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Prepareing for 3L swap

you sort of answered your own question. the t28 doesnt flow as much air at 11psi as Toms turbo does at 10psi. boost is almost an irrelevant term. what you really need to be looking at is the airflow.

as for turbo size the t28 is more suited to a smaller engine like the zetec.
 
I do see what you are saying, but wouldnt that depend on where the boost pressure is being measured? Well, maybe not, everything after to turbo on into the intake manifold should have the same pressure or close?

So yes, bigger flows more air, but to generate equal pressures in the same area wouldnt you require the same volume of air. Now the t28 would have to work much harder to get there and probably couldnt maintain high boost all the way to redline, but in the middle range where it can sustain the same boost preasure as a larger turbo why wouldnt output be similar?
 
because the smaller turbo has to spin faster to move the same amount of air as the bigger turbo incresing the temp of the intake charge. the hotter the charge the less actuall air there is therefor less power. by going with a bigger turbo you can move more air with less work creating a cooler air charge. cooler air means denser air which gives you more power.
 
Not to be the perpetual sceptic, but is there not a physics law that directly correlates temp to pressure, meaning that little turbo can work as hard as it wants but the only factor on temp it the increase in pressure.

I guess, what im saying is temp and pressure should vary directly, turbo size should not play a factor in intake air temp.
 
the size of the turbo itself doesnt but how fast the turbo has to spin does. the t28 has to spin at (making a number up here) 150,000 rpms to maintain the higher pressure. the bigger turbo can maintain the same pressure with less work (again just a guess) lets say 100,000 rpms. thats a pretty big difference.

now as i stated originally boost by itself is kind of irrelevent. what you need to look at is true boost. true boos is the intake manifold boost pressure minus the exhaust back-pressure. this is amount of boost the engine actually breaths in. since the bigger turbo has less back-pressure that means a higher true boost which means more air into the engine=more power.
HTH
 
SHOgoFast said:
Not to be the perpetual sceptic, but is there not a physics law that directly correlates temp to pressure, meaning that little turbo can work as hard as it wants but the only factor on temp it the increase in pressure.

I guess, what im saying is temp and pressure should vary directly, turbo size should not play a factor in intake air temp.
Click to expand...

Yeah you are forgetting friction. Air friction can generate a lot of heat.
Then there is the speed of the air as it approaches the speed of sound it does funny stuff...I'm no physicist.

The biggest T28 is only going to support 30#/min of air on a good day.

So 11psi is about 1.75 pressure ratio. Follow the compressor map to see what will happen
The T28 big 60:

-115K rpm at 1.75PR
-25#/min of air max before the air stalls out.
-efficiency drops to below 65% at that point

Efficiency is based upon how hot the air coming out of the turbo is due to compression, friction. The cooler the air out at a given pressure ratio, the more efficient. A bigger turbo is more efficient at moving more air at the same boost pressure. So cooler denser air coming out means a lot more power. Also the air is not moving as fast.
Volume and pressure can only be evaluated at equivalent temps. Obviously that is the major difference in the power.

Also, the T28 only made in the 290s HP on the 2.5L and it makes about the same on a 3L....makes sense because that is the max air it can flow.
Better to run lower boost pressure if you can and just increase the operational rpm range to make more power.
 
Striker, that was the kind of info I was looking for, thank you. Same for you Tom. That makes sense to me now. I was not even considering some factors.

Stazi, I have cruised Ebay almost daily looking for something like that and have had little to no luck. Oh well, I will keep looking. Worst case, is I have to wait to get the larger turbo and mess with some flange welding.
 
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