Let's talk air flow -


Air is a fluid.


Ok, I feel all those raised eyebrows, out there - but, no, I'm not any further around the bend than previously, so rest easy, folks.


Definition of fluid is ability of a substance to move thru, or fit into, a container without altering the shape of that container.


And, yes, I am familiar with the concept of a balloon, for all you hecklers.


Water, sand, flour, Diesel fuel, oil, air - all fluid substances.


Fortunately for us, air is right in there, as a fluid.


We can channel it, direct it, compress it, heat it, cool it, store it - most of us even breathe it, every chance we get.


As does this big stinky air pump we all are so addicted to.


Displacing 400 cubic inches every other 360deg crank rotation - it is an engine requiring 4 distinct cycles to do that deed, two per rotation -, it can pump ~400 cubic feet per minute at ~3000rpm.


Air flow is measured per unit volume, usually in cubic feet for automotive use - a dimensional area 12" by 12" by 12".


Flow rate is measurement of unit volume - cubic inches, cubic feet, cubic yard, etc - per unit of time - second, minute, hour, etc.


400cfm per 3000rpm indicates a minimum requirement for ducting and filtration which permits maximum attainable efficiency.


Simple math would indicate this pump cannot move 400cfm at idle, but we would not want to size an intake system based on the minimal flow.


Or, restrict it to only 400cfm - we have a turbine-driven compressor stuffing air into the intake at up to 2.5x pressure ratio, derived by (Boost + Baro) / Baro.


Which means, at maximum power and rpm, we require a system allowing ~1000cfm flow rate.


Would seem to require an enormous filter and ducting, right?


4", maybe even 6" diameter required here?


No girly-man 3"dia ducting gonna hold this big hoss back, that's fer sure!


Well - remove the rubber compressor inlet duct, and take a gander at the actual compressor intake diameter, where the blades are.


"Durn!! Whur's my tape maisure - thet cain't be right!!"


Remember - that's 1000 cubic feet flowing over a 60 second interval, about 17 cubic feet per second.


Now, think about this - if the flow rate of the supply air pump - the turbo compressor - is only equal to the demand air pump - the engine - no pressure will be developed in the system, and no advantage will be gained.


400cfm in, 400cfm out, 1:1 pressure ratio - may as well remove the compressor, as that's just Baro input, same as naturally aspirated.


However, if the flow rate of the supply pump exceeds the demand flow rate, excess flow will stack up in the intake as pressure - the greater the excess, the greater the pressure.


The GM-8, in recent tests, easily generated 20psi Boost on oem 63mm fuel rate, up to 2500rpm, where tests were limited to prevent DTC-78 and resultant power loss.


Factory '95 air box and curved intake duct, AC filter element - all stock.


So - 17cuft/sec - 1000cfm - doesn't seem to be limited much by that 2" compressor inlet diameter, so where's the need for 4" ducting? 6"???


The 3"dia where the inlet ducting is mounted\clamped should suffice for straight ducting, but a path requiring several sharp bends could benefit from the larger 4" diameter for less flow restriction.


Unless - the 3"ducting were plumbed up to the radiator bulkhead to take advantage of the cooler air available there, under high pressure.


Flow rate can be improved by increasing the diameter of the duct.


3"dia provides an area of 9.5sqin, and 4"dia provides 12.5sqin, where sufficient real estate is available for the increase. </

Ahh..

finally some tech information :)

This is the kinda conversation i like.

I have mentioned in another post about volumetric efficieny and high compression.

There seems always to be confusion about boost and the amount of pressure, and the volume of air at that pressure.

For some reason.. as soon as you mention boost, people seem to forget the fluid properties of air and how port flow characteristics and intercooler/upper intake piping etc affects the overall system. I have often wondered since getting this truck whether or not clipping the exhuast wheel a few degrees to allow more "slip" wouldn't improve the performance of the truck.

i'd be curious to see if anyone has actually flow benched one of these heads to see actual flow numbers.

The whole upper intake seems to be very inefficient as well. It seems to feed a large engine from a very small inlet before entering the lower intake manifold.

i don't have the time at the moment.. but will be redo'ing the entire intake manifold this winter most likely.

I just got some $ so i'll be putting a 4" exhaust (from the turbo all the way out) on the truck.

but yes.. flowing more air is better than having more pressure. turning up the boost is pointless if your not benefiting from it in terms of flow.

-- WheatKing

It's not realy important, WK - with only 3500rpm max, intake density is more important than volume.


When flow rate is limited - around 800cfm at full Boost - increased pressure will allow same oxygen availability as greater flow rates.


1cuft air contains twice as much oxygen at 30psi as at 15psi, and that's the secret to getting power out of this low rpm engine


More oxygen, more fuel, more power


No one has measured and posted head\valving data, so when you get around to it, it can be a welcome addition to posted info.

Nice post.


I'm sold on a ramair custom intake/exhaust with twin turbos.

