| arghx7 |
01-19-2014 01:23 AM |
A few things to think about with "leaving power on the table" :
The main tune and hardware are developed in a lab on an engine dyno. The engine doesn't behave quite the same way in a vehicle on a chassis dyno. Air and fluid temperatures are different, not to mention that frictional losses and especially backpressure can be different. You may have most of the design completed before the engine is ever in a car. This is because manufacturing something takes a couple years lead time.
The rich AFR at heavy load is to keep the cat and in some cases exhaust valves from overheating during heavy vehicle acceleration and in engine dyno durability tests.
Somebody somewhere had to pick what fuel to develop the hardware and tune on. If it's US certification fuel, it's 97 RON or roughly 92 pump octane. That's better than some gas pumps and worse than others.
There are NVH, emissions, and fuel economy constraints. For NVH, it can be the rate of combustion pressure rise (fast pressure rise is noisy). This relates to spark and injection timing among other things. For emissions, enough scavenging through cam phaser tuning can hurt the catalyst. For fuel economy (also CO emissions), the AFR can be tuned leaner than the AFR that gives best torque--think about closed loop delays.
Quote:
Originally Posted by pche
(Post 1462014)
I'd like to see a tune that can change compression...... Running rich is not as harmful as lean but definitely not "better off".
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Any time you retard the intake closing timing beyond intake BDC at a given engine speed, you are reducing effective compression ratio by blowing air back out the intake port. The effective volume decreases. However, later intake closing timing gives more time to fill the cylinder at high speed. This is due to inertia of the air. The big factor here is the pressure drop (restriction) across the intake valve... this varies with engine speed, load, valve position, etc. That's why a high lift VTEC cam lobe might close the intake valve at 60 degrees after bottom dead center, even though there's less effective volume and lower compression when you do that.
In a similar fashion, advancing the exhaust opening timing reduces the expansion ratio and hurts fuel economy. However, advancing the exhaust opening timing helps evacuate more gases and can affect pumping work. It's always a tradeoff. |
