Better Spot

[mygm]

which spot is better for a maf, before the turbo or after before the TB

 

[qwks6]

In theory they would both be the same, right? The mass of air should be equal, since you are not creating more air mass with the turbo, rather "pulling" more air and pressurizing it into the TB.

I think it looks better before the turbo, and it's probably easier to install there...

Just my thoughts

 

[DailyDrifter]

Install XFI and put the MAF on a shelf in you garage:smile:

 

[ijames]

The stock maf has to be preturbo or it will explode - can't take the heat and boost. With a translator and lt1 or ls1 maf you can do either. After the turbo gives better spoolup. I have my lt1 maf in my frontmount uppipe just below the top of the radiator.

 

[qwks6]

Interesting, why does it give you better spool up after the turbo? Does pressure have something to do with it?

 

[ijames]

Gets rid of a preturbo restriction. Turbos can blow through a restriction much more efficiently than they can suck through the same restriction.

 

[qwks6]

that makes no since to me, please explain. The way I see it, and I could be wrong, you only push as much air as you suck, so both values are equal. I still don't see how this helps spool up either. With a restriction on either side you are only pushing that volume of air, meaning you can't push more than you pull and can't pull more than you push (unless you are leaking somewhere).
I'm trying to see the point. Maybe it does help spool up. I'm not trying to argue, I would just like to see why it would.

Thanks

 

[Phoneguy]

Install XFI and put the MAF on a shelf in you garage:smile:

+1 :biggrin:

Bryan

 

[ijames]

that makes no since to me, please explain. The way I see it, and I could be wrong, you only push as much air as you suck, so both values are equal. I still don't see how this helps spool up either. With a restriction on either side you are only pushing that volume of air, meaning you can't push more than you pull and can't pull more than you push (unless you are leaking somewhere).
I'm trying to see the point. Maybe it does help spool up. I'm not trying to argue, I would just like to see why it would.

Thanks

I slept as much of Sunday as I could so this is a little late, and I'm going to be a little lazy and point you to wikipedia for background, and this is going to be a little terse but I hope it is sufficient so you can follow the logic . Go to Orifice plate - Wikipedia, the free encyclopedia and look at equation 2 (and the derivation if you want it). This is an ideal equation but for a small restriction and small pressure range it gets the point across. Basically, the mass air flow through an orifice is proportional to the square root of the density and the square root of the pressure drop, and rearranging, the pressure drop is proportional to the mass air flow squared divided by the density. At the turbo inlet the pressure is approximately atmospheric, or 15 psia, and at 30 psig boost the absolute pressure is 45 psia and the density is thus (assuming constant temperature to make it easy; yes the air will be hotter but let's focus on one thing at a time ) 3 times higher. That higher density means that for the same mass air flow the pressure drop will be one third as much, so a post-turbo restriction causes less of a pressure drop. Even more important, though, is the effect on turbo efficiency. The y axis on compressor flow maps is the pressure ratio, p(out)/p(in), and you can already see that making a small change in the numerator has a much smaller effect than a bigger change in the smaller denominator (remember p(in) will always be smaller than p(out) when making boost). Using p(out)=30 psig=45 psia and p(in)=15 psia the ratio is 3. Now put a restriction in the inlet that causes a pressure drop before the turbo of 1 psia so p(in) is now 14 psia, and to keep the boost the same the ratio now has to be 45/14=3.2, which is pretty big vertical move on a typical compressor map where you are already way off the island of best efficiency so you are working the turbo substantially harder. If you keep the ratio the same the boost will fall to 3*14=42 psia or 27 psig, a 3 psi drop. Put that restriction after the turbo and keep the turbo pressure ratio at 3 and the boost will fall to 44.67 psia or 29.7 psig, a hardly noticeable change. Or turn the ratio up to 3.02 to keep the boost at 30 psig, and again, the change is hardly noticeable. Hope that is enough, and understandable, since I have to get back to work now.

 

[Blazer406]

Carl,

You never disappoint......as a mechanical engineer..... I "read" your posts as always informed.... and methodical......you try and educate the people that want to be educated.... on these turbo Buick subjects....

Great post as always.

Keep it up.

 

[qwks6]

Very interesting...

Thank you for the detailed answer.

It brings up more questions now! Off to start a new topic...

 

[mygm]

I slept as much of Sunday as I could so this is a little late, and I'm going to be a little lazy and point you to wikipedia for background, and this is going to be a little terse but I hope it is sufficient so you can follow the logic . Go to Orifice plate - Wikipedia, the free encyclopedia and look at equation 2 (and the derivation if you want it). This is an ideal equation but for a small restriction and small pressure range it gets the point across. Basically, the mass air flow through an orifice is proportional to the square root of the density and the square root of the pressure drop, and rearranging, the pressure drop is proportional to the mass air flow squared divided by the density. At the turbo inlet the pressure is approximately atmospheric, or 15 psia, and at 30 psig boost the absolute pressure is 45 psia and the density is thus (assuming constant temperature to make it easy; yes the air will be hotter but let's focus on one thing at a time ) 3 times higher. That higher density means that for the same mass air flow the pressure drop will be one third as much, so a post-turbo restriction causes less of a pressure drop. Even more important, though, is the effect on turbo efficiency. The y axis on compressor flow maps is the pressure ratio, p(out)/p(in), and you can already see that making a small change in the numerator has a much smaller effect than a bigger change in the smaller denominator (remember p(in) will always be smaller than p(out) when making boost). Using p(out)=30 psig=45 psia and p(in)=15 psia the ratio is 3. Now put a restriction in the inlet that causes a pressure drop before the turbo of 1 psia so p(in) is now 14 psia, and to keep the boost the same the ratio now has to be 45/14=3.2, which is pretty big vertical move on a typical compressor map where you are already way off the island of best efficiency so you are working the turbo substantially harder. If you keep the ratio the same the boost will fall to 3*14=42 psia or 27 psig, a 3 psi drop. Put that restriction after the turbo and keep the turbo pressure ratio at 3 and the boost will fall to 44.67 psia or 29.7 psig, a hardly noticeable change. Or turn the ratio up to 3.02 to keep the boost at 30 psig, and again, the change is hardly noticeable. Hope that is enough, and understandable, since I have to get back to work now.

wow thanks alot for the detailed answer:biggrin:

 

[84BuickGNYorkPA]

Sounds like a great selling point for going to speed density.... thanks Carl.

Chuck