turbos are generally the most efficient with 3" exhaust but 2.5" is usually ok for low boost applications. Pretty much for any motor the less backpressure the better.

boltons on i4's, epecially hondas don't do much but v6 cars can get some pretty good gains. For example the comptech headres for the cls add about 30 whp.

 

I ran against a guy with one of those. That motor will never gain 30 hp from just an ECU. The guy I ran against had a stand-alone because he had hung a T3/T4 off it with a Porsche 944 intercooler, external wastegate and TIAL blow off valve. No way a factory ECU would be able to handle what he had going on under that hood. It was good for a 14.2 at 109mph! A laggy 2.7 60' will kill anyone's 1/4 mile time. He was going 12 mph faster than me, though... if he could just make a big enough fart to spool that turbo, he'd have been dangerous. He'd have destroyed me from a rolling start.

Without the turbo, there's no way it's tuned down that much by the ECU. That 8-valve motor's lucky to have the power it's got.

 

jesus...if he could get that 60 foot down into the 2.2 range he'd be in the 12's with a 109 trap. Haha i'd love to race him and pull a 14.1 @ only 99 mph, haha how sweet it'd be to have a lower et with a 10 mph less trap

 

Stock D16Z6 Civic puts about 107hp to the wheels on a 4th gear Dynojet 248C run.

Iceman adds 3 peak hp. CAI configuration adds about 6hp at 4400rpm but the 3hp peak stays the same.

GReddy SP exhaust is about 4-5 peak hp.

DC ceramic header also adds about 4-5 peak hp.

Total gain was about 13hp to the wheels.

Drop in K&N filter over stock didn't have any significant gain. However, I suspect that the drop in filter maintains its flow characteristics over a longer period than a stock paper filter. So that should mean 1-2 hp (if that) comparing say a 1 year old dirty stock filter and a 1-year old drop in.

 

so do more expensive spark plugs or wires help HP at all

 

I'd say no. Expecially ones with capacitors in them. That's a waste... it takes power to make a capacitor discharge, and it won't discharge more than the amount power you're feeding it. Quality low-resistance wires may not degrade over time the way others do, but there shouldn't be any horsepower gains from using aftermarket wires unless the ones you're replacing are bad.

That doesn't stop me from using them. Just because they don't add power doesn't mean they're not an important upgrade. Good wires will carry more spark power, and usually last longer. Just don't expect them to make your coil put out any more power than it already does.

 

Stock wires and stock NGK's ... turbo Honda guys still use them; that tells you something