Quote:
Originally Posted by Inge_F
OK, my explanation above was very simple and imcomplete, just to show typical different between NA and TC engines.
In theory you are right, given displacement and the differences will weigh up for each other, but in real life that will not be the case. I was a bit categorical when I stated that longer stroke equals higher torque.
Will try to explain a bit more:
Given two NA engines with different stroke but same volume, rpm etc. Also we say they have the same exh/inl-valves, timing, inlet and exh systems, (this will not be natural since they will breathe differently, but just now that will be the case just to simplyfie things).
The one with a slight longer stroke will achive better "filling grade" given the same conditions, (by filling grade I mean the engines ability to fill the cyl with fresh air, M engines vary between 100 to 110% filling, this means theoretically a cyl with displacement of 1 litre will fill up with 1,1litre air during inlet-stroke=110%)
The reason for this is that is that the "long-stroke engine" have better ability/longer period of time to create "speed" on the inlet air through the ducts, (due to the slight longer stroke) and thus continue to fill with air after BDC in the inlet stroke and again create a slight above atm pressure in the cyl. This again will create better combustion->higher pressure->increased force->higher torque.
Please bare in mind that this is just one of and a quite cost effective ways to create better breathing or filling grade of a NA engine, there are many ways to the goal as usual, many of them much more expensive. F1 engines for example are not very long stroked, but still create a filling grade up to 130% or more, (but those engines are far from cheap  |
I am not sure I am reading you right. My understanding is that for a given RPM and displacement, the quantity of air to fill a cylinder and the time it takes to do so are exactly the same regardless of bore and stroke. So the charge velocity in the intake tract would be exactly the same (if we assume same valve size as you stated).
However, the engine with the larger bore can have an advantage at higher RPM because bigger valves can be fitted, thus increasing volumetric efficiency.
I also highly doubt that the S65 has a volumetric efficiency higher than 100%. There was a good discussion in another thread on this topic. My guess is that a volumetric efficiency between 80% and 85% at power peak is more realistic.