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Tires,gears and turbo oh my

3K views 60 replies 10 participants last post by  RDY4WAR 
#1 ·
So i finally decided to ditch the centri and stock shortblock. Went with a forged bottom end, MHS heads,cams and still deciding on turbo. My question is what rear gears and tires should I run? I should be around 650-700 rwhp and running it through a 5 speed trans and will be using a n2mb. Rear rims are 15x10 and fronts are 17x4.5. Any help is appreciated.
 
#2 ·
Depending on what you really want out of the tire (street/strip/both) I would go with a qtp if only strip. Street and strip I would look into the Mt radial pros or the Mt et street r. 275/60s. Gears depend on your overall goal but 3.73 would be fine.
 
#3 ·
It's no longer a daily driver but will drive it on weekends. I will probably pick up a set of daily wheels and a track setup. At the track I have to run a dot approved radial. I want to get as wide a tire as I can . The 315-35-17s I had on the rear before looked good. I would like to get something at least comparable!
I
 
#4 ·
Why would you need a dot radial for the track? After living in va for 6 years, I know vmp and colonial don't care about tires as long as they aren't dry rotted or completely gone. Qtps are dot approved but are not a radial. A radial will not be good with a stick car fyi.
 
#5 ·
I checked and can run any dot approved tire so the qtp is acceptable. Now what gear and what size tires? I have heard run everything from a 327 gear to a 410.
 
#7 ·
I appreciate the link. How do I determine rpm since my car is still in pieces?
 
#9 ·
What are you building your car to make power to or what are you planning on spinning it too?[/QUOTE

I'm looking to make between 650-700 with the turbo setup. I will call MHS and find out what the peak rpm range is for the cams I have
 
#13 ·
Well just basing it off you will be going turbo amd the power you hope to make I am assuming you will spin it to around 7k. I would go 28" tire with 3.73s or 3.90s. If you are planning on more power later, 3.55s wouldn't be bad and may be better if it's more of a street car.
 
#14 ·
...I would go 28" tire with 3.73s or 3.90s. If you are planning on more power later, 3.55s wouldn't be bad and may be better if it's more of a street car.
If he runs it down the quarter with over 700 hp he'll run out of gear running a 3.90... assuming the car loses a little weight. With a turbo car I'd say 3.55's would be the best bet. Once you get the power, have you ever seen someone say "Hmm, that was fun, now I want to go SLOWER!" No. Even if he doesn't run out of gear with 3.55's on this build, he will the next time he ups the power level.
 
#16 ·
I will probably go with 3.55s. seems to be the happy medium. With 3.55s would you still run a 28" tire? What exact size would you guys run with a 15x10 rim?
 
#17 ·
#24 ·
Damn I wish I had a Trump meme to put in here, but.... WRONG!!!

1:1 in a straight drive is true 1:1. 1:1 through an auto is only 1:1 if the converter is locked up, which during a drag race, it's not. With an auto trans you have to take your stall into account when trying to figure out what your actual trap speed will be.
 
#29 ·
Not at all. Most guys racing with built motors tend to have built drivetrains. It would be stupid to not get a lockup converter. And you can do it with the stock converter but it has to be tuned for it. It's very common to lock up your converter when wot after you shift into 3rd
 
#30 ·
Guess I've been out of the game too long then. My best pass to date was a 9.99 in a Malibu running a 'glide.... no lockup there. I'm going to be running a C4 in my T. I guess I'm not familiar with running big power through an OD auto. I gotta say, this is interesting ****. I can't run anything bigger than a C4 in my T unless I want to amputate one of my legs. That would be cool as **** to be able to lock up 3rd in my C4 though. For some reason I'm going to bet they don't make that mod.
 
#32 ·
The C4 is a mechanical trans and to my knowledge it was never available with lock up. Pretty sure that started with the AOD. If there's a lockup available for the C4 it's a retrofit, and I'd buy that kit. I haven't purchased my converse yet, so options are on the table.
 
#33 ·
No lockup for a C4 or a TH400. C4's dont do well with heavier turbos cars! Broke mine 3 times. I ran my fastest and quickest times with 3.27 gears and 27" tires. I'll be swapping to 28" tires this years when I have to replace the 27" I now have.
 
#34 ·
When you say "heavier" how much are you talking? My T should be around 2k lbs.

Edit: I love your sig pic. Sweet shot. What stall you running?
 
