gm 7.5 rearend for g / str

[mitchell968]

after searchin the archives here a bunch  ,  and a little on the hamb , i need some advice on chosing a  gm 7.5 in rear end for my g/str , and i have a question for the roadster guys about body widths and rear axle track widths ect. i bought 32 frame rails from vhr , frame will be completed by them later this year and im wanting to put a 29 polyform on it  but cant find rear bodywitdh dimensions and havent contacted polyform on it yet ,  but im thinking my rear axle should in the 56 in neighborhood as far as an outer drum to drum width ? hambers say a 3in wheel off set looks proper and will keep the tires close to the fenders ?,is this right ? is that good for  bville ? how close does that leave me between the body and tire with goodyear 28s ?  and for the rear end guys(thanks tony)  , what  years and models of gm 7.5  2 series  would fit this kinda low buck , low tech, low hp build ? im hoping sparky will chime in on this , you seem to be the resident 7.5 guru.  i'll start a build thread after i get the completed frame from vhr some time after sw this year. meanwhile ,  please feel free to fill in any blanks in my questions here . its a great feelin for me to finally be purchasing parts and getting this project going in , well , some direction . dont forget your double smileys.    thanks for your  help.                                           mitchell

[Dr Goggles]

Hi Mitch.

Are you sure a G roadster is gonna pull 3.08's?.....the series 2 is 3.08 down to 2.14:1....that's a lot for a G motor, how fast you think you'll go? Depending on what kind of bullet you're planning on 28's and a 3.08 will give you about 26mph per 1000rpm, where does your motor make it's power? where does it redline? whats the current record in the class? Aim to have the motor at peak power at the current record speed in top gear.....then calculate the rear end ratio you need to achieve this.

I should have kept me gob shut but you can convert one way or the other, I think it's 2 to 3 with a spacer, Sparky will tell us...

I am a fellow sufferer, but half a world away..... we'll be running Ford 28 spline axles into a Torsen centre in a 7.5. If you can choose your width it's a lot easier, you've got a bit of work ahead of you......but I'm sure you'll love it....

[Stainless1]

Mitchell, your first issue is to be sure your body will be legal for STR.  The roadsters have a lot of rules, including more body measurements than NASCAR.  Search the site, there have been a couple of body measurment charts posted. 
Work backwards from motor info to figure out your gearing.  We usually try to gear the lakester for the speed required just below the HP peak RPM.  You can make adjustments with tire sizes.

[maguromic]

Don't forget to take into account the tire growth. On a LSR 28" Good Year you can have much as 1" growth and about 1/2" on the sidewall.  Tony

[Stan Back]

Tony --

We're nat'sass on the money with 28" Goodyear drag tires.  We figure tire growth cancels slippage.  Come within a mile an hour squinting at the Isky dream wheel.

Probly got around 700 ponies and took some real changes to get much over 200 with 3.10s with our '29.  But, then again, a real one isn't as slick as a Polyform (flush doors, almost no coupe' pillar, no hinges, no gas cap, etc.).

Got to be a heck of an Unblown G motor to do that.

Stan

[mitchell968]

records 132.6       licensing and learning car

[Dr Goggles]

4.11:1 x 6500rpm x 28's = 127mph  

x7000rpm =136.8mph

[mitchell968]

that helps . thanks

[mitchell968]

found 4.10s    2000lbs , 133 mph needs 367 hp.  is that weight realistic ?    300 + hp from 122 c.i.  not difficult         

[desotoman]

found 4.10s    2000lbs , 133 mph needs 367 hp.  is that weight realistic ?    300 + hp from 122 c.i.  not difficult        

May I ask how you came up with 367 HP needed to go 133 mph in a 2000 lb. STR? You might want to recalculate. Is this for Bonneville, El Mirage, Ohio Mile, Loring, or the Texas Mile?

Tom G.

[mitchell968]

calculator on a formula site . got a good one ?  bville only. is that a realistic weight !?

[Dr Goggles]

So, it seems like a doable thing. Having said that welcome to the world of LSR . Now is your chance to lie awake at night wondering what the car that set the record was like, how they got their power, what weight the car was , how long did it take for them to get that speed out of it?.... there aren't many records that are an easy get, some look easy.....


there was recently a story ( the link was posted by Mike)about one of the ol faves of landracing.com Jack Dolan and his challengers in a S2000 Honda....

http://www.caranddriver.com/features/not-so-fast-there-bud

it gives a great insight into this game............

[mitchell968]

been losin sleep over it for 3 yrs. only able to really get started recently. sw 2010-11. 2012 will be the last w/o  a car. with enough help, that is.

[desotoman]

calculator on a formula site . got a good one ?  bville only. is that a realistic weight !?

Something is not calculating that is for sure, here is my old setup. My STR I have for sale in the Classifieds, with a 355 Chevy that had a 3/4 hydraulic race cam, 600 cfm Holley, early Chevy FI heads, weighted 2500 lbs., Quick change Rear end, Automatic Trans, and never ran over 5700 RPM ran a best of 163 mph, at El Mirage. If the motor I had in the car put out 1 HP per CU, I would be amazed. Body is a 29 Ford Roadster.

Hope this helps.

Tom G.

[mitchell968]

it does. thanks. ballpark #s are what i need to get on track here. . i ordered a hot rod calc.