8" vs. 9" convertor

[roccodart440]

I am in the process of buying a new TC.

Everywhere I called offered me a 4K stall in a 9.5" convertor. THe last place I called offered me a 4,500 stall in an 8" or a 9".

I'm wondering which would be better, for what, and why it would be better. Same stall, different diameters.

The car is a 69 dart, 602hp @5,900 / 575tq @4,500, 4.10 gear, GVOD. Car see's a few track days a year. Mostly driven 25 miles or less, usually less.

 

[Garrett Ellison]

 

[66Valiant528]

For the few times you are going to run the car at the track I'd save the money and harsh street manners and put a much lower stall speed converter in it. Just my experience. It sure is nice to flash the converter and go but if it's used for tooling around town and regular errands I'd make it as comfortable a ride as possible.

 

[roccodart440]

I have a 3500 stall currently. It is too tight for the engine and pulls down on the engine at idle. I can only foot brake to 1200 or before it starts pushing.

I'm unsure what you mean by harsh street manners?

 

[MoparLeo]

It sounds like you don't really have a 3500 rpm converter in t now. I put a 9" converter in my cousin's 396 Camaro and it acted like it was in neutral until the rpm's hit 3,000 . No fun on the street at all.
Built a lot of heat in the transmission. Use a big cooler.

 

[roccodart440]

It sounds like you don't really have a 3500 rpm converter in t now. I put a 9" converter in my cousin's 396 Camaro and it acted like it was in neutral until the rpm's hit 3,000 . No fun on the street at all.
Built a lot of heat in the transmission. Use a big cooler.

My current convertor is a 10", it flashes to 3500 from a high idle and also flashes to 3500 if you do the 3rd gear test with it.

I data log my trans temps. They barely ever go over 160. I have a derale stacked plate cooler.

Modern convertors do not act like they are in neutral until they hit the flash stall.

 

[roccodart440]

Can someone answer my original question.

two different diameter convertors, same stall speed. Which is more desirable?

 

[B3422w5]

Can someone answer my original question.

two different diameter convertors, same stall speed. Which is more desirable?

Too me( and i asked this very question on Moparts years ago) the 8 inch is gonna be more efficient than the 9.5 at the same flash.
I have never used anything but 8 inch in all my street/ strip cars over the years with the exception of a Coan 10 inch years ago, which proved to be a dog when i ultimately swapped it out for an 8 inch.
Dont worry( at all) about heat, simply put a good cooler on it and go have fun.
I think you will be astounded how much better the car runs, with a “ real” convertor

 

[GTX JOHN]

The smaller the convertor = the more effficient for any give stall speed.
I routinely for years have run an eight inch of the street in my 10 Sec
A bodies! I have a extra 1/2 dozen = I will PM you.

 

[B3422w5]

Btw, based on your peak torque being at 4500, if it was my car i would want an 8 inch, and would want it to flash around 4800.
A tight 8 inch would be your huckleberry

 

[GTX JOHN]

The torque Multipication/Stall Ratio tends to be much better on the
8 inch vs the 9 inch although the stall speed is similar!
We often test a 1/2 dozen every time we build a new engine on our Stockers/SS
to find the last hundred in 60 ft. It is an expensive tedious game at National Level.
My shop is full of converters as you just have to keep testing.
Beware of the new expensive 7 inch converter - they will not fall out of a tree for us!

 

[roccodart440]

I appreciate the feedback guys. Sounds like the 4,500 8" will be the hot ticket.

What's this new 7" convertor you speak of? The latest greatest i've read about are the spragless convertor, but they make heat and are a race only, non-street deal.

 

[33IMP]

The larger the converter body is, the looser/sloppier it needs to be to achieve a certain stall speed. A bad converter shop can make a 11" converter stall 4300, but it would be complete garbage compared to a nine inch 4300. (I know, I had one. The single worst hotrod part I have EVER bought.)
Given the choice of a tight eight inch for 5000 stall, or a nine inch for the same stall...... I will take the eight inch 100% of the time! (My last converter swap picked up two tenths in the eighth,from low 10s to high nines 1/4)
Edit: if GTX John says buy an eight inch, buy an eight inch!!!

 

[roccodart440]

I'm thinking this will make a big difference for me. I'm leaving off idle, flashing to 3,500, which is 1,000 HP under peak TQ.

I'm also trapping at 5,200 and I make peak HP at 5,900 with a very broad curve, slow fall off the other side of the graph I should be trapping at 6,200. THat will require a gear change, unrelated, but between the two, I expect a pretty big change. My short time is 1.74 or so. I figure if I can get that into the 1.60 range, plus trap in the correct range I should see a 10.60 range, maybe better.

 

[33IMP]

If you are going 7.0/100 with a 1.74 60ft, and 600hp you NEED A converter!!!
Edit: .... but you'd better have the traction/suspension to hook it up.....

 

[roccodart440]

If you are going 7.0/100 with a 1.74 60ft, and 600hp you NEED A converter!!!
Edit: .... but you'd better have the traction/suspension to hook it up.....

I agree, I definitely need a new convertor, but i'm also geared all wrong. I'm going through the 1/8th in 3rd gear. THe car wrinkles the tires and hooks hard.

 

[GTX JOHN]

If you are going 7.0/100 with a 1.74 60ft, and 600hp you NEED A converter!!!
Edit: .... but you'd better have the traction/suspension to hook it up.....

Most 7.0/11.0 if they are sorted out and prepared right = go in 1.4s somewhere.
A 124 MPH well prepare car should go 10.5ish and 1.3 to 1.4 @ 60
5200 RPM in Traps = I would be looking for another tack unless you
were at 1/8 mile.

Our little Red 68 Valiant small block street car that won the MATS Street
Legal Class last week has 3:91 and a 29 tires at 120 MPH was 6250 thru lights!

 

[Bewy]

I have a hard time believing that tq multiplication is better on a small-er diam cter.
The tq is developed by the centri force generated of the fluid leaving the impeller fins. Bigger diam, more force.
What is intrinsically linked to cter design is the rotating weight of the cter.
So how much does rotating weight play in the change from a 12" to 10" cter that improved ET? The weight & the stall cannot be separated; & the reduction in rot weight occurs where it makes the biggest difference: on the circumference.
If there is an 8" & a 9" that are 4000 rpm stall, I would use the 8".

 

[RustyRatRod]

The 8" is lighter. That'd do it for me right there.

 

[roccodart440]

A 124 MPH well prepare car should go 10.5ish and 1.3 to 1.4 @ 60

5200 RPM in Traps = I would be looking for another tack unless you
were at 1/8 mile.

Our little Red 68 Valiant small block street car that won the MATS Street
Legal Class last week has 3:91 and a 29 tires at 120 MPH was 6250 thru lights!

I am going though the traps in overdrive. 4.56 + GVOD = 5,200@124

 

[mderoy340]

How are you shifting? 1-2-3-3od or 1-2-2od-3od. Either way it's a big gear change.

 

[roccodart440]

How are you shifting? 1-2-3-3od or 1-2-2od-3od. Either way it's a big gear change.

Currently I go 1-2-3-OD in the quarter.

I tried 1-2-OD-3 and it slowed.


I run 1-2-3 in the eighth.


For refference, I used to go 1-2-OD in the 1/8th at 6200 trap RPM

and 1-2-3 in th 1/4 and trap at 6200

The new engine makes more power and 4.56 is too steep for it