Quote:
Originally Posted by beatr911
Not sure what you're after with the throttle. It already has one that maintains vacuum in the intake. Would you say to increase the spring in the slide of the CV carb?
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What I mean is having a 14:1 compression
requires a low effective compression ratio in order to avoid pre-ignition (aka holes in pistons). Another way to look at a low effective compression ratio is a partial cylinder fill. The Atkinson cycle does this by delaying intake valve closing, pushing some air back out, as we all know. But custom grinding camshafts is beyond the ability of the average DIYer.
Another way to achieve a partial cylinder fill is to not let intake manifold vacuum drop below a certain threshold by restricting intake airflow with a throttle. 15"HG of intake vacuum is theoretically equals a half filled cylinder. My idea of a secondary throttle valve (in conjunction with a 14-16:1 compression ratio) is simply a means of automatically maintaining a certain vacuum level instead of having to stare at a vacuum gauge and modulate the throttle accordingly. Maintaining 15"HG of vaccum at lower RPM will require much more restriction than at higher RPM. And trust me, if you let the intake vacuum drop much below that with a 14:1 compression ratio on regular gas, bad bad things will happen quickly.
The ultimate question is whether a high expansion ratio will offset BSFC losses of a throttled partial fill.