243's on a 5.3 from the factory?
 

Not really an important, but I picked up a new 2008 Avalanche in December taking advantage of GM's Red Tag event. Anyway, I was under the hood doing it's first oil change and I noticed that it has 243 heads on it from the factory. It's the 5.3 with "Active Fuel Management" and it's 2wd which is rated at 320hp as opposed to 315hp for the 4wd. I thought the 5.3's had different heads with smaller valves and chambers? Anyone else ever notice this?

Yep, The 07+ Gen IV 5.3's will have either a #243 or #799 casting. Both are the same port, flow, etc... Just different castings. Most of the reason for the 07+ HP and TQ increase over the #853 castings.

Mine has the #799's and runs pretty nice for a stock truck.

I guess you really do learn something every day. Thanks!

Yeah, the 5.3's in Grand Prixs and Monte Carlos/Impalas also have 243 heads.

Anyone know if the chamber sizes have changed from a Truck #243 vs a Vette #243?

Or did GM just lower compression a bit but pick up power from increased flow capability?

yup mine has 243 also, and its flexfuel and dod

+1 ttt

Some 06 models got them too! mostly 799 from what I have seen. But The Vette or GTO's W/LS2 are the only ones that got the Hollow stem sodium filled valves. Same combustion chamber and valve size though

They are the same 64 cc chambers, and the same 2.00 intake and 1.55 exhaust valves as the vette 243s. Everything is identical to them except that the valves are solid and not sodium filled. I just bought a 2007 engine and sold the heads on it so i had to take measurements and such to be certain. I didnt measure the cc volume but i know they are.