Over the past couple of years, I have seen a lot of timeslips from a lot of different types of cars. I've also paid close attention to the times that NA g's and z's put down.

I have often read people assert that for the g/z, the automatic's gearing is damned near ideal for the 1/4. I have to say that I agree. After reviewing hundreds of timeslips, I have noticed something I find interesting: all else being equal(including 60 foots), 5AT's achieve the same ET's as the 6MT counterparts at a lower trap speed. By extension, 5AT's that trap the same as 6MT's will have lower ET's.

So the ~6% shorter final drive of the 6MT, and the shorter gears, may help it accelerate faster, but these do not help attain quicker ET's.

I've been thinking about this for a while, but wasn't confident enough to actually post. That is, until I had a short conversation with an acquaintance at the dyno day last week. This guy has an 03 Acura CL Type S, 6 speed. We were talking about power, and 1/4 times. His car puts down about 245 to the wheels. On the same dyno, I put down 265. A few weeks back, he ran a personal best of 13.5, at the same track where I've run a 13.6.

I was intrigued, especially because the CL is FWD. I assumed he must have had a monster launch. So I asked him what his 60' was, and he said it was a mid 2.0, which is outstanding in a six speed FWD with street tires. I have a hard time cutting mid 2.0's in my RWD with DR's. I then asked him what his trap was, and he said it was a low 100mph, about 3.5mph lower than my trap.

How in the hell can he run a 13.5 with that low of a trap and a mid 2.0 60 foot? For the sake of comparison, a well driven G35 6MT that launches with a mid 2.0 and traps ~100mph will be good for a mid 13.8, maybe a low 13.8, tops. The only thing I can thing of is gearing.

So considering all these points, I've been thinking about how to affect my gearing. I obviously can't change the gear ratios of the tranny, and I'm not going to do a FD swap. But I can change my tire height.

The stock tires are 26.3 inches tall. To achieve the same FD as the 5AT, I would need to make them taller by about 5.5%, which results in a tire height of 27.7 inches. It's nearly impossible to find DR's or slicks that tall in anything other than 15's, and 15's won't fit over my rear brakes. I did see that Nitto offer's a 28.2" tall 18 inch DR, but it's 305mm wide and requires a 9.5" rim minimum. In addition to weighing a ton, and probably being way too much tire for my application, I'm not about to spend another $800+ on wheels and tires when I already have DR's mounted to dedicated wheels.

So what are my options? I've considered scouring the local tire places for any old truck tires that are the right height that could be mounted to our extra 18" coupe rims presently in the garage. I know they won't offer the traction of my current DR's, but I want to take them down the track for a few passes to test out this theory.

Does any of this make sense, or am I totally off?

 

I think the guy is yanking your chain because I'm very familiar with the 3,500lb CL-S and with 245whp and 6MT, it's going to take slicks and a 1.8 60 foot to get there. I know of a few mid 13 second 6MT CL-S' on the Acurazine board and they're all on slicks.

I believe M&H makes a "street" slick in a 16" size that will fit your rim. Slicks are the key. They are super consistent and make DRs look silly. They're also far more forgiving if you don't dump the clutch.

As for the gearing aspect, don't forget that the 5ATs have a torque converter which greatly increases torque multiplication (far more than the 6MT gearing car) on the initial launch, but they bleed power a bit at speeds above 110mph or so.

My question to you is, what rpm are you crossing the finish line at in 4th?

 

How much does the CL Type S weigh? Ideally you want to be at the end of 4th gear in the 1/4 mile with your 6 speed. That means you should be crossing right before you hit the rev limiter.

 

You lost me here. Either im reading this wrong or...

Why exactly would you want to make your FD to that or equal to the FD of the 5AT's? You just stated they tend to have a lower trap speed, which I have also concluded.

Wouldn't you wan't to increase your FD of 3.5 to 3.7's or 3.9's rather than the FD of the 5AT which are 3.3's?

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I cross the line a little north of 6 grand. I've got a good thousand rpms left on the tach, considering how "optimistic" it is in 4th gear.

I figure if I go with a taller tire(and the tire fits), I could stay in gear longer. Granted, I won't accelerate as quickly, so it would be a tradeoff. The question is, how much of a tradeoff?

I found the tire that would be as close to ideal as possible: 225/55/18. I might be able to get a couple of those, used, for real cheap to mount on my extra wheels. We still have a full set of coupe 18's sitting in the garage collecting dust. That tire measures 27.8 inches, and from what I can tell, they weigh about 29 pounds. What's more, they are the stock rubber for a few vehicles including certain trims of the Honda Pilot and the Mercury Montago AWD. That means I might be able to source them from dealerships, if they have extras lying around. The key point here is that this is an exercise only; they will not replace my DR's, and I'm not willing to spend much money in this endeavor.

