Can you please explain to me how changing cams bumps up compression?

I've rebuilt a lot of motors, both car and atv/dirtbike, but nothing has or ever will change compression one these motors other than the piston shape and head height. I just don't get how a cam can do it...

 

I don't know eather I know going from a non PI head motor and putting PI heads on it will.

 

If I'm not mistaken, PI heads have a slightly shallower deck, which does bump up the compression and therefore power output. JMC, help me out here...

 

I think it has to do with the combustion chamber and intake and exhuast valves.

 

Valves don't have anything to do with compression...

 

The non PI heads are 51 cc per cylinder, and the Pi heads are 43 cc per cylinder.

 

the longer the valve is opend the more air can be sucked in increasing the engines compression because it has more air that it sucks in.. pistons and lower CC heads will increase the compression greater.. cams do to.

 

that just allow the intake ports to flow more air if they are undercut

 

Non Pi heads have a 52cc combustion chamber and Pi heads have a 49cc chamber. That is why Pi heads on a non Pi block raises the Compression ratio. Ford used 11cc dished pistons on the non Pi and 17cc dished on the Pi heads to control the CR. The Pi heads came with a hotter cam. So a less expensive route would be Pi cams in a non Pi head. If you find cheap used cams it could be an inexpensive alternate. According to SH the non Pi head runs out of breath at .350 lift where as the Pi head will go all the way to .600 lift. So choose your valve train and head combo carefully.

JMC

 

JMC - If you are reading the same SH book I'm reading, you will see that the non PI heads are 51 cc, PI are 42 cc ( page 42 ). It would take a bit to make the cr jump that much.
I am thinking abot the PI cams on a non PI head, as you suggested. Maybe the non PI $VO heads ( 53cc ) with boost. What do you hear about these?

 

U are tanken about how to built a max hp 4.6 right. If so I got it.

 

97- Yes, I mean the 4.6 heads and cams.

 

I got the 52cc from the shop manual spec page and you are correct I mistyped the Pi spec. It is not 49 it is 42. Sorry about the confusion.

JMC

 

double post

 

don't confuse compression ratio with cranking pressure.

the pi headed engine not only has a better breathing head but has a better intake and better cams so the hp increase is a result of not just the head.

if you have the skill or the money a swap from a non pi 4.6 to a pi head 5.4 will give you a dramitic increase in power.