I have a ls3 swapped frs with a t56 transmission. The trans is starting to go out and I have considered swapping to th400. My question is how much will the th400 slow the car down compared to the t56 in the 1/4. I plan to boost the car but as it sits I don’t want it to be sluggish compared to the t56. Car makes ~450whp at 2900lbs

 

An auto transmission is the quicker way down the quarter mile.

 

Depending on use... May want to look at a 4L85E if you like OD and any plans on hi-way drives ?

 

It will see the street occasionally for things like car shows but the main goal is just to get down the 1/4 mile as fast as possible.

 

Full manual shift automatic... not shifting faster unless you have a special race manual gearbox.

Like...G-Force Transmissions and Long Shifters (gforcetransmissions.com)
4-spd............................................... .................................................. ........5spd
.........

 

I think my bigger concern is that with the much longer gears in the th400 will it slow down the acceleration by a significant amount until the car gets a power adder? Ik the th400 will have the faster shift times but idk if that will make up for the difference as the car sits.

 

Optimized stall speed will be the key in this regard. For the stated purpose of this car, there is really no need to compromise on stall speed; go with whatever keeps you in the powerband at all times during WOT (shift extension landing no lower than peak torque with upshifts just slightly above peak HP). Of course, optimal stall speed and converter type will change once you install a power adder.

This will take care of most of your concerns, but trap speed may still be a bit slower with the auto - but ET will not, at least not if it's done right.

 

IF you go with an auto... GET A CONVERTER BUILT FOR YOU COMBO...."off the shelf" converters are a one size fits some and you won't be happy. BTDT

 

The converter will more than make up for the percieved gearing "disadvantage"
As has already been said...get a properly spec'd converter and have the transmission properly built (if planning on power adder, build it accordingly)
It will be quicker and faster in the 1/4

 

I absolutely LOVE this Post!!!

 

While I have only been to drag strip "test and tune" a couple dozen times, here is my experience. I estimate that only 10% of cars at a test and tune are manual trans, but the majority of the cars I have seen break down were those manual trans cars. Any tire spin and catch creates tremendous impact on the drivetrain when there is no torque converter to cushion the blow. On those manual trans cars at the drag strip, I've seen differentials break, u-joints snap and even transmissions shatter. And of course there is the danger of a "money shift".
I also think that an automatic with a quality converter will give a faster ET at a bit slower trap speed than a manual trans.

 

Even though the gear spread of the auto is not as ideal , With the torque multiplication factor of the proper converter all things equal IMO the auto will be the faster option.
I have over the years seen 350 hp autos with proper gearing and converter easily beat the time of an equivalent 500 hp with a manaul trans simpley for that reason. Just my humble observation and experience over the years and dealing with many customers who had both set ups or did the manual to auto swap.
Though if this is street driven much or highway I would suggest considering a 4 speed auto 4l60e or 4l80e depending on power with the up or down 1st gear options for either respectively and just FYI on my own car on he dyno being able to use lockup at the top above 4000 rpm I was able to gain 30ft lbs torque in 2nd and third.
But if its an around town car mostly for short cruises and strip the 400 with the right converter would be my choice. Though the fun factor of the manual is to be considered, But fastest in the 1/4 most certainly the auto.
Just my thoughts on it.

 

FMFP TH350 with a 5000 super efficient 9"

 

Until you are beyond 600-700rwhp just rebuild your T56... if you plan to make more than that... sell your T56 and buy a T56 Magnum.

My stock T56 Magnum F (same dimensions/bolt in as yours) is holding 1200rwhp ramped in. I don't launch it hard as I really don't like fixing broken parts... but it certainly moves out. It's mainly a street car and 1/2 mile. I can only do private track days as it doesn't have all the safety gear (full cage/parachute). An automatic is so boring... you can have an 8/9 second car... keep it a stickshift and negotiate corners unlike a full blown drag car. Plus you have overdrive.....



The pass from the strip was the best of the day on a new 88mm turbo with a ton left. The track wasn't prepped great and I launched it with zero boost. Turbo is rated at 1450hp+

 

That doesn't take the OP's primary goal into consideration though:

Quickest way down the quarter mile is always going to be a properly set up (meaning stall speed, etc.) auto. It's not really a matter of how much power the various iterations of T56 can handle - if it's a purpose build drag car where quickest ET is the "main goal" then a T56 is never the answer.

I know lots of manual folks feel that way, but it's subjective and, in this instance, irrelevant based on the OP's goal.

 

I would absolutely LOVE to have a T56 Transmission in anything that I could drive around in all day long, having FUN!!!
Nothing will replace that enjoyment!
I could not Trade it for anything!


However for a Drag Race Vehicle, going through the few Gears that the Manual Valve-Body provides after leaving off of the Transmission-Brake...
The Launch is AMAZING!! ...And the Uber-Quick, Positive and Firm/ Aggressive feeling of the Gear Changes are Incredible (Especially the 1-2 Up-Shift.)



It's another entirely different, yet absolutely amazing experience, and fun that I also would Not Trade away!
On top of that, it is simply the better way forward.
Faster and more Consistent, and more Reliable!
You Gotta do it!!!