davepl

I am building a 502, starting with a ZZ502 short block. The cam in that motor is only something like 214/220 duration and .540 lift, and the compression is around 8.6:1

I went with a 2000cfm throttle body and Trick Flow 340 rectangular port aluminum heads. Normally these would be too big for a 502 with that cam, but I am working towards adding an F2 supercharger.

My thinking was that having to flow two atmospheres is harder than just one, and therefore, maximizing the induction beyond what you'd do for an NA motor makes sense to me.

Thoughts?

BTW, yes, I know I could gain a lot by a bigger cam with more lift, but the ZZ502 lifters and so forth are limited to about 570 anyway, and I don't want to screw up the idle characteristics.

engineermike

Actually, your thinking is sort of opposite.

A cylinder head that flows enough for 500 hp at zero boost, will support much more with boost. For instance, a good ported LT1 head will support 550 hp without boost, but people have made 1200 hp with 22 psi boost.

Also, the big rectangle port heads have an advantage over oval port heads at high lift. With only .540", you probably won't even get there.

What are your power goals? You can probably make 1500 - 1800 hp with the stock oval port heads.

davepl

Now I'm no engineer, so don't be offended if I disagree, but:

I have to believe that pressure drop and flow drop are both a function of not only cross-sectional area (a simplification of port size) but also the pressure, density, and mass of the gas you're trying to flow.

If you increase the pressure and density (and hence mass), whatever restriction you saw while trying to flow through the port will be amplified. Thus, my line of thinking that at higher pressures and densities, you take take advantage of larger ports.

After all, by your own example, at 22psi, which is 2.5 atmospheres, the 550hp NA motor should produce 1375hp all else being equal (but let's not make that example a strawman to argue futher).

What I can't prove is that a head which needs .700 lift to take advantage of its flow capability can do so at some lower lift (like .500) when flowing at a higher pressure and density. I can feel it intuitively, but that doesn't count for much :-)

So far I've convinced myself that a blown motor can take advantage of bigger ports, but I'm still on the fence as to whether it make use of a high flowing port design at lower lift.

Roadie

if you can get boosted air back to ambient temperature, that 2.5 stmosphere example may be valid... But then you'd still have to account for the parasitic losses of the power used to pressurize that air.

free flowing will always make more power than a port full of turbulence. I think you're right in that some potential power is lost because of a flow restriction. You're never going to get all the "potential" power anyway. Forced induction guys just take advantage of pressures to make more power than NA guys and deal with the parasitic losses associated with them (blower drag/turbo exhaust backpressure/head flow/etc)

Though, it's not as much of a problem with boost due to the relative pressure (vacuum in cylinder induced by piston downward movement) and atmospheric pressure inside the intake. The pressure ratio of intake:cylinder is much larger in a boosted application and thus, more mass will flow when the valve opens and pressure attempts to equalize.

engineermike

In an actual running, well-tuned naturally aspirated engine, the pressure drop through the intake port will average around 1 psi. Even with 1 psi pressure drop, you can use port tuning to build up to 3 psi of positive pressure at BDC. This is close to the best compromise between restriction and velocity. With a turbo and, say, 15 psi boost, that 1 psi pressure drop will increase to about 1.4 psi (orifice calc). So, now you have an intake pressure of 30 psia and 1.4 psi drop in the port as opposed to NA where you have 15 psia and 1 psi drop in the port. The loss with a turbo is only 4.6% versus the NA loss of 6.6%. Directionally, the turbo motor is better off with the same head.

The 550 hp NA motor is a 383 cid and 12/1 compression with a large camshaft (240+ at .050). Whereas, the 22 psi 1200 hp motor is 355 cid with 8.5/1 and 220 at .050, which would probably only make 350 hp NA. If you were add 22 psi to the 550 hp motor, it would be 1500 hp.

As the valve is opened, the restriction in the port changes. At low lift, the restriction is the valve size and seat shape. As lift increases (D/4 lift seems to be the breaking point based on curtain area versus port cross section), the restriction is the port cross sectional area. I don't see this changing under pressure since the geometry of the flow path is constant no matter what the pressure.

Just for comparison, with a 2.25" intake valve, D/4 = .5625", so with a 2.25" intake valve, you really need more than .5625" lift to take advantage of a very large port.

Sounds like you've already made up your mind and were just looking for some people to agree with you.

There's a truck out there somewhere that's running in the 8's with a single T-6 turbo (101 mm maybe?), 509 cid, flat tappet cam, and oval port heads at <18 psi boost. Unless you're looking for a 6 second quarter mile, the stock 502 heads are fine.

Mike

davepl

It's possible, but then again it sounds like you're just being argumentative and can't substantiate your disagreement with physics. At least there's not even a hint that the D/4 "guess" should hold at greater than atmospheric pressures, and that's the crux of the whole argument.

