Effects of converter diameter at same stall rpm

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RDY4WAR
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Effects of converter diameter at same stall rpm

Post by RDY4WAR » Sun Jun 24, 2018 5:12 pm

I'm swapping a TH350 behind a bone stock LT1 in a '93 Camaro. I'm looking at a 2800-3000 stall, strip use mostly. (bracket racing) This stall seems to be offered (off the shelf) as small as 9.5" and large as 12". For strip only use, which diameter would be more advantageous and why? How does the diameter effect how the converter hits on the launch, how much it slips on the top end, the torque multiplication, etc...

1993 Camaro Z28
Bone stock LT1 w/ 160k miles (stock air filter to stock muffler), rated at 275 HP @ 5200 rpm and 325 TQ @ 2800 rpm from the factory
TH350, standard 2.52/1.52/1.00 gearing, will be footbraked
7.5" 10-bolt rear with posi, 4.10 gear, and solid pinion spacer
Factory stock suspension front and rear
255/50r16 M/T ET Street SS rear tires on factory salad shooters
3600 lbs race weight with driver and full tank, full weight car with full interior, AC, radio, etc...

The price of a custom converter isn't an option. This is a budget deal so trying to stay under $400 for the converter, might go to $500 if a converter in that range would make a night and day difference. Anybody willing to educate me on converter diameter differences at the same stall speed, and/or recommend a converter within this budget?

lefty o
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Re: Effects of converter diameter at same stall rpm

Post by lefty o » Sun Jun 24, 2018 11:01 pm

imo, basic fluid dynamics say the smaller converter is less efficient, but beyond that, and most importantly its all in how it was built.assuming quality is equal, for a strip only car id definately choose the smaller diameter converter.

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Re: Effects of converter diameter at same stall rpm

Post by RDY4WAR » Mon Jun 25, 2018 4:24 pm

We're also reconsidering the rear gear for this car. It was originally run with the factory 3.23 gear, swapped in the 4.10 gear, 60ft went down by .05, but 1/8th ET slowed by .05 and 1 mph, and 1/4 ET slowed by almost a full tenth and down 2 mph. That was with the 4L60 (700R4), 26" tires, and 3200 stall. We're thinking of putting the 3.23 gear back in and have considered a higher stall of around 3800-4000 instead.

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Re: Effects of converter diameter at same stall rpm

Post by lefty o » Mon Jun 25, 2018 11:33 pm

if your going to a th350, id get that in and try again before deciding on gears. that current trans has a deep first gear with a fairly poor split between 1&2. what you've already done suggest the 3.23 is not enough gear, but currently the 4.10 is maybe a bit much+ a 3.73 or for sure no less than a 3.50 gear.

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Re: Effects of converter diameter at same stall rpm

Post by Geoff2 » Tue Jun 26, 2018 5:58 am

Not a trick question.

You switch from a 12" c'ter with, say, 2400 rpm stall to a 11" c'ter that has, say, 2800 rpm stall. The 11" is faster by 2 tenths. Was it the reduction in rotating weight that made the difference, the higher stall, or both. And how could you determine which made what difference?

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Re: Effects of converter diameter at same stall rpm

Post by emsvitil » Tue Jun 26, 2018 7:12 am

I think the question being asked is:

Converter 1: 2800 stall 9.5"
C2: 2800 stall 12.0"

which is faster and why


What affect does diameter have on torque multiplication?

What affect does diameter have on slip?
Ed

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Re: Effects of converter diameter at same stall rpm

Post by In-Tech » Tue Jun 26, 2018 7:41 am

Geoff2 wrote:
Tue Jun 26, 2018 5:58 am
Not a trick question.

You switch from a 12" c'ter with, say, 2400 rpm stall to a 11" c'ter that has, say, 2800 rpm stall. The 11" is faster by 2 tenths. Was it the reduction in rotating weight that made the difference, the higher stall, or both. And how could you determine which made what difference?
Smaller carb, ya, that's it. :roll: :lol:
Heat is energy, energy is horsepower...but you gotta control the heat.
-Carl

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Re: Effects of converter diameter at same stall rpm

Post by RDY4WAR » Tue Jun 26, 2018 6:38 pm

emsvitil wrote:
Tue Jun 26, 2018 7:12 am
I think the question being asked is:

Converter 1: 2800 stall 9.5"
C2: 2800 stall 12.0"

which is faster and why


What affect does diameter have on torque multiplication?

