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Suck vs Blow HSFs : A poor study

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Inactive Moderator
Feb 13, 2001
Twin Cities
I spent almost an hour typing this once already, but it disappeared while I was spellchecking/editing it <(

So, here we go again <(

As the title implies, this was not a good study. My HSF, the Swiftech MC-462A is not your typical unit. It is way overkill for a 50W Duron setup. Both the HS and Fan are suited for more severe duty. The HS alone, without a fan will hold my Duron at 600 Mhz to 42C. Yes, just convective cooling. Alright, i admit, the weenie fan on the Northbridge chip next to it does stir a little bit.

My interest in pursuing this came from an experience I had two nights ago running a 156W peltier and using the 462A to cool it. The 462A was up for the job, but being a blow type HSF, it was washing my mainboard with hot air. My two Sunon case assist fans tried to maintain a flow-through environment, but the 68 cfm fan on the 462A simply overwhelms their effort. this happens all the time, but normally, I'm not trying to remove so much heat and it is not a day-to-day concern. As a matter of fact I simply love this big brute and highly recommend it.

HSFs for socket A boards come in two flavors. Suck and Blow. Any HSF will yield about a 3-5 degree drop in core temperature when in the blow mode as opposed to suck, but at a price to the system circulation and subsequently, temperature. The 462A is no exception to this rule.

In setting up the 462A to suck, I kept in mind that it was not designed that way from the get-go, though a separate model variation should be. Are you reading this Swiftech?

In order to successfully suck, an HSF needs a shroud. Any fan when sucking air off the HS will try to get it from the shortest path of least resistance. This typically means it will try to pull air from the area of the fins/pins immediately below it, where they are the coolest, as opposed to deeper down near the baseplate, where the heat coalesces. By using a shroud that protrudes partway down the outsides of the HSF, you encourage the air to be drawn into it nearer to the baseplate and then it travels up the fins/pins to the fan intake.

As a secondary function, the fan that sucks air from the HS and exhausts it upward, will actually work better in mid and full tower cases where there are two assist fans. These fans are typically deployed on the lower front and upper rear, with the lower front blowing air into the case and the upper rear sucking it out. On my KT7 mainboard, the location of the CPU socket is such that the HS fan is only an inch or two from the upper rear exhaust fan. When the HS fan sucks air off the HS, it exhausts it right next to the intake of the rear fan, which is more than happy to take that air and blow it out of the case, as opposed to both of them fighting one another for the air when the HS fan is blowing down into the HS. The result is actually synergistic. My rear case fan actually sped up a little when the HS fan was sucking.

So, now I hope I have given you a good mental picture ow what's going on in there. If you are fighting, trying to get your mid or full tower case temperature and subsequently system temperature down, consider either purchasing a HSF that sucks or modding the one you already own to suck. Fashioning a shroud is not difficult sheet-metal work. Materials are readily available from the junk box or the local hardware store. A ruler, tin-snip and drill are just about all the tools you need. Give it a try and see what happens. There are worse ways to spend an evening.

I wish to apologize for not having some fun playing with the terms suck and blow during the course of this rather lengthy exposition. ;-)

good work hoot. my fan blows (by which i mean blows onto the heatsink, however it is intel stock so it blows in that manor as well, anyways) so what your saying is For better all around cooling it is better to make a shroud and have it blow? Becuase if so thats what i'll do with my next puter. Thanks hoot, youve tackled the subject in a very through manor.
According to more than one hands on experiment with the fan on an Alpha PEP-66, the HSF cools the CPU more efficiently when it's set to draw air up and through the HS. As per the instructions that come with the HSF. If I remember right, the processor ran 1C hotter when the fan was set to force air down and through the fins of the HS.

I should include the fact that these tests were performed on the PEP-66 in its original configuration. That is, hanging on a Slocket.

Unlike far too many companies in the IT biz, Alpha spends a lot of time and money on the design and development of their products. While the arrow on the instruction sheet that comes with PEP-66's is dang near too small to see, it points up for a good reason.

From Hoot's post about the Swiftech MC-462A, it looks like this outfit takes their R&D very seriously too!
There was a time when the Alpha's were the most expensive HSF's. Am I that old?
I just remembered a part of my original post I forgot when I had to retype it all over.

You may not be able to simply build a shroud and flip your existing fan over in order to try sucking air from the HS. Adding a shroud and drawing air up through the HS adds a certain degree of resistance to moving the air and consequently back pressure. Unless designed to tolerate back pressure (read thicker) a fan in the 10-25mm thick range will break into turbulence, so some thought needs to go into the mod before you try it. If you have a suck mode HSF and want to try it in blow mode, remove the shroud first. They don't lend well to blow mode. To reitterate, suck mode works best in a mid or full tower case with an assist fan high up on the rear wall.


im only 14 so i dont understand 1/2 of what you just said.....so i think blow it shall be. Seems a lot easier and for me personally i dont think its worth the hassle for 1C improvement.

(dont bother trying to explain it to me im a lost cause.)
I'll try anyway :)

The issue is about improving overall case and mainboard temperatures. Where they lead, the CPU will follow. If you're in the market for a new HSF, consider one that sucks air out of the HS. In mid and full tower cases, it will naturally enhance heat removal from them via the high, rear exhaust fan. That's all.

I just remembered another thing.

The noise level of the fan will be greater on suck than blow because sound propagates in air and since on suck, the air is being discharged out the top, the HS fins/pins are not muffling it as much. With my 462A, it was pronounced enough to notice right away.