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Turbo Pressure - Dyno, this ones for you

Hehe I guess i'm the one who started this debate, so here goes.

Pressure is only one of many variables. You can't just simply say..."such and such engine can only handle roughly such and such psi." There are other factors that have to be taken into account. For example, any compressor of a given size, is going to have a specific flow rate at that certain pressure level (basic fluid dynamics). Not only are they going to have that specific flow rate, they are also going to have a specific efficency at that certain pressure and flow rate. If you size a turbo that can flow more air at the same given pressure ratio as another turbo, and have roughly the same amount of efficency, it's going to make more power.

Simply put. This is how people make large power numbers with large turbos. It's NOT solely because they run more boost (pressure). To proove my point, I made about 200whp with my T-25 at 12psi about 6 years ago on my honda. Now today at 12psi from my T3/T4 I am making 350whp. Same motor. My freind Geoff (owner of Full-Race.com) put down over 350whp at only 5psi with his GT40. Most turbochargers start to be efficenty at about 1.2-1.4 PR, which as you can see, can still provide great power at low boost. BTW, Geoff also made over 780whp at 35psi with the same turbo.

From a performance standpoint, when building a forced induction setup, your goal shouldn't be "how much boost I want to run?" It should be "how much power do I want to make?"
 
To help illustrate my points:

Some common compressor map's...
(small turbo)
gt25rcompress.gif


(relatively larger turbo)

gt30rcompress.gif


Notice the different efficency islands, as well as the different flow rates at the same pressure ratios.
 
How do you read these maps? On the y-axis, is the pressure ratio just of the (boost+ambient)/(ambient)? Say when it says 1.2 would it be just like 14.7psi x 1.2 resulting in 17.6 psi? Im pretty sure on the pounds of air per minute. I believe the 4.0 requires something like 350cfm, im not sure what that is in pounds per minute but still. So would running at say 700cfm, would that be the equivalent of running at 15psi?
 
BBeach said:
How do you read these maps? On the y-axis, is the pressure ratio just of the (boost+ambient)/(ambient)? Say when it says 1.2 would it be just like 14.7psi x 1.2 resulting in 17.6 psi? Im pretty sure on the pounds of air per minute. I believe the 4.0 requires something like 350cfm, im not sure what that is in pounds per minute but still. So would running at say 700cfm, would that be the equivalent of running at 15psi?
Click to expand...

if my memory serves me, its in bar on the y axis 1 bar is amdient, 0 psi

2 bar is 14.7 3 bar 29.4

the x axis how much air the motor is flowing at a certain rpm
 
Last edited:
tealcherokee said:
if my memory serves me, its in bar on the y axis 1 bar is amdient, 0 psi

2 bar is 14.7 3 bar 29.4

the x axis how much air the motor is flowing at a certain rpm
Click to expand...
From what i remember in high school chem, 1 bar of pressure was equal to about 14.5 psi. Im assuming that when you say "2 bar is 14.7" that you just mean 2 bar is equal to 14.7 (i think it should be 14.5) pounds of boost. Then again, i dont know too much about these, im just taking educated guesses.
 
91 Jeep Project said:
Turbo pressure ratings on a stock 4.0. Intercooled, and not, discuss.
Click to expand...

I know the 4.0 can take up to 8psi of maximum boost intercooled without any reliability issues but as with any forced induction engine, it's important to retard the timing with boost and optimize the A/F ratios (11.5-12.0:1 at WOT) to prevent detonation, since this is the no.1 killer.
Beyond ~8psi, the stock cast pistons might be the weakest link. If you build the engine with forged pistons/rods, nitride harden the crank journals, o-ring the cylinders, and use a copper head gasket, the engine should live with as much as 15psi and double the naturally-aspirated HP output.
 
BBeach said:
From what i remember in high school chem, 1 bar of pressure was equal to about 14.5 psi. Im assuming that when you say "2 bar is 14.7" that you just mean 2 bar is equal to 14.7 (i think it should be 14.5) pounds of boost. Then again, i dont know too much about these, im just taking educated guesses.
Click to expand...

yes, thats what i was saying 1 bar is ambient (or atmospheric) there for 2 bar is 14.7 psi (in the manifold) so yes, 14.7psi of boost. and yes it is 14.7
 
Dr. Dyno said:
I know the 4.0 can take up to 8psi of maximum boost intercooled without any reliability issues but as with any forced induction engine, it's important to retard the timing with boost and optimize the A/F ratios (11.5-12.0:1 at WOT) to prevent detonation, since this is the no.1 killer.
Beyond ~8psi, the stock cast pistons might be the weakest link. If you build the engine with forged pistons/rods, nitride harden the crank journals, o-ring the cylinders, and use a copper head gasket, the engine should live with as much as 15psi and double the naturally-aspirated HP output.
Click to expand...

I don't think you even read my post. :sure:

What your saying lacks a lot of information and takes a lot of important factors for granted.

BTW, My turbo kits, and manifolds are for sale on the Jeepforum.com. I havn't decided whether or not I'd like to become of vendor of this site yet.
 
You definitely know a lot more than me about turbos Bryson so I bow to your wisdom on that subject, but I still think that avoiding detonation by optimizing the fuel and timing curves across the rpm range at various loads is the main key to engine survival regardless of the level of boost used.
Cast pistons are more likely to break the ring lands if the engine does detonate so that's why I err on the side of caution when suggesting a maximum level of boost for a stock 4.0 (or a cast piston stroker).
 
Dr. Dyno said:
You definitely know a lot more than me about turbos Bryson so I bow to your wisdom on that subject, but I still think that avoiding detonation by optimizing the fuel and timing curves across the rpm range at various loads is the main key to engine survival regardless of the level of boost used.
Cast pistons are more likely to break the ring lands if the engine does detonate so that's why I err on the side of caution when suggesting a maximum level of boost for a stock 4.0 (or a cast piston stroker).
Click to expand...

I agree 100%. :D . People just have this falocy about boost pressure when it comes to different compressor sizes. I would also not reccomend "high" boost for anyone who is not willing to take on the possible risk associated with forced induction. While you can tune agains't detonation by targeting a very low AFR, and retarding timing, there is always a point where the risk of detonation even in the slightest amount will have drastic negative effects on the engine. I'm sure Dr. Dyno knows, but for those who don't, retarding timing will increase combustion temperatures, so if timing is retarded too much then your back where you started...with a blown motor. :conceited
 
1 Bar = 100 kPa

1 atm = 14.7 psi = 101.3 kPa


N.B.!!!!

1 Bar != 14.7 psi

1 Bar != 1 atm



:D
 
BBeach said:
1 Bar doesnt equal 14.7 psi....
1 bar is 14.50377psi.
http://www.convert-me.com/en/convert/pressure

The physical atmospheric pressure considering standard temperature and pressure (STP) is 1.01325 atm = 101.325kPa = 14.69595 psi (basically 14.7psi) :moon:
Click to expand...
Exactly!

Note: "!=" means "Not equal to".

I won't comment on the usage of six signifigant digits. :p
 
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