Need Help L29 Build Questions

[Erik the Awful]

True duals is not the best setup. A properly done single will outrun duals all day every day.

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I'll bite.

True Duals > Stock
3.5" Single > True Duals
Duals w/ Crossover > 3.5" Single

The long tube headers are a headache, but worth the effort.

Yes.

Be sure to only use mandrel bends in the exhaust.

That's the conventional thought, because the '80s era dinky crinkle-bent exhausts were so bad. Engine Masters just had an episode with crush bends versus mandrel bends, and the difference on the dyno was within the margin of error. Personally, I buy mandrel u-bends because they're a cheap and easy way to weld my own exhaust.

 

[yevgenievich]

Blocking off one side of the port--from port wall to valve guide boss--IS a restriction.

I was just thinking of different tactic that I seen gm employ before. Intake runners on a sonic

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[Carlaisle]

I'll bite.

True Duals > Stock
3.5" Single > True Duals
Duals w/ Crossover > 3.5" Single

I used to know someone who had this exact engine with long tube headers with true duals and a crossover. At some point he ended up replacing everything from the headers back. When he did it, he kept the duals to a dual in single 3.5" out muffler. I don't recall if he had any numbers to back up his claim, but he said the single 3.5" was noticeably superior to his original true dual with crossover set up. Of course, this is only hearsay.

 

[Schurkey]

I used to know someone who had this exact engine with long tube headers with true duals and a crossover. At some point he ended up replacing everything from the headers back. When he did it, he kept the duals to a dual in single 3.5" out muffler. I don't recall if he had any numbers to back up his claim, but he said the single 3.5" was noticeably superior to his original true dual with crossover set up. Of course, this is only hearsay.

The problem being, unless the engine tune is optimized for a particular exhaust, changing the exhaust may improve performance because it causes the existing tune to be closer to optimum--but folks will assume that the improved performance is due to a "free-flowing" exhaust.

Conversely, a "better" exhaust system may DEGRADE performance because the engine tune is farther off with less back pressure, or different pulse tuning/extraction of the new system.

There's ten thousand ways to build an exhaust system, given variables such as dual/single/dual with H crossover, dual with X crossover, one muffler, two mufflers, four mufflers, tailpipe size and length, crush-bent vs. mandrel bent, on and on and on.

HOWEVER it's built--the engine needs to be tuned to work with the exhaust system.

This is why I laugh at folks that pay money for "exhaust cutouts". Do you tune the engine for the 90% of the time it's running through the mufflers? Or do you tune the engine for the 10% or less time it's got the cutouts open? Or you you figure out both tunes, and switch back and forth? Of course, the real answer is that most folks open the cutouts on the fairgrounds or at "cruise night" to impress people they don't know, who's opinion doesn't matter, making the time, money, effort, and enthusiasm of exhaust cutouts totally wasted.

 

[1952Chevy]

1. You have "Net Lash" rocker studs. They're not adjustable. Either you buy a cam with OEM base circle, or you upgrade the rocker studs and nuts to the older, adjustable style. Problem is, GM didn't drill and thread the rocker stud bosses in the head to 7/16-14 like in the old days. You'd need special "conversion" studs, or you have to have the heads drilled 'n' tapped to the larger size. When it was me, I just bought the ARP conversion studs.

2. You'll have valve rotators under the valve springs. Aftermarket springs won't like the rotators. You'll need two sets (16) rotator-eliminator shims. BEWARE of the shims not seating fully on the intake guides--the radius of some intake guides at the spring pocket is too large, has to be ground-down by hand. I had two guides on each head that needed to be touched-up.

3. Cut the block decks so that they're square to the crankshaft, and so the piston has proper quench/squish when using a typical-thickness head gasket. The amount that has to be cut from the block depends on the gasket you select. I'd be looking for 0.035--0.040 quench distance--and that's the thickness of typical gaskets, so the block needs to be cut so the piston tops (not the dome, if used) is level with the deck.

4. A large piston dome will hit the "widow's peak" in the combustion chamber of the L29 heads. May have to relieve that "widow's peak". I used small-dome pistons so there's wasn't any problem. Those pistons had valve reliefs intended for stock-size (2.08) valves. If you use 2.19" valves, beware piston-to-valve clearance. (I don't know if 2.19 valves will work in the L29 heads.)

5. The L29 heads have a ski-jump in the intake port. Very much like the crappy TBI small-block "swirl port" heads. They're biased for low-rpm power and fuel economy. There's no power above 5000 rpm. Maybe someone can port them so they make power. My engine was pretty lame. I got rid of the L29 heads on the replacement engine.

Thank you for all of this info! Any CAM recommendations?

 

[TexasAggie]

Great info here. I will be doing the same thing in a year or so. Not looking for big power, just a bit more than stock. I will be putting long tubes on this summer and need to figure out the best way to run the exhaust. I'll have the PCM re-tuned by black bear as well.

 

[Schurkey]

Thank you for all of this info! Any CAM recommendations?

All my experience with L29 heads was on a non-computer-controlled boat engine from a decade ago. I had a Lunati roller-cam, but aftermarket "conversion" roller lifters. It was pretty mild. I don't remember the specs.

I believed the magazine when they said the L29 heads were a fabulous improvement on the round-port junk I had on the engine before.

I was largely disappointed in the results.

 

[L31MaxExpress]

The ski ramp in a 454 head is like the ski ramp in a 305 Vortec head. It really doesn't hurt high lift flow, but if you remove it, the lower lift flow will nose dive. I bet Shurkey was running too much timing for the heads to run correctly.

 

[L31MaxExpress]

The problem being, unless the engine tune is optimized for a particular exhaust, changing the exhaust may improve performance because it causes the existing tune to be closer to optimum--but folks will assume that the improved performance is due to a "free-flowing" exhaust.

Conversely, a "better" exhaust system may DEGRADE performance because the engine tune is farther off with less back pressure, or different pulse tuning/extraction of the new system.

There's ten thousand ways to build an exhaust system, given variables such as dual/single/dual with H crossover, dual with X crossover, one muffler, two mufflers, four mufflers, tailpipe size and length, crush-bent vs. mandrel bent, on and on and on.

HOWEVER it's built--the engine needs to be tuned to work with the exhaust system.

This is why I laugh at folks that pay money for "exhaust cutouts". Do you tune the engine for the 90% of the time it's running through the mufflers? Or do you tune the engine for the 10% or less time it's got the cutouts open? Or you you figure out both tunes, and switch back and forth? Of course, the real answer is that most folks open the cutouts on the fairgrounds or at "cruise night" to impress people they don't know, who's opinion doesn't matter, making the time, money, effort, and enthusiasm of exhaust cutouts totally wasted.

I have dual 3" cutouts in front of the muffler on the Express van. Power does in fact increase on it when they are open and the engine takes on a noticeably rougher idle when the backpressure of the muffler is removed. There is absolutely no reason for two different tunes though. The PCM tuning with the MAF and 02 sensors keep the air/fuel mixture right where I command it with them open or closed. When the cutouts open the MAF increases fuel delivery for the increased intake airflow. I tend to open the cutouts on 2 lane country roads because I get more torque and I like the V8 rumble.