Ran into a few fellow f-body guys today. Got to talkin and when I told them I wanted to get a 9" and said what gear ratio, they all told me no.He said they had a 6 speed with 4.10 and it fell flat on its face in 5th gear They also said a 3.73 in an auto is a mistake I was going to consider a 3.89 or a 3.70 for some mileage and the track.; I am working with Lloyd Elliot on some trick flows and switching my pistons to 5cc flat tops so I can raise compression again. I want to run at LEAST a 150 shot of nitrous. More depending, so while the 9" may eat HP I think it will stand up to the abuse longer than the Moser 12 bolt. I will be able to reuse the current cam I have , but even know it feels like it wants some gear. Get the car wound up and it pulls like a ****. I don't have any 60 ft times but I think there is certainly more to be had once the 3.23 gear is gone. Am I wrong or just missing something here? I figured a 4.10 would multiply torque more so than a 3.73. The car shifts at ~6700rpm and with a 3.89 gear , what gear would I be in shooting thru the traps?
Any help or input from folks woulkd be appreciated

 

/QUOTE=94zgreenmachine]Ran into a few fellow f-body guys today. Got to talkin and when I told them I wanted to get a 9" and said what gear ratio, they all told me no.He said they had a 6 speed with 4.10 and it fell flat on its face in 5th gear They also said a 3.73 in an auto is a mistake [/QUOTE]

I have no clue on what they were telling you but they were wrong. With a numerically higher gear ratio, you will be picking up some torqe multiplication. As far as it being a dog in 5th?. Who races in 5th gear?!. do a search, with you putting in 3.90's in your car will feel like a new car. Id recomend them especially if its a m6.

 

Gear it for the trap speed its capable of running, and crossing the line in your 1:1 gear - reflecting engine peak HP rpm, max engine rpm, tire diameter and growth and converter slip. Then put enough tire under it to hook. I run 4.10's with an automatic (TH400). Not sure why its a waste.

 

I run a 3.42 based on the logic Fred expressed. With my tire size and hp I cross the line at a bit past peak power in a 1:1 gear. Actually, with the TH400 it was a bit too far past peak due to converter slip. So, the correct ratio will very much depend on your combo. Luckily, it isn't hard to change or go to a different tire if you get it wrong. Still, it'd be cheaper and more convenient to get it right in the first place.

Rich

 

The more I go to the Real Street Drags, the more I want to race the car than just have it a street car. I just finished this motor and I am still wanting more so I am getting ready to dump another 5k or so between the rearend, rebalancing the motor for new pistons and some worked over trick flows. Lloyd is telling me I'll gain another 25 hp or so peak but will make LOTS of midrange power . Making peak power between 6600-6700 but will pull thru 7k.
The heads will get a lt4 "race port" along with a lt4 intake ported to match the heads.
What is going to be the best way(read cheapest) to determine my gear ratio. Is there a way to determine or is it going to be a "trial and error" type thing?

Fred,
I went to your page and noticed you are using a 12 bolt. I assume it is holding up well. Your power levels are above mine, is there a reason why I shouldn't get a 9"? I like the idea of keeping my GM all GM.

Rich,
How do you figure converter slip. I do have datamaster, would that tell me if I log a run?

Mean LT1,
Even in 5th I would still think you could accelerate better than a 3.42.

Thanks for the replies

 

The amount of slip will vary from converter to converter and also depend on how power you putting through it. Mine is about 5% in the 6,500-7,000rpm range. It is the ratio between crank and DS rpm but can be calculated by using rpm through the traps, gear ratio, and tire size to calculate a theoretical mph with no slip versus the actual trap speed. For example, if you calculated trap speed is 140mph and your measured speed is 135 there is 5/140 = ~3.5% slip. The converter manufacturere may be able to provide an estimate in advance.

You need dyno numbers and weight to estimate trap speed. If you know your traps now and figure you will be adding 25hp you should be able to come close using on of the many ET calculators. As far as 12-bolt v. 9", do a search. This has been discussed recently.

Rich

 

I did a search yesterday. The main reasons "I" saw was the 12 bolt is strong enought to handle power up to 900-1000HP. The 9" is stronger BUT, it's alot heavier and as I seen in one thread it's simply overkill..........extra weight for not much to gain. That's just my take on what I read, please correct me if I took the post wrong.

 

Thanks Rich, A buddy and I were messin around with gear ratio's, tire size etc, just to see what rpm's I would be pulling. I realized tire size can affect it even more. How is tire growth and is there a big difference between a drag radial and a slick? Is there a constant that they grow at or is it different between tire manufacturers?

I guess I will be going with the 12 bolt, most likely. I'll do a search in the morning, I just want to be 100% sure of my choice.

