Rover v8 build: 4.0 block, 4.0 pistons, 4.6 crank and rods, 3.9 heads

[juha_teuvonnen]

Hi All,

I am contemplating the following Rover V8 build: 4.0 block, 4.0 pistons, 4.6 crank, 4.6 rods and 3.9 heads. I was told that 4.0 pistons combined with 4.6 crank and rods will yield a compression ratio somewhere in the neighborhood of 10.9:1, which IMHO is a bit high. I am wondering if I use the 3.9 heads (37 cc) instead of 29 cc 4.0/4.6 heads, what will be the compression ratio in my build?

[juha_teuvonnen]

I did the math  based on http://en.wikipedia.org/wiki/Compression_ratio

With stock 4.6 pistons,rod and crank, assuming
bore=94 mm
stroke = 82 mm
Compression ratio Cr=9.35

We get
Chamber volume Vc = 68cc  

According to clever and resourceful folks on discoweb, with 4.0 pistons, 4.6 rod and crank, the Cr turns out to be10.9, which in turn means that the chamber volume turns out to be Vc ~ 57.27  

If we slap a 3.9 head instead of 4.0/4.6 on that puppy we increase the Vc to approximately 65.27

Which yields us a compression ratio of approximately 9.71

While 9.71 != 9.35 it should be workable with premium gas only, especially given the fact that GEMS has knock sensors and knows (how to/when to) retard ignition timing and enrich the mixture.

[Phillip G]

You guys are amazing,.

Are you all former GM & Ford engineers ?

Where'd you come from ?

keep them on the track,

phillip g