My point is that while the 255 is taller, its still not enough for me on some courses.
[/quote]

I hear ya. I was attempting to point out to the OP that the 275 to 255 difference in diameter is about 2X the 255 to 245 difference, for whatever that's worth.
I'm also starting to wonder more about the benefit of the shorter tires on courses that require a bunch of 3rd gear acceleration. I haven't run my S2000 all that much, yet, but most every course I've run has had me in 3rd gear for prolonged periods of time. Maybe it is a west coast thing, specific to our Crows Landing and Alameda NAS sites. How much were the BS S2K guys in 3rd at Lincoln last year?

 

My point is that while the 255 is taller, its still not enough for me on some courses.
[/quote]

I hear ya. I was attempting to point out to the OP that the 275 to 255 difference in diameter is about 2X the 255 to 245 difference, for whatever that's worth.
I'm also starting to wonder more about the benefit of the shorter tires on courses that require a bunch of 3rd gear acceleration. I haven't run my S2000 all that much, yet, but most every course I've run has had me in 3rd gear for prolonged periods of time. Maybe it is a west coast thing, specific to our Crows Landing and Alameda NAS sites. How much were the BS S2K guys in 3rd at Lincoln last year?
[/quote]

There is a 27% drop in revs on the 2-3 shift...that is probably close to a 25% drop in power. Add to that shift time, there are places where all the GoPro videos show S2000s pegged against the limiter for a second, two, or three. Then the extra 1 or 2 mph has a small impact. 3 seconds on the limiter may be worth a bit less than 0.1 sec on 255s vs 245s.

However, shifting earlier...if you never get to redline in 3rd, is probably not an advantage. However, below the limiter the smaller tires will be at higher rpms with more power; an advantage. For example, at 50mph the 245 will have another 100 rpm. That can be as much as 2hp at 45mph. A lot of cars have their power peak well short of the redline...but that isn't the case here.

The key would be to have a fast automatic or auto-shifting manual...something that snaps of shifts so fast that they can't be seen in a speed trace, does it without distracting the driver, and leaves a free left foot and a big left pedal for braking. PDK Porsches, the new automatic Corvettes, and the Alfa 4C would probably fit in that group. And most of them are in SS.

Net: this is one of those very small factors. The longer the course benefits bouncing off the limiter, the bigger the advantage. But the 245 is effectively shorter gearing and except when a 255 is in second and the 245 has shifted, the 245 has a power advantage.

Question: How may Street class S2000s have shocks with adjustable ride height/cross weight so that they can be lowered to stock ride height after the taller 255s (or for that matter 225s) are installed?

Sidebar: in the 1960s many high performance cars had high numeric axle ratios (remember 6000 or 6500 was considered a high limit then) for acceleration. Sports Car Graphic tested a 1970 LT1 Corvette with 4.10 gears (and F70-15 bias ply tires) that had a top speed of 122mph. My 1970 Boss 302 had 3.91 gears and a similar top speed (on F60-15 Polyglas tires). For drag racing (most cars were under 100mph or at least 110mph in the 1/4 mile) 4.33, 4.56, and 4.88 gears were available .I think one of the magazines (Hot Rod?) tested an L88 Corvette that had 4.56 gears.

 

The key is how long you are accelerating in 3rd gear (given an upshift at peak power, not shortshifting). When we did datalogging on the Lotus Elise I helped set up for SS "back in the day", we found 2-3 pops on the limiter in second was about the limit before an upshift would prove faster...given that there was a meaningful opportunity to accelerate in 3rd following the shift. Since then, I've run a Z06, RX8 and Shelby GT in Stock/Street, all of which had sufficient ceiling in 2nd for most every course I ran...as did the WRX I won DS in at Nationals. My CM car was another story...4 gear set up and used them all every run

I'd say for nearly any local course you'll run, the slight additional speed in 2nd with the 255 will be meaningful and the best option. On a course like last weekend, where you accelerated foot to the floor straight to 3rd gear and stayed in 3rd the entire time going out and coming back, the shorter effective gearing of the 245 had to be an advantage for the S2K...which just got me thinking about whether that also could translate to the types of National courses I've seen over the last few years.

 

Why not 245/45? Or for that matter a 255/40-18? Or even a 245/45-18? In the Street classes the shocks could be threaded to restore ride height, which in the factory specs is referenced from the chassis to the ground. However, remember that larger diameters increase rotational inertia and the power differences are on the order of 1/2%-to-1%?

If that little power increment was decisive, autocross prep would be in the engine builders shop. I'd be very surprised if 5hp couldn't be found in a rebuild...I can think of that much of the bat: overbore, ring tension, tight clearances and low viscosity oil, reciprocating components at their minimums, minimum deck height, etc. Does the SCCA have a ruling on coatings? Has there ever been an engine teardown?