What affect does diameter have on slip?
This is correct. That's what I'm asking. I see both offered, both around the same price. Just wondering what advantage the smaller converter would have over the larger one other than just size/weight.

Right now we're running circles around ourselves trying to decide whether to rebuild the 4L60 or swap in a TH350. It's a 90% track car.

Peak HP is at 5200 rpm per the chassis dyno. With the 3.23 gear, it was crossing the 1/8th mile at 5350 rpm in 2nd and crossing the 1/4 at around 4700 rpm in 3rd. With the 4.10s, it was crossing the 1/8th at around 4600 rpm in 3rd, and 1/4 at around 5800 rpm in 3rd. Shifts at 5800. The short 1st gear is over before the 60ft with the 4.10s. Still fairly consistent.

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Re: Effects of converter diameter at same stall rpm

Post by Garry in AZ » Wed Jun 27, 2018 7:52 pm

Generally speaking, the larger the torque converter, the more torque (and horsepower) it absorbs. Because of this, a larger converter will usually have less stall speed than a smaller-diameter converter. The Catch-22 is the fin angle inside the converter. Because of fin angle changes, it is possible (both in theory and in practice) to produce a converter in something like an 11-inch size that actually has more stall speed than a 10-inch converter. The stall speed changes gained by reworking fin angles obviously have some limits. But, you can only increase the stall speed so far before you're forced to go with a smaller-diameter converter.
The bottom line is, diameter is not a final predictor of footbrake or flash stall speed, nor track performance. And some companies advertise footbrake stall RPM, while others use flash stall RPM. You can't easily compare them. There are a TON of other variables that all have an effect, right down to things like what sort of brakes are on the car and the available traction for the tires to hold the car in place! A converter that works great on a prepped track won't work as well on a less prepared surface, or a slicker track. I know some pro-mod guys who will even change converters based on track temps.
Basically, what you’re trying to achieve during converter selection is to ensure that the converter itself has enough volume to handle the power level, while still retaining a physical size that will not be too large – a delicate balancing act. In very general terms, the smaller diameter converter should be slightly more efficient, and therefore quicker and faster. But none of this is that simple. You can't properly select a converter as simply as you would a set of heads, or an intake manifold or connecting rods.

Call a company that builds converters, and ask them for their recommendations. Be prepared for a long list of technical questions about your cam timing, lift, duration, LSA, headers, trans gear ratios, suspension type, weight bias, etc. The company I buy my converters from has a five page questionnaire to fill out before they will recommend a converter. But when given the correct data, I have never been disappointed with the results.

Give them as much info as you can, and listen to what they say. That's my method for selecting the proper converter for any drag car.

Garry

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Re: Effects of converter diameter at same stall rpm

Post by F-BIRD'88 » Thu Jun 28, 2018 1:07 pm

For drag racing w a th350 get a 10" "3500 stall" converter.
Change the camshaft. Get a camshaft with more duration @.050"
The stock Lt-1 camshaft is TINY. There are many off the shelf hyd roller cams that will wake this up Like the GMPP LT/4 hot cam.

218-228 112 .525" w 1.6 rockers.
it is also fine with your stock 1.5's
use the good LS blue valve springs and SBC conversion retainers
Now the engine torque curve is much better for the used power band with a high stall converter and 4.10 gears w the th350
And it will make more HP too.

Now you will get the bang for the buck on the converter swap and go great.

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Re: Effects of converter diameter at same stall rpm

Post by RDY4WAR » Fri Jun 29, 2018 1:38 am

Garry in AZ wrote:
Wed Jun 27, 2018 7:52 pm
Generally speaking, the larger the torque converter, the more torque (and horsepower) it absorbs. Because of this, a larger converter will usually have less stall speed than a smaller-diameter converter. The Catch-22 is the fin angle inside the converter. Because of fin angle changes, it is possible (both in theory and in practice) to produce a converter in something like an 11-inch size that actually has more stall speed than a 10-inch converter. The stall speed changes gained by reworking fin angles obviously have some limits. But, you can only increase the stall speed so far before you're forced to go with a smaller-diameter converter.
The bottom line is, diameter is not a final predictor of footbrake or flash stall speed, nor track performance. And some companies advertise footbrake stall RPM, while others use flash stall RPM. You can't easily compare them. There are a TON of other variables that all have an effect, right down to things like what sort of brakes are on the car and the available traction for the tires to hold the car in place! A converter that works great on a prepped track won't work as well on a less prepared surface, or a slicker track. I know some pro-mod guys who will even change converters based on track temps.
Basically, what you’re trying to achieve during converter selection is to ensure that the converter itself has enough volume to handle the power level, while still retaining a physical size that will not be too large – a delicate balancing act. In very general terms, the smaller diameter converter should be slightly more efficient, and therefore quicker and faster. But none of this is that simple. You can't properly select a converter as simply as you would a set of heads, or an intake manifold or connecting rods.

