I was recently looking at a slimjim's dyno "overlay" comparison between GT35r and GT30r. And was wondering what is best for me, and how to figure that out.



1) I wanted to find out if the max # of PSI of a turbo is determined by the compression or max torque the motor can take?
2) Does compression determine what the max torque a motor can handle?
3) is compression a deciding factor for what turbo to use?
For example : Is 9:1 compression a good compression only for higher psi. Is compresssion a factor in how much psi I can run or a factor of how much torque/HP the motor can handle.
...Then, is a GT35 on lower psi or GT30 on higher psi is best for me? and on what compression?

Because I already have a Benson sleeved block but I'm still deciding on what compression is best for drivability and 450+ HP.

Or should I run a higher compression 10:1 for better drivability?

My goal is to have a good turbo and compression combo for good drivability and 450+ HP.

I hope I'm making sense. Please excuse my noob q's to FI. I just want to get it right the first time. Please feel free to educate me on any misinformation.

Thanks in advance.
Paul

 

Everything is related in one way or another and there are a large number of variables that need to be considered to answer your question properly.

First off, the compression ratio doesn't have a whole lot to do with the stresses inside the cylinder. That is, if you increased or decreased the compression slightly, the overall stress on the internal components wouldn't be significantly altered.

Where compression ratio comes into play is engine management. Higher ratios will yield better torque and power numbers as more energy can be extracted from the fuel air/mixture during higher pressure combustion. The downside is that predetonation (knock) occurs sooner, at lower boost levels. The effective "tuning window" is decreased, sometimes to the point where extremely conservative fuel and spark maps must be used. Lower compression allows a much larger tuning window, leaving a nice margin for error should the ambient temperature rise to 100+ degrees or someone accidently fill the car up with 87 octane. The maximum amount of power that can be achieved with reduced compression will be less than with higher compression.

The correct turbo depends on SO many factors. Both the compressor and the turbine must be sized appropriately. The key thing to remember is that the amount of power you can get out of an engine is generally related to the amount of airflow you can force through it. Larger compressors can achieve the same flow rate as smaller compressors at a lower pressure ratio (the difference in pressure across the compressor stage from the inlet to the outlet). The downstream pressure minus any losses through charge pipes and intercoolers is often quoted as "boost pressure".

Referring back up to the third paragraph, high boost pressures and high compression ratios are more likely to lead to knocking. An engine making a certain amount of power with a large compressor wheel will require a specific amount of boost pressure. Replace the large compressor with a smaller one and the pressure rise across the compressor stage required to achieve the same air flow rate (and therefore power) will rise. Sometimes it will rise above the point of "safe boost".

Turbine selection determines how fast the compressor will spool and how much backpressure will be imposed on the engine. Adjusting the turbine selection can help one "place" the torque curve, one can dictate when they want their compressor to be spooled. Smaller turbine wheels and housings will help a turbo spin quickly at low flow rates (lower RPM). They are very restrictive, however, and will cause the amount of boost necessary to reach a specific power number to rise. Larger turbine wheels and housings don't accelerate the flow as much, but work very well at high RPM when the air flow from the engine is moving quicker than at low RPM.

Power versus boost pressure and what it all means...side note...

In comparing the plot in the original post, the cars are nearly identical and were run at very similar boost levels. The major difference between the two setups is just the turbo selection.

The GT30 has a very restrictive turbine housing that leds to really high exhaust gas velocities into the turbine wheel at lower RPM. It's super restrictive nature can be seen by the torque drop off at higher RPM. The GT35 has a much larger and freer flowing turbine housing, as well as a larger compressor unit, so not only does the turbo spool slower due to a larger turbine selection, but the compressor wheel takes longer to accelerate up to speed.

 

SlimJim, great write up. Very informative.

 

James thanks again for taking the time to post a thorough answer and the link to the power vs. boost pressure write up.

I just finished reading both posts and I'm trying to get a better understanding of what was said.

taken from the P. vs. BP thread
Is it correct to assume the following: (all else being equal)

1) The setup with the boost pressure/back pressure of 20psi/10psi with 9:1 CR will run hotter and have a smaller "tuning window" because of the higher psi and temps?

2) If I had a less restrictive setup (tubular mani) with a same delta but with 15psi/5psi, I can run a higher CR of possibly 10:1 but with lower temps to yield more power. The trade offs would be a similar smaller tuning window similar to setup #1 because of the higher compression ratio and a lower "safe boost psi #"?

- Would #2 also help with better "drivability" before spool due to higher compression ratio?

 

Yes to all of your questions. You hit on the head!

 

awesome! now to figure out which turbo (compressor/turbine) combo to use...

 

What are your plans HP/TQ wise?
A lovely place to learn a little,
http://www.bullseyepower.com/turbos.asp
A lovely place to get prices,
http://www.bullseyepower.com/catalog.asp

Tony from T1 is a cool dude who uses their turbos and is a sponsored driver who runs 10's and under. Now I know you aren't looking for drag strip material. This should however help you with understanding turbos a bit more. Obviously looking in google will get you more info. S200 series are going to be your best bets obviously, have fun and let us know what you pick up!

 

Thanks dalink, I'll take a look at those links!