383SB or 400SB retro into a L31 1998 Suburban

[b454rat]

400 all day. I’ve had a few, and all tan great. One was .040, and hard time keeping it cool at idle/low RPM’s. But highway was fine. I had a block ready to put together but money got in the way n sold it. Now that things are different, might look but prolly be fine with a fresh 350. 400’s are awesome torque motors. Doesn’t take much to get even more outta em. Any machine shop that knows what they’re doing will drill bypass n steam holes.

 

[L31MaxExpress]

400 all day. I’ve had a few, and all tan great. One was .040, and hard time keeping it cool at idle/low RPM’s. But highway was fine. I had a block ready to put together but money got in the way n sold it. Now that things are different, might look but prolly be fine with a fresh 350. 400’s are awesome torque motors. Doesn’t take much to get even more outta em. Any machine shop that knows what they’re doing will drill bypass n steam holes.

Cool is a relative term. The factory electric fans on the 3rd gen and LT1 do not even turn on until they are ~220F. A GM 195F thermostat is not fully open until 217F. Even with the added electric puller fan, my 1980 L82 Vette swapped to a L30 305 ran ~230-240F in the summer heat idling around town. With a properly pressurized cooling system and a 50/50 antifreeze mixture it will not boil coolant until it gets a good bit warmer than that. On that 400 G30 I set up, used a 454 G-van dual core radiator with the OE oil cooler and an 11 bladed GM fan. It never ran hot, actually ran cooler than the 350 it replaced that had the smaller single core radiator core and 5 bladed steel finger chopper. When you add power you have to add cooling capacity. Cannot use the old half plugged up radiator core from the old 283 or 305 and expect a 400 to stay cool. Case in point, even the L31 350 I put in the 87 G20 has a 454 radiator and upgraded mechanical fan. Cooling it will not be a problem even with more than double the power of the stock 305 that was in it. Remember something like 30% of the heat from however much gasoline you burn to make X amount of power gets shed into the cooling system, double the power, double the heat. 30% of the gasoline burned is power at the crank, 30% is shed to coolant, 30% leaves out the exhaust, and 10% is friction which heats on the oil. That 10% to the oil is also why I run the oil cooler in the radiator in all my builds.

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[Nad_Yvalhosert]

Are you retaining the OBD2 system and the Vortec intake manifold? You'll need a tune to feed the larger displacement of either engine.

Using the 400 block though youll lose the crank sensor. That's no bueno..

Using the 400 block, the knock sensors will detect additional noise from the hydraulic flat tapped lifters (as it's programmed for roller lifters) and seriously retard ignition timing.

I'd suggest building a stroker with your OE L31 and parts from the 400...

 

[L31MaxExpress]

Are you retaining the OBD2 system and the Vortec intake manifold? You'll need a tune to feed the larger displacement of either engine.

Using the 400 block though youll lose the crank sensor. That's no bueno..

Using the 400 block, the knock sensors will detect additional noise from the hydraulic flat tapped lifters (as it's programmed for roller lifters) and seriously retard ignition timing.

I'd suggest building a stroker with your OE L31 and parts from the 400...

Flat tappet cams were run with knock sensors long before roller cams came out. My 83 G20 had a knock sensor and it was a flat tappet cam. I have never seen valvetrain effect the knock sensor even on the engine I tuned that had a solid roller in it and on the Express that had noisy Rhoads V-Max lifters in it. My L30 305 in my 99 Tahoe also had both regular and Rhoads flat tappet lifters in it, neither had false knock retard out of the knock sensor. I rarely have to touch the knock sensor sensitivity settings on a SBC.

Putting a crank sensor on a 400 is pretty simple. The Holley cover bolts right on and seals to any SBC. The reluctor slips on the crank snout in front of the timing set

 

[Schurkey]

Putting a crank sensor on a 400 is pretty simple. The Holley cover bolts right on and seals to any SBC.

I'd expect an OEM plastic cover, or a new aftermarket plastic replacement, to also bolt on and seal.

 

[L31MaxExpress]

I'd expect an OEM plastic cover, or a new aftermarket plastic replacement, to also bolt on and seal.

The newer L31 block has more material where it seals. EFI connection sells a plastic unit with metal added to it to seal, but the Holley cover is cheaper.

 

[1998_K1500_Sub]

Thanks for all the responses; more are welcomed.

How many of the above concerns are side-stepped / altered by using the Dart 400SB block noted by @L31MaxExpress instead of a bona-fide GenI 400SB?

- OBD2 reluctor wheel on crank?
- Provision for crank sensor?
- (edit) these seem answered by @L31MaxExpress 's post above.