Call a company that builds converters, and ask them for their recommendations. Be prepared for a long list of technical questions about your cam timing, lift, duration, LSA, headers, trans gear ratios, suspension type, weight bias, etc. The company I buy my converters from has a five page questionnaire to fill out before they will recommend a converter. But when given the correct data, I have never been disappointed with the results.

Give them as much info as you can, and listen to what they say. That's my method for selecting the proper converter for any drag car.

Garry
Great info. Thanks!

F-BIRD'88 wrote:
Thu Jun 28, 2018 1:07 pm
For drag racing w a th350 get a 10" "3500 stall" converter.
Change the camshaft. Get a camshaft with more duration @.050"
The stock Lt-1 camshaft is TINY. There are many off the shelf hyd roller cams that will wake this up Like the GMPP LT/4 hot cam.

218-228 112 .525" w 1.6 rockers.
it is also fine with your stock 1.5's
use the good LS blue valve springs and SBC conversion retainers
Now the engine torque curve is much better for the used power band with a high stall converter and 4.10 gears w the th350
And it will make more HP too.

Now you will get the bang for the buck on the converter swap and go great.
After discussing it with others, I think we're going to go with a higher stall. We're considering the FTI ESR8082 3800-4000 stall.

There's plans for this car down the road. My wife's cousin drives it. She's just 17 and new to the sport so we're not trying to go too crazy just yet. This is the second time we've had to yank the 4L60 out for a 3-4 clutch failure so we're ditching it for a TH350.

Next winter, I planned to pull the engine and freshen it up with a point more compression, 1-3/4" headers, K&N cold air, and upgrade the fuel system with a Walboro 255 pump and 30 lb/hr injectors to run E85. (consistency) I had spec'd a cam for it myself pretty similar to the hot cam but with a bit more lift. 219/227 @ .050", .584"/.584" lift w/ 1.6s, and 111 LSA.

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Re: Effects of converter diameter at same stall rpm

Post by bigblockmopar » Sat Jun 30, 2018 5:42 am

Smaller diameter convertor = less reciprocating weight, which means quicker engine acceleration.

Not really scientific but some years ago I changed transmissions behind a small streetuse-only 318ci.
Torque convertor changed from a factory (1960s) 12" to a stock factory 11" in size, and I could tell from just revving the engine in neutral the engine liked the smaller diameter convertor.

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Re: Effects of converter diameter at same stall rpm

Post by Geoff2 » Sat Jun 30, 2018 8:02 am

BBM agree.

Nobody has been able to give a definitive answer to my earlier post, just some dope with a stupid answer.

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Re: Effects of converter diameter at same stall rpm

Post by lefty o » Sat Jun 30, 2018 10:45 am

Geoff2 wrote:
Sat Jun 30, 2018 8:02 am
BBM agree.

Nobody has been able to give a definitive answer to my earlier post, just some dope with a stupid answer.
since some dope asked a stupid question without enough variables layed out, you get a stupid answer. to put it really simply for you, there is a lot more to a converter than the diameter!

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Re: Effects of converter diameter at same stall rpm

Post by Geoff2 » Sun Jul 01, 2018 5:04 am

Lefty o,
Just because YOU didn't understand the question doesn't mean somebody else doesn't. So with your answer now, that makes two dopes....

I am WELL aware of how a converter works & probably knew it before you were born......

Here is the question again:

If a smaller c'ter with a higher stall speed [ & no other changes ] produces quicker ETs, is it the reduction in rotating wt, or the higher stall, or a combination of both that produced the better #s?

I raised the original question because there are 'thinkers' [ excluding said two dopes ] on this forum. Has anybody been able to come up with a formula or method of determining what percentage of the improvement was due to the stall increase & what percentage was due to the reduction in RW?

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