 

Radials have minimum growth. Typical 10" bias-ply tire, figure 1-1.5" or so.

Rich

 

Thanks, I appreciate the help.

 

I figured the tire growth by realizing I was scraping rubber off the slicks with the leading edge of the fender when trap speeds went over 125MPH. I figured about 1/2" growth on radius, which would be the 1" that Rich (I believe) is talking on diameter. Also moved the rear axle backward a bit

I figured the convertor slip by comparing engine RPM with rear wheel RPM calculated from the MPH, on the chassis dyno, which involves an assumption on tire diameter (and of course reflecting the rear axle ratio). That makes it a bit difficult to calculate, because its hard to figure the actual tire growth on the dyno. They should "grow" exactly the same way they do on the track, except for the fact that the rear of the car is tied down to the floor. You can see the back end of the car rise up as MPH goes up. Does it grow as much, or do the sidewalls bulge out more as the growth tries to occur, because the chains are holding the rear of the car down, due to the angle they make with the floor. I chose to ignore that issue, and assumed the tires were growing about the same as they would on the track.

After playing with a bunch of numbers, I built a spreadsheet that includes "growth" by entering it as a constant of 0.50" (radius) at 120MPH, then prorating the growth to any other MPH using the ratio of the square of the MPH. For the converter slip, I got 11% using this method. All of this jives with actual track performance, although that involves using the trap speed, allowing for the fact that the true finish line speed is about 1MPH above the trap speeds, and then matching up the finish line to the engine RPM from the data logger, based on the number of seconds from the time the throttle went to 100% + ET to get the correct frame of data representing the rpm at the finish line.

As you can see, its possible to totally over complicate the whole issue. Its easiest to just run the car down the track, and see what happens. But then you might find you bought the wrong set of gears, so its a crap shoot. Sometimes I think if I spent as much time running the car down the track as I spend doing calculations, I'd have a lot more fun.

And, even in my own case, I no longer have the right gears. With the 125-shot the car trapped 128mph at about 6,750rpm, with a 7,000rpm nitrous cut-off. But now that its set up for a 300-shot, its obvious (fingers crossed) I'm going to run out of rpm short of the finish line. Rather than swap in a set of 3.90's, I'll just hit the button on the Gear Vendor O/D and let it roll through the last 100-ft at 5,400-5,600rpm.

 

I was also a bit curious about air pressures and there effect on tire growth. I can understand your thoughts on less calculations and more time at the track. I still have a hard time believing the calculations but when I try to check them they are coming out or are damn close so I am getting to have a little more faith in them.
I wonder if I should be looking more at the new combination then and not so much at what I have now. I'll be swapping to some worked over trick flows and a 5cc flat top piston to bring compression back up close to where I am now. 11.8:1 on 92 octane without any spark retard . (The last part makes me want to laugh at everyone who said it couldn't be done with out race gas. Lloyd and Bret know there stuff and it shows. People still don't believe me but oh, well.........)Right now Lloyd is saying we'll be able to make power thru 7k rpm and if I don't it will be the cam and NOT the heads being the restriction. We are figuring on a shift point of about 6800rpm. I have been toying with the idea of a strange 12 bolt with a 4.10 gear.Pretty much locked in on the Strange, still toying with the gear ratio.Limited street use so I am not so worried about the extra rpm's.
Not quite sure how tall of a tire can be tucked under the car but it seems like a 29" tall tire lowers my rpm's a few hundred compared to 4.10's with my current 275/45 ZR17(25.7" tall) I definately understand what Rich was talking about by being able to use tire size to adjust accordingly, but like you said I don't really want to play the "crap shoot", in a perfect world we'd nail it right the first time, but live and learn......
thanks for the help I appreciate it.

 

Now that I think about it, tire pressure should play a part in tire diameter but there would be a point were too little or too much tire pressure would begin to affect the contact of the tire on the track, right?

Without modifying the fenders, what is the tallest tire that would fit, assuming 1 1/2" of tire growth?

 

When you're looking at tire pressure, you're generally playing with numbers in the 10-15psi range. Not sure how much difference that little change would make. I've also changed from tubeless to tubed tires, so who knows how that might affect it.

A 29" tire is not going to fit without body surgery. 28.0" is about the limit with the stock fender opening. I've seen a 29.5 and it took a lot of body work at the front and rear of the wheel opening, and some cutting back of the inner fender liner.

http://cjcfo.fbody.com/members/injun.../DCP03836a.jpg

http://cjcfo.fbody.com/members/injun.../DCP03863a.jpg

 

I don't want to do any body work to it so looks like I'll be checking into the 28" range there some more. Thanks for the help