Go do a search on lateral-g.net and pro-touring.com, where you will find numerous posts about the 502 heads being a major bottleneck and restriction for blown applications. Maybe they're all wrong too, but they're at least much cheerier in their posts about it.

engineermike

Please re-read the first 2/3 of my earlier post. D/4 is not a "guess". When lift reaches D/4, the curtain area (that is, the flow area between rim of the valve and the rim of the seat) matches the cross sectional area of the port just upstream of the valve seat. Is it coincidence that most ports stall at around D/4 valve lift??? This flow area relationship is not affected by air pressure or velocity.

We just made 870 hp with a bone stock 502 and a Whipple supercharger at low boost on pump gas. Again, if a small block can make 1200 hp at 22 psi with heads that flow 260 cfm, I'm sure a big block can make more than 1500 hp at 22 psi with heads that flow 300 cfm.

Again, what are your goals? 2000 hp and 6 second quarter miles, the 340's will be great. 1500 hp and 7 second quarter miles, stock heads with a cleaned up bowl and valve job.

Mike

davepl

No, its likely not coincidence, but it sounds empirical to me, and likely from experience on NA motors. When you say that relationship is not affected by air pressure or velocity, I guess what I'm looking for is supporting evidence.

The best forumla I could find for gas flow was this:



...and as you can see, flow varies in inverse proportion to the root of the density.

LameRandomName

I wonder if you're fully taking into account that a naturally aspirated engine is dealing with roughly the same pressure on the exhaust and intake side, whereas a mechanically supercharged engine has a significant differential.

I am not enough of a physicist to fully understand the implications, but I do know it's there and it does matter.

engineermike

You're absolutely right. Go back to my earlier posts. I skipped over the calc's and jumped straight to the answer.

Pressure drop is proportional to the square root of the inverse of the density, as the equation shows.

Therefore, with double the density, the pressure drop increases by a factor of 1.4 (square root of 2).

So, if a NA engine has 1 psi pressure drop in the intake port, then at 15 psi boost (roughly double the density), the pressure drop becomes 1.4 psi. As a percentage, the pressure drop is actually lower on the boosted motor.

davepl

Uh, not only am I not an engineer (ok, I actually am, I lied) but I'm also not a mathematician. In that equation, both pressure and density are under the root, so I'm not sure where you see the 1.4

engineermike

You're right. However, if you do the math using mass flow rate instead of volume flow rate, you find that the pressure drop goes up by a factor of 2 when you use the compressible flow formula and double the density and flow rate. Now, you're right in line with an optimized NA engine.

For the 3rd time, what are your goals?

RealQuick

(a^2) + (b^2) = (c^2)

Pythagorean theorem...........ok, thats my mathematical contribution...even though it has nothing to do with anything I was feeling left out

Subscribing

5thGen

simplify your thought process.

The efficiency (powermaking ability) of an engine entirely depends on how much air the engine can intake, combust and exhaust.

Forget about Boost for now.

Boost changes the engine in the same way different altitudes will change it. For example, at high altitudes, there is lower pressure (down to 11 psi I think), and therefore the engine will run worse, and lower altitudes, the pressure is higher, and the car will run better.

Then temperature changes it as well. At two extremes, if you have a hot day at a mile over sea level (say 90 degrees), your car will never run as good as it would at sea level on a cool day (say 50 degrees). Even with tuning, because there is only so much air it will be taking in.


If atmospheric pressure is 14.7 psi where you live, this is what your car runs at. Say it makes 500 hp, if you port the heads intake and run bigger valves and a bigger lift cam, and exhaust, you will make more power, say 600, but still be at 14.7 psi. You will be increasing the cars ability to bring in air, burn it, and expell it.

Now if you add 14.7 PSI or 1 bar of boost to your car, you will increase it's pressure by 100%, in theory it will allow it to make 2 times the power. So if you made peak power of 500 hp, in theory you could make 1000 hp at 14.7 lbs of boost. If you ported and moded everything to 600 hp, at 14.7 psi, you could theoretically make 1200 hp.

now, that is with keeping the intake air temp at the same as the outside air. If you keep it identical, you can make approximately double the hp. If you make it cooler, you can make more. If it runs hotter, you will make less. I am sure you know if you compress air, it heats up.

Hope this helps.

Most of the Mystique in turbocharging is from the need for stronger components, lower compression ratios, etc. Remember in race cars like indy cars that are not boosted, they also need to run stronger components, due to the amount of stress on the parts. With boost, you are adding more stress than the car was built to handle.

Turbocharged race cars run ridiculously high compression ratios. This is because they run race fuel. You can also run 14.7 psi on 14:5 to 1 pistons, if you have 110 or higher at the local shell. I don't.

At the lower octane levels available to the public, 8:5 to 1 for higher boost levels, or 9:0 to 1 for lower boost levels are needed to make up for it.

This all has little to do with the subject, but these are all factors that make turbocharging a "beast" in most people's perception.

5thGen

sorry for the over simplified explanation, but no one else was doing it.