#35 ·
My car was over 3500lbs. Yeah, you shouldn't have and issue. But the th400 are stronger than the c4 and cost about the same. Mine will stall around 5000rpm with 12psi.
 
#36 ·
Gotta go C4. A Model T has very limited legroom as it is. I need the smallest trans I can get that will hold the power.
 
#44 ·
I'm looking at the 275/60/15 but like the width of 325/60/15. Has anyone ran the 315 in a 15"? Here are the wheel specs.

Wheel Specs

Front
Size: 17x4.5
Offset: -25.4mm
Backspacing: 1.75"
Weight: 17.16lbs

Rear
Size: 15x10
Offset: 21mm
Backspacing: 6.25"
Weight: 14.52lbs
 
#45 ·
Anyone?
 
#49 ·
I dont think it will change the width much just the height.. I will say this my current 305/40/17 are wider then my previous 315/35/17 both tires were mickey Thompson ET
 
#52 ·
There are variations between manufacturers but if both were MTs then the 315 should be wider than the 305 by about 10 mm--the numbers indicate the width in millimeters. 315 mm is about 12.4 inches. I have run both Nitto 315s and Sumitomo 315s. The Nittos run small. The Sumitomos have a wider contact patch by about an inch. But that really doesn't mean anything as far as traction goes. The rubber compound means as much as the tire width. ET Streets in 315 have a lot more traction than Nittos but they last 1/3 as long. As far as traction goes the Sumitomos suck.
 
#53 ·
Sounds like I need to stay with the 275.
 
#54 ·
I never personally ran the MT 275/60-15's, mainly because I had 18" Bullitt wheels, buy my buddy did, and they hooked awesome. I ran just the standard Nitto 555's because I did about a 35 minute drive to and from work every day. From a dig they hooked pretty well, but from a dead stop, you couldn't launch for ****. Maybe the Nitto 555R's or the NT07 (I think that's what they are called) would hook better, but I have no experience with either to say yay or nay. I do know the MT's hooked great.
 
#55 ·
I can tell you MT et streets hook very well on the street at my power level with 275/60/15s. Currently i have the same size radial pros and those are even better. From a dig it can get a little squirrely if you just expect to mat the pedal down and hook but you should know that you have to have some sort of driver mod for digs on the street lol.

That being said, i also had nitto 555rs and for a street tire they werent bad at all. Tire life is a lot longer but at the cost of traction. From a 30 roll or up i would hook just fine. Very good street tire, not so good track tire.
 
#56 ·
Also, there's a ton of variables that have to be considered in all of this. The tire's width alone does not affect the total contact area, only the contact geometry. A 275 tire and 315 tire at the same height and pressure provide the exact same contact area, but a 275 tire will put down a more square contact area with more front to back contact and less side to side contact while the 315 tire will provide more side to side contact with less front to back contact. You want the tire that will lay down the longest (notice I didn't say most) contact area in the direction you are traveling. This is directly tied to Newton's 3rd law of motion regarding every action having an equal and opposite reaction. The torque applied to the road is getting an equal reaction from the road in the direction of travel. Given the same sidewall height and tire pressure, a 275 tire will complete this task better than a 315. A 315 tire will distribute the torque to the ground more laterally left and right but you're not going left or right, you're trying to straight ahead. The engineers at GM switch to a 315 rear tire in the Corvette (though I forget when they made that change) not to gain straight line grip. The 275 had plenty enough of that. They made the change to shift the contact geometry to favor lateral movement to make the car handle better. This is the same reason newer Mustangs went to 18+" wheels and wider tires is to increase lateral grip.

A lot of people get confused by this topic. Two tires under the exact same pressure will put down the exact contact area, regardless of any other variables. As you decrease pressure, you increase the coefficient of friction which increases grip.

Now some people will say "well I got better traction with 315s than I did with 275s so you must be wrong cause my grip increased."

No, your grip did not increase. The immutable laws of physics says your grip did not increase. You got more traction because the 315s were 2" taller as well which decreased torque applied to the ground. When you increase tire height, you decrease the total gearing ratio. Going from a 28" tire to a 30" tire decreased the ratio of axle to ground surface by 7.1% and thus the torque applied from input from axle to output at the ground was decreased by the same amount. You got better traction because you were putting less power to the ground. The grip remained the same.

