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gear ratios

Discussion in 'Chevy Trucks' started by chev4life, Jun 3, 2003.

  1. chev4life

    chev4life Junior Member
    Messages: 29

    what would be a good gear ratio for racing?

    thanx for the help on my other ?'s :)
     
  2. wyldman

    wyldman Member
    Messages: 3,265

    There is no actual number that is good for every car\truck.

    The main goal is to keep the engine in it's "powerband" or torque range when accelerating,and keep it right near the HP peak when running at a constant RPM.

    If you drag racing you want to pull the most gear you can to help the car accelerate,but not over rev it in high gear at the end of the track.

    When circle track racing,you are usually in one gear all the way around,so you want a gear that will give you your max RPM (peak HP) at the end of the straightaway.

    There are numerous factors which will affect the gear you choose,or the number you use to calculate the correct gear.Torque range,peak HP,max engine RPM,torque convertor stall speed and efficiency,tire size (outside diameter),and gear ratios in the transmission.

    A truck engine,which makes good bottom end,and peak HP is probably 3500-4000 or so will want a much taller gear,to keep the RPM's down.If you throw in a hot small block screamer,that pulls hard to 9000 RPM,you will need a much lower gear,otherwise the engine will never get up into it's powerband,and it will be a dog.We see this all the time when guys spend huge dollars on a motor,and throw it in a car with a 2.92 rear.

    Another way to get the engine up into it's powerband,is to install a higher stall torque convertor.It allows the engine to "rev" up alot more before it starts to move,so the engine makes more HP.The convertor tightens up as RPM's increase,so you don't lose as much top speed as compared to going with a lower gear.

    The last option,is going to a trans with more gears,or an overdrive.This will allow you to run more rear gear,and still keep the RPM's down on the top end.The tighter gear spacing keeps the engine more in it's powerband too.

    Clear as mud,right ?
    :D

    A typical Chevy truck will have a 3.07 rear.You add bigger tires (35's),you will need to go with a lower gear (bigger number),to keep the RPM's the same.Let's say a 4.56

    Now you add a cam,intake,carb,headers and an exhaust.This will probably move the powerband up some,so you'll want to again lower the gear some more to keep the RPM's up where the engine is making good power.A looser torque convertor will help too.You may want to look at a 4.88 or so.

    Now you swap the 3 speed trans for a 4 spd OD,you will want less gear,as the OD gear is taller,so you would probably want to go back down to a 4.56.
     
    Last edited: Jun 3, 2003
  3. snowjoker

    snowjoker Senior Member
    Messages: 283

    Chris: can a high stall torque convertor be used in a 4x4 or plowing? I was just wondering because of your post. Walt
     
  4. wyldman

    wyldman Member
    Messages: 3,265

    Yes it can,to a point,but it's not normally done on a plow truck.The main reason is as you go to a looser convertor they lose effeciency,so your not transferring all the potential torque.It will help if you've got too much cam,and not enough gear.I wouldn't go anything more than a few hundred RPM over stock though.A slightly looser convertor makes it easier on the trans and drivetrain when shifting from drive to reverse.

    The other drawback is heat.The effeciency lost in a higher stall convertor is turned into heat,so you need a real good cooler setup,and an extended fluid capacity,to help get rid of and control heat buildup.
     
  5. snowjoker

    snowjoker Senior Member
    Messages: 283

    Thanks for the info. Just has been a thought in my mind because i never heard of anyone doing it. :dizzy:
     
  6. wyldman

    wyldman Member
    Messages: 3,265

    No problem.

    We do it on all our gas trucks,but they are only a wee bit looser,to aid in the drive to reverse,and back,shifting.Otherwise they shift really hard,as they are pretty cranked up inside.Most have mildly done motors,and the extra little bit helps,as it's cheaper than regearing them.
     
  7. Earthscapes

    Earthscapes Senior Member
    from WNY
    Messages: 577

    >When circle track racing,you are usually in one gear all the way around,so you want a gear that will give you your max RPM (peak HP) at the end of the straightaway.<

    Sorry but i have to disagree with you on that one.
    You want max rpm's between 1/2 to 3/4 down the straightaways depending on the size of the track (3/8th's to 1/2 mile short tracks i'm talking here),,,to give yourself enough braking time to enter the corner. Other wise you could pickup the nickname "wallmeat"
    :D
     
  8. wyldman

    wyldman Member
    Messages: 3,265

    OK,OK,I stand corrected :D

    "wallmeat",I like that one !
     
  9. chev4life

    chev4life Junior Member
    Messages: 29

    what i wanted to know was gear ratios for drag racing ive heard of guys using 3.73's and 4.11's and so on and i wanted tot know which would be better.
     
  10. wyldman

    wyldman Member
    Messages: 3,265

    Neither one is "better" than the other,again,it depends on the combination your running.You have to gear the car according to what you have in it.

    If you put 4.11's in a car with a stock motor,3 spd trans,and small street tires,your going to run out of RPM at the end of the track.So you either have to back out of it,or overrev it.

    Re read the original post I made,I think it describes it pretty well.You can also read over this post,as it has some references to gearing,and Chuck posted a good link for picking gear ratios.

    http://www.plowsite.com/showthread.php?threadid=10205
     
  11. John DiMartino

    John DiMartino PlowSite.com Veteran
    Messages: 2,154

    chev4life,in 99% of the cases the 4.10 is better for drag racing than a 3.73. I have 4.10 in my Ram,and it is great for drag racing,until recently,now I went and added a lot of horsepower,and I need to put 3.73's in or taller tires.I am running out of RPM at the end of the 1/4 mile,since I added a bunch of HP.
     
  12. chev4life

    chev4life Junior Member
    Messages: 29

    ok thanx that all i wanted to know and the other in put was good and helped alot thanx.:)