Lbar

This is a question you probably don't get often. I'm thinking about sending the heads on my 05 CTS-V to Advanced Induction for a 226cc port and have them milled .0020 or .0030. I'm on the fence though, I do a good bit of driving and I'm really into optimizing mileage. If I could pick up a couple mpgs I would go through with it. If I'm going to lose some then it's not worth the trouble to me.

I know normally a small compression bump will increase efficiency. I'm more curious about the head flow changes hurting me while lugging around. If that is even a legitimate concern. It has 156,000 miles and the mods are 220/226 580/580 cam, fast 92, ported ls2 tb, long tubes, cai. I have the tune set with aggressive dfco and my best full tank average has been 27mpg. Pure highway it would be at 30 or higher as it sits I believe.

01_SuperSlow

iTrader: (7)

That is a great average mpg! I get 27 mpg highway only with my 05 V.

My guess would be with a mild cam like what you have now and a bump in compression, it would stay the same or add 1-2 mpg hwy only. I'm considering AI as well in the near future.

Lbar

The reason I get the extra is I made sure the deceleration fuel cut off is set really aggressively in the tune. It comes on in every gear except first and stays on until 1200 rpms. I can get some serious distance with no fuel being used. From what I understand Cadillac didn't use dfco in the factory tune, it takes away from that smooth coasting they want. Most tuners don't bother with dfco much because the customer doesn't care about gas mileage.

CAMSTER

iTrader: (10)

I would not mill the heads at the mileage point you are in, to further punish your compression rings....

Whistler

iTrader: (46)

The difference in economy with that small a change in compression will be unnoticeable if anything. You may be burning a more complete fill triggering a slight change in O2 readings but any observable changes are more likely attributed to your driving habits as a result of the extra power provided by the heads.

My guess? You get worse around town because you'll drive more agressively, and the same on the hwy with the cruise on.

SEMI-RETIRED

Milling .002-.003 would be basically a gasket surface clean up barely any milling so the compression bump would be minimal for sure.

Mike454SS

iTrader: (4)

In my experience, with enough attention to detail, ANYTHING that helps the engine breath easier helps MPG...there's less "pumping loss" to create the power necessary to maintain speed pushing the car down the road.

As Whistler pointed out, we all tend to get heavier on our right foots as we make the cars more fun to drive though.

My bet is that since it sounds like you tune the car yourself, you will properly optimize the VE and MAF stuff when the heads change so that the car is able to predict and transition fuel properly (what it has to do before closed loop can correct the steady state stuff) and you will probably also properly tweak in the spark advance tables for the change in the combustion chamber...so my bet is you'd increase MPG. I'm going to send you a PM.