Oil Pressure Loss in Braking

[Norm Peterson]

What is the "load" while compression braking? I can't imagine it's zero. But it can't be as much as being WOT either.

"Downward" loads - think compressive loads in the con-rods - will be less. But the upward inertial loads on closed throttle as the piston passes through TDC will be essentially the same at the end of the exhaust stroke and higher at the end of the compression stroke than during WOT. The peaks of the upward loads are likely of shorter duration than the WOT downward loads, though.


Norm

 

[Department Of Boost]

"Downward" loads - think compressive loads in the con-rods - will be less. But the upward inertial loads on closed throttle as the piston passes through TDC will be essentially the same at the end of the exhaust stroke and higher at the end of the compression stroke than during WOT. The peaks of the upward loads are likely of shorter duration than the WOT downward loads, though.


Norm

Thanks for explaining that Norm. Lots of things make more sense when you explain them.

 

[ddd4114]

It really depends on the engine geometry and how high you rev it.

For our engines, here is a rough estimate of the connecting rod load in WOT firing (blue) compared to closed-throttle motoring (pink) at 5500 rpm:

It's not really a concern.

For a very high-revving engine, the inertial load under motoring might be significantly higher, and connecting rod load (and therefore bearing load) might actually be higher than in WOT firing. The inertial force is proportional to the square of engine speed, so the higher the engine revs, the worse it gets.

I agree that 40 psi shouldn't be much of a concern if it's not sustained. However, it is worth mentioning that due to the "delay" of the oiling system, the oil pressure tends to be lowest right around when I hit the gas on corner exit.

 

[Norm Peterson]

I really came back to this to ask if the lap data charts are from an Aim Solo/SoloDL. I just got a Solo, and with no data (yet) to download and analyze I'm sitting maybe at square zero. I'm guessing that the red and blue bands at the top of the chart in Post #1 identify corners (am I at least on the right track here?), and I may have a few other questions that I'd ask via PM if you wouldn't mind.


But back to conrod loading and the above graph, I was looking at a tension-side difference between closed throttle and WOT being 2600-ish vs maybe 700. The bearing loads are still enveloped by WOT compression, but things like conrod tension (and rod cap loadings and rod elongations) are not. On a 'built' engine, that could affect your choices of P/V and P/H clearances.


Norm

 

[ddd4114]

The data was recorded with an AiM EVO4, but it still uses the same analysis software (Race Studio) as the Solo. Since we don't have oil pressure or oil temperature available via OBD, I needed the EVO4 to add external sensors. You're correct that the red and blue sections indicate corners. Feel free to PM me if you have other questions.

 

[Norm Peterson]

Thanks, and I will be taking you up on your offer.

I did drive down to the store at my home track (NJMP) and turned the Solo on as I got there to see if it would recognize either of the courses. Left it on for a couple minutes, and just having something - anything - to download and analyze has cleared up a couple of the questions I'd have had otherwise.

Hopefully I won't be completely over my head come December 12 when I expect I'll be taking lap information for real.


Norm