Rear Gear Ratio

[SMOKD U]

heres the question. i have 3.73 in the rear and i have 27 inch tall tires. last yr at the track i ran 28 tall tires and i was hitting 6300 at the end. how can these guys with 4.10s make it through the traps? my cruising rpm now is 2900 at 60mph and 3100 at 65mph. to recap this year i put in a C4 o should i have the same rpm at the end of the track. just wondering how the guys with a C4 and 4.10s make it down?

[Greg@GLD]

Mark, the "3rd" gear in a AOD is the same as a C4, it's 1:1, just like 4th is on a T5 or Tremec, so it should not change anything, unless you were shifting into overdrive while running down the 1/4 mile. If you run out of gear then you will have to go back to a 28" tire... Having a C4 should not be a factor.

[SMOKD U]

yes i know this but how do others witrh a c4 and 4.10 or higher make the trap?under 7k

[Greg@GLD]

I guess I am wondering why you reference "guys with C4 and 4.10's" because any transmisson that has a 1:1 "top" gear (virtually all of them) will see the same RPM/MPH relationship relative to the final gear ratio, know what I mean? In other words, ok.. I have a Tremec 5-speed, but I obviously don't shift into OD while running down the 1/4 mile. So if I have 4.10's and you also have 4.10's, each of us will see the same RPM thru the beams if we have the same size tires. The fact that you have a C4 and I have a Tremec don't figure into the equation... Follow me??

[Kirk]

So in other words it comes down to tire size.

[Mark302]

with 4.10's I was turning 6700rpms though the traps at 125mph with a 26" tire.

[Greg@GLD]

http://www.midwestmustangassociation.com/analyzers.html

I punched Mark 302's numbers in and it comes back a 4.13 (4.10!)

Mark S- play with that calculator once, it works very well...

[SMOKD U]

AXLE RATIO CALCULATOR

"RESET" Before each combination!
SPEED IN MPH 124
DESIRED RPM 6300
TIRE DIAMETER IN INCHES 28
TRANSMISSION HIGH GEAR RATIO to-1.1





AXLE RATIO NEEDED 4.65

it says i need 4.65 and i have 3.73's my point is that it seems like my rpms are way up there for 3.73s but maybe thats how everyone else is also. i should get taller tire i guess. how tall of a tire can you fit under this thing?

[Mark302]

I have 3.73's now with a 26" tall tire I like it MUCH better than the 4.10's. I only had the 4.01's for about weeks when I first superchargered my car.

Those calculators are GREAT!!!

[Greg@GLD]

When I first installed my blower I had 4.10's and it was too much gear. I did go to a 28" tire for awhile (ET Streets) but when I went back to my drag radials I had problems touching the rev limiter thru the traps. 3.73's were/are perfect for me also, with a 26" tire.

[NitrousEdge]

Mark S - the only additional thing that you have to take into account is convertor slip. That will slightly increase your trap rpm's also. :burnout:

[Greg@GLD]

Eric, don't they have lockup in 1:1 or is there constant slippage???

[NitrousEdge]

Maybe Dave can elaborate a little more on how it all works, but there must be constant slippage. My car always revs higher than any calculator I have seen. It doesn't rev quite as high as Marks car did with the 4.10's - but at 7-8 mph less I was very close to his trap rpm.

I have only seen one calculator that had an allowance for convertor slip (input in a decimal percentage (.01=1%)) and I can't find that one anymore...

[Greg@GLD]

Well from my old school days, when I had T400's in my GTO's, if your converter slipped in 1:1 you got heat and that was bad??? They were supposed to go to "full lockup" at some point?? I also thought with AODE's there is a point where they lockup and this can be controlled via chip???? Sorry to go off-topic guys.

[NitrousEdge]

Well from my old school days, when I had T400's in my GTO's, if your converter slipped in 1:1 you got heat and that was bad??? They were supposed to go to "full lockup" at some point?? I also thought with AODE's there is a point where they lockup and this can be controlled via chip???? Sorry to go off-topic guys.


The way I was told - the higher stall you go the more it will slip - one of the reasons I stayed where I am for now (~3800) Yes mine runs on the warm side, but was told that was part of the deal.

I too am running a C4 Greg... :p

Yes, in the aode's and 4r70w's you flip a switch and will get full lock up = one of the big advantages in going with one of those tranny's.

