-5s or -7s

[Monster GT-R]

I‘m catching up with this thread and I would like to also comment on the two different turbos producing the same Horse Power but at different pressures topic. The internal stresses of the engine are the same for both turbo setups, but the real difference is in the thermal efficiency of the engine & turbo and the efficiency of all heat exchangers involved (Air / Water / Oil). Yes the higher boost pressure will generate more heat and less dense air charge but in the end has the same number of Oxygen molecules to generate the same Horse Power at the same cylinder pressures. It’s the heat generated and the heat that is needed to be dissipated that is the real difference and problem.

One of the biggest topics anywhere is always the turbo response or spool up time and how much horse [power you can make. But if people gave it more thought about heat dissipation and air density and air temperature you can generate more Horse Power and longevity only when you have less heat. Less heat means less boost required for a given horse Power. Look at I think its Ludders Car, Massive horse power at low boost. Did you see the Intercooler he’s got on the thing?

For a truly good setup you need to consider the NASA calculations matching the parts to the application. That’s the hardest part cause as time goes by the application changes as you get used to more and more power and make more and more changes to the car.

[paul cawson]

Originally Posted by SamuraiSam

Sorry, this is not true at all. Did you learn how turbos work before making that post? Turbochargers compress air. What happens when you compress a gas? That's right, it heats up.

Two turbos running the same amount of boost, one larger than the other, will NOT result in the same mass of airflow going into the engine because the more efficient compressor will heat the air less.

You can model the amount of airflow into an engine using the ideal gas law; PV=NrT where P is pressure, V is volume, N is the number of molecules, r is a constant and T is temperature. From this you can see that increasing the pressure will increase the molecules or Mass of air, increasing the volume will increase the mass of air, and decreasing the temperature will also increase the mass of air! Pretty basic high school science concept.

Now, apply it to the sizing of turbochargers:

Take a GT2860-5 at 30lb/min airflow and a pressure ratio of 2.5 (1.5 bar boost). That compressor wheel will be operating at 76% efficiency (look it up on the compressor map at turbobygarrett.com

Take those same specs and look at a GT2860-7. You are going to be outside of 60% efficiency - it's pretty close to the edge of that 'land' so we'll call it 58% efficiency.

76% vs 58%.

Now if you look up compressor efficiency vs ΔTemperature, you can see how the inlet air temperature will change. The chart I am referencing (page 29 of Maximum Boost) is in Fahrenheit, so bear with me.

According to the chart at 75% efficiency you would be heating the air about 305 degrees. And at 58% efficiency you'd heat it about 365 degrees. So that's 60°F hotter air at the same pressure! Your intercooler is going to work to bring that charge temp down, but it makes sense that if you start with colder charge air in the first place, the intercooler will be able to bring the charge air temp that much more.

What does all this mean? Everyone knows that colder air is denser air and denser air makes more power. There it is, your incontrovertible proof that larger (more efficient) turbochargers will flow a greater mass of air, and therefore make more horsepower, than a smaller turbo at the same boost pressure.

Good info, a small turbo at high boost will produce a lot of heat which a intercooler will not be able to get rid of
This in turn causes detonation which will destroy your pistons.

[Monster GT-R]

There are a few other things that cause detonation like timing and fuel / air ratio and fuel octane. If a turbo gives you variable displacement then Alcohol or Toluene injection gives you variable Octane. Variable Octane gives you variable boost capabilities and variable fuel selection at the pumps. Personally I think the compressor map is more important then the turbine selection. Pick the horse power level you want to build to or pick the response and lack of lag for the job. The rest is all along for the ride. Be realistic / conservative in your expectations and you will be happy with the results. Mines has shown that the fastest response is not the biggest horse power.

[[N1ZMO]]

Originally Posted by frostmotorsport

hi all,

am planning to buy some new Garrett 2860 turbos in a day or two - but question is whether to go for the -7s for response or -5s and risk it being a bit more laggy.

Remember that this is a currently standard R33 GTR with factory turbos and will always remain a road driven car - so a big single is completely out of the question (have drive canmans one and while it makes fantastic power, you have time to make a cup of tea waiting for the boost to arrive!!)

so - popular consensus on -7s for response or -5s for power?

Ive -5s on car at the moment..They are not laggy at all.
I also had turbos that were very close to the -7s/gt ss before these ones and yes they spool very fast but run out of puff at the top.
The -5s just keep pulling and dont want to run out,start moving from 3k and dump you in your seat at 4k ..Im in Wellington if you would like to have a looksie.

[Sub Boy]

I Have -5's in my car, I drive it everyday to work (This will have to change soon!) and they are not laggy at all, Like N1SMO said I wouldn't bother going for the -7's....The extra power of the -5's is well worth it. Mine start producing boost at 2500rpm and are really into it by 3000rpm. I am also running Tomei PonCams.

[gspot10]

what sort of power would you reckon out of 2560-7s runnin 1.1 bar of boost? getting my car mapped next week and just wondering!o and it has stock internals... would i maybe be better goin to 1.0 bar of boost?? or just get -5s???

[n boost]

I have gt2860-5's with stock cams /engine @ 1.1 bar and they are not laggy at all. Currently still finshing of mapping but they start spooling around 2k and by 4-4.5k ish your rocking all the way to 7.5-8k.

[Sub Boy]

Just got my car back from the tuners.
With the 2860-5's and Tomei Poncams it makes 512hp at all four wheels with only 20psi at 6800rpm and 450 lbft at 4800rpm. Boost starts at 2000rpm, and basicly just goes straight up from there!


I don't think I would bother with -7's or anything else smaller than the -5's with the response I'm getting, car is a little "cammy" sounding now with the cam timing....But it has won the "monster response" award from the tuners.

[heller44]

Any chance you could throw up the cam gear settings? out of interest.

[[N1ZMO]]

Originally Posted by Sub Boy

Just got my car back from the tuners.
With the 2860-5's and Tomei Poncams it makes 512hp at all four wheels with only 20psi at 6800rpm and 450 lbft at 4800rpm. Boost starts at 2000rpm, and basicly just goes straight up from there!


I don't think I would bother with -7's or anything else smaller than the -5's with the response I'm getting, car is a little "cammy" sounding now with the cam timing....But it has won the "monster response" award from the tuners.

Great isnt it
They are a "tad" more responsive than 2530s but do give away a few kws up top, but who cares when they respond fast and slam hard..perfect street low mounts on a RB26DETT

[Sub Boy]

Originally Posted by heller44

Any chance you could throw up the cam gear settings? out of interest.

Can't see them sorry...Would have to pull the cover off

[Sub Boy]

Originally Posted by [N1ZMO]

Great isnt it
They are a "tad" more responsive than 2530s but do give away a few kws up top, but who cares when they respond fast and slam hard..perfect street low mounts on a RB26DETT

Yup, they sure produce the goods down low Andy! just wish the car didn't drink as much gas as it does....Time to buy a Prius

[DarkChild]

Sub Boy,
20psi? That's like 1.3/1.4 bar right? Wow. I am hope I achieve those figures when mine goes in.

[Sub Boy]

Would be about that.

I can post a pic of the dyno sheet if anyone is interested?

[Monster GT-R]

I'd love to see them !