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MG MGB GT V8 Factory Originals Technical - Compression Ratios: Static and Dynamic

Hope this link comes up, it's a good one.

http://members.uia.net/pkelley2/DynamicCR.html
Glenn

Glenn,
Yes the link does work and yes it IS a good one.
Thanks for posting.
Cheers, Pete.
Peter Thomas

Hey -

"The Compression Ratio (CR) of an engine is the ratio of the cylinder volume compared to the combustion chamber volume. A cylinder with 10 units of volume (called the sweep volume) and a chamber with a volume of 1 has a 10:1 compression ratio."

- Oh no it doesn't! it is 11 to 1 CR (swept vol + chamber vol divided by chamber vol)

Other statements are misleading - compression can, and does, start before the inlet valve closes, moving inlet air has momentum which has to be overcome before air in the cylinder can escape.

Detonation (pinging or pinking) is not dependant on the DHR but on cylinder pressures which are dismissed early on.

An intereseting but misleading calculation!
Chris at Octarine Services

The main feature of the link is the DCR calculator, which allows you to determine if an engine with a seemingly high static CR will operate on pump gas, and if not, how to tweak your combination of parts by means of cam ICA, HG thickness, valve relief/dome/dish volume, combustion chamber volume, etc so that it will.

I put it here because of differences of opinion on the GM 60* V6 and using aluminum heads with iron head pistons. I figured others building an engine could benefit from it too.
Glenn

Sounds like a helpful tool. I think Chris has a point though, really I wonder if it isn't mainly a matter of definitions. Whatever the static compression, actual effective compression with the engine running is an entirely different matter. Big difference with the throttle opened and with it closed, but it's still the same engine. In fact, any component in the inlet or exhaust tract can affect the effective ratio, and it is quite true that the proper combination of velocity stacks, tuned pipes, and valve timing can make a dramatic difference in a specific speed range. Whether or not effective compression under those conditions can exceed static compression is something I do not know for certain, but I rather suspect it can.

Jim
Jim Blackwood

Dynamic Compression IS variable. As engine speeds increase, the ram effect from the increased velocity of fuel/air stream increases and expedites cylinder filling at higher RPM's. As the amount of air/fuel crammed into the cyclinder increases, so does the dynamic compression. This is why the NASCAR guys get such high volumetric efficiency.
Kurt Schley

This thread was discussed between 06/11/2003 and 13/11/2003

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