I like trap speeds as a measuring stick, who cares about et? trap speeds are not all that dependant on launch and are easy to reproduce. What about et's, you try and try, finally get that number that you will never get again but your speed is the same as your slower times. Just a lucky (or skilled) launch. Personally I am not into dragstrip racing anyway.

I do agree with Kyle, I think the pettit will run the turbos fine as long as boost is not through the roof.

Matt

 

I have a set of BNR 3's that I will eventually get around to putting on, so I am following this thread with more than just some interest.

I have also seen similar discussions in other threads regarding various turbos and comparisons of their relative capabilities; however, there is a lot of conflicting data. It is easy to debate merits based on types, styles, setup, mfg., etc., because these are readily identifiable features. But what are we actually comparing? Maybe some of you will think about it in the terms I present below and offer your comments.

I submit to you, the real power factors:

A = Air density at the intake port, as defined by:
1) absolute pressure
2) absolute temperature

B = Exhaust factors, including:
1) pressure AND temperature drop across turbines
2) absolute pressure at turbo outlet
3) pressure drop in exhaust manifold (port to turbine)

C = Volumetric efficiency (VE) factors:
1) effective displacement
2) RPM
3) engine type
4) porting (and/or cam, where applicable)
5) tuning, etc..

NOTE: VE is actually what determines CFM flow, not the turbos--if they are sized correctly. If a turbo changes CFM flow then it is has reduced (or added) a restriction that affects the subject engine's volumetric efficiency. An example of this might be overrunning the compressor capabilities at X RPM or an excessive turbine restriction. A clipped turbine may produce more power at high RPMs because it can reduce exhaust restriction and increase an engine's effective VE. This is why maps are created and used to carefully define a turbo's correct operating point and range.

The power relationship is basically of (A - B) * C (above). This is not the literal equation, but it illlustrates the point. Any turbo that can produce the same intake manifold pressure at the same temperature with the same exhaust gas pressure (at the engine exhaust port) will produce the same amount of power--period. This assumes that you are comparing on the same engine under the same steady state conditions and thus VE.

Dynamic conditions such as spool time and turbo configuration (single vs dual vs sequential) will affect power under acceleration and the all important HP vs time (aka area under the curve), which IMO is too often neglected in favor of peak power numbers.

I hope this might give another perspective to the usual debates. Your comments please...

 

Yea, but slipping down the track and only putting power down through 2/3 of the track is going to have an inpact on your trap speed regardless of taking it easy on the launch.

However, trap speeds are a closer indication to the car's performance rather than the 1/4 time which is mostly dependant on the reaction time and launch.

 

Kyle I agree with all except about the reaction times. They will not show on the timeslip and don't matter unless you are competing. If it is just a test and tune type event your clock will not start until you cross the line even if you sit there for 3 seconds. I have done it before.

 

Yep, unless you are competing. For just bragging rights, reaction time is not a huge deal. However, I'd be more interested in lap times, not 1/4 mile times.