guidelines for choosing converter stall speed. . .

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dizuster
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guidelines for choosing converter stall speed. . .

Post by dizuster » Wed Apr 11, 2007 12:54 pm

Hey guys,
I think that a lot of people have a general intuition of picking stall speed for a motor. But what is really the best way to do it. How can you choose a converter based on a dyno sheet. How much does gear ratio/car weight play into making the choice.

My gut feeling is that you should choose the flash stall so that the car has the highest average HP operating window between flash stall, and shift point. But eventually as the converters get looser, they get less efficient also. A car may average the HP with a flash stall of 6200 and a 7000 shift point, but the 6200 converter may be so in-efficent that it would actually be faster with a 5500 stall.

What do you guys think?

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Post by F1Fever » Wed Apr 11, 2007 3:13 pm

I think for a street car choose one a little below the cruise RPM, for an all out street/strip car one that flashes just under peak Tq and for a drag car one that flashes just under peak HP so as to achieve the average highest Hp #, similar to what you said.
I was no longer driving the car consciously. I was driving it by a kind of instinct, only I was in a different dimension.
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Post by dizuster » Wed Apr 11, 2007 6:05 pm

Yeah I think you're like me, you pretty much have set what guidelines work for what situation. But why does it work? When you really get down to it, whats "a little under peak HP rpm".

My guess is a converter that actually averages the highest HP is actually probably slower then one that is a little bit tighter to make the converter more efficient?

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Post by randy331 » Wed Apr 11, 2007 11:01 pm

dizuster wrote:
My guess is a converter that actually averages the highest HP is actually probably slower then one that is a little bit tighter to make the converter more efficient?
Dizuster, I'd bet on the one that keeps the engine around peak hp. the most time.
I took out a 3000rpm converter, and put in a 4400rpm converter. The ET droped 4 tenths of a second in the 1/8 mi, even though I now cross the finish line 1700rpm past peak power. Two tenths of that time was in the 60' time.
The 4400 rpm converter is way losser all the way down the track. My finish line rpm is up several hundred rpm.
This engine made peak power @ only 4700 rpm. For best et this is the way to go, but for the street this converter wouldn't be the way to go.
I think a more efficient converter would help my et, but not if I had to give up stall speed to get it.

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Post by dizuster » Thu Apr 12, 2007 12:00 am

Well in your case, I bet the converter is still pretty efficient at 4400, so I can see why it would pick up. I guess I'm asking more about higher RPM motors. Say I have a motor that makes peak power at 6700, by the general guideline maybe it needs a 6200 stall? But the problem is that by nature the converter is inefficient at 6200 stall. (lets say in my case the 3400lb car probably needs an 8" not a 7") So if you put a tighter 8" in it (say 5500 or so) if you're only loosing 10hp on the average run rpm, I bet the car would probably be faster because the converter will be more efficent design for an 8" at 5500 then an 8" at 6200.

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Post by johndougherty » Thu Apr 12, 2007 2:37 am

I pulled a TCI 10 in that flashed 3400 and put in a 9.5 in that goes flashes 4800.
The car is a 71 nova (3440 lbs) with a 327cu in / th350 shifted at 7000 rpm.
With the 10 in convertor I ran 11.72 @ 115 with a 1.62 60 foot time. The average 1/8 mile time was 7.44.
WIth the 9.5 in convertor the best has been 11.85 @ 113 with a 1.62 60 foot time, the best 1/8 mile et has been 7.59.
When the car leaves it sits at 4800 or so for a long time. I get the feeling the car needs a 4.56 gear instead of a 4.10 to run the looser stall speed. I think the tighter convertor locked up sooner in 1st gear causing the car to accelerate better to the 1/8 mile.

I am in the process of reinstalling the TCI 10 in :)

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Post by dizuster » Thu Apr 12, 2007 7:28 am

Yeah that's exactly what I'm getting at there. The 9.5" is much closer to the peak HP rpm (more average hp range) but the car is slower. The MPH shows the 9.5" converter is no where near as efficient on the big end. How much did the finish line RPM go up with the 9.5"? (or down?)

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Post by johndougherty » Thu Apr 12, 2007 12:33 pm

The rpm went up 100, with the loss of 2 mph.

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Post by randy331 » Thu Apr 12, 2007 12:43 pm

At what rpm did your engine make peak power? Also as the converter stall speed gets closer to peak hp, you would need to shift at a lower rpm now.

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Post by johndougherty » Thu Apr 12, 2007 2:00 pm

Never ran it on a dyno.
With the 10 in convertor shifting at 6800 was fastest, shifting at 7000 made no change. With the 9.5 in, shifting at 7000 is fastest, shifting at 6800 is .1 or so slower and lost .5 mph.

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Post by F1Fever » Thu Apr 12, 2007 3:29 pm

that's the downfall, less Tq multiplication at RPMs where the converter is slipping/stalling when going to a smaller diameter.
I was no longer driving the car consciously. I was driving it by a kind of instinct, only I was in a different dimension.
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Post by BLACK BART » Fri Apr 13, 2007 12:29 am

One thing I havn't heard anyone mention yet is that you may have a peak torque and horsepower figure to look at, but these curves can be very different from engine to engine and still have the same peak numbers. Just look at a radical small inch motor compared to a mild big inch motor, for instance, your typical SBC -vs- BBC scenario comes to mind here.

There are just too many factors such as these to consider when you pick a converter to try and apply a general rule. Talk to any converter company and they will almost want to know the entire combination you are running before giving you a recomendation. Even then, you still may not get it 100% right the first time. This is why some companies will give you a free adjustment on the converter when you buy their product.

I wish there was an easy way to pick a converter like you are thinking, but in my humble opinion it will only get you close :( , which may be good enough if you are not seriously racing the vehicle and looking for every last bit of performance. Converters are kind of like cams, I think they are a big bag of compromises in many ways. The person that can compromise the least will have the best performance, and that is not easy to do. I would suggest working closly with a good converter company or looking at proven combinations that are very similar to yours before buying a converter. It's hard to go wrong this way and will put a :D on your face when it works. CJ

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Post by Cammer » Fri Apr 13, 2007 12:38 pm

Good post Black Bart!

You need to consider the whole package when choosing a converter.

Just matching a converter to an engine will not produce optimal results!

A converter manufacturer will want to know everything about your package before building a converter specifically for your vehicle!

Just a little tip- Choosing a good converter manufacturer is your number one priority!

All converters are not built the same!
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