RickBetterley

My short-ratio gearbox is headed off to G-Box for a rebuild, so this gives me an opportunity to rethink the ratios.
Current ratios are a blast, but I am definitely top speed limited (about 130 mph is the best I can see on the back straight at The Glen, for example. Back straights at Tremblant and VIR are just ridiculous; I feel like I should bring a book).
Since I am 'just' driving in red run group, I am not worried about class rules, but I would like to gain some top speed without spreading out the lower gear ratios.
Here are the current ratios (I believe that these ratios were selected by the previous owner with Mid-Ohio in mind):

Ring & Pinion 9/31 (3.444)

1st gear 11/42 (3.8182) (39.24 mph)
2nd gear 21/43 (2.0476) (73.171 mph)
3rd gear 19/31 (1.6316) (91.830 mph)
4th gear 27/38 (1.4074) (106.456 mph)
5th gear 22/27 (1.2273) (122.081 mph)
6th gear 32/34 (1.0625) (141.014 mph, but I haven't seen more than 130 or so indicated)

Engine is stock and staying that way.
My thought is to stretch out the space between 5th and 6th a bit.
Any suggestions for how to get a bit more top speed without losing the fun of the close-ratio lower gears?
Thanks in advance for suggestions.

bobt993

Rick, I am not sure if you are limited by the main shaft, but if your 2nd is not welded then you want to stack higher second (80 to 85mph) then space accordingly. If your really ambitious do a tall 1st too, but I did not bother. I have not found a corner on a track yet that the apex speed is below 60mph. T5 to T6 should be pretty close. If your chipped, then the redline should be a little higher than 6800rpms. If your apex at T1 is a min of around 75mph, then you should hit 135 to 140 by the bus stop driving with your foot down.

Larry Herman

I agree with Bob (of course). You need an 80 mph 2nd gear and then space out from there. No reason that you can't have a 160 mph box with a 6 speed. I did that with a 5 speed and it was great.

mark kibort

remember, you gain no hp with going shorter or taller. you only optimize the HP available for the tracks you visit. you want it to match the long straights with gears that end up at redline, and provide shift points in critical points the track that dont hurt control.

since you have a 33mph first gear, that is for the paddock. , you would NEVER use it on the track, so call your car a 5 speed. go taller,especially since you are not reaching over 130mph as you stand today, you might as well gear up so that your 5th gets you to 130mph and then if you need 6th, you can get the 155mph you want. this way, your 1st can be used on tight turn exits, to 55mph. you might gain a lot of HP optimization with a change that only makes the entire gear box taller. remeber, you would not be changing your gear spacing, only shifting the gear set taller.
just going to a taller 6th, might be a disadvantage overall . you dont keep your "close ratio "gears, you just make a a gear that is more widely spaced. you still keep the close ratio, if you just make the entire set taller. same RPM drop per shift, same closeness. the only thing is the "feel" you have on the city streets runing, say 50mph down the expressway. you now will be at a lower rpm, but on the track, this wont matter, because its about going through all the gears, or at least several. its about optimization. put up some video of you and your car at at the track to look at gear selection and time spent in those gears.

Mark

bobt993

Mark, I don't think Rick stated he is at redline. Looking at his gearing chart and assuming 285 R6's with the standard R&P he still has a little room especially if the redline is a little over 7000 (stock 6800). Rick I can send you my ratios or ask Larry since he wanted a slightly different setup than I ended up with. If you are actually at redline (6800) then consider chipping the car, improving the valve springs/keepers on the top end. You will gain more top speed for less than a re-gear on the trans.

BTW shifting at redline in a close ratio box is not always optimum. Using data and Long G's you can derive different shift points that slightly alter off redline and improve acceleration. This requires someone who knows data and a couple of simple math equations. If you not sure argue with Buddy Fey on this one.

RickBetterley

Really good advice; thank you.
Since we are redoing the box, I expect we have the option to redo each gear, so leaving 1st as is and moving 2nd and up higher up the scale seems to make sense. Keep the spacing, just start 2nd up higher is what I think I am hearing.
I have some headroom on my skill/bravery scale (still can't keep my right foot fully engaged from turn 1 at The Glen, but other than a bit of breathing before turn 2, I'm flat out until breaking zone for bus stop) as evidenced by my 2.18.xx personal best at The Glen. May as well get the car right, though, while I'm working on the driver.
I would appreciate seeing the gear ratios of those that offered; if you prefer to email them, please send to rbetterley at gmail.com.
I am really looking forward to getting the transmission right, as the car is lovely to drive.

mark kibort

Rick put down the max speeds in each gear, with 141mph being his limit in 6th, so I would think this was redline for him. If not, it would be interesting to hear where these range MPH numbers came from. (usually, they are redline, which is the right way to analyse optiimization of his gear sets for a given track)
Now, chipping it to 7000rpm will give him only 2% more speed in each gear, which is less than 5mph at 140mph, so that wont buy him much at VIR, for which he referenced.