Of course, the Dart's not particularly cheap, but benefits include:

- One-piece seal
- "Chevy 350" mains (being as I already have new 3.75" stroke crank w/ "350" mains )
- "Provisions for OE stock roller lifters & cams" (what's really meant by this?)
- "Uses + .300” tall stock 87-95 roller lifters" (quoting Dart's info, here)

Of course I need to consider the value (utility/cost) of having 400-406CID vs. 383cid... <5% more displacement. Too, there are some sunk costs in my 383 project that is already underway, so maybe the proper path 383/400 is rather clear...




With either engine:

- retain/reuse the truck's existing L31 induction / fuel delivery (Delphi MPFI) / spark (coil/cap) / radiator / black-box ECU.

- tweak the ECU config. Maybe an 0411 ECU will come down the road.

- consider a 4L80E (the current 4L60E was GM reman about 80k-miles ago)

- tweak the exhaust (OE L31 log manifolds or... Holley? / larger downstream pipes)

 

[Schurkey]

- "Provisions for OE stock roller lifters & cams" (what's really meant by this?)
- "Uses + .300” tall stock 87-95 roller lifters" (quoting Dart's info, here)

Same thing--the Dart block uses the inexpensive and easily available OEM roller lifters and step-nose cam, the OEM dogbones, and OEM spider to hold the dogbones in place.

The old 400 block needs expensive aftermarket tie-bar roller lifters, and unless you perform some inconvenient modification, you'd use an aftermarket roller cam plus the stupid aftermarket thrust-button instead of the simple and wonderful thrust-plate and step-nose cam core.

So if you're using a roller lifter/roller cam, some of the cost of the Dart block is recovered by using cheaper lifters and the simple and reliable thrust plate, which you'd already have from the L31. (Cam too, if you aren't going custom.)

^^^ All of this is icing on the cake. The BIG benefit to the Dart block is that it's twice as sturdy as the production 400 small-block. Thicker cylinder walls, stronger main saddles, potentially better-grade of iron poured into the mold.

I've wanted a Dart (or Bowtie) 400 block for a decade or more. If I could tune a computer, that's what would be in my '88 K1500; and then I'd be 4L80E-swapping, and then I'd be driving something else for two or three years until I got all that "right".

 

[L31MaxExpress]

Personally I would not worry about the 80E until you break the 60E. It will break at some point in the future regardless if it is behind a 4.3L.

Dart block is twice as strong as either the 350 block or 400 block and already comes with splayed main caps and it is 100% new so it has that going for it.

The bigger the engine the milder it will act with the same camshaft. The stock intake manifold and vortec heads are going to work against high RPM so better to build for torque. Keeping the cam timing mild also contributes to fuel mileage and off-idle torque. It is just a more docile, fun vehicle to drive, especially when you can keep the drivability near stock. Without headers an 87 octane pump gas 383 will probably make about 425 ft/lbs of torque. A 400 would make about 450. The 400 will basically make more power everywhere.

From a cost perspective the 383 wins. From a grunt and longevity perspective the Dart 400 wins. In the past 20 years I have built about 8 flat tappet engines. I have yet to have one wipe a cam. I have steered away from Comp Cams flat tappet stuff and always used the Delphi hardened foot GM lifters or in the case of the L30, Rhoads lifters. The Rhoads lifters in that were even put on the cam that had already run 5,000 miles with the Delphi lifters.

A stock 400 block that has been magnafluxed and sonic checked especially one that is starting at standard bore and will clean up by 0.040" would not scare me even with a flat tappet cam in it in your application. External balance on the 400 would not scare me, but I would put atleast a 5.7" rod in it. The HT383s are externally balanced. Actually thought about that earlier this evening. The Mercruiser 383 in my G20 van is the same as a HT383E. It has a counterweight bolted to the balancer. I bet that balancer could easily be balanced to a 400 crank since it is counterweighted for a 3.80" stroke to start with and it has the snout cut down for the reluctors width if you want to build an OE style 400.

I know one thing for certain, whatever engine I built would have a Holley Aluminum cover on it. For about 2x the cost of a crappy aftermarket plastic cover that is gauranteed to leak in 1-2 years I will never not use one so long as Holley makes them or I can find one.

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[Schurkey]

Personally I would not worry about the 80E until you break the 60E. It will break at some point in the future regardless if it is behind a 4.3L.

I'm on my third TH700, the second two were rebuilt by an age-old and well-regarded local trans shop.

Both failed transmissions had broken planetaries.

Engine was essentially-stock 5.7L.