Power Valve Selection: The Definitive Answer
It all depends on the combination and the carb.
I've told this story before. Had the car on a chassis dyno and just made jetting changes and experimented with a little more timing (no good). Two pulls were about a equal so my on the spot decision was leave with the slightly smaller PMJ setup.
On the freeway type highway, the engine started revving and then dying. It only happened when over 60 mph or so. So my solution was drive home at 55-60 max (which was OK on that road).
Moral of the story was those jets were the lean limit for cruising. Additionally, while maybe just as much power on the dyno, likely a little too lean for the track. So on that carb and application, the PVCR could be left alone.
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With Respect to finding PV opening by checking for increased acceleration when crowding a vacuum.
Here's an example:
Blue line is vacuum. Black is RPM, Orange is forward accelation. Violet is AFR derived from WBO2.
The first two steps down in vacuum are matched with step increases acceleration and rising rpm.
In this example, done at relatively low speed (25 - 50 mph or so) the next increase in throttle, dropped the vacuum as expected but there was no increase in acceleration. This dead spot around 12" HG was not because the AFR was lean. In this case it was because the timing was too advanced and possibly also due to an incorrectly located set of kill bleeds. I've corrected the timing but haven't had a chance to put that carb back on. Maybe some time this fall.
You may not have to.
Test first, drill second.
It all depends on the combination and the carb.
I've told this story before. Had the car on a chassis dyno and just made jetting changes and experimented with a little more timing (no good). Two pulls were about a equal so my on the spot decision was leave with the slightly smaller PMJ setup.
On the freeway type highway, the engine started revving and then dying. It only happened when over 60 mph or so. So my solution was drive home at 55-60 max (which was OK on that road).
Moral of the story was those jets were the lean limit for cruising. Additionally, while maybe just as much power on the dyno, likely a little too lean for the track. So on that carb and application, the PVCR could be left alone.
------------------------------------------------------------------------------------------------------
With Respect to finding PV opening by checking for increased acceleration when crowding a vacuum.
Here's an example:
Blue line is vacuum. Black is RPM, Orange is forward accelation. Violet is AFR derived from WBO2.
The first two steps down in vacuum are matched with step increases acceleration and rising rpm.
In this example, done at relatively low speed (25 - 50 mph or so) the next increase in throttle, dropped the vacuum as expected but there was no increase in acceleration. This dead spot around 12" HG was not because the AFR was lean. In this case it was because the timing was too advanced and possibly also due to an incorrectly located set of kill bleeds. I've corrected the timing but haven't had a chance to put that carb back on. Maybe some time this fall.
