Alright, by springs on N2O I will be trapping in the 130s, mid to high hopefully.

I figured it all out in the car on the way to Chicago the other week what traps I would be crossing in what gear with different gears, and Im kinda stuck on the decision.

With 27" tires and stock T56 gearing

3.73s = 4th gear+6500 rpms = 140.9
4.11s = 4th gear+6500 rpms = 127.9
4.56s = 4th gear+6500 rpms = 115

Someone correct me if im wrong on those calcs

I figure its mostly a street car, so I probably dont want the 4.56s but if they are doable I might, someone with them help me out.

3.73s i will still be in 4th at the traps so that is good, but my 60ft will be way down and possibly et

4.11s seem to be a good choice, I wil be shifting into 5th so that kinda hurts, but my 60ft will be quite a bit better and 8th will be too.

Any opinions? Car will make approx 450rw NA and 700ish juiced, these gears will be in a 9" obviously, 6500ish revlimit, can go slightly further, but thats where the cam will top off

 

I'm not going to doubt your calculations but the gear change is also changing the amount of torque applied to the wheels. A heavy car needs lots of torque to get moving however too much torque can easily overpower the tires and you'll just spin off the line.

The trouble I can see with your calculations is the distance. Using a static number for your rpm and tire size the total distance traveled will decrease as the gear ratio drops. Using 3.73 gears you may cross the 1/4 before reaching 6500 and 4.56 gears you may reach the red line before reaching the 1/4.

Don't use OD in a drag race. The gear is small and weak plus it means power needs to travel through another gear set. You want to cross the 1/4 mile at or just before your shift point in high gear (1:1).

 

hmm... here's what I got. I assumed a FULL tire inflation and a true 84.82" circumfrence. With a axle to pavement distance of less than 13.5" the speeds below would be lower, and the rpms' higher).

Here's what I get for 27.000" tires and 1:1 4th gear:

3.42s @ 6500 = 153 mph
3.73s @ 6500 = 140 mph
4.11s @ 6500 = 127 mph

More importantly:

3.42s @ 135 mph = 5747 rpm
3.73s @ 135 mph = 6269 rpm
4.11s @ 135 mph = 6909 rpm

I wouldn't even consider an extra shift and looking for OD at 127mph. You could always "hold it" at red line but 10mph is an aweful lot to give up like that.

3.73's sound best if you'll be in those speeds and shifting at or before 6500rpms, but that's JMO.

 

The only reason that 3.73s dont seem right is looking at Taners setup, 4.56s = 9.60s! I think most of a heavy torquey car like ours is the launch, they dont do quite as well down the line like a turbo car would, I think the main thing is to get the car moving good, it seems that in a 4th gens case, as the 60ft goes down the ETs drop exponentially almost.

I know I had a midish 12s car before trapping around 110, but it ran a 13.2 on a 2.4 60ft, seeing cars running 12s with same type setup, those traps, but 60ft seems to be the huge thing, I mean Taner pulls what, 1.29s 60fts, thats retarded. Guess the ultimate thing would be to put a set of 3.73s in his 9" and see what it runs

Maybe it would be best to consider a little taller tire, around 29 or 30" and 4.11s,

29" at 6500 in 4th with 4.11s puts me at 136, thats without any diameter increase with speed though, so a little higher

 

What rpm are you making peak hp at? In any case, if 6,500rpm is your ideal shift point, then I'd agree with Steve that 3.73's seem like the best bet. You definitely don't want to shift into fifth. With a 28" tire you could likely use 4.11's (I didn't actually do the calculation). But 3.73's are a nicer street gear, IMHO.

Rich Krause

 

I have 29" tires with 4.56 gears. I currently shift and cross the finish line at 7000 rpm. This winter I'm installing a set of 4.30 gears. My shift point is actually too high so I'm going over the HP curve. By using the 4.30 gears my shift point should come down into the 6700-6800 range. Since I use a powerglide I only have to shift once.

Theoretically since I'll be shifting closer to my peak HP, my ET should pick up another .1-.2. I won't know until I get back on track in May. Every dyno/drag software program I've tried all say I have too much gear. The alcohol BBC produces so much torque that I don't need the deep gears to get me moving.

