Thanks. I have a revised list of crushes as people measure differently and torques are different.

I'll make the changes.

So while were talking about this stuff... Has anyone here actually measured the chamber volume of the P79 themselves? The online calculators use 53.6 cc, but can anyone here attest to the accuracy of that number?

I've not measured it myself, and I don't currently have the equipment.

I have one from a chap named Jeff that is 53.5cc. One sec and I'll find the source.

Might that be one of our resident racers @Jeff G 78 ? Seems like the kind of thing he'd be all over.

I also found another error source... If you look carefully at the piston design, you'll see that the tops of the pistons don't actually touch the cylinder walls. There's a small gap between the piston tops and the walls. You can see the effect of this by the carbon ring left at the tops of the cylinders where the rings don't scrape. Nothing scrapes there because there's no contact.

I measured the volume of that area to be about 0.63 cc. Doesn't sound like a lot of volume, but since it's uncompressed, it changes the compression ratio by almost 0.1.

So if you're at 8.9 and want to get to 8.8, you can account for that.

I'm at 8.83 on my spreadsheet and am about to call it a day. Now I need to find someone who's measured the P79 volume AFTER they cut the head.

Edited September 12, 20195 yr by Captain Obvious
Fixed the wrong tagname

@Jeff G 78

18 minutes ago, Captain Obvious said:

 Now I need to find someone who's measured the P79 volume AFTER they cut the head.

From the great Brian Little:

" The one on the right is a P79/P90/P90a chamber. It's actually larger/deeper in volume than the N42/47 at 53.5cc. Compared to an open chamber design you can see how the flattop piston "quenches" the mixture into the sparkplug as it reaches the top of it's stroke. This is a pretty standard shape for most modern heads. The photo is after shaving it .080", I measured it at 46cc in volume. Note the extra water hole on the bottom right vs the N47. "

 

Source: http://datsunzgarage.us/p79/

6 minutes ago, siteunseen said:

@Jeff G 78

Thanks for that. I had the wrong one! He's probably wondering.... "Huh?" 

7 minutes ago, 240260280 said:

The photo is after shaving it .080", I measured it at 46cc in volume.

Cool. And thanks for that too!

Edited September 12, 20195 yr by Captain Obvious

?  The Hoover bag will never let you down ?

 

14 minutes ago, Captain Obvious said:

Thanks for that. I had the wrong one! He's probably wondering.... "Huh?" 

@Diseazd would be another P79 king.

Opinions here please.  I have access to an N42 and a P79.  Without much drama, which would be the easiest to "hop up"?  I've read the P79 shaving and older valves work supergood but is there that big of a difference in a N42 head.  I like square exhaust ports. 

Me too...