Quote:
Originally Posted by mike gilbert
Interesting thread. I have a B20A engine that I was goiing to install in my 1800S (which has a tired B18). I was going to upgrade it to B spec first but I'm also more interested in low end torque below 4000rpm and I notice that the torque figures are not much different between the B20A and B. In fact according to this chart on Wikipedia the DIN torque figure is actually higher for the B20A. Is that really true and if so what advantage in the real world is there for someone like myself in changing the cam and compression ratio (or even retaining the twin carbs, though obviously I will)?
https://en.wikipedia.org/wiki/Volvo_B18_engine#B20 |
My immediate impression of what's going on with the B20 data in that table is that the compression ratio is probably playing the bigger part in produced power. The torque number is kind of staying about the same but the position of peak torque happens at very different rpms.
The bore and stroke of the B20 engine remains the same 88.9 mm bore and 80mm stroke making it a slightly oversquare engine. If you really wanted more torque from an engine design perspective you'd start (probably) with an under square engine design.
https://en.wikipedia.org/wiki/Stroke_ratio
I can only assume that the position of the peak power (which on that table varies from 4800 rpm to 6000 rpm) and peak torque (which on that table varies from 2300 to 3500 rpm) is mainly due to a combination of compression ratio and cam profile. There's a lot of chat about B18/B20 cam profiles on the interweb but from what I can make out there seems to be very little difference between the factory fitted versions.
More information can be found here =>
http://www.1800philes.com/ianr/_superlist_grinds.html
In the pre-blurb on that site it is stated that the longer the duration (when the valve is open) "the peak torque is not raised, merely moved higher in the rpm range"
I conclude if you wanted the peak torque to be lower in the rev range then you'll be best off sticking with a smaller duration on the camshaft as you're gonna have trouble trying to make the engine an "undersquare" engine...
...thing is most people want to get the drop off in torque as the rpms increase to coincide with the rising power curve so you don't get the feeling of a flat spot (I'm not sure how that works for the B20 - need to speak to someone with a bigger pay grade!)