I've done all the online calculator's and hoping some of you guys could could post some real world RPM numbers at the end of the 1/4. I got a SBC406 turning 11.55 at 117.83 with 3.90 gears finishing up at about 5900 rpm's at the end. Race weight is 3415 lbs. Some more specs are 3500 stall, 28" tire, Comp solid flat tappet, 250 duration, 110 lobe Sep .589 lift with the 1.65 rockers I'm running, power band is to 6500 on the cam. I'm thinking I could pick some ET buy going to 4.56 gears but I didn't want to spin it out of the power band for the cam. I know there are so many variables between setups but anybody running anything close with 4.56 gears and a TH350 that could post what they're finishing at. What do you guys think? Should I go for it!

 

Calculations with:

117.83 mph
28.1" for the ET streets
1:1 high gear
3.90 diff gears

Calculates out to a finish line rpm of 5503. Your guess of 5900 is a little high.

Now keeping all that but switching to 4.56 gears calculates out to a finish line rpm of 6434. Now that's also assuming you'll keep the same mph after the gear swap.

I'd say that a much better gear ratio than your current 3.90's.

 

Thanks for the reply Stephen. That's what's got me wondering because I'm finishing more than 5500 that the calculators say I should, maybe I guessed a liitle high at 5900 but 5800 is for sure. I'm finishing more like 4.11's, without pulling the cover and counting teeth is there a way I could check next time the car is on stands? I'm saying 3.90's because that is what the guy I bought the car off of said he had in there, I never had any reason to doubt him until now. Unless I got some slippage somewhere but the car is real consistant. All feedback is appreciated.

 

Well 4.11 gears puts you at 5799 rpm and you said 5800. From the calculations, I'd say the car has 4.11 gears and not 3.90's. 4.56 would still be better. It's best to cross the finish line at or just above the shift point. The extra torque multiplication of the deeper gears will also get you off the line a little bit quicker.

 

Hey SSCAM very nice car! Sorry this is off the subject .But did you have to massage the inner fender well any to get the 28x12.50 ET streets to work.Im just curious ive been considering going with 28x11.50 ET streets.I use a 15x8 dragstar with 5" backspacing.And have no problem running a 275/60/15 BFG drag Radial (28x9)appox. tire.Main reason for asking is specs on the ET streets are slightly smaller than advertised size.Thanks for any input.and again sorry for getting off subject.

 

11SEC91Z, I recall the previous owner of the car mentioning that the passenger rear was massage slighty for clearance but at the time he had 29" tires under it, it looks like some work was done their but it's hard for me to judge how much with the undercoat covering it. The only thing I did was to cut off and smooth over where a bracket use to stick out that wasn't being used, it wasn't touching but too close for my comfort. I'll show you where it was in attached photo. This is looking up from the floor. There's also a 1/4" spacer on that side. This probaly don't help much, if you were local I'd let you trial fit one of my wheels.

 

You need to do some modifications like this if you need more room.

 

Nice Pic, that's worth a 1000 words!

 

Not my car but I used the picture to do similar modifications to my own. For the last season I've had Goodyear 29 x 10 slicks. After installing them I could see just how much more clearance I had. Next year a set of ET Drag 29.5 x 10.5 go on and a little more modification may be required. I won't know until I'm ready to install them.

 

Thanks guys for the good pics and info!

 

I'm guessing the converter is a non-lock up converter? Take that into account on the top end. The motor will actually be turning at a higher RPM than what it would calculate out to due to converter slippage.

 

That's possible. Using 3.90 gears and your best results calculates to 6.86% converter slip. 8 - 10% is usually normal.

If in doubt, pull the cover off and count the teeth.

 

exactly, a lot of people fail to account for converter slippage

 

Ok guys, now I'm understanding, given my converter slip is 6.86% approx. If I went to 4.56 gears I would be turning about 6875 at the finish line. Does that seem right the way I figured.

