pressure vs volume

[Jim Blackwood]

On another (blower) forum we got to discussing boost pressure vs HP increase. Disregarding for a moment intercoolers and such, one fellow's theory was that to double flow volume pressure had to increase by a factor of four, and he based this on some flow bench work. I may not entirely agree with his theory, but I figured you guys would know if he was on the right track or not. Is there a reasonably close way to estimate flow volume in relation to pressure? THX

Jim

 

[Ormand]

Sounds like a mixture of apples and oranges and turnips. The pressure drop across a restriction, like the intercooler, or the throttle body, will increase by a factor of four when the VOLUME flow rate doubles. That's because pressure drop is proportional to Vsquared, and velocity is directly related to volume flow rate.
But boost pressure increases density, so you get more lbs of air, at a give volume flow rate. At any given rpm, the volume flow through the cylinders is fixed. (look it up, it's not hard to find) An increase in boost will raise the density of the air, but it won't change the volume, except on the suction side of the blower. If you want more VOLUME flow, you have to increase either rpm, or the displacement of the cylinders.
Think about it this way. Every time the piston gets to BDC, the cylinder is full of air/fuel mixture. And every time, the volume is exactly the same as every other time. The density will vary because the pressure in the cylinder will vary, but the VOLUME will be exactly the volume of the cylinder. The only way to get more VOLUME flow is to A) have bigger cylinders, or B) fill them more often- higher rpm.
Bottom line- increasing boost increases density, and mass flow- Increasing rpm increases volume flow, and that, in turn, increases the pressure drop across flow restrictions. More rpm, you might need bigger valves and ports. More boost, you just need better gas!:smile:

 

[VadersV6]

Bottom line is, get more O2 molecules in the cylinders. Volume flow isnt restricted to more rpm or more boost. If you can get more volume flow out of the heads/intake, then at boost for boost (same boost before and after), the improved heads, which are giving more volume flow than before because of their larger ports/valves, etc, will automatically provide more mass flow, or O2 molecules. Think of the intake runner volume. You have 200cc's of air at 20psi vs 150cc's of air at 20psi. The larger runner automatically has more mass flow because of the increased volume. Say under 20psi, there are 1 million o2 molecules in a given cc of intake runner, and this wont change no matter what size your heads are. Say you increase the size of the intake runner from 150cc's to 200 cc's. Now those molecules have somewhere else to go, so now you have 700,000 molecules per cc of intake runner. As a result, the boost, or pressure, or restriction has dropped, to say 16psi instead of 20. Crank the boost back up to 20, so now you're back to 1 million molecules per cc, but now you have a 200cc runner. At 20psi, you will have 50 more cc's of 1 million O2 molecules per cc, being pushed into the cylinder. The mass of air has increased, because the volume was increased. Now if you had never touched your heads/intake, the only way to increase the volume would be to increase rpm. Turning the boost up wouldnt do it. It would only increase the mass of air, not the volume. BUT, given the additional compression of the air, (higher boost) you would heat the charge up significantly, because now you're trying to squeeze 1.3 million o2 molecules into a given cc of runner volume. given the additional heat, the charge will be "thinner"..given the math you would think you have 1.3 million, but its more like say, 1.2. Add even more boost for another .3 million...but its not working. this time it only increased by 1.0 The next increment in boost yields another .5. It gets less and less efficient as you try to squeeze more air in through the same small intake runner as before. Thats what the compressor map will show. It will show that you can make say, 600hp at 38psi, and you can also make 600hp...on 23psi...with the same exact turbo. Which is better? Obviously the lower boost number. Much easier on parts and engines. Less octane requirement, less IC requirement, more timing (which = more power) etc. But the only way to tap into the efficient range of the turbo, is to tap into the efficiency range of cylinder heads. You'll need to open things way up and get those heads and intake to flow flow flow. The more power you can get out of an N/A engine, the greater the gains will be under boost. The rough formula for boosted power is boost divided by 14.1 + 1 times N/A power. 20psi/14.7+1= 2.36. This is your pressure ratio, which you will see on a compressor map. Multiply that ratio by your N/A power, and thats roughly what you'll end up with. I used this formula to find out that our engines, if the 275hp number was real (not the advertised power), then our engines would be making roughly 137hp in N/A form. Increase the boost on that stock engine to say 20psi. 2.36 x 137.5= 324.5hp. Now if we opened things up so that the engine makes 180hp in N/A form, then the power at 20psi would be- 2.36 x 180= 424.8hp. Same engine at say 15psi- 15/14.7+1 = 2.02 pressure ratio, which says 2.02 x 180= 363.6hp. So by increasing the power of the N/A motor by 42.5hp, we have an increase of 363.6 - 275= 88.6hp...all at the same stock boost level.

 

[RUQWKNF]

Both of you, those were excellent replys. Very informative and easy to follow.
Gotta love those pressure ratios and compressor maps. Now the phones will be ringing off the hook at all of the turbo suppliers, from people wanting compressor maps for the new series of turbos. :biggrin:

Good job guys.

Patrick

 

[turbo2nr]

Look at any speed-density ECU.

MAP is used to help figure out the VE of engine, for fueling, kinda.

 

[VadersV6]

By the way, the cylinder volume is fixed, and what Ormand said is true about that, but assuming that the cylinders are being filled to capacity, in terms of volume, before and after, is misleading. The boost reading, or measure of restriction, is in the intake plenum, not the cylinders. The cylinder heads and intake are the bottleneck, no matter how good they are. This is what creates the restriction, which is what gives you your boost reading. You only have so many milliseconds to fill a cylinder. In say 5 milliseconds, you can push 1 billion o2 molecules in the cylinder. Up the boost to increase mass flow, and the heads prevented the cylinders from filling as much as they could, had the intake runners been opened up.

 

[Jim Blackwood]

Man,.. I knew you guys would have this stuff down. Yer good, no question.

Might as well give you the scenario just for fun: I built a 215 Olds blower motor using an Eaton M-90, EFI and an experimental intercooler using a 2.32:1 overdriven blower drive ratio. I used stock Olds heads and a stock cam and the result is that boost comes in fast and early, and builds to 15psi+ as I'm getting up in the revs, but with 8.5:1 compression the motor is handling it fine even with a massive power output and a very wide powerband.

Now if all that boost was getting to the cylinders it'd blow that motor sky high I'm pretty sure, but the Olds heads were the most restrictive of all 215/Rover heads and my theory was that the port restriction was keeping cylinder filling low enough at speed to save the engine but offering little restriction at low speed and lower boost levels and causing the powerband to be unusually wide, in effect giving a considerably larger volume "under the curve". Those guys didn't buy it though, claiming what I really needed to do was port the heads and reduce boost. I'd have gone along with that if I was anywhere near the capacity of the blower but I'm not. It's good for up to a 350 cu.in. engine.

So what do you guys think? Did what I built account for the results I'm seeing, or am I just mis-interpreting the results?

Jim

Oh, another issue was that the high pressure air would be hotter when passing through the intercooler thereby dropping more heat, and also that there would be an orifice effect as the air left the runner and entered the cylinder causing some amount of cooling, so that the net heat buildup would come from eddy recirculation around the rotors which might be offset by increased intercooler efficiency. Can't wait to hear what you guys think about all this.