This is also not taking into consideration the unsprung weight of the 315s which will add to the rolling resistance and inertia losses of the car.
 
#58 ·
If your hypothesis was correct, we'd all be running bicycle tires at the drag strip for maximum longitudinal contact patch.

The fact of the matter is tires do not behave in the classical Newtonian sense; between deformation and their elasticity it can't adequately be expressed by F(friction) = Fn x u. If the tires were locked in place and you were dragging it, then maybe you would see different widths (with pressure and sidewall held constant) approximate one another. But for a dynamic condition where the body is moving and the wheel is rotating about the axle? It goes out the window. In reality it's going to be at least a 2nd order differential equation, with respect to both location (however you want to define that in a coordinate system) as well as time.
 
#57 ·
You are mixing a lot of variables. I'm assuming that you are talking about two specific tires when you say the 315s were a taller tire. I've run 245s on 17x8" wheels, 275s on 17x9" wheels, and 315s on 17x10.5" wheels that all have the same circumference. The thing that I have noticed is that I cannot run the same tire pressure in the 315s that I run in the 275s. In order to get proper tire wear I can only run about 22 psi in the 315s. (This is true for both Sumitomos and Nittos.) If I don't I wear the centers of the tires out. My best guess is that because the weight of the car is spread over a wider tire there is less weight per square inch. By reducing the air pressure to get proper tire wear I am also increasing the contact patch, not sideways, but forward. So I think that would mean that the 315s have a bigger contact patch than the 275s. But I'm not an engineer. Just a shade tree mechanic.
 
#59 ·
f = sum(p*da) (assumes constant pressure)

Assuming most of your force is caused by the air pressure in the tire, it will conform to have an area such that it produces enough force to hold the car up. So if you have the same pressure in a wider and narrower tire, each tire will conform such that enough area is in contact with the ground to hold the car up. A bicycle tire can't produce enough area to hold a car up. Real world throws sidewall stiffness and other factors into the equation but this is the basic theory behind it. There's variables with molecular forces which are affected by the rubber's deflection, like Wicked mentioned.

In any case, the coefficient of friction is directly related to pressure and changes with it. This is why it's important to run as much pressure in the tires as you can get away with so you can maintain traction while minimizing rolling resistance.

In my last post, I was not insisting that a 315 is always 2" taller than a 275. I meant to say that most of the guys running a 315 are running a similar profile making them taller and thus leading to my rant. I didn't convey that very well in my wording.

---------- Post added at 01:19 PM ---------- Previous post was at 12:54 PM ----------



Yes, you increased the contact patch when you lowered the pressure. You also increased the coefficient of friction and thus the grip of the tire. If you kept the same pressure (say 30psi) in both the 275 and 315, the contact patch would be the same.

---------- Post added at 01:29 PM ---------- Previous post was at 01:19 PM ----------

Let's take a car (for simple math) that weighs 4,000lbs with perfect 50/50 weight distribution front to rear. That's 1,000lbs per tire. That's 1,000lbs of pressure pushing against the ground. In order to counter this, the tire must counter that pressure with its own pressure. The difference is your contact patch. Let's say the tire has 30psi. 1,000lbs / 30 = 33.333... square inches of contact area. Increase the tire pressure to 40psi and you get 25 square inches of contact area. Decrease the pressure to 20psi and you get 50 square inches of contact area. Now this is assuming a slick tire without tread.

Let's say you have 20psi in a 315 tire and a 275 tire, both Mickey Thompson ET pros. They both will put down 50 square inches of contact area. The 315 tire will have a contact area of 11.4" wide by 4.38" long. The 275 tire's contact patch will be 9.8" wide by 5.10" long.

You can do this math backwards. Say you want to maintain the same length of contact patch (5.10") when switching to a wider tire. You'd simply take 5.10" times the new width (11.4") to get 58.14". Then divide 1000 by 58.14" to get 17.2psi as your new tire pressure.
 
#61 ·
If you know the coefficient of friction of your tires, you can (roughly) calculate the contact patch needed for a given amount of power. The roughly part comes in because the coefficient of friction is constantly changing. The temperature of the tire will affect the friction of the compound. The stickiness of the track is another factor. Concrete has a higher coefficient of friction than asphalt which is why drag strips have a concrete "launch pad" that runs out to the 100ft at least and then asphalt from there on as the demand for grip decreases as acceleration decreases going down the track.
 
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