Head Swap vs. Porting

[therealsethallen]

Alright. I’ve got a 2005 97x. It’s a 4.2 with the 275bhp version of the LL8. I feel like it’s really lacking some oomph. It’s a lovely drivetrain, but I feel like it needs some more. I’m thinking of swapping in the head from a 2006-09 truck, Or having mine sent out to be ported. Along with this I’d get a brand new exhaust manifold and port that out as well. I’ve heard rumors of 50hp gains from this. Sometimes with a tune up to an extra 75hp. Although this is just from what I've heard.


Thoughts on this. Or should I just do the tune and call it a day? I mean, it is a ten year old SUV with 123k on the clock.

 

[gmcman]

Competition Cams will provide a cam set or a least grind a set of cams and they have the specs for the aftermarket cams and their performance grind. I believe these cams with the port and polish on the 06+ heads can net around 50 HP.

This is heresy but I have read this a few times.

The cams are pricey, a little north of a grand.

 

[therealsethallen]

Competition Cams will provide a cam set or a least grind a set of cams and they have the specs for the aftermarket cams and their performance grind. I believe these cams with the port and polish on the 06+ heads can net around 50 HP.

This is heresy but I have read this a few times.

The cams are pricey, a little north of a grand.

Can't say I'm really in the market to spend that kind of dough. Right now I'd like to focus on suspension upgrades.

 

[Pittdawg]

The larger heads were responsible for the majority of the hp increase to 291. That's 16 more hp than yours add a tune to the heads and that's good for at least another 15 and now your at 306 from 275. Noticeable difference.

 

[HARDTRAILZ]

Not sure where you heard those kinds of numbers, but I would just tune it and spend the money of the suspension you want.

 

[therealsethallen]

Not sure where you heard those kinds of numbers, but I would just tune it and spend the money of the suspension you want.

That's the plan. If (lets pray it doesn't happen) the head gasket goes, I'll probably swap in an 06+ head. But that's it.

 

[limequat]

The numbers come from efi-diy who dyno'd (ENGINE dyno) a 4.2 with a ported head, hot cams, NO vvt, and low compression. His numbers were ~350 hp. Compare that to a stock engine dyno of 290.
We don't really know how much of that is cams and how much of that is exhaust porting. We also don't know how much MORE it would have been if he had used stock compression.

I will say this, with all due respect: replacing the head for a 16 hp gain is a fool's errand. First, you won't even get 16 hp, as some of the 16 hp from 2005 to 2006 comes from increased compression. Second, that increase in power only comes north of 3500 - 4000 RPM. Under that, there is a torque loss. Most people never venture north of 4000 RPM. Finally, swapping heads on the atlas is no small job, even for skilled mechanics with the proper tools. You will break head studs. You'll probably break exhaust studs too. Are you prepared to redrill and re-tap the block? Pull the oil pan to reset the timing chain?

I love the 4.2, but in the OP's shoes, I'd be selling it and getting a 97x aero with the 6.0 and 400 hp out of the box.

 

[triz]

Not sure where you heard those kinds of numbers, but I would just tune it and spend the money of the suspension you want.

I agree there. I would probably think somewhere around 25-30HP with a proper tune.

 

[midnightbluS10]

Just a honest question but...


What makes you think you need more flow through the intake and exhaust? Have you had your stuff on a flow bench? Does it start backing up around .400 lift? Stop gaining and outputs the same cfm numbers for .300, .400, .500 lift?

So what made you think you need to open it up? I'm curious. It's quite possible that porting a stock manifold for a stock type build will do more to hurt than help. Especially if an amateur does the porting. Lose enough airspeed and it won't matter how many cfm it flows. No airspeed means no power.


Porting intake manifolds, heads, etc is a science. It's not just about hogging it out to the biggest you can. I've seen Intakes come back flowing less cfm, yet make more power. It's all in how you do it. You can take an intake from the best porter in the world and try to copy it... And never come close to what they've done. I know a lot about cars. But I also know when I don't know enough and it's time to let the pro's take over. Porting is one of those things. Even trying to gasket match the entrance can have negative effects if you don't know what you're doing.

Which all comes back to why would you think a 50hp increase would need intake/exhaust porting.

 

[northcreek]

I love the 4.2, but in the OP's shoes, I'd be selling it and getting a 97x aero with the 6.0 and 400 hp out of the box.

The popular saying in my Grandpa's day was "six in a row, will never go".