 

Converter slip could be as much as 10% with a TCI. I would stick with the 3.90's if you drive it often on the street. I have ridden in a truck that had 4.56's and it was ridiculously annoying driving around.

 

You're shifting at 6500 and going through the traps at 5900 with a 28" tire and 3.90 gears.

There's a few things you could do. Switch to 4.10/4.11 gears which would probably be just about what you need.

Get a tighter converter. Not really needed.

Get 4.56 gears and a slightly taller tire.

If for any reason you do more changes to the engine and more the power band up higher, through all these numbers out the window.

With my 4.56 gears and 29" tall tires, I shift at 7000 and cross the finish line at 7000 rpm. My 8" converter is also not as tight as yours since it stalls at 5700 rpm.

 

Thanks for the input. It drives me a little nuts when I get a little fun money and I'm trying to figure the best way to spend it. Ok correct me, please, if I'm wrong. If I go with 4.56 gears and 29.5x10.5 ET Drag (29.9 overall diameter) I should finish at about 6420 rpm figuring in 7% converter slip (which seems about right based on what I'm running at present) as long as I keep my 117 MPH. The other way I could get there, which I've been resisting, is spray a 125 shot and pick up 10 MPH and keep the setup the way it is. I started out trying to maximize the car N.A. but it's so damn tempting. Decisions! Decisions!

 

Ok, after thinking about what I can do, I decided to check into seeing if Richmond Drag Race Pro gears will go in to my 12 bolt. I'm thinking 4.33 gears, spool, and C-clip elininators. Does anyone know if that setup will go into my 12 bolt that came out of a 71 Chevelle years ago. I hoping it will because that ratio would put me right where I need to be at the finish line, without changing anything else on my current setup.

 

OK Guys, Made the appointment for next tuesday at A&W performance in Levittown, PA for the 4.33 Richmond standard gearset, Full Spool, and C-clip eliminator. Will be heading to Atco Dragway, Saturday Oct 2 to break them in.

 

Just make sure to let us know how the changes did.

 

I'll be postin' up Sunday morning the 3rd. Should be interesting to see how the car responds. Thanks for the help!

 

uhhhh not to be the mean guy but you should drive your car for like 500 miles change the rearend oil and then itll be good to go draggin otherwise youll be breaking stuff

 

Got the 4.33 gears in and went to Atco Saturday despite the fact the weather wasn't lookin good. I was hoping to get a couple passes in, was in staging lanes all buckled up and helmet on and the rain came. They called it a day and I didn't get to make a pass, so I don't have anything to report. I guess I'll have to give it a try next weekend and start a new post.

 

Except that doesn't work for race only cars. And properly set up, there won't be a problem.

 

SScam,
When I was running the 400 the first set up had 28x10.5 M/T ET Drags with 4.56 gears, TH350 and ATI 8" conv. I shifted it at 6800 and it trapped at 7000. The last season I ran that motor I switched the rear to a 12 bolt with 5.14's and went to a 30x9-92.5 Hoosier radial slick. With this setup I still shifted at 6800 and it trapped at 7200. It et'd in the 11.40's with the 4.56's and with an accompanying rear suspension upgrade and a 2.75 low gear in the trans it dropped to 11.20's with the 5.14's. I had reworked a set of 5.7" rods when I originally built this motor in order for it to run the rpm range. It had a 294S Comp Cams Magnum cam with a set of 1.6 rockers. The rear suspension change helped the car et/mph range become closer, it used to mph at 117-118 when it ran in the 40's and then 115-116 when the et dropped into the 20's after getting the tire spin issue handled.

 

mod313, thanks, good info that's close to what I figured I would of finished if I went to 4.56 around 6800 and we have similar setups. I went to the 4.33's so I would be finishing a little lower. I would'nt mind dropping some MPH if I pick up some ET. I'm itchin' to try 'em out. I'll get a chance next weekend hopfully.