"10 Psi" Spring opening at 4 psi - Backpressure issue?
12-07-2016, 12:47 PM
#21
If you do the math, it sure looks like 2.5" piping is better suited. I'm not saying all the complex bends and 90's you had in place before were correct. Or that what you have now doesn't work better than the previous setup with your installed parts. Just that a properly laid out 2.5” setup (with the correct IC) should increase response while not causing turbulence/drag issues on most mild SBE LS engines.
Also read you size the piping based on the engines NA flow. As you said… because of turbulence around 450 FPS (.4 mach) is the commonly accepted do not exceed point for airspeed in a cold side pipe. Tube size can roughly be checked by calculating the maximum airflow attainable, dividing by the area of the tube in square feet, and dividing again by 60 to convert to feet per second. So If we use a 400hp NA engine (roughly 600CFM) as an example. Airspeed is about 293FPS (.27 mach) in a 2.5” pipe. Of course more restriction/turbulence will be added when you add additional bends/length.. But for the typical LS making sub 400hp NA, it sure seems like 2.5” piping should lower the volume of the system and improve response. According to rough math anyway.
Also read you size the piping based on the engines NA flow. As you said… because of turbulence around 450 FPS (.4 mach) is the commonly accepted do not exceed point for airspeed in a cold side pipe. Tube size can roughly be checked by calculating the maximum airflow attainable, dividing by the area of the tube in square feet, and dividing again by 60 to convert to feet per second. So If we use a 400hp NA engine (roughly 600CFM) as an example. Airspeed is about 293FPS (.27 mach) in a 2.5” pipe. Of course more restriction/turbulence will be added when you add additional bends/length.. But for the typical LS making sub 400hp NA, it sure seems like 2.5” piping should lower the volume of the system and improve response. According to rough math anyway.
I did the math when originally selecting 2.5" when I first put it together, but I found through experience that the airspeed math is for straight pipe, and although I can guess it is tough to nail down just how much resistance/turbulence all the bends add.
My combination of 2.5/3/3.5 should put me safely under the 450fps number even when considering bends.
I read that for water flow at least a 6" radius bend of 2.5" can be equivalent tot 7 feet of straight pipe? I need a source and exact numbers, but I remember the bends being fairly extreme.
I did try referencing at the intake vs the housing before redoing the piping. I could certainly get the boost up to XX number, particularly if I held the gate shut with my simple manual controller, but the power was not comparable to current.
Next I am going to try the same, fairly low boost with cxracing's 4" intercooler vs the 3" and see if I have to add fuel to keep up with increased airflow.
12-07-2016, 12:50 PM
#22
The Cleveland is a very cool engine.
On the LS turbo application I am using a factory truck driver's side manifold and a single 2" OD crossover pipe under the engine, so I know my EMAP is very high. I have already blown one gasket, but spool time is excellent, almost instant any time you dip into partial throttle.
12-07-2016, 02:03 PM
#23
How did you verify the gains? Don't get me wrong I'm not disagreeing with you. I'm all for finding out a more exact method to determine the correct cold pipe sizes. I hate just tossing on a 3" pipe because that's what everyone else does is all. (which is what I've done. 
)
I'd think you could use the equivalent tubing/pipe pipe length per bend charts. Need to find one for gas flow though, not water. Tally up the entire system length and used a calculator for a guess. Couplings can also cause tons of restriction. Not sure how you'd add that.
Chart like this maybe...? Nice they actually list ID for us. I'd assume if these are screw in fittings they will be a little higher if anything restriction/flow wise.
http://www2.iccsafe.org/states/seatt...pendix%20A.pdf
Then tally it all up with something like this? Using 1.0 for SG and air.
http://www.freecalc.com/gasdia.htm
) I'd think you could use the equivalent tubing/pipe pipe length per bend charts. Need to find one for gas flow though, not water. Tally up the entire system length and used a calculator for a guess. Couplings can also cause tons of restriction. Not sure how you'd add that.
Chart like this maybe...? Nice they actually list ID for us. I'd assume if these are screw in fittings they will be a little higher if anything restriction/flow wise.
http://www2.iccsafe.org/states/seatt...pendix%20A.pdf
Then tally it all up with something like this? Using 1.0 for SG and air.
http://www.freecalc.com/gasdia.htm
Last edited by Forcefed86; 12-07-2016 at 03:08 PM.
