gear ratio

WHAT REAR GEAR RATIO BE GOOD FOR MY NEW CAR I JUST TRADED FOR?LOCAL TRACK IS 1/8 BUT THE OTHER TRACKS WILL BE 1/4...I'M THINKING HP RPM BE AROUND 7500RPM TO 7800RPM[I'M GUESSING]IF ANYONE CAN TELL ME THIS[RPM RANGE]WOULD BE GREAT ALSO..

SPECS;
540bbc blown alcohol engine
4.50 bore
4.25 stroke
6.535 rods
11.8 static comp.
731/726 lift roller //114 lobe center //running dur 322--326//dur at .050 284--288
brodix 2-plus heads W/FULL flow job
12-71 blower 25%OD
big&ugly injector hat
port nozzles
990 pump
fuel system flowed by spud miller

trans;;pg with brake
1;80 straight cut gears
10 disc high gear

car is a CM 23T altered
35x17x16 slicks
driver weights 180lbs [don't have car weight]

 

Just curious, but what kind of flow numbers do you have? I would start with a 4.10 gear and see where you are at In the traps.

 

378cfm on intake and 261cfm on exhaust

 

I would look towards a 4.30, especially since you are moving some weight. Had similiar BBC set up (4.30) in altered and it was turning 7800 @ 200mph. Don't think you'll get quite that high as it's going to take you longer to pull the rpms up.

 

430 is where i would start

 

What Gear Ratio?

Somebody will probably slap me, but I would think that having one "best" rear gear ratio for both 1/8 and 1/4 mile would be hard to determine. I have two different ratios for 1/8 and 1/4 mile (526 hemi, 40 psi, roots, alcohol, 1900# TAD, 34.5's, etc). It seems to me that some of the faster cars in the 1/8 mile don't (won't) even try a 1/4 mile because they can't run the rpm's. Depending on your horsepower (and the track conditions), you might consider 4.56, it gives around a top mph of 176 at 7,800 in high gear. If you have the horses to do that number in the 1/8 mile or where that mph number happens in the run depends on your horses. IMHO, I think the 7,800 max rpm is a little low to have one gear ratio for both 1/8 and 1/4 mile. I'd suggest a max rpm of 8,200with your set-up, with a 4.56 for 1/4 mile, 4.88 for the 1/8mile. Just my thinking. Forgive me guys.

 

I have been ranting about less gear ratio on this site several times....my experience has been that if you have an overall 1st gear ratio of 7:1 or more, the car will be violent off the line....spin - shake - etc. A 4.30 X 1.80 = 7.74:1 (way too much) With a 3.90 rear X 1.80 trans = 7.02 you might get off the line without having a timing controller of some kind.

Edited to specify the above comment is with a converter transmission only as the converter multiplies torque - I have no experience or comment on clutch cars

 

my converter is a PTC 10.5" Spragless converter..tradeing my doorslammer for a 23T altered..it doesn't have center section so got to get one..tight budget so trying to buy one time so can get feel of car and engine..the altered has a M/W floater rear end with 40 spline..was told i had to use M/W center section and they are very spendy..thanks everyone for the help on this..
Don

 

I would do a 4.30 or maybe even a 4.56, depending on how much those slicks grow and how fast you are going to be. Its gonna be violent launching with either gear, I'd figure the gear right off of RPM and MPH through the traps.

I'm also going to agree with TADhemiracer, I bet that motor is gonna like more than 7800 max rpm, but I'm no expert.

 

my first blown alcohol injected engine..is there a way to know about where engine likes rpm?or is it more a try and see or dyno it being only way?

 

I agree. Last year ran 4.10 with 1.80 trans. Very close combo to the altered in question. I couldn't get the shake out of it to save my a**. Tried different slicks at all different pressures, different launch rpm, tune the motor up- tune it down.... nothing helped. This year 3.89 rear gear (as per Nitro Madness formula). smooth as silk.... now I can finally start tuning up the motor. Already went 1.02 60ft @ 7.02 at Boise on a very soft tune. Just my 2 cents

 

I run a 557ci bbc 4.5 bore x 4.38 stroke, bb2 heads combo in 3100lb car with 3.90rear & 1.69trans and still need to pull timing to get down. 6.69 @ 207mph 7800rpm.I think the trick is to keep the tight convertor loaded in multipication for as long as you can.Maybe a lighter car my be able to get away with more gear without smoking the tires.
I think a 4.11 would be a good ratio for a convertor driven combo on 34.5 tire.

 

I agree. But if your on a budget and can't afford to try a bunch of different parts and combos, I would go to the softer side and have fun. BTW went 6.98 at 4500ft this weekend. still way soft tune up. Just my 2.

 

gear

a 430-456 will work ok with the 1.80 low. I've never run a car that light but I ran a glide in a promod and the 4.56 would go 173 in the eighth and over 200 in the quarter without insane rpm.
The best thing you can buy is a timing control. If you have a msd promag get a used 6 shooter or run a regular msd ignition with retard features.
The best thing about the converter is it will NOT weak shake. Mike C was sounding a little like a clutch guy with the supersonic metaphor. With the automatic dump about 15 degrees for 2 seconds and let it eat. It will be a turd but it won't shake and you'll get seat time. Slowly bring the timing in quicker until it starts to rattle. You may need to dump a little timing around the shift but it will probably be ok for starters.
I don't know how much money I wasted screwing with gears,fuel system,tires etc thinking I was always on the wrong side of the curve.
If you get a wedge automatic running good with no timing control the tuning window will be so tiny it will never repeat at a different track or in weird weather.
BTW we are testing a new screwblown promod with a converter in a couple weeks and we will do the same thing I described. Back into with soft timing and turn it up as the driver gets comfortable and tweak the fuel system a little each pass.

 

Let's get something straight here. "Weak" shake is usually caused by the car not accelerating, sometimes because the motor is weak suck, but normally caused because when the tire tries to settle back down @ 1.2 or so, the speed of the car isn't quick enough to equal the amount of acceleration you're trying to pull at that point in time. You can have the motor screaming bloody murder and still "weak" shake because you spun the tire too hard and weren't accelerating the car, which you can also do just fine with a converter. We call it weak shake because it tugs the motor down from the tire starting to stick, but normally its because you didn't get the car moving fast enough for the tune-up you're trying to pull off. The difference is with the clutch, if you run it on 4 cylinders early, you've welded the clutch to the motor and when bring it back in, you have to be more delicate or you "weak shake" from hitting the tire too hard or do a burnout about 150 feet from the starting line.

Not trying to nit-pick at you, just saying anything can "weak" shake.. That and it sounded like you said clutch guy with a little resentment.

 

I have been at this a long time and I should probably know this but I have never understood the 1/8" mile gear deal.
If it's been answered my apologies.
I have always felt that if X gear gets you your best 60 and 330 why would you change to Y gear just because it's a shorter track?
OR
Does this only apply to cars that are over-tired?