How much HP will a th350 handle? New motor!

[1979 K10]

1979 K10
original 400 sbc, headers, dual exhaust, 1406 carb
stock th350 with shift kit, deep pan, external cooler, max life synthetic atf
12bolt rear 3:73 gears
NP203 converted to part time

Trans shifts fine now, only issue when outside temps below 40, no first gear until warms up.

Sometime in its life it’s had a shift kit installed as it shifts firm and will chip rear tires when floored 1-2 shift. Not sure what purple paint means on valve body.

I live at high elevation so new engine will be more like 400 hp which still is more than double the original hp that was rated 185hp but 300 tq

Would like to convert to 700R4 or 4L60E when th350 gives up the ghost.

Ordered Blueprint 400 with 508 hp and 473 Tq
https://blueprintengines.com/products/400-ci-stroker-crate-engine-small-block-gm-longblock-bp4002ct1

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[Curt]

Congratulations on your new motor.Will like to hear how it does after break in.If you can do a TH400 and a 205 that’d be way sweet,however I do believe the TH350 will be fine.

 

[1979 K10]

Congratulations on your new motor.Will like to hear how it does after break in.If you can do a TH400 and a 205 that’d be way sweet,however I do believe the TH350 will be fine.

Thanks! Motor has Been a longtime coming!

 

[Waylon]

I drag raced a ‘54 Chevy pickup with a 454 that would be a very conservative 500hp and never had an issue with my built TH350. Everyone said it wouldn’t last. But it did.

7.74 in the 1/8th at 90+ mph in a full bodied 54 Chevy truck with a cheap converter and no electronics.

 

[Matt69olds]

I have built lots of TH350 for low 11 second cars. Properly built, 500hp is no big deal.

Use a rebuild kit with the high energy friction material. The stock paper clutches are a light tan color, the high energy stuff has a geeenish gray color. Order an extra direct friction and steel. Machine the direct piston the equivalent of the friction and steel, you can fit 5 clutches instead of the factory 4.


Install a wide direct drum bushing a 700 low/roller clutch is a good upgrade. The roller clutch has larger rollers than the stock 350 part


The stock valve body gaskets won’t work with the shift kit without adding a couple extra holes to match the spacer plate. If the accumulator piston is plastic, replace it with a older aluminum piston. Use a 3/8 freeze plug to block the 2-3 accumulator feed in the valve body, under the accumulator piston.


Careful assembly, pay attention to the end play and wear items. That’s all really needed.

 

[sirweesarunch]

Where in the RPM range is the Torque. ? High Torque down low breaks parts more readily. Driving habits are the biggest determiner of vehicles.


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[1979 K10]

Where in the RPM range is the Torque. ? High Torque down low breaks parts more readily. Driving habits are the biggest determiner of vehicles.


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[sirweesarunch]

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Ask for the whole torque curve from idle. You may be surprised at how weak the engine is off idle where you need most of your power on a truck.


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[Matt69olds]

Where in the RPM range is the Torque. ? High Torque down low breaks parts more readily. Driving habits are the biggest determiner of vehicles.


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With the exception of the big block Cadillac engines, nothing beats the raw low end torque of big block Buick, Olds or Pontiac engines. Those engines don’t really have a torque curve, it’s better described as a torque plane. The torque starts high, hits its peak, then settle back to high. Of course, by 5000rpm those torque monsters are weezing and out of breath. Two of the quickest TH350 I have built are in a 70 Buick Sklark (11.15) the other is a 66 cutlass (11:40s)

The biggest weakness in a TH350 is the intermediate sprag. The roller clutch will explode if the shock load of freewheeling to lockup is too much to handle. The biggest influence on shock load is either shift firmness, or rpm.

Look at it this way, the shock load at 7500 rpm is going to be far greater than at 5000. This is why it’s critical to leave the intermediate clutch wave plate in the clutch pack, the wave plate absorbs some of the shock load. Same thing with keeping the 1-2 accumulator functional, anything to absorb the shock load of the roller clutch will help.

