grade and have to downshift to 3rd from 4th , or 2nd from 3rd ( damn 305 ) it goes to 3000 rpm , and I will not sit there spinning 3000 rpm all day long :thumb:
Click to expand...

Why not? Thats how I drive my 305/sm465 with 4.10s and 35"s. I've done about 2 1/2 hours at one time doing 65-75mph roaring at about 3000rpm.
 
zero cool said:
Running the setup with the 700R4, 4.56's and 35's, you're going to be under 2K RPM at 65. That's really too low, especially hauling a heavy truck over passes. Get you some 5.13's in the axles and you'll be running just a hair over 2100. That's a better range overall and will give you a little more perceived power up and over the hills.
Click to expand...

4.56 is fine for 35's on the street. Why regear the axles when you can just pull the shifter down from "D" to "3"? This makes it sound like having OD is a bad thing. Who says you have to use it all the time?
 
Let's say the speedo and tach are off. The biggest problem though is a serious lack of power going up hills. The tranny shifts down through the gears as the angle increases and the power is needed. However, after I'm down in first gear, I'm still lugging badly. I think on the last trip in first gear I was able to do about 10 mph and the RPM's were below 1000. That is why I shifted the rear to 2 low. Once I did that, I was able to make enough power to at least climb up the grade. Obviously I couldn't go that fast in 2 low, but I was able to pull it. I am wondering if the engine just isn't cutting it anymore. I was going to take it in and have a compression check done on it. DrEmu, I'll take you up on the tach check when you have some time. It has the factory tach in it and it could be off.

I know the speedo is still slightly off. When I paced another car at various MPH's, I wrote the speeds down and then ordered a ratio adapter. My average percentage was around 40-42% slow. The company I ordered from sold a 38% and a 44% increase ratio adapter. I went with the 38% so I know I'm still a little off, but I should be pretty close. I'm less concerned with the speedo as I am with the tach. If the tach is way off, and I'm actually turning the proper RPM's at speed, then it would lead me to believe my engine is crapping out.
 
Yeah you got something going on , I pulled a trailer through the mountains with all my gear in the Blazer with the 305 , and it went slow , but never that slow :eek1:
 
Does anybody have a triple stick doubler setup that has the ORD instructions with it? I want to make sure I have the TC set up correctly. I believe it is right but you never know. I believe the proper settings are having the 205 rear in H, the 205 front in neutral, and the 203 in H.
 
Isn't Sonora pass about 10,000 feet elevation? That could have a little to do with your power loss. Also how old is the fuel filter? ignition system? . Mabe you have other issues besides drivetrain.
 
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