For your car the 3.73 gears would be nice for the street but 4.10/4.11 gears are just as nice since you do have an OD in the tranny. My daily driver truck has 4.10 gears, 28" tires and a 4L80E tranny. It's not the best gas mileage with a BBC but it could be worse if it didn't have the OD. The shift point on the truck is 5600 and it wants to go into OD a couple of hundred feet before the finish line.

 

I want to make peak power at 6500, the motor isnt finished, but Im going to have Joe Overton grind me a cam, and Ill want it to quit around 6500, ill rev higher if I need to, but would rather not.

I wouldnt mind 4.11s on the street, so if Im able to stay in 4th with 4.11s, is that the way to go then?

 

If peak power is going to be at 6500 rpm, and unless your HP curve is perfectly flat for 2500 rpm (just doesn't happen), then you're going to need to shift on the far side of 7000 rpm for best ET. You'll also want to gear the car so that it will go through the traps in 4th gear 500-800 rpm above that peak.

The good news is that this gives you a lot more gear to play with.

Good luck to you.

 

I don't know if they make them for the 9inch but I would consider 3.90's. The should put you pretty close to the mid 130 at 6500

 

Why would I want to shift 500Rs past my peak power, that doesnt make much sense to me, Ive always heard to shift 100 or 200 before peak power. The power band is going to be quite wide, so i dont need to be put at a higher rpm when shifting into next gear, if i shift at 6500 ill be around 4000 in the next gear, which is still right around peak power, especially on gas.

 

I'm sorry, but that is incorrect. The idea is to maximize the average HP throughout the rpm band. Unless you have a perfectly flat HP curve (not gonna happen), then the only way to get the most average HP is to shift several hundred rpm beyond peak HP.

You are making a LOT less HP at 4000 rpm than you are at 6500 rpm, assuming that you were correct in your HP peak being 6500 rpm. You are also likely making a lot less HP at 4000 rpm than you are at 7000 rpm - even though HP is going down by that point. Thus, shifting prior to 6500 will absolutely kill your ET.

Again - all this assumes that 6500 rpm is your actual HP peak.

Do you have a dyno graph? You can do a rough plot of shift rpm vs the HP curve and get a reasonable idea of where to shift.

 

Well, the problem is I am not shifting at 7000, period, the motor will handle it, but I dont want to push it. So 6500 is the max rpms i want to shift at.

Some reason this still doesnt make sense to me why i would want to go beyond peak power, like when i was just H/C, if i took it past the peak (5900), it just felt stupid, i was slowing down it felt like.

 

Your peak HP will depend on how the engine is built. Cam grind, compression, ignition timing plus lots of other variables determine where the engine makes the most power. Saying you want to shift at 6500 doesn't mean an engine that isn't built yet will want to shift there.

You want to launch at peak torque but all your shift points should be around the peak hp level. It all depends on tranny gear ratios since when you do a shift, you want the rpms to fall back to the start of the hp curve not far below it (700R4 1-2 shift). At the finish line you want to be at or just over your shift point.

Your best shift point and the engine's red line is not the same thing. You can easily go over the shift point to find a better sweet spot without red linning the engine.

HP is power that is doing the work. peak hp at higher rpms can do work faster. 2 engines both make 400 hp. One peaks at 4500, the other at 6500. The one that peaks at 6500 can do work longer and will be quicker.

 

Then I suggest building a motor that peaks significantly less than 6500. It will work much better for what you wish to do, be a better street car, and be faster for how you want to drive it.

I guess I don't know how better to explain it. Perhaps someone else can.

I do wish you the best of luck.

I don't necessarily agree with this. Using my car as an example, I get my best 60 fts (by far) launching more than 1000 rpm above peak torque (like 6500 or more), and my shift points are well beyond peak HP (though I think you were meaning to imply that peak HP was just a starting point for finding optimum shift points).

Aside from your last statement (which I simply don't wish to discuss), I generally agree with the rest of your post....for whatever that is (or is not) worth.

 

I understand that shift points, peak power, and redline are all separate things. But what im saying is that i dont know EXACTLY where the peak will be, as the motor is not done, but when I tell joe what I want, i will tell him i dont want to rev past 6500, so if i have to shift at 6500 that is fine, i am not exactly sure where peak will be, i guess i kinda screwed up a little in the first post.


So i guess i should wait and ask this when the motor is complete huh?