Now, shifting at redline. Yes, shifting at redline is ALWAYS better IF you have a rising HP curve or one with a narrow (narrower than the gear spacing) peak HP arc. IF, at any rpm drop due to the "close ratio" you have more hp by leaving it in gear to redline, than the POST shift RPM HP, then it pays to redline. This is pretty basic. Dont misinterpret what Buddy Frey is pointing to with his advice. (of course we are talking straight line acceleration, not areas of the track that might be traction or control limted due to shifts at inconvenient times ) In otherwords, you dont need G-values, complicated data. all you need is your HP curve and the spacing %-wise between gears.

If you have a hp curve that does point to better acceleration by short shifting, I would like to see it. its not common and certainly is the excpection. more common to redline to maximize acceleration.

mark kibort

why not just change the final drive? that way, everything is shifted up. 1st would then become a usable gear in tight turns, where now its basically useless.

were those MPH's for each gear based on redline or some other reference point?

RickBetterley

Yes, the max speed in each gear is at redline (which is 6800).
I have yet to see the 141 mph, but if I had a long enough straight...

mark kibort

Thats what I thought.

So it sounds like a rear end change might be the only thing you need to do, unless you have some hard data from the tracks you visit for speeds on all straights and turns. that way, you gear up things, you still have the same close ratios, and you end up with top end speeds to meet the faster track requirements.

with regards to shift points, if you post your dyno run, you can accurately see where shifting would occur. generally, its at redline.

I dont see the top of my last usable gear 4th. its max is 155mph, but the greatest I see is 140mph . at my tracks, i only hit 4th for 2-3 seconds its not a huge deal, trading off the advantages of 3rd, optimized for most all the key straights and turn exits. I gues Ideally, if I could, i would just shorten 4h to 140mph, but then, would not be able to visit a track like cal speedway or willow springs. thats the kind of decision making it takes when looking at the gear set selection.

by the way, you do have a close ration gear box! is that normal? anyway, its a 15% rpm drop (resultant rpm after shift near 85%). so, to look at shift points, if you have a HP arc that runs from peak hp to max rpm of 6800rpm, the post shift RPM would be 5700rpm. if there is more HP post shift, then shift earlier until there isnt. If you have a curve that looks like a GT3, then redline all shifts, no matter the gear spacing.

bobt993

Rick, I am certain with a little bit of car tuning and some suggestions on how to attack T1 exit to the bus stop will be worth at least a second a lap at the Glen and another 5mph down the back straight. If you send me a run file I can look at the data make some suggestions. Mark, I am not going to argue with you about data results on gear selection. When you finally dig deep into this your going to realize how this is derived and why. Your dealing with long G comparison across each gear before and after the gear changes. The is a golden ratio that is why DAS guys make money. I picked up .5 secs a lap on a 2.5 mile track by following the math around the track, but as usual I must doing something wrong. Rick the biggest issue with a standard G50 trans is 2nd rear is often welded to the main shaft and becomes expensive to re-tool. Some boxes this is not true. Don't change the R/P the stock set is reliable and works well with a custom box.

Veloce Raptor

Bingo

Veloce Raptor

Bingo #2

mark kibort

Bob, its pretty simple physis. It should back up any simulation you can provide. If you look at the two dyno runs for GT3's Ive attached, just provide me the logical reason why shifting before redline would provide any advantage.
when you speak of golden ratio, are you talking about optimizing for a particular track or straight?
now, when you picked up .5 seocnds a lap, what did you do different? are you saying you short shifted by the VR classic advice of "300rpm before redline" in spots? if so, do you have a hp curve we can see, or does it look like something you can describe? falling torque and HP doesnt buy you an advnatages to shift, especially if the next gear provides LESS hp than staying in gear. this is just basic stuff and I dont want to see any advocation of BS science here. If your curve looks like this one (stock 993, and you have the 85% shift change gear spacing), then yes, the bell arc of this curve is 78%, so this graph would indicate short shifting for an advantage. BUT, this is not the look of most race car curves.

If you dont think i understand " how this is derived", please let me know what I might be missing here. Bottomline, if you have more hp after a post shift you can shift earlier. if not, as in the 2 curves above, then you best shift at redline, ALWAYS when going in a straight line.
In otherwords, unless you can correct me of a possible situation, the 2 graphs above dictate shifting at reline, and the one below, could be shifted earlier than redline if a close ratio gear box with less than a 20% drop in post shift rpm is found.

If you look at the "long Gs" of pre and post shift, all you need to do is follow the HP. it keeps you from needing to calculate thrust forces, because they are inextricably tied together and proportional. again, at any greater HP at any vehicle speed, you will have greater acceleration. Even Fey willl not argue this!


Mark

Veloce Raptor

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