So I want to port and polish the head on my 05 Civic with a D17A VTEC engine in it. My questions are:
1. Is this a DIY project?
2. How many horsepower could I gain by porting the head on a Civic with an upgraded intake pipe, D16Y8 intake manifold, DC Sports header, 2.25 inch exhaust and MAYBE a Crower Stage 1 camshaft in the future?
3. What effect will porting the head have on gas mileage?

Thanks in advance as always!

 

1. It can be done DIY if you really, really, know what you're doing or spend tons of time doing research and making sure you do it right. Mistakes will be costly in several ways.

2. I'd guess 20 HP or less. Most definitely less.
3. Probably reduce your fuel economy as you would likely change the engineered air flow characteristics at lower engine RPMs - making for more poorly mixed air/fuel charge - which leads to worse horsepower - and worse fuel economy.

You're unlikely to gain any kind of desirable amount of horsepower from doing this. The D17 is not a good platform for all-motor builds. You will spend tons and tons of time, probably a lot of money, and will end up being unsatisfied and underwhelmed with the results because you will not see a good improvement in power.
If you want performance - especially if you have a D16Y8 intake manifold - go forced induction (turbocharger). It will be cheaper, and faster, and give you an actual performance improvement over these bolt-ons with a port and polish.
Forced induction spits in the wind to a port and polish – it’s the only real way to make any good improvement in power on a D17 - or really any engine.

Here's some reasons why this is a waste of time.
Porting and polishing is a much more difficult and scientific process than it may seem. You have to understand flow rates, fabricate a flow bench, be extremely careful to make all ports as close to one another a possible. Don't remove too much material or you're buying a new replacement cylinder head etc.



Messing with the way the cylinder head was designed by actual engineers is a great way to ruin your car's drivability. It's possible you'll have even less low-end torque, and a very small top end improvement. I take it this is not a dedicated track-only car you are thinking of modifying, and you just want more performance out of a daily-driver.


Think about what porting and polishing does. You are trying to optimize the intake path so that you can maximize the amount of air that enters/exits the engine.
You need to be able to measure EVERYTHING to get ANY results. If you don’t, you WILL optimize the wrong thing and get no benefit from your work.


How bad do you think the current flow is and at what RPM? Are the intake/exhaust ports actually the bottleneck point for flow? Are the valves in the cylinder head even large enough to permit a 100% volumetric efficiency at maximum engine speed? Where does it need to be optimized? Would you benefit from larger exhaust ports or larger intake ports? And on and on..

Let's just say for example (these numbers are totally made up) that all before 4500 RPM your engine is running at 100% volumetric efficiency. Meaning - it's getting the maximum amount of air into and out of the cylinders. Thereby making a port and polish 100% useless as you can’t normally exceed 100% volumetric efficiency with a naturally aspirated engine (and especially NOT with a economically designed gas-sipper that is the D17).
So if this is the case - how much do you expect the efficiency would drop at higher RPM? How much can it be improved by porting the head and adding bolt-ons?
Again - making up numbers - but using basic math/logic.
Let's say the volumetric efficiency tapers off to 80% at 7000 RPM. So the maximum improvement that could be made for volumetric efficiency at the very end of the RPM range would be 20%. A port and polish job will not be 100% efficient - so let's be generous and say you gain 10% more volumetric efficiency at 7000RPM. That would mean that your actual gain from 4500 RPM - 7000 RPM slowly increases from 0% to 10% - the gain is inversely related to the original loss. Doesn’t sound very great, does it?

So the question is - is it worth it to MAYBE gain up to 10% at the very top end of your engine speed? To likely sacrifice other things as well? Your fuel economy and torque may be reduced at lower RPMs? You will spend a lot of time and over a thousand dollars for what I would say is no reasonable gain.

If you want real performance and real improvement in driving enjoyment, get a turbocharger. It will improve performance at basically every part of the rev range, give you significantly more horsepower than these suggested parts and labor, give you a much more enjoyable car to drive at any speed or RPM. And will not require a port and polish job.

 

Thank you for the extremely detailed response. That is pretty much what I thought, after all if it was that easy to improve the performance and it had no significant downsides, they would probably be doing it from the factory.