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#1
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What rear gear ratio? 355, 373, 391?
I am going to replace the 323 rear gears I have with something, but I need help determining the proper ratio. The 323's just aren't enough for me, but I don't want to wind out the rpms on the freeway, the 323's turn a reasonably low Rpm at 70 mph, but I could tolerate more. My current setup is as follows:
69 Charger, 383 with all the Eddies RPM bits but the carb, 700 holley dbl pumper, 727 auto, 323 rear, 2800 stall TC, 15" wheels with to be measured tire height. I am guessing at 373 gears, but I will eventually run the gear vendors overdrive. I will probably be running wild tall gears with the gear vendors, but it would be nice if my choise now would work later. I may have to live with the gears I choose now for a while, so too tall would suck, and my gut says 391's might be too tall. I did gleen this from an old post; Simple math will give you the answer. If 74 MPH = 2300 RPM and you want 74 = 1900 RPM, then you divide 1900 RPM by 2300 RPM and get .826, then multiply that by the current gear ratio to get the ratio that you will need for the desired RPM and speed. So, 3.73 * .826 = 3.08 So I figure if my 323 gears are running 2200 rpm (just a guess) at 70 mph the 391's would run around 2541 rpm and 391's would be around 2664 rpm at 70 mph... is my math right? I really need to go for a drive and verify my 70 mph rpm data. What would you guys do in my situation. |
#2
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What are you trying to achieve by running lower gears other than less top speed and more rev's on the highway? You are running a higher stall convertor so what is the cam, this makes a big difference? If you are going to do some straight line driving 1/4 mile at a time then you will need to list all motor mods so an informed answer can be given.
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#3
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Well according to what I can find in order for you to pull 2664 rpm @70 mph with 3.90 gears you'd need about a 35" tire. If you have 27" tall tires the engine would be turning 3400 rpm, @ 70mph, and that isn't even taking into account torque converter slippage, so if you take that into account you would get a final tally of 3700 rpm. Now just something that I ran into when I built my first car, I had a 1965 Polara with a 383, 9.5 compression, a Isky 280 mega cam, and a stock 4bl intake, 750 Edelbrock, headers, 2.5" dual exhaust, and a hemi torque converter. The stock ratio I had was 3.23 gears, and it would run 15.1's in the 1/4 @ 5800' of elevation. Everyone told me that I needed lower gears, so I switched to 3.90's. I did not get the results that I was expecting. I was able to break into the 14's... I.E. 14.9's, but just barely, and my 60ft times were worse. It made my gas mileage much worse, and took my top speed down, (not in the 1/4, but in terms of actual top speed). It wasn't until the science of a torque converter was explained to me that I understood what happened. I just reduced the load my torque converter saw, so it didn't multiply torque as much as it was with the 3.23 gears. Anyways I can't give you a real solid answer, and it all just depends on what you want... here is a good resource that you can use to bench race with gear ratios, just give a ratio of 1.1 for the transfer case ratio... http://www.4lo.com/calc/gearratio.htm
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#4
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Ah ha... that's the kind of answer I was looking for! As for the cam, I didn't list the specs, but it is the Edelbrock power pack cam that goes with the RPM intake and heads, specs are:
Duration at 0.006" Lift: Intake: 300° Exhaust: 308° Duration at 0.050" Lift: Intake: 238° Exhaust: 246° Lift at Cam: Intake: 0.320" Exhaust: 0.330" Lift at Valve: Intake: 0.480" Exhaust: 0.495" Timing at 0.050" lift: Open Close Intake: 14° BTDC 44° ABDC Exhaust: 58° BBDC 8° ATDC Centerlines: Lobe Separation - 110° Intake Centerline - 105° I am starting to think leaving the 323's in there and finishing my 451 low deck stroker will be the better choise. |
#5
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Well from that combo, I would say to leave the gears alone, but I would go with a little looser converter, say a 3200 to 3500 stall. I think that would get you the results you are wanting a lot more than the gear swap... Plus the 451 combo will work a lot better than the 383...