12-07-2016, 02:08 PM
#24
On the LS turbo application I am using a factory truck driver's side manifold and a single 2" OD crossover pipe under the engine, so I know my EMAP is very high. I have already blown one gasket, but spool time is excellent, almost instant any time you dip into partial throttle.
Run the smallest piping possible to keep velocity up. Then run largest exhaust housing/turbine wheel possible that still meets your spool needs to provide the lowest back pressure. That's the theory at least. And assumes your engine isn't flowing so much air that the exhaust gas is going sonic in the pipe. I'm around 1000hp at the crank with mine. So unless you are making more than 900hp 2" shouldn't be an issue. Your T4 78/75 turbo necks down WAY smaller than my exhaust housing as well.
Last edited by Forcefed86; 12-07-2016 at 03:07 PM.
12-07-2016, 04:35 PM
#25
How did you verify the gains? Don't get me wrong I'm not disagreeing with you. I'm all for finding out a more exact method to determine the correct cold pipe sizes. I hate just tossing on a 3" pipe because that's what everyone else does is all. (which is what I've done. )
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With the 2.5" setup I was only seeing 510lbs torque delivered to the trans at 5-6 psi. Denmah's Colorado build, which is almost identical but with the LJMS Stage 2 cam instead of the Stage 1, reported ~500/500 at the wheels at similar boost.
12-07-2016, 10:20 PM
#26
Don't forget the stem area in your calcs... Then there is the radius from stem to valve head then a inverse radius from that to seat.
Keeping in mind dinamic changes in manifold pressure.
Keeping in mind dinamic changes in manifold pressure.
Last edited by gtfoxy; 12-07-2016 at 10:26 PM.
12-08-2016, 10:42 AM
#27
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Exhaust Valve Seat Pressure YES, you are correct.
MY WORK here is directed to the little known fact, Valves are Valves, the same poppet valve, if on the gate OR head, has the same pressure present, changing SEAT Pressure Requirements.
As you see, NO ONE in recent posts, is concerned.
Most here would rather "six bolt" the heads BEFORE fixing the original problem, a CAUSE of head lift.
Lance
MY WORK here is directed to the little known fact, Valves are Valves, the same poppet valve, if on the gate OR head, has the same pressure present, changing SEAT Pressure Requirements.
As you see, NO ONE in recent posts, is concerned.
Most here would rather "six bolt" the heads BEFORE fixing the original problem, a CAUSE of head lift.
Lance
12-08-2016, 11:30 AM
#28
I’m would like to know how to more accurately calculate the actual pressures acting on the valve. I don't see how your example/formula can be possible. It’s like there's a missing factor. I seem to have proven it incorrect on many builds. If that formula was correct I would constantly floating valves…which isn’t happening.
Exh. valve is only 1.55” on most LS OEM heads. If I really had 180+psi acting on the valve why would it not float with a 130psi valve spring? It was perfectly stable up to 6800rpm and 26lbs of boost. Makes me think there is something we are missing? Or something I am leaving out? Cam was a 211/230 @.050 valve lift and .558/.552 lift with 1.7 rockers and a 122.5 centerline.
The problem IMO is most run into excessive reversion and detonation at higher exhaust back pressure ratios. That spikes the cylinder pressure thru the roof and pushes the HG. Stay out of detonation with stable cylinder pressures and you’re golden.
Exh. valve is only 1.55” on most LS OEM heads. If I really had 180+psi acting on the valve why would it not float with a 130psi valve spring? It was perfectly stable up to 6800rpm and 26lbs of boost. Makes me think there is something we are missing? Or something I am leaving out? Cam was a 211/230 @.050 valve lift and .558/.552 lift with 1.7 rockers and a 122.5 centerline.
The problem IMO is most run into excessive reversion and detonation at higher exhaust back pressure ratios. That spikes the cylinder pressure thru the roof and pushes the HG. Stay out of detonation with stable cylinder pressures and you’re golden.
Last edited by Forcefed86; 12-08-2016 at 11:43 AM.
