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Old 07-25-2011, 08:45 PM
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Gear Selection Question

Hi everyone. I have an 83 S15 jimmy. 350, vortec heads, mild comp cam, rpm intake and double pumper carb. 3000 stall, th350 trans with 10 bolt rear end. I am currently running 3.73 gears. 295 50 15 tires. The truck runs decent for a very mild build but it doesn't feel like i have near enough gear. Everyone has told me 373 is a great street gear but im not at all pleased with it. My question is will a 4.10 gear be noticably different, or should i go to a 4.56. I dont want to be reved out going down the street with it, but i want more gear for sure. any suggestions please.
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Old 07-26-2011, 10:34 AM
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Sounds like you already have too much stall and gear for the motor you've described.
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Old 07-26-2011, 02:08 PM
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Im no expert on this stuff by no means, thats why im asking for advice. It only turns about 2500 rpm at 70 mph. I want it to get into higher rpm's quicker.
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Old 07-26-2011, 05:01 PM
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Lower gear

By all means, get the 4.10 gears. You will like them after you get used to quicker acceleration. You will pay the price because your engine will run somewhat hotter, especially with a 3000 RPM stall converter and you can kiss good gas milage goodby but it's worth it.

If you want better gas milage put the V6 back in it.
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Old 07-26-2011, 06:43 PM
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I care nothing about mileage. Its just a toy. I want quicker acceleration and to get the motor into the range where it makes the most power faster. I run a large 4 core radiator and while im driving the truck rarely runs hotter than 150-160*. Thanks for the reply.
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Old 07-26-2011, 10:33 PM
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j ab, something doesn't make sense to me. With 295/50r15 tires, you should be running about 3300 rpm at 70 assuming no trans slip and assuming you don't have an overdrive behind your th350. 2500rpm would equate to 2.82 gears.

Here's how that works: 70 miles/hr divided by 60 min/hr gives 1.167 miles/min. Your tires should be 26.6" in diameter. Your tire should have to roll 884.6 times to hit 1.167 miles, so at 70 it's turning 884.6 times per minute. Divide that into your rpm at 70 and you should get your gear ratio.

Makes sense because I have a th350 with a 3k converter on my Camaro, and it runs to a 12-bolt rear with Richmond 3.73s (I know they're 3.73s because I ordered them and installed them). I turn over 3000rpm at 65.
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Old 07-27-2011, 03:22 PM
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X2^^, sounds more like 2.73 gears than 3.73.
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Old 07-27-2011, 03:40 PM
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Gears

To check you gear size lift the rear of the truck place a white mark on your tire then turn the tire a full turn while looking at the drive shaft. Count how many times the rear turns with the full turn of your tire and you'll get your rear size. 3.73 will be 3 and 3 quarter turns. A ten bolt 410 rear will blow your motor on the highway over 55 mpr. not a good move 373's are good driving and dragging. Unless you want to get serious put a full range cam and 456 gears and you'll be hauling ***.
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Old 07-27-2011, 04:27 PM
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Im gonna go drive it and take my little handheld gps with me to get accurate speed reading. My speedometer is way off because i never changed the little platic drive gear to match anything. My wife was behind me on the highway and said she was running just shy of 70. Ill check it with gps. I know the trans isn't slipping. Fresh rebuild this past winter. All B&M clutches steel and band. New hughes converter. Im nowhere near done with this motor. What is a good cam that will work well with vortec heads. I have a comp now bt its just a factory spec cam. I wouldn't care to upgrade heads but i've been seriously thinking about a 383 stroker kit for this motor. And ive heard the 383 likes vortec heads. Only thing holding me back is local machine shop quoted me $600 to machine the block. I thought that was a little steep.
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Old 07-28-2011, 09:20 AM
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What cam to use kind of depends on what your rear gears are. 67nova suggested a good method to find out. If you really have something like 2.76 gears then your stock-replacement cam is probably a good thing. It won't have any power left at high rpm. If you plan to end up with 3.73s or somehow do have them as it turns out, then I'd suggest a nice flat-tappet cam with sensible lift numbers and a lot of duration. Maybe something like 230/230 @ .050, maybe around 300 advertised, and perhaps 108 lobe separation. My cam is 230/230, 312 and 106. It can be a little hairy but it idles well (at 1000) once it's warm. But Vortec heads, which can make up to about 500hp, have limitations. First, unless they are remachined for 'normal' intake bolt pattern (some are), you have a limited choice of intake manifolds. Second, generally the springs don't tolerate more than about .490 gross lift, though again some are sold with springs that will take up to .520 or so. I don't use Vortecs, but my lift is only .465. It's a good thing. Shorter lift combined with a slow ramping flat-tappet cam with wide duration is real easy on the valve train, unlike a roller, and has great power too. I would suggest a single-plane, but they aren't plentiful for vortecs (again, unless yours are tapped for the old style angled intake bolts) though they do exist. Edelbrock rpm heads are readily available, but those are dual plane. And it looks like you can get Victors etc., but those might be overkill. I use a little weiand xcelerator.

One last hint... don't try slapping on 1.6 rockers for more lift unless you know your clearances, pushrod heights etc. It's not the 'easy' way to make your cam bigger in most cases.

Last edited by Leoman; 07-28-2011 at 09:33 AM.
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