Thread: HAI vs CAI. CAI wins

Originally Posted by Mike Kopstain

Sure but as I mentioned earlier, it's all out there, both actual data and empirical data and not just with 3800s/ m90s.

BlueGTP91's assessment of hotter/ less dense air translating to less knock is just not accurate. If it was then we'd just all shut off our intercoolers in the summer and be able to run smaller pulleys because now the air is hotter and less dense... lol. It's just not accurate and it goes against basic common knowledge.

The only time I'd expect someone to have to pulley up in the winter is if they built their car in the dead of summer, when it was hot and the setup was built to be right on the cusp of knocking. Otherwise, if it's setup properly you really shouldn't have to pulley up in cooler weather.

And if we don't have an intercooler?

Then, well...you're ****ed by the laws of everything, Chris.

Originally Posted by Nixon

And if we don't have an intercooler?

It's still not true. Reduce the whole issue of knock down to this: heat. Everything you do as far as modifications go is to reduce heat/ pressure. You open up your exhaust to reduce cylinder pressure which reduces heat which reduces knock. You intercool your air charge to reduce heat, you add an aftermarket intake not only to supply the engine with more air but to reduce the amount of resistance the blower experiences when sucking in air which in turn reduces heat, you run a cooler thermostat to... you guessed it... reduce heat. Just about every supporting modification serves to introduce more air into (and out of) the engine but more importantly, reduce increased cylinder pressure and temperatures as a result of more boost.

Just make sure you guys understand what's causing knock though. It's heat... It's a fuel mixture igniting because of cylinder pressure/ temperature driving it to ignite prior to the spark plug igniting. When I see someone running a 3.0 pulley and negating the importance of inlet temperatures it just tells me that they don't get it. Not only is it important but exponentially more so on a setup running a 3.0 pulley non-intercooled. It's just the opposite of reality. Maybe the sky is purple in your world. :-)

So when the outside temperature drops, the drop in temperature should offset the increase in density, thus negating the need to up a pulley size or two. So yes, all things equal, the denser air is more likely to result in knock but in order for the air to be denser it also needs to be proportionately cooler which offsets any issues on all but setups that are either a. improperly tuned or b. on the edge of experiencing knock even in the best of conditions.

Here's an article I wrote on this a bunch of years ago:

Knock Retard and you... - W Body Store

At the end of the day I'd encourage everyone to do their own research. Don't listen to me and don't listed to others. Go out and read, learn how this stuff works on your own, what causes what and all of the sudden you understand how A affects B and B affects A and how one can't change without affecting the other.

Originally Posted by SnowFlake

There's more to consider here than just a bad tune or a car that's setup on the verge of knocking. Which is basically where you want to be since the car will make the most power there... anyway, there comes a point in an m90s life where it'll heat the air more than the inlet temps change. Not to mention there will be more drag on the motor and more boost pressure, further exacerbating the situation.

Sure but you should be constantly scanning. People make the mistake of setting up their car one day, tuning it, not seeing any knock and then assuming they have a knock-free setup. It literally changes day to day and you actually don't want to be that close because it leaves no room for error. As far as the M90 heating air more than the inlet temperature changes, agreed but again, this is precisely why you want to ensure that your inlet temperatures are as low as possible to begin with. Why would anyone intentionally handicap their own setup? :-)

In the winter, you can pulley up and STILL make more power than you did in the summer.
I remember my genV setup made big numbers uncorrected in the winter on a 3.5 pulley. In the summer I dropped to a 3.2 and could still never come close to the uncorrected winter numbers. Although the SAE numbers were all very similar, with the edge going to the summer numbers.

Be that as it may, it still isn't contrary to the point I'm trying to make. I think that just goes to show that a non-intercooled car is almost always going to have a larger performance change from temperature differences than an inter cooled car.