 

Ah, I see. On the east coast I've experienced the opposite. I've run mostly courses that have short bursts where 3rd could be used. This has me wanting for longer second gearing. The third gear sections long are enough to question the 2nd->3rd->2nd shift and could result in a loss of time/control/momentum if not executed perfectly. So its ride 2nd out at redline a few extra seconds or take the risk. I'd rather not have to make the choice with a taller wheel/tire combo.

Also, are you taking the car up to redline in second or are you watching the tach? When I first started running the S, I had to get a modifry shift indicator because I noticed in my early videos I was shifting way early. This may not be the case for you but I figured I'd mention it.


OP,

You've referenced "restore ride height", in reference to threaded and adjustable spring perches and restoring ride height, a couple times in the last few days. This is not street class legal.

13.5.A.3 Adjustable spring perches are allowed, but the spring loadbearing surface must be in the same location relative to the hub as on the standard part. Shims may be used to achieve compliance.

but,,,

13.5.A.4 The fully extended length must be within +/-1" (+/-25.4mm) of the dimension of the standard part.

Basically, if your car sits higher with a taller wheel/tire you're SOL on ride height.

 

My point is that while the 255 is taller, its still not enough for me on some courses.
[/quote]

I hear ya. I was attempting to point out to the OP that the 275 to 255 difference in diameter is about 2X the 255 to 245 difference, for whatever that's worth.
I'm also starting to wonder more about the benefit of the shorter tires on courses that require a bunch of 3rd gear acceleration. I haven't run my S2000 all that much, yet, but most every course I've run has had me in 3rd gear for prolonged periods of time. Maybe it is a west coast thing, specific to our Crows Landing and Alameda NAS sites. How much were the BS S2K guys in 3rd at Lincoln last year?
[/quote]

A big part of the reason I got beaten by a significant margin by Lane's corvette day 1 while day 2 we were effectively tied was because the day 1 course was very fast and I should have shifted to 3rd but I made a bad call to stay in 2nd (which he didn't have to worry about with the vette gearing). Looking at data later I would have benefited significantly from a shift. 3 runs is tough, I decided to stay in 2nd my first run but then took a critical turn better on my later runs which meant I had enough speed to shift to 3rd but I stuck to the plan.

Whereas day 2 course was tighter and while we had a few quick bounces off the limiter I think the gearing was good just staying in 2nd.

It wasn't like some of the west coast courses I've seen though, there was really no opportunity to stay in 3rd for very long. Probably having a tall tire and staying in 2nd would have been ideal if I had had that choice.

 

troooof!

 

My point is that while the 255 is taller, its still not enough for me on some courses.
[/quote]

I hear ya. I was attempting to point out to the OP that the 275 to 255 difference in diameter is about 2X the 255 to 245 difference, for whatever that's worth.
I'm also starting to wonder more about the benefit of the shorter tires on courses that require a bunch of 3rd gear acceleration. I haven't run my S2000 all that much, yet, but most every course I've run has had me in 3rd gear for prolonged periods of time. Maybe it is a west coast thing, specific to our Crows Landing and Alameda NAS sites. How much were the BS S2K guys in 3rd at Lincoln last year?
[/quote]

A big part of the reason I got beaten by a significant margin by Lane's corvette day 1 while day 2 we were effectively tied was because the day 1 course was very fast and I should have shifted to 3rd but I made a bad call to stay in 2nd (which he didn't have to worry about with the vette gearing). Looking at data later I would have benefited significantly from a shift. 3 runs is tough, I decided to stay in 2nd my first run but then took a critical turn better on my later runs which meant I had enough speed to shift to 3rd but I stuck to the plan.

Whereas day 2 course was tighter and while we had a few quick bounces off the limiter I think the gearing was good just staying in 2nd.

It wasn't like some of the west coast courses I've seen though, there was really no opportunity to stay in 3rd for very long. Probably having a tall tire and staying in 2nd would have been ideal if I had had that choice.
[/quote]

Yep, makes sense. I think the bottom line is the 255 is the right choice. The situations where the very slightly steeper gearing and very minimal drop in rear CG of the 245 would be meaningful are probably rare.

 

I stand corrected. Where do they get the reference measurement from? I'm not sure I understand 13.5.A.4. The extended length is at full droop. Many (most? all?) aftermarket shocks are designed to handle an inch or more of lowering which results in a longer shaft (including the Bilstein PSS I referenced in another thread). The extended length is never specified for the shock. How is a competitor going to know that dimension? Bilsteins, Penskes, and Ohlins it may be possible to do with a shaft change...but that ins't free. On Penske and I presume the Ohlins it could probably be ordered that way.

 

i read it as the length between mounting surfaces, i.e. top hat to control arm bolt, fully extended.

I don't think you'll have trouble meeting that rule, and you would want to meet it anyway for optimal performance.