GT25 or GT28?

[Shades]

If you were to pick one of these turbo's which one and why?... List of Turbo's

I know that with the GT28 there will be more lag, but if I go Disco Potato they claim that it's higher flowing.
If I go with the GT25 it will be quicker to pick up and go, but with the GT28 it will produce more top end horsepower.

I looked at Garrett and did their application search... most Miata's (1.8L) used the T25... and one or two applications someone used the GT28.

I have Maximum Boost by Corky Bell, and I've tried to figure out those crazy compressor maps which looks like a graph with a big fishing net drawn on it. They just don't make any sense to me.

Muchly appreciate your 411 on this topic... thanks!

[Lex]

The compressor map just tells you how much air the turbo will flow at a certain psi and how efficient it is at doing so. The x-axis is airflow (they use different units depending on manufacturer but this can be converted into the usual CFM. The y-axis is the absolute boost pressure ratio. Here is the ratio versus atmospheric pressure. A "2" means a 2:1 ratio and equals 14.7 psi which in absolute pressure is 29.4 psi since atmospheric pressure is 14.7 psi already.

Now what you need to know is how much air the BP flows at a certain RPM point. This can be calculated based on bore and stroke boost pressure (fairly accurately). So once you figure out how much the BP can flow in CFM at a certain boost pressure, look at the compressor map and see where that point sits on the map. If it's right in the middle of the centre island, the turbo is very efficient at that point but if that is at your max power point, it is going to be laggy everwhere else.

Look at the sticky in this section : http://www.wihandyman.com/forum/showthread.php?t=21003. It has a flow calculator and everything that comes with maps. Turns out the BP at 14.7psi at 7000rpm flows close to 400CFM. So depending on how much psi you want to run and power you want to make and how much lag you want you can pick a turbo.

Hope this helps.

[Shades]

You are most helpful... thanks for the explaination!

I feel like a

[Shades]

Ok, after reading that whole FAQ up and down... I've come up with this (I'm asssuming you picked 7000rpm cause that's when you make the most power... so that's what I used):
@ 8psi, 7000rpm - pressure ratio: 1.58, air flow 17.12
@ 14psi, 7000rpm - pressure ratio: 1.99, air flow 21.03


Both of these put me in the center of the island (pretty damn close to the center line) on the GT25 and GT28... this is what I want correct?

Now that I know how to read these, I noticed that the difference between the GT25 and GT28 is that the island is longer on the GT28 but lie about the same position on the grid. So that means that the GT28 will be better when I crank the boost up past 14psi, since the center of the island on the GT25 stops around the pressure ratio of 2.00?

It's looking more and more like Disco Potato for me...

[BuGS]

Ya, disco potatos are da bomb.

[FE3-323]

to me it looks like the disco potato is only truely efficent 15PSI+. Am i reading correctly?

[FE3-323]

curiousity would get me to ask how quickly the disco potato would spool in.

[Lex]

Yes, the larger turbo comes into its efficiency zone much later which means it is laggier. When we have a VJ20 (or other small turbo) we usually run off the right side of the map - in other words the turbo can't flow enough for max RPM and maintain a certain PSI at that point.

However, when you get a larger turbo, you don't want to be close to falling off the left side of the map (past the surge line).

The larger turbo will be efficient at 15psi at 7000 RPM, but it won't be efficient at 15psi at 3500 RPM. This will result in lag and poor throttle response.

The bad thing about turbos is that they are most efficient in a narrow region. So whether you like it or not, you are either going to have poor efficiency at the top end or bottom end and will have to make some sort of compromise. A small turbo will do very nicely for quick acceleration and daily driving. A large turbo will rock in the top end but you don't want to be screaming at 7000RPM to make your power if your car is a daily driver.

A disco potato would be a good turbo if you plan on running more than 15 psi and plan on having the fuel and management to back that up. The car will be a beast in the quarter mile but not as much fun in traffic. A smaller turbo is easier to manage and will make for a more fun AUTOX car.

Another thing to consider is your management. If you run a smaller turbo you will make do with a plug and play GTX ECU. If you run big power, you will need to go standalone and once again lose some of your day to day driveability.

It all comes down to what you want out of the car.

[Shades]

Yeah it does come down to what I want outta a car... :microwave

Correct me if I'm wrong... but the surge lines on both of the maps I post are pretty darn close to be at the same point on the grid.

Using the calculator I plotted some points from 3000-7000rpm, and they seem to be about the same % on the island. It looks like the disco potato has a pretty big island, not being only longer but just as wide as the GT25.

I calculated that the lag would only be about 200rpm different from both turbo's... sounds good to me.

[Denis]

I wouldn't worry about lag, those are all ball bearing turbo's and quite pricey.

[Shades]

curiousity would get me to ask how quickly the disco potato would spool in.

Here's a graph I found comparing the GT2560 to the GT2860 (DP)... Disco Potato plots are bold red (power) and blue (torque), GT2560R are faint red (power) and blue (torque). Btw, this was installed on a 1.8L BP.


Seems like it laggs a bit from 3000-4000rpm... but not that much.

[Lex]

Neither of the turbos look maxed out - if you turn up the boost on the GT25 you can match the flow of the 28 and do without the lag

[BuGS]

Hmm, if I remember the article, the GT28 spools as good as the GT25. You almost don't notice the difference. But that ya a long ago article out of Turbo magazine...

[GTXWGN]

guys sorry im a newbie but i do have some information to offer.

i recently fitted a gt2860rs(potato) to my mates BF sedan with a bpt and microtech, it made 240hp at the fronts on 18psi...however the day after the bpt threw a rod...im thinking 180kw or 240hp is the limit for a stock bpt cos the others ive seen have blown around this time , somea re lucky depends on your motor (mines alive still with 247)

anyway , the potato comes stock with a .86 hot side...

this turbo was very linear on the 1.8 , it did go hard but dont expect it to spool like a vf12 or a vf10 at all, id recommend getting the potato with the .64 hot side for some more response and kick , the .86 got full boost after 4500-4600rpm. the .64 will get it before 3800

[Lex]

Good info and welcome to the club!

You're one of the FordLaser members aren't you?

Im surprised the BPT threw a rod at only 240whp. Was it revved pretty high? I know some guys running with 300+whp BPTs for some time