NEED HELP - Cruising RPMs for J-10 with th400

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tigger4x
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NEED HELP - Cruising RPMs for J-10 with th400

Post by tigger4x »

I have a '74 J-10 and am looking at a pair of J-20 axles that have 4.10s. I'd like to know what the typical cruising RPMs and power band are for the 360/401 with a th400 so I can best determine what size tires to run. I attempted a Search on here, but didn't find compatible results. IIRC my current ratio is 3.73 with 30" tires, but the speedo is off and mileage isn't typical.

I doubt it has much if any relevance, but I have an EB Performer intake, 2131, waiting on install and am looking at installing a cam/lifter set. Snoopy will see light trails and more often than not be packing a slide in pop-up camper in the bed.

Really appreciate your help :thumbsup:

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Re: NEED HELP - Cruising RPMs for J-10 with th400

Post by will e »

Best calculator around:
http://www.grimmjeeper.com/gears.html

How high you want to rev on the highway is up to you. I sport 4.56 gears and a tf727 so, like the th400, no overdrive. With 37 inch tires my highway RPM'S at 65 are around 3000.
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Re: NEED HELP - Cruising RPMs for J-10 with th400

Post by tigger4x »

Hey there Mike! Good to hear from you.

I've used the Grimmjeeper calculator before with good results, but I knew my target rpms and power band. I don't want to run at the bottom of the power band and not have anything behind it when I hit an incline or step on the skinny pedal. I couldn't find the numbers in any of the tech info I've found so far. It'd be nice to cruise 65-70 and not watch the gas needle move. Tire size will determine if I can do this now or if I have to sit on the axles a while as I scrimp to save for a lift kit or something. Is 3Krpm where I have to/need to be?
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Re: NEED HELP - Cruising RPMs for J-10 with th400

Post by Stuka »

With 4.10's and no overdrive I would say go with 32/33 tires. My J10 with 4.56's and 33's runs a whole lot of RPM on the HWY, but I am glad I went with those gears since I have an i6. With a V8 I would have gone 4.10's.
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Re: NEED HELP - Cruising RPMs for J-10 with th400

Post by tigger4x »

Using the calculator this is how it shakes out as suggested. Running 35" instead of 33 will drop the RPMs from 2867 to 2703 respectively. The RPMs still sound high to me.

Maybe I didn't attach the pic right. It says "Not downloaded yet".
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Re: NEED HELP - Cruising RPMs for J-10 with th400

Post by tigger4x »

Looking at it further if I run 37s I will only get down to 2557rpms at 65mph. And that is not likely to happen on my rig. At least not without a good lift to clear them which would increase my COG especially with a slide in camper, even if its a lighter pop-up model.


Its looking like the only way to get the RPMs down is to run 3.73s on 35s which would have me running 2459rpms at 65mph. That now sets me on a whole new search for a set of axles. It took me quite a while to find these. UFF DA!!
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Re: NEED HELP - Cruising RPMs for J-10 with th400

Post by Tatsadasayago »

In my experience a stock 304/360/401 does it's best at around 3000 RPM under light loads. It's all about the heads.
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Re: NEED HELP - Cruising RPMs for J-10 with th400

Post by Stuka »

One thing to note, besides cruising rpm, is ramp speed. IMO, ramp speed is more important than cruising rpm. With only 3 gears, I would go with slightly higher cruising rpm to get a better ramp speed.
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Re: NEED HELP - Cruising RPMs for J-10 with th400

Post by tigger4x »

That's what I was trying to get at asking about the power band to be sure it had some get n go / ramp speed when needed yet decent RPMs for best as it can be mpgs. Not so good with the 'splaining sometimes. :oops:

When you say "With only 3 gears, I would go with slightly higher cruising rpm to get a better ramp speed." which numbers are you referring to?

If I go with the 4.10s at 65mph ...
1) on 33s is 2867rpm
2) on 35s is 2703rpm

Or if I can find 3.73s at 65mph ...
3) on 33s is 2608rpm
4) on 35s is 2495rpm
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Re: NEED HELP - Cruising RPMs for J-10 with th400

Post by Stuka »

Ramp speed is basically how fast the engine will go through its RPM range in relation to the increase in speed.

4.10's will have a better ramp speed than 3.73's. What I mean by only three gears, is the gears are spread apart more. Since those ratios being wider means you need lower axle ratios (high numerically) to get the same ramp speed that a transmission with more gears would offer with higher axle ratios.

Oh, and when using a gear calculator, you need to use rolling diameter, not advertised diameter. So for a 33x12.50, which actually has a diameter of 32.5, will have a rolling diameter that is closer to 31.75. Different tires have slightly different amounts of squish, but its worth noting.
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Re: NEED HELP - Cruising RPMs for J-10 with th400

Post by tgreese »

Squish does not change the height, so don't measure the height on the car. You want the advertised height, not the number on the sidewall. The manufacturer's specs for the tire will have a height specification which will typically be a little less than the sidewall marking height. You can always measure the circumference of the tire and divide by pi - that will be exact.

JMO - you don't need to be so exact for this purpose. Finding the RPM to the nearest 100 is as exact as you need to be. Realize that the TH400 in particular will have some internal slippage, and your calculated RPM will always be a little lower than what you see on the road.
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