s1ngle

Yes, people have been saying the same thing over and over, but none of them really explain it and give good sources. but I will admit the last few posts are a bit better.

I have been doing some reading on the topic, and here's what i have found: the main thing about smaller turbos is the restriction in the exhaust housing. That makes it harder for the engine to get the exhaust out fast, thus more backpressure and wasted power.

However, I can't find any sources that say that a larger compressor makes denser air

s1ngle

read this and tell me what you guys get from it:

http://turbonation.com/turbo.htm

projekteg

just shut up and get a bigger turbo

s1ngle

yeah, dumbasses like you would do that wouldn't they.

projekteg

no, i already have a big enough turbo. you keep egging this thread on by saying "all of you are saying the same thing, but you're not explaining it", well, i just read this post for the first time and saw it explained to you about as many ways as it can be, all the way from an electrical fan down to a ------- coffee straw. buy 'max boost' if you want to educate yourself on cfm's, a/r's etc.

bettonracing

Ok. Lemme take a shot at this...

I'll split them up into different posts to make it easier to read.

There are a few factors that come into play:

1) Compressor efficiency at a certain airlow/psi.

eg. @ 100% efficiency and 14.7psi, the compressor would double the amount of air injected into the engine. Most compressors run at about 70% efficiency at their peak (note: most & about). This means that they'll give u about 70% of the max [effective] air at that psi (and CFM...) due to heating of air when compressing (laws of physics: it will heat up). (this is my horrible attempt at Layman's terms...) (If u disagree, we can argue it out in another post. PM/email me)

Take a look at a compressor map and u'll see that at a given airflow and psi (u have to calculate airflow using rpms and engine size [mainly] in order to use the map), the efficiency of a small turbocharger compared to a larger one, is usually lower. However, this isn't always true (being that the efficiency line is not linear). It is possible that a smaller turbo has a higher efficiency (at that rpm & psi) than a bigger turbo, and given that the CFMs are close enough for the same psi, the small turbo should out perform the bigger turbo (at that rpm/psi). (comparison criteria: Rpms are equal. psi's are equal. CFMs are close (700 to 195 will not work). small eff is higher than big eff.)

BTW, some compressor maps are given in cfm... they account for the mass by assuming constant weight or something to that effect. Similar effect to lbs/min map.

If everything I just said is jibberish (which it seems like to me after rereading...), refresh your memory on reading compressor maps then read again.

verdict: Bigger usually= better, but not always. However vast diff in size, bigger wins.

(SEE #2 in next post)

bettonracing

2) CFMs as explained before, does make a difference.

The straw analogy applies but is somewhat incorrect. What if both turbos are breathing through the same 2.5 diam inlet pipes? Well technically, if u somehow could hook a firehose to a straw, it would still flow more than a garden hose thru the same diam straw (both at the same psi). Neglecting factors such as turbulence, etc.

Same thing w/ the fans analogy. If they both blow into the same diam pipe, the bigger one will still flow more at teh same psi.

Here's a bad example that might help put things in perspective:
A building hallway. A small fan blowing air straight thru the hall at 15psi. A gust of wind blowing thru the hall @ 15psi. (bad ex but kinda conceptual).

Verdict: Bigger = better

dewme5

this argument has turned into more of which turbo is capable of more hp. Your engine has requirements. so long as those requirements are met, it doesn't matter which turbo you use to get there. Boost is created by stacking air tighter then before. 5psi in a thimble, or 5 psi in a swimming pool is 5psi. you have to remember to compare apples to apples. The turbo that is going to create that 5psi where you want it, most efficiently is going to create more power then the one that over heats the air.

so far as exhaust restriction, that's another ball game too. Just remember apples to apples here. A tighter exhaust housing .42 vs .69, is going to be more of a restriction. But, you have to have this restriction to create boost. The .42 is going to create boost sooner, but it's going to limit rpm more.

These are very simple things to figure out, you just need to know which numbers to put in place. A compressor map is going to tell you what cfm and boost a certain setup is capable of. Find out what you need, and then find the compressor to fit the bill. Then, figure out where you want you power. This is going to be a give and take situation... find out what you want. all on by 2500, and end at 5000, or all on by 3500, and good till 8000rpm. Useable torque, or racing power?

There are no right turbos for the job, but there is one closer than any other, then make sure it has ball bearings.

does any of this matter if you are going to use the first one you find in a junkyard anyway?

hotrex

s1ngle is you dont understand now, you may be handicapped.

warren83

2 mins on google and then reading a surprisingly well written article on a Saturn forum should explain it for you s1ngle if it doesn’t then no one here can help you
http://www.turbosaturns.net/articles...sor%20maps.htm

its all about the right size turbo and how to pick it ..
3 pages happy reading