 

[1979 K10]

With the exception of the big block Cadillac engines, nothing beats the raw low end torque of big block Buick, Olds or Pontiac engines. Those engines don’t really have a torque curve, it’s better described as a torque plane. The torque starts high, hits its peak, then settle back to high. Of course, by 5000rpm those torque monsters are weezing and out of breath. Two of the quickest TH350 I have built are in a 70 Buick Sklark (11.15) the other is a 66 cutlass (11:40s)

The biggest weakness in a TH350 is the intermediate sprag. The roller clutch will explode if the shock load of freewheeling to lockup is too much to handle. The biggest influence on shock load is either shift firmness, or rpm.

Look at it this way, the shock load at 7500 rpm is going to be far greater than at 5000. This is why it’s critical to leave the intermediate clutch wave plate in the clutch pack, the wave plate absorbs some of the shock load. Same thing with keeping the 1-2 accumulator functional, anything to absorb the shock load of the roller clutch will help.

Thanks for the detailed reply’s. I’m guessing my trans is stock with a modified valve body since it shifts firm. Who knows if it’s been rebuilt in the past 40 years. Since I’ve owned it for the past 22 years it’s always been fine except for no first gear when freezing temps.

Hopefully it will continue it’s good service with new motor but when it blows would like an auto with OD except will be more work with moving crossmember, shorting rear driveshaft and possibly lengthening the front.

Any thought on the NP203 transfer case? I’ve heard they are pretty tough but not as strong as NP205.

I don’t do any rock crawling or anything extreme. Dirt road, pavement pounder and occasional light 3500lb towing.

 

[Matt69olds]

I know next to NOTHING about transfer cases, other than what they do. I don’t know the specifics, or advantages of one model over another. I’m the wrong person to offer advice, I’m more than willing to step back a d let other more knowledgeable people comment.

 

[Camar068]

Ask for the whole torque curve from idle. You may be surprised at how weak the engine is off idle where you need most of your power on a truck.

I know really......every day drivers arent going to see over 4500.....they always start at 2500 with their ****. Start it at 1200-1500.

 

[1979 K10]

Here’s the original 400

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[bft305]

The NP203 is pretty strong, the chain is what I believe usually causes people to ditch it. Plus a lot got converted to part time and people forget they have to drive around in 4x4 every 200 miles or so to get oil to splash into the back of the case. If you don’t then it gets destroyed. I read somewhere to also use 10w-30 to help it lubricate everything easier. When I got my off-road truck it had a np203, I swapped to 10w-30 and said I am going to flog it until it blows up and then swap in a np205………I am still waiting for it to blow up! Though I did blow up my stock th350 when I upped my engines HP. So this might be when The NP205 goes in finally!

 

[1979 K10]

The NP203 is pretty strong, the chain is what I believe usually causes people to ditch it. Plus a lot got converted to part time and people forget they have to drive around in 4x4 every 200 miles or so to get oil to splash into the back of the case. If you don’t then it gets destroyed. I read somewhere to also use 10w-30 to help it lubricate everything easier. When I got my off-road truck it had a np203, I swapped to 10w-30 and said I am going to flog it until it blows up and then swap in a np205………I am still waiting for it to blow up! Though I did blow up my stock th350 when I upped my engines HP. So this might be when The NP205 goes in finally!

I put Mobil 1 full synthetic in the transfer case. Can’t remember if it was 10-40 of 10-30. I should prob change it again anyway.

I did the full time to part time conversion prob 12 years ago. It was the cheaper conversion kit with needle bearings not the better mile marker kit where you change out the shaft.

I either lock the front hubs and drive around for a few miles in 2wd or leave front hubs unlocked an put in hi loc at least once a month to lube the rest of the case.

Noticed no fuel economy improvement but my front axle end u joints should last forever and it turns better on pavement vs full time 4x4. Plus I can light up the 12 bolt rear end now. It is a posi but clutches are nearly gone so mostly a one wheel peel.