What kinda times you want/ mph in the 1/8

 

Aiming to run low 5s/mid 130s. The car ran 5.7 @ 125 before on a very sketchy track with a Shafiroff 565 that made 1008hp N/A. Even back then I was wishing I had gone with a glide.
Since it‘s already set up for the TH400, I’m thinking of either changing the first and second gear set or going with 2 speed TH400 setup.
I suppose the easiest thing to do is change the rear gears, but then I wouldn’t be using the top part of third gear in the 1/8.

 

I have a Coan 1+2 valve body. I have yet to leave in second gear but so far the brake works great leaving in 1st. That might be easiest way without changing rearend set up.

 

Thanks. I’ve spent some time overnight running the different gear ratio combos through the RPM drop on Shift calculator on Wallace Racing and then I realized the obvious:
Based on what little I know, I think the bigger concern on a nitrous car is the RPM drop on the shift, and with a 2.48:1 first gear and 1.48:1 second, it‘s the difference between 1st and 2nd ratios that matters.
Put another way, it don’t matter what the rear gear ratio is, the 2.48 - 1.48 shift is going to produce an RPM drop of around 40%, whether I put a 3.89 or a 5.14 rear gear in it.
So, if I’m shifting at, say, 7500 rpm, with a 33” tire and a 4.57 rear gear and a 2.48 first gear, that equates to around 65mph (assuming 0% slip, of course), and when I shift to second, 65mph equates to just under 4500RPM with a 1.48:1 second gear, which will likely drop my RPM below the stall speed of my converter. That may in and of itself solve the RPM drop problem, depending on how much the converter slips when I make the shift.
BUT, if I’m running the typical “tight“ converter (which I will be, unless I change the stator) that will put a lot of load on the motor and drivetrain (“pulling the motor down”, as some folks call it).
That load can’t be good for a motor that’s ingesting a sizeable whack of nitrous.
So, it may be more important for me to change my first gear to something like a 2.08:1, which reduces the RPM drop percentage to ~28%, since the car is going faster (~77mph) on the shift (again assuming % slip) and the first gear ratio is closer to the second gear ratio.
That all assumes I can get it to hook with an 11.3:1 SLR, of course.
And I realise that converter slip and tire growth can throw these numbers off somewhat.
So, I’m probably going to try it the way it is, program the curve in the Launcher to reduce the nitrous shot below, say, 5000 RPM, just to see where I’m at with gearing and converter, and go from there.
Here’s some screenshots of the charts I made on the Wallace racing calculator (that site is truly one hell of a resource) to show what I’m talking about:

 

Changing gear ratios in the trans was the hot thing for awhile but it seems like to me lots of guys are going with the second gear leave deal. I believe you have enough gear to over come that lug at the hit so it should not backfire. Again just my opinion.

 

You might be right. But I suspect the guys that do that are making much more power than I will be though. IIRC, there’s another thread on here where a guy with 2,000 hp tried it on his big tire car and said it was sluggish.
It’s easy enough to try it. But that 1.48 only works out to a 6.76:1 SLR. That‘s pretty tall, and would most likely require a big hit of nitrous off the line to get the car moving.
I have some time to figured it out though, since I’m not expecting my motor to be finished until right before Christmas.

 

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I’d think you do best around 7.5 SLR

 

Thanks. That gives me something to work with.

EDIT: Funnily enough, I just calculated what a 1.48 gear would be with a 5.14 gear (I have a set of those already) and that works out a 7.6:1 SLR.
Maybe after trying it with the current 4.57s I’ll stick those 5.14s in the diff and forget about using first gear, except for burnouts. I’m only racing 1/8 anyhow.

 

Here’s how it looks on RPM drop on the shift with using the 1.48 ratio as “first “ gear:

 

I would stay with 3 gears to keep the engine in its happy spot in the rpm range. There are plenty of gear sets available it just depends on how much you want to kill the SLR.

 

Thanks. That’s what I’m thinking too. I’m not going to be anywhere near the power output of most (if not all) of the guys that are using a second gear start, and I’m running a big tire and race weight will be around 3200lbs as well, so starting in second might make it a dog, I think. I’m thinking of something like a 2.10 first gear and retaining the 1.48 second and trying to keep it above converter stall speed (say, 5,000 rpm?).
When I run those numbers through the Wallace Racing calculator, it shows my RPM drop to be fairly close and above 5,000 rpm on both shifts.
Obviously, this calculation doesn’t involve converter slip, but I think it shows in principle your point about keeping the shift drop in that “sweet spot” on both gear changes.

 

It's gonna fall back to converter stall speed after gear change unless it's lock-up trans/converter.

 

Thanks. It’s not a lock-up. I’m going to aim for a max of 5,000 rpm on stall speed though, or maybe even a little lower?
Are you saying that this is the problem with using a three speed setup, or with the second gear start?

 

It seems to me that there’s an obvious factor to be taken in account on this question of SLR, which is, what is the impact of tire diameter on selecting the “right” SLR for any given setup?
Because the effective gear ratio changes significantly with tire height, does it not? So the optimum SLR for a car with a 29” tire might not be the right one for a 33” tall tire, no?

 

Aim for converter stall speed to be 800 rpm less than shift no matter what your transmission ratios are.

 

Thanks. Do you mean 800 rpm less than the shift point or 800 rpm less that what the rpm drops to when you make the shift?

 

If shift point is 7500 then converter stall speed around 6700-6900 should work that's what rpm will fall to after gear change

 

Thanks. I’ll have to wait and see what the stall and slip is on the converter I have now (an old PTC bolt together) with the new setup. It was definitely too tight for my previous setup, but I figure it will be a bit looser with more power.

 

For custom gears / planetaries, who do you guys go with? Coan?

 

So far I’m considering buying a gear set and associated parts from Coan (who did my trans) or Hutch. I’ve also looked at Rossler’s site. One of those would be my choice.

 

if your gonna stay at the 1300hp level a 2.10 gearset for the th400 is what i would do...gearing is good for 1/8 mile and close to what i run..4.56 on a 32in tire at 3270lbs..i run a 1.58 glide but have few hundred more hp...i run a hughes 9.5 converter that drops 900 when i use 1 system.

but i do not think it would slow down with a glide with 1.80 low...and if you turn the power up later you would still be good...i was 1.80 for a very long time but was violent with both systems on.