| Dee had emailed me about making 30 PSI on these turbos and I figured I would share my thoughts here as well. Basically you need to consider that Garrett has released this new map for a reason. The Map is on Garrett's own web site and is what they want people to see.
Garrett has always been private with information. The fact that they released this information should tell us they have learned something.
The map they originally released did not make any sense for a turbo this size. Something to remember when pushing the limits is that at some point your going to go past the limit at the expense of your equipment.
If your willing to sacrifice your equipment you will find out what the limit is at your expense. If you look at either the old map or the new one it shows the greatest efficiency of the turbo at 250 HP per turbo and it is rated to go to 320 HP per turbo at max.
This tells us that garrett designed the turbo to flow the MAX air flow making 640 flywheel HP. Anything past this is over what Garrett intended. We obviously have found ways of extracting more power for any set up, But the higher we go the less safe it becomes and the greater the wear and stress on all the parts in the given set up.
With Seb's car in a controlled environment at JWT we hit 669 RWHP at a max of 21 PSI. Seb's car has hit 23 PSI on the street, But the EGT temp get to 950 quickly and you can not stay in it for long and expect the engine to live running it that hard.
My car hits 25 PSI because I have smaller cams. The PSI number is not the important number.........You have to look at the Air flow it takes to hit a certain HP. Once you have hit the HP number a given turbo is good for you have hit the shaft speed the turbo was designed to work in. At some point you will actually stall the turbo from hitting the turbo's choke point by pushing it too far.
The more efficient you make the engine and turbo the lower the boost number will be to achieve a certain HP. What I am saying here is that the better the engine breathes the lesser the boost pressure and cooler the air charge.
In order to create pressure you need resistance......without resistance pressure can not exist.
Seb's engine is extremely efficient because it has larger exhaust valves, larger ports and larger cams.....Seb has less resistance and moves more air then a stock engine and hits the max efficiency of the GT28RS at a very low boost pressure. A stock Z engine would have more resistance and could make more boost with this same turbo, But it would make much less power and it would make more heat.
Basically you want to make the lowest PSI boost with the highest HP......This would mean you have achieved the highest efficiency and your equipment will hold up the best at that given power level. You want to make more with less. Having a goal of hitting a high boost number is not wise IMO.
If you want to make safe High HP numbers you need a turbo that is capable of moving the air it takes to get to that HP range.
The GT28RS is still a small turbo and is very efficient, So it makes it's HP/air flow numbers at low boost pressures.
If you wanted to make 30 PSI, I suggest using two GT2867R turbos.
If you look at the size of this turbo and the map with it, You can see how large the turbo needs to be to really hit 30 PSI safely.
[ http://www.turbobygarrett.com/turbobygarrett/catelog/Turbochargers/GT28/GT2876R_705330_2.htm ] You should not look to make the absolute most power or PSI from a given set up or you will be on the edge of the equipment and should expect it is probable you will have problems at some point or at least limit the life of your parts.
In competitive racing where money is no object or in testing to find the extremes with the understanding the equipment could fail, pushing the limits is important and can be fun. I do not recommend this to someone that wants to enjoy a street car. Hope that helps
Greg
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