Why Cold Air Intakes Matter
Thanks to Brian Turner of Dynotune Motorsports (Dyno Brian) for getting my thinking cap on. Also a shout out to Dan (dsmuts), Clay (CapnBreakAlot), Matt (AlphaGP), and others who were there at the 3800 dyno day and spoke to me afterwards, plus others I’m sure I’m forgetting.
Put your nerd caps on, and hear me out.
It’s common belief on here and other 3800 boards that the intake doesn’t really matter. Even the safely modding your 3800 thread recommends you not spring the $200+ for a JMB/Wizaired/K&N intake and instead take a trip to your local Home Depot and build your own intake for $30. The dyno backs this up, as it proves little difference in wheel horsepower between an “open cone” and a Thrasher intake box.
But does that tell the whole story? My inclination is no. This is for one critical reason – the intake air temperature sensor [hereinafter referred to as IAT] is located in the intake tubing, forward of the throttle body. This is almost always the case for any 3800 motor – even my setup with a Northstar throttle body and LQ4 MAF places the IAT sensor forward of the throttle body. It is axiomatic of any tune that the computer would advance or retard ignition timing via a post-main table modifier of some sort to compensate, as a high IAT would be more prone to predetonation inside the cylinder than a lower IAT.
This is fine on naturally aspirated engines, where the incoming air comes through the throttle body, through the plastic upper intake manifold, and into the engine from there. It has little chance to heat up from either engine heat or any other reason.
However, when the car is supercharged and especially intercooled, things start to get strange. It’s been discussed ad nauseam here that the Eaton M90’s are inefficient and radically increase the air intake temperature, simply on account of it quickly compressing the air and increasing the stored energy inside of it. The problem arises here because GM didn’t relocate the IAT sensor to be after the supercharger (or after the intercooler if you have one). Meaning the stock tune can only compensate for high IAT’s in front of the throttle body, but the computer has absolutely no idea what the “true” IAT is – that is to say, the temperature of the air after it’s been compressed by the supercharger.
There’s some connection between IAT’s in front of the throttle body and the IAT at the point where it enters the engine, but short of drilling a hole through my intercooler core and inserting the IAT sensor there, I will never know for sure exactly how effective the intercooler is at cooling the incoming air. All I will know for sure is when I get KR after a sustained WOT pull and the intercooler heat soaks, the intercooler is no longer doing it’s job. But if I were to relocate the IAT post-intercooler, I could plausibly have the computer automatically yank timing once it sensed high IAT’s, precluding the need for KR. Of course, without that sensor and attempting to tune, the tuner is simply shooting in the dark – adding timing and fuel until he runs into KR and then backing off.
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To bring us back to earth – and why I will make my case for a cold air intake versus an open cone – the lower the IAT is in front of the throttle body, it obviously stands that the IAT will be lower post-supercharger (all things equal). Lower IAT’s mean less predetonation and less KR. That allows me to add timing, and thus add power for as long as the IAT remains low.
And now the fallacy behind the “Dyno Science” thread is explained – of course the car will make roughly the same power with an open cone versus a Thrasher box or other CAI setup – because the tune remained the same during all pulls. Using a CAI, the owner of the car failed to re-tune his car and add more timing to take advantage of the lower IAT… of course he’s not going to make any more power.
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At the 3800 dyno day, I had a Wizaired box on my car. It’s something I’ve always poked fun at, saying “I can’t believe the previous owner cranked out $200 on this thing when I can build my own intake for $40.” But on the rollers, I had IAT’s in the high 60’s while Dan (dsmuts) had IAT’s somewhere around 110* using an open cone.
He and I have similar builds as of November 2014 (S1x, Gen V, FSIC, headers, catbacks, 3.29 gears) but he also has additional mods (1.8 roller rockers, stage 3 heads, and a 2.8 pulley vs my 3.0 pulley). But it was my car that put out 8 more wheel horsepower on the same dyno… 323 versus 315. There are independent factors that contributed to this (he encountered significant blow-by during WOT pulls) so it may be impossible to draw any firm conclusion from this that “Wizaired added power.” But I think the opportunity to add timing due to lower IAT’s is definitely a window of opportunity to add power solely on account of a cold air intake.
Do I really think that changing your intake alone is worth the horsepower? I wouldn’t go out of my way to do it, because you would not only need to buy the cold air box ($200+) and then would need to retune the car ($100+) and may well only add a degree or two of timing. While on the dyno each degree added on my car added 6 wheel horsepower… say if a CAI enabled to add you two degrees of timing at WOT would it be worth the $300 to add 12 wheel horsepower? The butt dyno is unlikely to notice the difference, but your wallet certainly will - unless you build your car from the ground up with the intent of having a CAI and tuning accordingly.
I can think of numerous other ways to get a better return on your money, but just that it’s out there – I think a CAI can increase power over an open cone, provided you re-tune afterwards.
Originally Posted by 91parkave
