454 (7.4L) swap / repair options - 1999 Suburban K2500

Pull valve covers, find out what cyl has slop in pushrod/rocker. Check compression, find out what cyl is wrong. Go from there.

stutaeng said:

No advice to OP, but just a general question since topic is 7.4:

Does anybody have any experience with the L21? It sounds pretty cool with forged internals and coil-on-plug ignition. Never seen one in person. Looks like only the Kodiak/Topkick had them? Not sure what HP/TQ those made. An why did they make less power than L29? Seems it would make more? Maybe it was vehicle-specific factory tune?

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Yes, the L21 was used in the medium-duty trucks, but was also optional in 3/4 Suburbans and the 2500/3500 vans.
A bit less HP/Torque, but set-up for long term heavy-duty use.
Those engines were also used as marine engines.
A great engine for a rig that does a lot of towing.

The L21 was medium duty only. The l29 was in the Suburban and van.

L21 also had coil on plug and I believe put power through an Allison transmission.

Gibson said:

Yes, the L21 was used in the medium-duty trucks, but was also optional in 3/4 Suburbans and the 2500/3500 vans.
A bit less HP/Torque, but set-up for long term heavy-duty use.
Those engines were also used as marine engines.
A great engine for a rig that does a lot of towing.

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Interesting. I did not know they were optional on the 2500 Suburbans or Express Vans. Is it possible to "convert" a L29 to L21? Swap ECU and add coil packs and remove distributor? Of course, with out adding the forged stuff and opening up the darn thing.

I'm sure there's more power to be harnessed without the (distributerless) wasted spark system, no?

454cid said:

The L21 was medium duty only. The l29 was in the Suburban and van.

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The Vortec 7400 (RPO L21) was a Commercial version of the Chevrolet Big-Block engine used in the Medium Duty truck platform and Workhorse Custom Chassis. It shares the much from the L29 454, with the addition of forged pistons and crankshaft. It has slightly reduced hp/torque than the L29 454 and uses a different PCM than the light duty trucks did. And was used with the early 4 speed Allison automatic transmission or manual transmission depending on application.


L21 Applications:

• 1998-2001 GMC Kodiac 4500 5500
• 1998-1999 Chevrolet Suburban 2500/GMC Suburban 2500 (option)
• 1998-2000 Chevrolet Express 3/4 or 1 ton

1998-2001 Kodiak/Topkick 1998-2001 P21 Workhorse Chassis

As I have said in an earlier post/thread,, if one knew how to read thru the option book, you could order all kinds of stuff that a dealer wouldn't even know about.

Did you get that from Wikipedia?


https://en.m.wikipedia.org/wiki/Chevrolet_big-block_engine

Maybe Wikipedia is wrong! No wonder my kid's teachers don't allow Wikipedia to be listed as works cited, LOL.

Don't know how I managed to de-rail this thread again!

Anyhow, I admit I don't know much about these trucks, so don't listen to what I have to say

stutaeng said:

Did you get that from Wikipedia?


https://en.m.wikipedia.org/wiki/Chevrolet_big-block_engine

Maybe Wikipedia is wrong! No wonder my kid's teachers don't allow Wikipedia to be listed as works cited, LOL.

Don't know how I managed to de-rail this thread again!

Anyhow, I admit I don't know much about these trucks, so don't listen to what I have to say

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Yes,, but they do exist.
In the Suburbans, if you wanted to get one they would generally have been ex-government vehicles, fire departments, Dept of Forestry/BLM, etc.
The kinds of Suburbans that had crank-up windows, vinyl bench seats, and rubber floor mats, an AM radio,, and FF 14bolts, with 3500 front suspension.
The vans were mostly "fleet sales" for U-Haul, Penske,, that type of application.
Like I said, something that the local Chevy dealer would never have had on the lot.

You'll need a doner for the 8.1 or 6.0. 6.0 is slick. I had a 6.0 3/4 ton Chebbie and it would smoke my stock 454 Vortec. Hands down.
If going 8.1 or6.0 you'll need all electronics. Big PITA. I'd do a 6.0 before the 8.1. However, your NV4500 setup will be complex to hang on a 6.0.
A 502, or 489, or mild 454 is ideal, easy to do with carb or whatever. The NV4500 is a bolt on with 454 based engine. Vortec perf parts are everywhere.
All instruments will function even with a HEI distributor if carbed.
Shorty headers will fit even with rear AC. Gibson's will.
The ECM will have one job...control the fuel pump. Which it will do.
With the 454, all accessories are good including AC with minor plumbing mods.
Going into a 6.0 or 8.1 will require a ton of underhood creativity with power steering, water plumbing, AC, and electronics for either. Not worth the effort in a 'burb.
One biggie negative, it will not pass emissions and will be destined for off-road use only [emoji6]
Only showstopper is the size of your wallet.
Dig around in my old posts and the road map is there.


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I'll disagree with you here on taking a 6.0 over an 8.1. I get the impression that the 8.1 has better legs than the Vortec 454 does after talking to someone who has owned one. With the Allison trans he bested my empty dually by 1 mpg as well. I'll concede the Vortec 454 has no legs, may as well call him Lt Dan, but when the thing can get it done in the midrange it does so a million times better than a small block anything. Sure the 6.0 has a wicked downshift to look forward to, but that's not how I like to tow and that's why both trucks have Whipples. I tow in one gear higher than I did NA and there's way less noise to listen to. My temps stay rock solid when cruising as a result. Stop and go is another story.