Lower compression pistons = lower vacuum pressure?
 

I recently had my engine rebuilt and installed lower compression forged pistons. I've noticed that my vacuum at idle is a bit lower than before the rebuild. Could it just be the pistons?

Yes, but the effect shouldn't be too miuch. What was the compression ratio before/now? Tuning has A LOT to do with it. IE-I have more manifold vacuum here than most (idle 23-24 kPa) but it's from running rich with advanced timing in the 20-21* range.

I switched from whatever stock is (10.5:1?) to 8.5:1. I haven't changed the tune significantly yet because I'm still trying to chase down a few gremlinsafter the rebuild.

There's not much of a vacuum difference, but it is noticeable on my vacuum gauge.

Thanks!

Stock is like 8.8 or 9:1 IIRC. Not enough difference to loose any vacuum at idle. If you have retarded base timing that will reduce vacuum.

Cool, I'll check.

Thanks Pat!

9.4:1 manual and 9.0:1 auto for a stock 1.6 CR, but yes it can cause a change in vacuum.