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| Best gear ratio for 225/904 https://slantsix.org/forum/viewtopic.php?t=24699 |
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| Author: | Feather Duster [ Wed Aug 29, 2007 8:31 pm ] |
| Post subject: | Best gear ratio for 225/904 |
I have a '75 Duster with a stock 225 and a 904 automatic transmission with the stock convertor. It currently has the small bolt pattern 7 1/4 rear end in it. I plan on driving it on the highway at 70 mph and bracket racing it in the 1/8 mile next year. I have a big bolt pattern 7 1/4 with 2.94 gears in it. I have a set of 255/60 x 15 inch rear wheels and tires with a 28" diameter for this rear end. I am not looking for the fastest ET for my combination, but I want the correct all around gear so that it will run at 70 mph without over reving it, but low enough so that it will still get out of it's own way. Will the 2.94 ratio and 28" tall tires be too high a ratio? Would a different torque convertor be worth the extra money? Will a 7 1/4 hold up behind a stock 225/904 or should I find an 8 1/4 with a sure grip? Any help will be appreciated. |
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| Author: | Joshie225 [ Wed Aug 29, 2007 9:11 pm ] |
| Post subject: | |
That's a pretty tall tire. I'd think 2.94 to be the minimum and 3.23s to be about right for something mostly street and highway oriented. 3.55 or 3.91 would help on the strip, but 3.91 would probably be pushing it somewhat on the freeway. If it were my car I'd probably go with 3.23s and get shorter tires like 225/50R15 for the drags. A higher stall converter will make the car quicker, but I would start with a 2 bbl and better exhaust first. |
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| Author: | Romeo Furio [ Wed Aug 29, 2007 9:11 pm ] |
| Post subject: | gears |
2.94 and a 28" tire will be great on the highway, not so in the 1/8 mile. I have no problem with 7 1/4 rears and with your combo you will be fine. No neutral drops though.......... |
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| Author: | emsvitil [ Wed Aug 29, 2007 10:39 pm ] |
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The normal tire diameter is around 25", so to just correct for the 28" tire you need a 28/25 * 2.94 = 3.29 rear.... The good all around ratio with the 25" tire is probably 3.23, or 3.62 with the 28" tire |
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| Author: | Charrlie_S [ Thu Aug 30, 2007 5:50 am ] |
| Post subject: | |
Quote: The normal tire diameter is around 25", so to just correct for the 28" tire you need a 28/25 * 2.94 = 3.29 rear....
How about a 3.23 or 3.55 rear? Use the 28 in tall tire on the street and change to a 22-23 in tall tire for the track.The good all around ratio with the 25" tire is probably 3.23, or 3.62 with the 28" tire My 66 Valiant has a 3.91 SG 7 1/4 rear and 22 inch tall tires. It goes through the 1/4 mile traps at about 6,000. The 3.91 would be a little to deep on the highway. You could try and find a complete 7 1/4 or 8 1/4 used rear, with to ratio you want. 8 1/4 would be better. Chances of finding a sure grip are pretty slim (especilly a 7 1/4) , and a 8 1/4 open rear would be less prone to breaking then a 7 1/4 open. |
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| Author: | casey70dartreed [ Thu Aug 30, 2007 8:11 am ] |
| Post subject: | |
I have been bracket racing (1/4 mile) a 225 slant six with a modified 904 (shift kit) for three years now...Logging at least 600-700 passes on the stock open 7 1/4 (2.76 gears). This year with a new combination btw...4-BBL, Mild RV cam, double rolling timing chain, custom forged racing pistons, mild head etc. etc. The car will do high 16s w/o a problem and I have yet to experience anything that would lead me to believe that the 7 1/4 is inadequate...Especially with a stock combo in just the 1/8 mile. Of course everybody will tell you that the 7 1/4 will go away sometime (and they may be right), but on the contrary...It is not worth messing around with unless you really start messing around w/the 225. But the moral of the story...1900 stall on the line every time, stock open 7 1/4...Hundreds upon hundreds of passes...And not one problem. You be the judge. |
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| Author: | Feather Duster [ Thu Aug 30, 2007 4:31 pm ] |
| Post subject: | |
Thanks for the info all. My '75 Duster is a slant 6 with a 3 speed manual transmission, column shifted. It has the 9" brakes and small 4" bolt pattern and 7 1/4 rear end. I haven't checked the ratio and I have never driven it as the engine is locked up. What were these slant 6 automatic Dusters usually geared? I just bought a '73 225/904 that runs great. I plan on installing this engine and transmission into the Duster and driving it as it is to see what I have exactly as far as the brakes and suspension are concerned. I have a 7 1/4 big bolt pattern rear end with 2.94 gears and a set of 15 x 8 rallye wheels with 255/60 x 15 BF Goodrich TA radials on them, that I can install if need be. Maybe I can check the ratio of the rear end that is currently in the Duster and since a 7 1/4 open rear end should be strong enough to not break and the slant 6 not strong enough to spin the tire, maybe I could just try it like it is and leave the 14 x 5.5 rallye wheels with 225/70 tires on the rear. Thanks again. |
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| Author: | slantzilla [ Thu Aug 30, 2007 5:15 pm ] |
| Post subject: | |
My Duster had the 7-1/4" for the first couple years we raced it. I did big John Force funny car burnouts with it and always expected the "BANG" every time I did it. It never happened.  With a 28" tall tire I would not be a bit frightened of running 3.55 or 3.91 gears. A manual trans car may already have 3.23 gears in it. An auto would have 2.76 or 2.94. |
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| Author: | Feather Duster [ Thu Aug 30, 2007 7:23 pm ] |
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I will check the gear ratio of the rear end that is already in the car. It will be nice if it is a 3.23 ratio. I may just keep it as it is with the 9" drum brakes and small bolt pattern and open rear end. What kind of 1/8 mile ET should I expect from this combo? Will it be in the 11's or 12's? I am currently racing my '67 Valiant with a 340/904/8.75 w/2.76 gears and running 8.80's @ 80 mph and getting 14.6 mpg on average. I figure I can cut my fuel and maintenance costs by racing a slant 6 instead of a 340. I will then be able to afford to race at more events and buy back more often if needed than I do now. I want to get it running before the season ends to see if I can do OK with it. There was a guy that bracket raced for years around here that had a '67/68 black Barracuda with a 225/904 named Eugene Ross. He had the slowest car at the track but he won more than his share of events running in the Footbrake class. It really made many of the other racers mad because they had so much $$$ tied up into their fast cars and Eugene would leave with the $$$. |
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| Author: | casey70dartreed [ Thu Aug 30, 2007 10:13 pm ] |
| Post subject: | |
It all depends on where you want to shift it. If you shift it yourself at say 4000...11s should be no problem I would imagine. You probably should leave on the converter as well. W/my old Super six setup (2.76 open gears)...It would run in the 12s no problem. I suspect the 3.23 to help a lot. |
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| Author: | Charrlie_S [ Fri Aug 31, 2007 6:33 am ] |
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Quote: The car will do high 16s w/o a problem and I have yet to experience anything that would lead me to believe that the 7 1/4 is inadequate...Especially with a stock combo in just the 1/8 mile. Of course everybody will tell you that the 7 1/4 will go away sometime (and they may be right), .
I was not knocking the 7 1/4 rear. I am running one in a 13 sec car with slicks. Just saying if there is a choice of 7 1/4 or 8 1/4 I would go with the 8 1/4, do to the extra beef, and availability of parts (gears and sure grip)
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