Any dyno comparison info on 4.8 with 799 vs 706/862 heads?
03-13-2017, 08:07 AM
#1
Any dyno comparison info on 4.8 with 799 vs 706/862 heads? Which combo do you think would make more low end power (say below 4k)? 9.5:1 and small valve heads? Or 8.8:1 and large valve heads? Curious if the 799 is an “all around” upgrade or if you lose low end power compared to the smaller valve/CC heads.
It looks like the newest 4.8 (L20) comes with 799’s and is 8.8:1 compression VS the older 4.8’s (LR4) running the 706’s at 9.5:1. I’m guessing if GM decided to bolt the 799’s on the newer engines (even with the compression loss) that they didn’t give up any low RPM performance?
Reason I ask is I have a 4.8 short block. I’ve got a set of 799’s and 862’s. Will have a 216/222 @ 111 cam. .56x lift with 1.8 rockers on an LS6 intake. This will be a turbo setup. I’m running a pretty darn tight converter though (9.5 PTC with 14 stator) and a large 1.32 T6 S480 turbo.
My previous 4.8 (same tight converter) with 706 heads would barely light the 1.25 T4 S476. With a 212/212@112 .565”. But I ran a 2.5” hotside then. Now I run a 2” divided hotside. I’m not really worried about overall peak power as much as being able to get into boost initially. (trans brake drag car)
It looks like the newest 4.8 (L20) comes with 799’s and is 8.8:1 compression VS the older 4.8’s (LR4) running the 706’s at 9.5:1. I’m guessing if GM decided to bolt the 799’s on the newer engines (even with the compression loss) that they didn’t give up any low RPM performance?
Reason I ask is I have a 4.8 short block. I’ve got a set of 799’s and 862’s. Will have a 216/222 @ 111 cam. .56x lift with 1.8 rockers on an LS6 intake. This will be a turbo setup. I’m running a pretty darn tight converter though (9.5 PTC with 14 stator) and a large 1.32 T6 S480 turbo.
My previous 4.8 (same tight converter) with 706 heads would barely light the 1.25 T4 S476. With a 212/212@112 .565”. But I ran a 2.5” hotside then. Now I run a 2” divided hotside. I’m not really worried about overall peak power as much as being able to get into boost initially. (trans brake drag car)
03-13-2017, 12:46 PM
#2
Not exactly apples to apples... but, I will be putting 241s on our Gen III 4.8L. I will take a lot more cylinder filling ability over a minute amount of compression.
Also, I plan on doing home brew thick MLS gaskets using the guts of the old gaskets. Drill out the rivets and stack 'em up. Might lose a bit of compression there too.
Also, I plan on doing home brew thick MLS gaskets using the guts of the old gaskets. Drill out the rivets and stack 'em up. Might lose a bit of compression there too.
03-13-2017, 01:08 PM
#3
I would too generally, but this is a temporary engine being dropped in front of a converter that’s WAY too tight. Speced for a motor that easily made 400ish ftlb at low rpm. It might still be fine with the smaller 2” hotside. Hoping I can make it work without nitrous. I didn’ta want to have to deal with the weight and hassle.
Heck I wouldn’t have hesitated to throw my 317 heads on it if I could drop a loose converter in there. Hoping to have a LSX block in it come next spring though so I didn’t want to buy another $1000 converter.
-thks!
Heck I wouldn’t have hesitated to throw my 317 heads on it if I could drop a loose converter in there. Hoping to have a LSX block in it come next spring though so I didn’t want to buy another $1000 converter.
-thks!