Actually, a single turbo is more efficient than twins. Generally you only want to go with twins if packaging makes a single impractical. Because of the huge crossover pipe (which I'm sure loses exhaust velocity due to heat loss as well as the bends associated), you might see an advantage to having one turbo on either side of the block, but I'd still be inclined to keep the single.


I'm thinking 17:1 compression with a stroker crank (around 7.0L would be nice), ported intake/heads (since if the heads can't flow you're just wasting energy with higher boost), throw on a PowerStroke turbo with a huge intercooler and 25 psi boost. Should be good for enough power and torque to put some Duramax/Cummins/PowerStroke folk to shame. You'll also want to do something about the exhaust manifolds... I'm sure the factory ones don't flow as well as they could.


Hardest parts would be tuning the engine properly (one of the guys who knows how to program these computers would have to be in on the project), and getting a stroker crank. I'll gladly offer my truck as a guinea pig. http://www.dieselplace.com/forum/smileys/Big Smile.gifEdited by: tdupuis

A single turbo might be more efficent under 1500 rpm but certainly not above. I'm gearing up to be able to reforge things here on the farm. On the list is making a true BB head for the 6.5, recasting exhaust manifolds for twin GM 8 turbines (cuz there so small) recasting intake with heat exchange core inside (no ducting) and ram air through the hood. I am willing to guniea pig mine too.


No more crossover, or restictive manifolds or small valves to kill the motor. I'm on a mission to take the 6.5 beyond it's current power restrictions. After all the mechanical is completed I will work on a custom flash for my eprom.


And

It's not realy important, WK - with only 3500rpm max, intake density is more important than volume.


When flow rate is limited - around 800cfm at full Boost - increased pressure will allow same oxygen availability as greater flow rates.


1cuft air contains twice as much oxygen at 30psi as at 15psi, and that's the secret to getting power out of this low rpm engine


More oxygen, more fuel, more power


No one has measured and posted head\valving data, so when you get around to it, it can be a welcome addition to posted info.

umm.. flow rate is limited.. you said it yourself.. so increase the flow rate..

800cfm at 15psi is a lot cooler air charge than 800cfm at 30psi.. same volume (same amount of oxygen) at lower pressure.

will the ford turbo bolt to our manifold or no? i don't care about boost pressures or what not or turbines or anything else.. will it bolt up?

i'd rather run it at 8psi.. than the gm4/gm8 at 20.. volume.. and efficiency :) anyone got a compressor map for a GM4 turbo?

-- WheatKing

You would probably have to make an adapter plate to fit the ford turbo on your stock exhaust.


There was a map of the GM8 around last year.

The GM8 turbos are pretty small... I plan on having a pair of them on my 5.6L Jag V12, my side project. That said, I think that you will run into spooling problems running a GM8 on only 1 bank. Perhaps 2 GM4s would work. Remember, we're only working with up to 3500 rpms here, and off-idle torque is most useful indeed. The GM8 will spool up virtually instantly in its current configuration. Now, if you have the new heads and new intake manifold that will allow for more flow that will help alleviate the problem (especially if you can figure out how to bore or stroke the block... I still like the idea of a 7.0L stroker motor... the crank isn't compatible with a standard BBC, is it? That would be nifty...), but you'll still want to run the maps and figure it out. A single turbo is actually more efficient at higher rpms vs. lower rpms, but as I mentioned because of the nature of the 6.5's crossover pipe, smaller twins may work. What about a pair of GM4s or GM6s? I haven't looked at the maps, but those may work.


Incidentally, I looked at a picture of some 6.5 heads for sale on eBay. They looked like the combustion chambers had plenty of room to enlarge the valves, or was I just looking at the picture wrong? From what I could see, there's not much of a combustion chamber there. Anyone have some more detailed pictures to look at?


Too bad Chevy didn't make this motor the same as a standard BBC... that would make our lives simpler in terms of internals.

I think others should know the compatibility of internals. My guess would be everything's specialized.


The use of twins should allow for more boost pressure in the system overall. Each turbine would make half the boost(15psi x2 max), with half of the exhaust pressure(gmctd estimates 1000 cfm max stock). Hexcode manipulation can increase fuel delivery and acceptable boost levels where spooling shouldn't be a problem.


My conceptualization of this is placement is putting it all as close together as possible on top of the engine. Exhaust headers would reach and do a 90 on each side. The turbines placed right on the new intake manifold.


I will keep displacement at 396ci. Increasing cc volume for 19:1 compression overall and larger valving/ports requires that the head get bigger and stronger. The stock head is limited to small valves due to internal weakness that manifests as cracks between the water passages and exhaust valves.


Anyone who has a pair of 6.5 or 6.2 heads, cracked or not, I would like to acquire a set to have.

QM - My heads will be pulled next week if their cracked and i'm in need of new their yours for the shipping. But all guestimates indicate head gaskets so far.

I sent an FSD cooler to Elky up there in the great white north.


The 12 ounce package cost 33 dollars. I don't know if I can afford to ship the heads.