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#6
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the 451 stroker will be cammed with a Comp EX275HL, and should make massive torque, which is what I'm realy after more than anything. I'm not tryin' to set ET's or nothin', just want that arm socket tuggin, freight train pullin, eyeball suckin' torque!
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#7
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Since you`re going to a 451, I would leave the 3.23`s. The 3.55 wouldn`t make much of a difference. The 3.73 or 3.91 on the street with a 451, to me, is counterproductive. You want the engine to use it`s torque band to "pull through the tach". Low gears will have the engine buzzing through the torque band, thus not using it. Leave the low gears for small blocks.
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#8
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Or get a manual transmission...
One with overdrive, of course, like the Passon modification of the A833. |
#9
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Hey Ray, a manual would be cool if a motorcycle crash in 2004 hadn't left my right leg paralysed... I've got the quad muscle working and a couple small muscles on the sides of my thigh working, but nothing else. I put the throttle on the left side of the brake and work it all with my left foot, you would be shocked to hear how many car guys buy my explaination of it being a Canadian car, and didn't they know Canadian cars had the throttle on the left? I crack me up sometimes!
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#10
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Next thing you'll revert to the old style where the throttle was in the middle...
This system was actually used in Grand Prix cars up until the fifties! Those motorbikes are dangerous things. |
#11
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#12
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My buddies that drive my truck think it's wierd at first but they all get used to it... suprizingly easy to drive that way. I just couldn't live with the hand contol throttle that the rest of the world gets, so I just fab up what I need... I am spoiled with a full machine shop at work. I even machined out a fully reversed stock style throttle to match the 69 charger pedal, but eventualy I went with a locar pedal so I could position it easier.
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#13
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The question has more to do with how long of trips do you drive with the car, and how often? If it's just a weekender, short trip car, and you want the best out of it, you could go as low as 4.56's in the car. A 451 that's built up at all, will spin to 6500 - 6800 pretty easily, and even if you run 3200 rpm down the highway, other than less than best fuel economy, that won't hurt the motor a bit. If it is purely a show car and you drive long distances with it, 3.23's or 3.55's are the gears you want. I had a '67 Barracuda with a 383, a true B&M 9" "J" converter, 4.56 gears that ran 11.20's in the quarter, and I drove it around with no problems. Now I didn't take 3 hour buzzes down the interstate with it either.
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#14
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yeah... I do run the ocasional couple hour long freeway bomber run... chasing my old biker buddies around. I think I'm gonna stick with the 323's for now.
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#15
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with a 2800 stall converter and 3.23 gears will he need to beef up his trans cooler for extended highway trips?
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#16
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Well that is always a good idea, but if he is travelling down the road it isn't absolutely necessary like if he were pulling a trailer, or loading the converter a lot such as T-Brake or stalling situations....
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#17
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gears
Quote:
im guessing your math is a little off but i could be wrong anyways maybe i can be some help to answer your question im running 391s in my 73 challenger and at 60mph im at 3000 rpm and thats with a tall tire my setup is as follows 400bb with close to 400 hp 727 with around a 2400 stall im getting 12 miles per gallon im switching to 355s hoping to get 14 and a little more top end 323s aren't enough and 391s are to much i road in my bosses cuda with 355 gears and there the perfect gear for a car you wana use for performance and some daily driving if i was just going for performance id keep my 391s hope i was some help |
#18
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He hasn't been here since mid-2011. Posting to a dead thread rarely gets a response unless it was put up by a currently active member.
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#19
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Unless you wanna get wupped at the stop lights by all the little pocket rockets, and newer mustangs and camaros, go with 3:91s!. I went from 3:23 to 3.91 on my Cuda (3500 stall) and have never regretted it. Then get some sticky MT ET streets!
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#20
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His math is correct. Although there may be a little slippage in the driveline with the automatic and the loose converter.
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#21
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#22
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Obviously, others are interested in the subject too.
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