With a fuel cut of 7200, those tires would let my 3rd gear top out at 103.73mph. If I don't suffer any acceleration penalty with those tires(unlikely), I would be able to stay in 3rd gear for the pass. That means I'd be between 255 and 265rwhp for several hundred feet.

Just thinking out loud here guys. Thanks for the feedback.

 

If I can increase my ET at the cost of some mph, I'm willing to give it a shot. The larger question is which is the larger factor in the 5AT's performance: the ratios of the first three gears, or the FD?

 

I think I see where you are going with this.

I would naturally tend to agree with others that the FD in the 5AT and other factors contribute to a slower trap speed, and potentially a slightly slower E.T.

If anything, I think you might be better off with a shorter FD. It just seems logical to me.

Goodluck with this experiment.

PS. Out of curiosity: why are you not willing to change to a shorter FD? Loss of practicallity for daily driving?

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It's all about money. I'm done spending major money on this car, at least for a long time.

 

I think you are on point with your theory. However I think you will also be sacrificing traction for an attempt to extend your 3rd gear. I guess the only way to find out is to give it a try.

I know in the few runs I've made with my automatic, I've accomplished my best time if I held 3rd gear all the way until I crossed the finish. However I had my gears swapped out in place of the 3.5 FD so I can't say that my car behaves like an automatic 100%. My question is then, being that I have one less gear than the 6-speed, should my times compare to automatic or manual?

 

If I recall, the automatic G35's still have different gear ratios than the 6MT. So even though you have the same FD as a 6MT, I think it will still behave differently than a 6MT.

I'm not exactly sure of this. Just a thought.

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Trey,

If you're crossing the line at just north of 6K, then you're gearing is pretty ideal. The general rule of thumb is that you want to be crossing the finish line through the heart of your powerband and in a gear near a 1:1 ratio. You wouldn't want to gear any lower (ie a 3.7+ gear) nor would you want to go taller (3.3). Your MPHs are in the upper 103mph range and honestly, I think that's about the limit your car will ever see unless you start digging into the motor itself. The MPH is there, but the ET is not. As you know, your ET problem is due to your 60' and I also think it's due to the light weight flywheel which is killing the moment of inertia on the initial launch and bogging the motor down when the tires eventually hook about 15' out. My recommendation, and I believe your only viable option, to fix this ET problem is:

1) Slicks
2) OEM flywheel assembly

There are plenty on people on the 350Z site that use full blown slicks or cheater slicks mounted on 16" wheels that clear the same size brakes you have on your 05 G35. I'd research the wheel and tire types these Zs are using. I believe most of these guys are running 26" tall slicks which would work out perfectly because they would fractionally increase your gearing to bring you closer to crossing the line at an even more ideal 6500rpm and the extra gearing would overcome some of the drag associated with slicks on the far end of the track. I don't know about the Z crowd, but I know the Maxima crowd that uses 26" slicks typically see an ADDITIONAL 1mph gain with slicks because the car dead hooks (mid 1.7-low 1.8 60 foot) and all the power is placed to the ground in every gear even on the shift.

 

The DR's that I currently run are already 26". That's one of the reasons I'm interested in trying out 28" tires. If I can get the tires cheap, and it looks like I can(~$25 each used from a local stop), then it would behoove me to at least give it a shot.

And I know I'm close to the end for power, but it's got a mid to high 104 in it. That 103.6 came on my first and only run of the night. I usually pick up about 1.5mph and 2 to 2.5 tenths from my first run to my best run of the night.

At the 1/8th mile last week, my first run was a 9.0@79.9mph. My best trap of the night was an 81.5mph(1.5mph difference), and my best ET was an 8.775.

I've already spent the money on DR's. I'm not going to by slicks until I wear these out, or in the unlikely scenario where I find some one to buy just my tires. I'm not going to continue drag racing at the rate I have over the past two years, so it's just not worth it to buy slicks.

 

You've got a uphill battle then Slicks are the only way that car is going to go sub 13.5s. I know it's got a 13.3 or even a 13.2 in it with a low/mid 1.8 60'.

A 28" tall tire is going to kill your gearing. With that light flywheel, the reduced gearing is going to make for bog city and a sluggish first 1/8 mile.

 

You're probably right on the tires, but this car has a 13.4 in it, with me driving, even on the DR's.

Bet you a set of aluminum pedals that I'll do it by Nov 1st(just because of the heat), with no more mods to the car. Any interest?

 

Found this on another site, don't know how accurate it is.

Elapsed Time X MPH = 1335
Examples:
13.35 x 100.00 MPH = 1335
12.00 X 111.25 MPH = 1335
If the factor is more than 1335, add more rear gear.
If the factor is less than 1335, reduce rear gear