Author Topic: LS turbo sizing  (Read 2139 times)

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non-intercooled flatty

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LS turbo sizing
« on: December 24, 2012, 01:21:35 PM »
This is from an article in Hot Roddings Engine Masters, Summer 2012, Volume 12 Number 2.  It's using an LS3 but it provides very nice evidence of correct turbo sizing.  Sadly, it doesn't go into the details of the turbines used, only the compressor inlet sizes.  Precision Turbos provided the two turbos used, identified as a 67 mm and a 76 mm, and they clearly had different sized turbines as the 67 mm was only run to 6.5 psi while the 76 mm was run to 13.5 psi boost pressure.  The engine was run NA, then with the 67 mm and finally with the 76 mm and the chart below shows the HP and torque for each.  The same cam was used for all, a roller hydraulic dual pattern with 0.617/0.624 lift and 231/239 duration @ 0.050 with a 113 degree LSA.




So, in NA trim peak torque came at 5000 rpm (551) and peak power at 6200 rpm (576).  Some very nice numbers given the compression ratio was reduced to 9.9:1 for the turbo application.  The 67 mm turbo gave peak torque at 4400 rpm (815) and peak power at 5400 rpm (766).  Clearly the turbine is small as its spinning up the boost fairly quickly but limiting airflow in the upper rpm range causing the torque to drop off rapidly and hence power to peak earlier.  Does that look familiar?  Look at what happened to the absolute value of the power and torque numbers, in the NA case power was higher than torque (576 vs. 551) and in the 67 mm (small) turbo case the power is lower than the torque (766 vs. 815).  Does that look familiar?  Further, the small turbine has masked the effect of the hot cam.  Said another way, the lack of increasing airflow with rpm has negated any benefit of a higher overlap cam and there is likely gains to be had in the torque numbers by swapping in a more conservative cam.  Does that sound familiar?  Now this isn't all bad if that's what you want and you're not planning to rev the engine much, it would be a good street set up, BUT you really can't turn up the boost to make more power like so many people try to do because it doesn't increase airflow where the engine is making maximum power, it decreases it!

Now look at the 76 mm (large) turbo, the peak torque is at 5200 rpm (956) and the peak power comes at 6000 rpm (1035), those are within a couple hundred rpm of the NA engine output.  That is a properly sized turbine!  It lets the cam work properly and makes some serious power increases, almost doubling the total power output at 13.5 psi boost pressure.  Given that the static compression ratio of the motor is 9.9:1 that is some substantial boost pressure, I would have liked to see the timing curve but they didn't mention it.
The White Bear - MPI fatty
Oscar - TBI flatty
85.5 - yet to be named, yet to even run

FunkyPhil

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Re: LS turbo sizing
« Reply #1 on: December 24, 2012, 04:30:23 PM »
mmmm, without knowing turbine specs we can only speculate. This is the same problem everyone runs into. This compares 2 Comp Turbo units with VERY different cold wheels. How they react at a given pressure will vary drastically.

No way would I ever recommend something as small as a T67 in a single turbo setup over 300ci. That would be like running a 12a Td05 on a G54B........
87 Conquest Tsi: 519whp 519wtq 7.09@102--10.95@122
CLICK TO SEE BUILD

non-intercooled flatty

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Re: LS turbo sizing
« Reply #2 on: December 26, 2012, 09:24:35 AM »
Yeah, the disappointing thing about that article is that they don't do a full disclosure, they perpetuate the notion that the compressor is the only decision maker in the equation.
The White Bear - MPI fatty
Oscar - TBI flatty
85.5 - yet to be named, yet to even run

FunkyPhil

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Re: LS turbo sizing
« Reply #3 on: December 26, 2012, 10:21:53 AM »
yeah, its a shame. Article says Precision but the pictures show a comp turbo.

As long as we know the cold side, we have an "idea" as to what the hot side might be.
from CT
a 67mm comp will be mated with a turbine wheel with a 65,66,67mm exducer
a 76mm comp will have a 79mm exducer wheel. Which is nearly the size of the 65mm turbines inducer.

Whereas a PTE 76 could have a turbine sized anywhere from a 75mm exducer up to an 85mm.
HUGE flow difference.

That single 67mm was prolly part of a stg 1 "street" turbo kit. And prolly designed for a near stock LS.

Truthfully the turbos are in completely different classes and dont really show a good comparison.
87 Conquest Tsi: 519whp 519wtq 7.09@102--10.95@122
CLICK TO SEE BUILD