Engine and Transmission Tech

I was running some calculations today pondering piston and rod options for my 4.6 block. I happened to run what I have as the stock figures for the 4.6 and the lowest compression ratio I could come up with was 9.44:1! The standard published figure I have seen for the 4.6 is 9.35:1, in fact it is printed right on my block. So here are the figures to get 9.44:1, anyone have an idea why the difference? 0.09 is not a lot but it is a difference.
Bore: 3.7008" (94mm)
Stroke: 3.2284" (82mm)
Chamber vol: 29cc
Head Gasket Thk: .05"
Head Gasket Bore: 3.8"
Piston to Deck Hgt: 0.0387" (8.96-(3.2284/2)-5.8937-1.4134)
Piston Dome Vol: 22.29cc

Thanks
Mark

I'd be happy with 9.35:1, Mark.

Most ones I see are 8.16:1.....Your engine is UK market?

The engine came from Atlanta in the US but I am unsure of the origin. The Rover it came out of was stripped pretty much beyond recognition when I stumbled on it in the bone yard.

My puzzlement is why the figures I have gleaned from reliable sources don't add up to the factory rating of 9.35 stamped on the engine when you run them through a compression calculator. And I actually had to play with the figures a little to get to 9.44. If you use .046" thickness for the head gasket and a 3.74" bore which one source says is factory and, you wind up with 9.575!

I tried three different calculators and they all came up with the same answer. I hoped someone might be able to shed some light on why the numbers don't add up. If I have a mistake in my figures I would like to find out what it is. 9.35 is obviously the published factory figure but I can't make it work.

Any ideas?

It is puzzling, but apparently not troublesome. Without having given it much thought, I had guessed that the difference might have been in the compressed thickness of your head gasket... but you've already been thinking about that, eh? Anyhow, such a small difference probably won't effect piston/rod decisions.

I guess you've already checked your heads for flatness and determined whether or not yours need to be skimmed, right? Mine needed skimming when I was ready for a valve job, but I was happy to get the extra little bit of compression.

Have you played with "dynamic compression" calculators yet? Here's one: [www.wallaceracing.com]

Thanks Curtis,

Yes I agree it is not a critical difference. 0.2 is probably not going to affect the project significantly. I was just surprised that I could not come up with the stock figure using the stock specs which we all seem to take for granted.

I have significant work planned for the heads. Joey Martin at High Velocity Heads was working on a setup for me with Ford Windsor valves (1.78, 1.45). He got up to 204 at 0.6 on the intake before he decided to quit doing heads and go back to school. He is still available for consultation however.

I have two sets of heads, a 3.9 set and a 4.6 set. What I was playing with was when I found this problem was whether I could get a stroker combination I was happy with using the 4.6 heads since they would be less work to port. Right now it looks like a toss up.

The dynamic calculator is a good idea. I have not tried that and will see what happens.

Oh, another issue I have not seen addressed anywhere. My 4.6 heads do not have the usual valve centers of 2.66." They vary from 2.635 to 2.65. Joe says this is not uncommon. A guy comes in from a rough weekend at the pub and sets the gang drill for the week's run a bit off. I guess the lesson is always check your valve centers before you put a lot of time and money into a set of heads! My 3.9's are spot on at 2.66".

Mark