Gearing question

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GUP

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Here's the rundown on the evolution of my car.
1st trip to the track:
street tires that are 27" tall, 4.56 gears. 60' was 1.88, 1/4 mile was 112mph and 12.8 seconds. I spun pretty bad, was on 6400 rpm crossing line.

2nd trip:
I bought street slicks that measure .75" shorter than street tire. Same gear. 60' is 1.58, 1/4 mile was 107 mph and 12.3 seconds. I crossed the line at 6500 rpm.

I am thinking I need to change gears to be 3.90. The car is a 440 w/727 trans, 3400 stall, holley HP750, M1 intake, eddy heads, 9.2:1 compression, mopar 284deg .539" lift mechanical cam, TTi headers. I had hoped the car would be in the high 11's but it has fallen short a little. I am thinking I need to change the gears to better utilize the torque of the motor and get the mph up and the ET down. Any suggestions?

Here's more info if you want it.
-timing is 25 degrees initial, 36 total
-jets are 80 front, 86 rear with the power valve plugged in rear only.
-valves are lashed at .018" I & E
-plugs are gapped at .032", and they read good as far as jetting.
-Went up 2 jet sizes and slowed down, went down 4 sizes to where I am at and gained 2 mph.
-use competition engineering slide-a-links. Leaves straight and hard.
-Could probably use a 950 carb, but I can't afford it!
 
Changing the gear really won't change much in the ET but would probably help with MPH if you are running out of carb and cam, 1.58 60' is about normal for a low 12 car. I don't think you are going to improve much here with any gear change, although it does seem like it is over geared a little. Shedding weight is the easiest way to pick up ET, 100lbs = .10, removing weight from the front or changing your front to rear weight distribution will pick up ET also. I thinking more inline with a 4.30 or 4.10 gear.

Chuck
 
i would think that an 850cfm holley would be better for the motor. 6400 rpm's is real nice going into the traps. are you running open headers? if so add at least 18" of collector to it and advance the cam 4 degrees. if you don't want to go into the motor loosen the lash up .004"-.006" but not over .008"
 
Something seemed a little odd regarding your rpm vs trap trap speed. I did a quick bit of math and noticed that the amount of converter/transmission slip was .7% on the first day at the track and rose to 4.08% on the second day. Just wondering if you noticed any additional slip lately and it may be something you want to check out.

It would seem that most of the 5/10ths that you did pick was in the first 60" and the remainder shortly thereafter, altough it would be impossible to tell without seeing the 330' times. I would think that was due to the reduction of wheel spin from the softer compound of the slicks, rather than the slight numerical advantage afforded by the shorter tire.

As 340Mopar said, I agree that you are somewhat overgeared for your combo and are running out of carb and cam, so you have two ways to overcome your present problem. You can drop your gear ratio numerically which will cut down some on your tire spin, increase your gas mileage if you care and give your vehicle better street manners too. Or, you could give it more cam and carb and go even faster, but you may have to bump up your compression ratio.

If you were to go with a gear change, a 3.90 gear would get you 120 mph in the 1/4 @ 6000 rpm w/ 0% converter slip or 6240 rpm w/ 4% converter slip. Assuming you have the power of course. :)
 
I thought the same thing about the slip. It would seem the tranny is letting go. I do need to pull the motor and get the block decked to get the CR up to 10:1, but don't see that happening until winter at the earliest. (I bought the car this way, I would've done it from the get-go.) I appreciate the input. I will get the full run tickets and post it tomorrow.
 
Hey GUP,
It is hard to figure anything without knowing what the weight of your car is. If you could post that, it would help. I will give you a little run down on the car I have been driving for a guy that might boost your thoughts a little.

1973 Plymouth Duster, 2750 to 2800lbs...depending on driver
446 cu. in. 9.5:1 compression...Was a stock rod street motor
915 heads 1.6 ratio crane roller rockers, some porting
310 x 585 straight up...Big block has all the torque it needs down low, don't advance your cam. If anything retard it.
Holley Street Dominator intake, been milled and handworked for Dominator Carb
1050 Holley Dominator Carb
727 torqueflight w/ reverse cheetah valve body, 4800 stall
2" headers w/ 7" collectors...Long collectors build torque (not needed w/ big block)
4.86 pro gear, coil overed, 8 3/4, ladder bars
12.2 x 32 Hoosier Slick
This car runs consistent 10.80's all day long..Last time out we run 1/8 mile and it run from 6.94 to 6.97 all day >>> maybe this will help...Sounds to me like your car has a lot left in it w/ better combo....Probably as 340 dart and ramcharger said...Your car is running out of breath (out of cam and cfm) gears would probably help a bunch or a taller tire to get the motor backed up some...Just my 2 cents...Lee
 
I had the car on a chassis dyno about 2 years ago. Tell me what you think about this. The pull started at 2400rpm and eneded at 6200rpm in 3rd gear. It had a level 300hp all the way thru that range. It never varied more than 2 or 3hp. That tells me the trans or converter has issues, but I don't know whether to believe the results or not. I had dismissed them as bogus until now. You don't notice it when driving, but maybe???
 
I still think the car has more in it also. I am discouraged about the results so far, but hey thats part of it. I think the car will go mid to low 11's in the end. The TTI headers I run are the shorty tri-y style. The flow could be better, but thats wjhat came on the car. I know that's hurting me a little. The trans may be slipping, I am not sure how to tell 100%. The gears and/or the tires need changed out to better utilize the rpm range and grunt of the motor. The motor runs GOOD! It revs hard and fast!
 
70dartman,

Does this 440 car your driving have any cooling issues? I seem to fight my car to keep it cool. I have a aluminum 2 row radiator with dual 11" electric fans from Be cool. 180 degree thermostat, hi-flow water pump. I almost wonder if the coolant isn't moving too fast thru the radiator?
 
So, how much does the car weigh...No, we don't really have any cooling issues unless we don't have time to cool it between rounds...CSR Aluminum elec. water pump and alum. radiator (can't remeber what brand, I believe grant), 1 elec. fan... Usually on a hot day and not getting cooled good it doesn't get over 200...Usually with cooling, by the time we get back to the trailer, it is around 180...Lee
 
Does the car nose over towards the end of the track... Getting hot? Are you sure it is not way to lean...What part of the country you in?
 
I am in St. Louis, MO. The car pulls pretty hard all the way down the track, I don't think it's leaning out. The plugs don't show a lean condition. I don't know what the car weighs, I would be guessing. It weighs what a stock dart does only with a big block. My cooling problems are on the street, not so much at the track. If I drive it for long periods or in 90+ degree heat are when the troubles start. I think everything becomes heat soaked, and it's alot of motor in a small area. I stayed up late working on my fishing boat last night and forgot to bring in my timeslip. I will do it tomorrow for sure.

Thanks,
GUP
 
here is the run.
60' 1.69
330' 4.973
1/8 7.765 mph 87.49
1000' 10.211
1/4 12.308 mph 106.94
 
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