Remember that aluminum heads typically need more compression to make the same power as an iron headed engine (aluminum does not retain heat like iron). On our engines, if we don't have the additional compression, it can hurt the power output-most noticeably on the bottom end.
We can get around that somewhat by running more timing in the lower gears to get some additional heat into the combustion chamber at launch and more boost on the top end.
If we were racing theory, we would run iron heads, but aluminum has some good qualities....s
ave some weight, cheaper to cast in low volume numbers, and easier to repair.
Thinking about boost one more time-boost is a measurement of resistance to flow. It's like a garden hose. Turn it on full blast and water comes flowing out the end with only the resistance of the hose diameter/length and the flow input at the faucet to slow it down. Put your thumb over the end of the hose and suddenly you feel a lot of pressure on your thumb and the volume passing your thumb takes a lot longer to fill a bucket even tho it will spray further from the end.
If we have a turbo and a stock intercooler and we run 25 psi on the boost gauge which is tapped into the plenum, but we would see 30-31 psi at the turbo outlet if we had a gauge plumbed there.
Take a good intercooler with more volume and bigger in and out pipes and we might have to make only 26 at the turbo to get 25 in the plenum. That would mean the turbo did a lot less work to get the 25 and the air would be a lot cooler. Cooler is denser so we might actually get a lot more air molecules stuffed into the plenum than we did with the original ic that pushed the turbo side up to 30 psi which heated the air much more.
Now that we have air in the intake, we have to get it into the engine. If we port the heads well then the heads don't cause as much resistance to the flow of air out of the plenum so we are opening the plug on the other end and we may see the boost drop a pound or two yet we are actually getting more air into the engine. Bigger valves will help to let the air in...and/or a longer duration/higher lift cam that lets more air into the cylinder in a given time will do the same. If the valves did not open at all, a small turbo could make a heck of a boost in the plenum...if we took the valves out of the heads, it might not make any boost. I remember a friend of mine with a Stage engine about ten years ago that put a TA49 on it until he could get something bigger. All he could get was 17 psi of boost with the wastegate closed. The additional port side, valve size, and cubic inches was taking so much flow that the turbo could only generate that much back pressure.
These improvements may not be as obvious on a forced aspiration engine as it is on a conventional engine because there is a lot to be said for the power of a good turbo to force air past restrictions, but, it still makes it easier for the turbo to get its job done while keeping the air charge temperature down.
We have all seen a mild combination pick up a few tenths with a good set of heads...just unplugs the circuit.
Where we get into problems is by screwing up the combo with a mismatched set of parts. Put a big intercooler on the car with a small turbo and we lose velocity which hurts throttle response and may increase compressor surge at light throttle because the air velocity to the throttle body is lacking. Compressor maps seem to have a lot to do with what turbo may stall out with a given combo and we can really screw up a driver with the wrong combo. I went from a Craig 45 (T63E) that shook like a wet dog unless the pedal was pushed 3/4 down to a 66BB that has no surge at all....nothing else was changed. Front Mount IC.
On my son's car, with a slic and a TE44, it went from no surge with a stock ic to some at light throttle with the slic.
The difference between the two was on the first car, it was impossible to drive thru the surge without completely lifting and then dropping the hammer whereas on my son's car, I can come down on the gas while it is surging and it will take off.
There is a lot I don't understand about the process. I did make it better on the first car by leaning it out at part throttle...but it was no fun at the best. I think it is just the nature of the compressor map-what rpm it stalls at and the steepness of the stall curve, but I really am guessing
On a race car, we can put a higher stall converter that makes the car slushy to drive, kills gas mileage and all around kills the fun. I think race cars are easier to build because we just have to make it run well over 800-1000 rpm and we are not concerned how it runs if you drove it down the street.
I have seen more than one smart guy that went slower than guys who did not understand the theory but used the combinations of parts that worked for guys that were going fast.
