Pros And Cons Of Doing A 243/799 Cylinder Head Swap On The LM7 5.3L

At LS Performance Racing we understand that there are a lot of newcomers eager for Knowledge. That's why we try to give you the best information possible, so you can get the best setup possible for your vehicle application.

At LS Performance Racing we understand that the Gen3 LM7 5.3L engine is a very common engine that people like to do cylinder heads and camshaft swaps with and one of the most commonly asked questions we get is, is it worth swapping over to a set of factory 243/799 cylinder heads, and the answer is yes and no. Yes, it'll give a little bit more HP and TQ but not enough to justify doing the swap if you're not going to mill the cylinder heads a minimum of 0.025 to compensate for the 3cc bigger combustion chamber size. Here is an LM7 5.3L engine being dyno'd with a set of factory 706 and 799 cylinder heads.

As you see the results are far from impressive but why is this? To figure this out let's take a look at the two-cylinder heads and compare the differences, the LM7 5.3L engine comes with a -8cc dish piston from the factory making it very important to have a smaller combustion chamber to achieve a good compression ratio. The 706/862 cylinder heads come with a 1.890" intake valve, 200cc intake runner, a 1.55" exhaust valve, 70cc exhaust runner, and it has a 61.5cc combustion chamber. The 243/799 cylinder heads come with a 2.00" intake valve, 210cc intake runner, a 1.55" exhaust valve, 75cc exhaust runner, and it has a 64.5cc combustion chamber. The 243/799 3cc bigger combustion chamber might not seem like much but let's look at the factory Compression Ratio as seen here.

And then compare it with the 243/799 cylinder head swap as seen here.

As you can see the compression ratio drops to 9.10:1 but can this make that big of a difference? To a degree, but there's another variable that we need to look at, and that's the combustion chamber diameter. The combustion chamber diameters can play a big role in hindering flow and performance. The 706/862 combustion chamber diameter measures out at around 3.670" and the 243/799 combustion chamber messages out at around 3.880” and giving the fact that the LM7 5.3L engine bore is 3.780" this more than likely has something to do with the power loss. This is why we strongly recommend using the factory LM7 5.3L 706/862 cylinder heads and just simply have them ported. We have been able to get 312CFM @.600" lift on average with our stage2 port job without going over 215cc on the intake runner volume and we are able to achieve a 52cc Combustion chamber bumping up the LM7 to 10.55:1 compression ratio this will help keep the engine very responsive and strong down low for daily driving, and improve your 60' track times. If you're still interested in running the 243/799 cylinder heads on an LM7 5.3L I would highly recommend milling the cylinder head at a minimum of 0.025 to bump up the Compression Ratio back to the factory as seen here.

We mill our cylinder heads in 0.025 Increments because most of the push rod companies cut their pushrods in 0.025 Increments and this makes it easier for the customer to purchase the correct pushrod length. Here's a set of 243/799 milled 0.050 as seen here.

And for the maximum compression ratio possible using the 243/799 cylinder heads, 0.075 is the limit we recommend go as seen here.

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