4.8 to th350 troubleshooting

[Jstock85]

Got a couple quick questions for you guys out there.

First I swapped a 4.8 for a 305 and reused the 350 trans which ran fine with no issues. After the swap there seems to be some kind of delay when I push the accelerator. It will rev up then begin to move. Kind of like driving one of those go karts at a track. There is nothing electric on the trans so i'm assuming I need to change the converter. The new motor idles and runs fine.

Can anyone confirm the difference is the power band is my problem?

Also can anyone recommend a converter I should use or what stall speed I should used if the is in fact my issue?

Thanks
Jeff

[skyphix]

Couple of things: Are you sure the trans has enough fluid when hot? Did you remove the torque converter from the trans at any point during the swap? It may not be fully seated.

How high is it revving before it engages? Does it slip at all when it shifts?

[Jstock85]

I checked to fluid level first and it has enough. No I left it on there the entire time. Im pretty confident its fully seated but could be wrong. It idles around 500-800 and it revs up to about 1200-1500.

[Jstock85]

No slip.

[skyphix]

1200-1500 is probably the stock stall rpm for that converter if its the stock converter, I think you're probably fine but hopefully someone else will chime in as well. I was thinking it was revving up beyond, say, 2500-3000 with the stock converter.

It could be that the stock converter is getting worn, so its not doing any partial engagement.

[72GreenMonster]

Interested as I am swapping a 5.3 with my th400.

Which flex plate spacer did you use?

I read somewhere the flex plate holes don't always match torque converter
holes? You didn't pull yours?

Good luck and please share your solution.

[Jstock85]

I used the dished flex plate not the flat one that was with my old engine. So I did not need a spacer. I also used the ring to center the engine and trans. The flex plate holes will not line up but that is a easy fix. I simply reamed out the hole about a 1/4 inch.

[72GreenMonster]

Originally Posted by Jstock85

I used the dished flex plate not the flat one that was with my old engine. So I did not need a spacer. I also used the ring to center the engine and trans. The flex plate holes will not line up but that is a easy fix. I simply reamed out the hole about a 1/4 inch.

J,
I have what seems to be the dished fp on my 02 5.3, does that mean I don't need a spacer to mate my th400?
Thanks for confirming the fp holes.
What was your old engine? before the 4.8?

[Jstock85]

Old engine was small block 305. You shouldn't need a spacer with the dished plate. Is your engine and trans out of the swap truck? If so the easiest way to tell is to set the starter in place and see if the plate will line up on ignition. Then mate the trans and check for a gap in between the flex plate and converter. No gap = your all set

[Jstock85]

Skyphix
Took the truck out the other day and I was in fact having to rev the engine past 2000 RPM to get moving. Do you think the converter is the cause of my problem? Again it is a mechanical transmission. The shift linkage and kick down and actual shifting are all in working order best I can tell. The only problem seems to be the delay in moving the truck.

Thanks

[skyphix]

It sounds to me like the converter is whooped - it probably had no issue with 120hp but 280 is too much for it.

[Jstock85]

You have any recommendations on a stall speed I should look for? This is a daily driver and not looking to spend a lot on this upgrade.

[skyphix]

1500rpm - 1800rpm stall speed should be adequate and if that 4.8 is staying stock, pretty much any converter should work, even the stock one for your vehicle. Something like a 2000 may make it feel a little more peppy but anything over that, in my opinion, wouldn't be worth it. If you think you're going to put a different cam, even a mild one, at some point, go with 2500 now.

I'd almost consider this more a repair than an upgrade, given your use case. I don't really have a budget recommendation for a converter, but myself, I'd probably test out the summit converters like this one https://www.summitracing.com/parts/sum-g2699/overview/ or even the B&M 20400.

[Jstock85]

Ill start looking into that. Thanks again!