[SLOWZ]

Yeah the converter slip was a biatch in my '98 Z. I don't know if there is a lock up point in a C4. I don't think there was one in a TH400. there was one in the TH350C's that came in full size van's in the 70's if i remember correctly. In my 4L60E i ran a lock up switch. In the computer, all it does is ground a certain circuit when it reaches the parameters that are set. But, if you splice in a wire and then run it to a switch and from the switch to a ground, you have a lock up switch. I went from 12.79 @ 102.x to a 12.65 @ 104mph when locked up at the right rpm in 3rd gear.

The only time the converter locks up is under part throttle driving, and consistent throttle driving, like when on a freeway. When you go WOT, it unlocks and flashes to a certain point on the downshift.

Mark, with that calculator, the numbers you put in would be correct for a manual transmission car and you could run a 4.56.

Do you have any datalogs from last year in which it showed what you were running through the traps at? If you do, you could adjust it for a 4.10 swap and get a pretty good guess of where you will be.

oops, need to read your post a little closer. Do me a favor, what tires exactly were you running last year(e.t. drag, e.t. street?) and what tire are you gonna run this year?

[SMOKD U]

well i can tell you for sure there is no lock-up. you can get a lock-up in the aod from lentech but mine from tsi has none. not sure if it even an option either. well its hard for me to see the line and my tach at the end but i think its around 6300 rpm at the end. i have 28 slicks for th track and 27 ets for the street, i wanted to just use my et streets at the track but im afraid that ill blow my motor up over 6300 rpm in third gear. i was told that a taller tire will give you more mph if you have the hp to turn them. so i was thinking i should go to a taller tire to try to save my motor. what rpms are you guys trapping at the end? and how dangerous is it to rev over 6300 with an E303 cam thats only suppost to go to 5500 rpm max. on the dyno i was still making power over 6300 and there was no float as far as i call tell. couldthis high reving wreck my heads?

[SLOWZ]

yeah, if you can turn a taller tire, you'll have more rpm to play with or atleast bring it down if you feel you are too high right now. Good thing is you will also have a larger contact patch on the launch. BUT, if you can't turn the tire, it may affect you negatively. But it may be worth it to sacrifice a tenth for the sake of longevity?

for the question on high revs affecting your heads, i guess it would be determined by the valvetrain being run on those heads. a good enough spring? rocker arms? do you have the bottom end to support the rpm? my stock rod bolts did not like anything over 6800rpm :)

[dave bandt]

Mark,

The big thing that will kill your engine is RPM. At 6300 you've got a pretty good margin of safety. When you get above 7000 it gets risky. Like Z28 said....all converters have slip built into them unless they are specifically designed to be a lock up. If you read through the rules for heads up series racing you will see that lock up converters are banned in a lot of classes. A true race lock up is expensive....but it is worth some ET.

[SLOWZ]

and, if you are talking about a 28x11.5" e.t. street that has a total height of 27.5" and comparing it to a 28x10.5 E.T. drag that has a total of 28.1", i don't think there would be TOO much of an rpm raise from last year, but there'd be something. If you are running 26x11.5 E.T. streets that are like 26.3", then you would be pushing it if you are REALLY concerned on the 6300.

Chris

[MAT88GT]

you need to call PI and ask them what would be more efficient on the big end of the track with your weight, trap rpm and tire size

a 2" difference in tire size doesn't sound like much, but with my tcs I would have picked up 5% or more in efficiency due to less "slip" in the converter by swapping to 28's...just do the "sweet spot" in the converter at those rpms (6700-6900)

you can kinda think about it as a bike, cruising downhill in a numerically higher gear...you're going to be pedaling like a bastard but not appling much power to the ground. The converter is the samething. Its transfering mechanical force via fluid coupling, when your engine side is spinning too fast it surpasses the effective coupling range of the transmission side...tossing too much goo :)

[onewickedstang]

i'm running 28x11.5 ET Streets with 4.30 gears behind a C4. my convertor is an 8" 4000 stall. behind my stock lower end 302 with a .512 hydraulic cam i've shifted anywhere from 6300 - 6700 and haven't noticed any difference at all in et or trap speed. i'm leaving the line @ 2500 on footbrake and if i remember correctly i'm going through the traps at about 6100.

[Chuck]

I am thinking of trying 26" instead of my 28"s. I was trapping at 6300 @ 117 with C4 and 28"s.