Gimme Fuel
Well-known member
For a few years now I have had about 90% of the parts needed to convert the 8.1L in my Chevy Avalanche 2500 to basically the marine variant 496 MAG HO (425 hp) specs plus a few extra. Well, I am finally going to do it when my truck is torn all apart to receive a complete new suspension system that has also been sitting in my garage for two years. With the boat motor basically complete I finally am able to get back to this project. There is a bunch of misinformation floating around on this engine or at least not the complete story told because a certain company wants to peddle their custom parts for it. There are a few bottlenecks that when addressed can open up a bunch of extra power. They are an extremely reliable engine and aren't a complete bastardized child like some believe. There are common interchangeable parts. It is essentially half LS and half Gen 6 BBC. camshafts, rockers, pushrods, roller lifters are identical style and interchangeable with the GEN 6 BBC. The engine is set up with a tight piston to valve clearance so anything more than the HO camshaft (.510/.510 lift) can cause interference without enlarging valve reliefs in pistons or changing to aftermarket pistons. Another misnomer is the cast crankshaft is weak. This is not true and just like the LS series motors, the factory cast crankshafts can now hold as much as the old forged cranks could so upgrades here are not needed until well over 600 hp. Another misnomer is that the pistons are junk and can't handle boost. Once again, they are hyperutectics like LS motors, the ring gap clearances need to opened up to accommodate for the additional expansion otherwise it will bust the ring lands. Other than that the stock pistons are very stout. The fuel and electronics are all same era LS components and interchangeable once again.
Although I will not be pulling the engine at this point, I am still expecting to make some substantial gains (70+ hp) over what I have already made from stock. The avalanche had a detuned version at only 310 hp to begin with. Through tuning and exhaust work, I gained around 60 RWHP on dyno mostly on tuning and torque management deletions. The stock truck camshaft is identical to the 370 hp marine MAG version which is around .480" lift. which is why I was able to get the tuning gains equivalent to put me at that crank HP.
My plan is to add a ported intake, marine (3 hole) injectors, LS2 Vette throttle body (105mm), Ls2/3 MAF sensor conversion, HO marine cam swap (uses same valve springs so no change needed there), and a new fabricated 4" dia. air intake to complement LS2 TB. This should net me right around 450 crank hp with a good tune. It should be a very nice bump but yet not hinder driveability and towing.
I talked to Bob Madera a bit before undertaking this project to see if it was even worth doing. He said that it would be "Well worth my time" which means it should be substantial. He also says that even with the tight valve clearance there are decent gains to be had from camshafts. For now I am sticking into the MAG HO camshaft so I don't have top tear apart and modify the heads. Some day down the road maybe.
First thing I started addressing was the intake. It has huge long runners with a consistent cross section that should make nice power right? Wrong, as you can see in this cut-away there is a huge upstream air restriction where the air enters the plenum. See how the intake air goes under the intake then up into the center of the plenum? This kills the flow into this intake and is why the power is done by 4700 rpm on this motor. It just cannot flow. By removing that plenum divider with a die grinder and introducing the inlet air straight into the front of the plenum, this bottleneck is almost completely eliminated.
Now I could not reach inside to get all of it, but the opening straight into the plenum was created and is a very large opening now. The other challenge is the flange and bolt pattern between the ls1/truck throttle body (3 bolt) and the larger LS2 style (4 bolt). With my truck being electronic throttle, I have no idle air motor and circuit to worry about so that simplifies things. I bought a simple $25 adapter off of Ebay and will blend and port the opening of the flange. I put some good epoxy on the outside of the flange because there isn't very much meat there to make a gentle blend. This way, I can take it a bit further without making a new hole somewhere. I will be blending that section tonight.
On Sunday, I ported the plenum opening. I just did the rough port as I need to pick up more cartridge sanding rolls to clean things up.
Before:
Here is a same size cable TB I had laying around for reference:
Plenum opening rough ported:
LS2 to LS1 adapter plate before porting and blending:
Although I will not be pulling the engine at this point, I am still expecting to make some substantial gains (70+ hp) over what I have already made from stock. The avalanche had a detuned version at only 310 hp to begin with. Through tuning and exhaust work, I gained around 60 RWHP on dyno mostly on tuning and torque management deletions. The stock truck camshaft is identical to the 370 hp marine MAG version which is around .480" lift. which is why I was able to get the tuning gains equivalent to put me at that crank HP.
My plan is to add a ported intake, marine (3 hole) injectors, LS2 Vette throttle body (105mm), Ls2/3 MAF sensor conversion, HO marine cam swap (uses same valve springs so no change needed there), and a new fabricated 4" dia. air intake to complement LS2 TB. This should net me right around 450 crank hp with a good tune. It should be a very nice bump but yet not hinder driveability and towing.
I talked to Bob Madera a bit before undertaking this project to see if it was even worth doing. He said that it would be "Well worth my time" which means it should be substantial. He also says that even with the tight valve clearance there are decent gains to be had from camshafts. For now I am sticking into the MAG HO camshaft so I don't have top tear apart and modify the heads. Some day down the road maybe.
First thing I started addressing was the intake. It has huge long runners with a consistent cross section that should make nice power right? Wrong, as you can see in this cut-away there is a huge upstream air restriction where the air enters the plenum. See how the intake air goes under the intake then up into the center of the plenum? This kills the flow into this intake and is why the power is done by 4700 rpm on this motor. It just cannot flow. By removing that plenum divider with a die grinder and introducing the inlet air straight into the front of the plenum, this bottleneck is almost completely eliminated.
Now I could not reach inside to get all of it, but the opening straight into the plenum was created and is a very large opening now. The other challenge is the flange and bolt pattern between the ls1/truck throttle body (3 bolt) and the larger LS2 style (4 bolt). With my truck being electronic throttle, I have no idle air motor and circuit to worry about so that simplifies things. I bought a simple $25 adapter off of Ebay and will blend and port the opening of the flange. I put some good epoxy on the outside of the flange because there isn't very much meat there to make a gentle blend. This way, I can take it a bit further without making a new hole somewhere. I will be blending that section tonight.
On Sunday, I ported the plenum opening. I just did the rough port as I need to pick up more cartridge sanding rolls to clean things up.
Before:
Here is a same size cable TB I had laying around for reference:
Plenum opening rough ported:
LS2 to LS1 adapter plate before porting and blending:
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