Each turbo will have to do the same boost but half the flow. The pressure is not added up, the flow is. At one time I had found a compressor map for the GM8 (also known as the IHI RHF6), but forget where and it was. I'll see if I can find it.


So from what you're saying, because of the stock head's weaknesses there is room for larger valves and porting, but doing so will cause almost sure failure? In that case, what about the idea of recasting the stock head with thicker walls and from a better material, and then do work from there? I would think it would be a lot less effort to do it that way, but I could be wrong.

It might be a lot of work but beefing the head in the cast process seems like the way to go. GM reduced the valve size between 91-92 because they realized the smaller valve didn't need such a big hole. 6.2 heads have bigger valves than the 6.5.


I happen to have two GM4 turbos. and from what I hear a twin turbo 6.2 or 6.5 will make 500 hp.


I'm thinking of putting a dodge or ford turbo on my 93 with custom headers. That will free up that gm4 for the other project.

500Hp is quite a stretch...even if you built one up the way Ruben did his, I don't know how you would ever get enough fuel delivery out of a DS to make that possible. 500HP w/18:1 pistons would require atleast 45psi boost and probably about 140ccs of fuel. Edited by: Texas Diesel Guy

I guess propane would fill that heat/energy gap if I can't get enough out of the pump. I'm not sold on 18:1 pistons. I like the stock bore/stroke enough to work around it by recasting the heads.

Here's a rough sketch of the concept.


http://www.dieselplace.com/forum/uploads/19D_concept396tt.jpg


with GM4 turbos one would have to sit reversed of the stock or have it's aluminum housing upsidedown.

You can keep the stock bore/stroke but just dish the pistons to get the 18:1 (or 17:1, or 16:1, whatever you want) compression. Engine displacement is a function of bore and stroke, nothing more. Compression ratio is then a function of everything.


Crude drawing, but gets the point across. I think it would probably fit. Would have to do some shrouding on the driver's side to protect the brake system.


Then dual exhausts. Mmm... sounds yummy. Now all we have to do is supply heads that are up to the task. I'm appalled that the stock heads can't take bigger valves considering just how much room there is. Not good in my book.

Dishing the pistons probably weakens them. What do you think about adding a little compressed O2. Atmospheric is 17% O2. What kind of burn would you get at 30%, 50%? Based on my experience with O2/propane, O2/acetylene i'd say it would burn hotter and with less soot. NOx might actually be reasonable even with the extra heat. Might help the turbines spool at low RPMs. carbon fiber tanks take the weight out of carring it.

Carbon fiber tanks also = big $$$. The problem I have with any kind of injection of something into the engine for any vehicle is that you run out. Of course, fuel is the one exception to the rule because you need to run your vehicle on SOMETHING, but if you inject pure oxygen or oxy/acetalene or oxy/propane, you need to refill the bottle. I don't like refilling bottles. When I modify my vehicle, I want it to be capable of making the power I've modified it to make on nothing but the fuel I put in the tank. For me this is the whole point of using forced induction. But to each his own.


As I've said, I'd like to make a stroked 6.5 (How's 427 cid sound? That's a magic number for sure) with lower compression, improved heads, intercooler (air to air for me) and more boost on a single, larger turbo.

Where would you want to increase the stroke? Crank, rods? Wouldn't it require machining to fit a bigger crank?


The 6.5 crank and mains are known for failing.

Hey QM, does this look like the picture you were trying to draw?


http://www.dieselplace.com/forum/uploads/F2D_TTurbo.jpghttp://www.dieselplace.com/forum/uploads/A26_TTurbo2.jpg

that's perty.


I'm putting the turbos closer and more on top of the valvecovers, but that's about it as long as that's an intercooling core in that intake manifold.

*drool*


Just so you know, its not even diesel, Banks new BB chevy twin-turbo kit...available in 800, 1200 and *gulp* 1600 HP!


http://www.bankspower.com/twin-turbo-products.cfmEdited by: Texas Diesel Guy

The sparkplug wires kinda gave it away.


If you look at how the down pipe attaches to the back of the turbos, I bet something like could be done with the GM4 that our trucks came with. I'm talking about removing the turndown and fab'ing a downpipe that bolts on instead. it would save some space.

I'm sure its possible, but the turndown houses the wastegate as well, something would have to be done about that.

No, the waste gate come parallel to the exhaust turbine. I'll find the pictures I took awhile back and post it here.





http://www.dieselplace.com/forum/uploads/AZ2_turbo12.jpgEdited by: quantum mechanic

QM yeah, your right, that was the GM-8 with the vertical mounted actuator I was thinking about.



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<TD class=bold>gmctd wrote:

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<TD class=text>1cuft air contains twice as much oxygen at 30psi as at 15psi</TD></TR></T></TABLE></TD></TR></T></TABLE></TD></TR></T></TABLE>You were talking absolute pressure right? so thats Atmospheric pressure and 15psi boost? twice the boost doesnt make twice as much air flow into the engine, the air just has more push behind it to overcome drag. More boost does fill the cylinder with more air, but its not a linear comparison.Edited by: Texas Diesel Guy