Compression

THANKS >";"< BUDD

Clooney,

The atmospheric correction factors for "below sea level" are probably only valuable for submarine racing -- for Russian or third world navies still using internal combustion at this point. --sorry, couldn't resist.

Just for reference, there's a guy who's already posted two similar charts on the web - one a simple graph for stock land travelers which probably answers Budd's question fairly well. The second one introduces air pressure correction factors which will be of interest if he's moving from Denver to New Orleans or vice versa. Used to worry about altitude performance factors on my Dad's Mooney bird, but never thought much about it in regards to tractors here in the flatlands.

Anyway, here's the link -- and with some luck, the basic chart will even show up here.

Steve

• Compression ratio vs. cranking pressure

Sounds good, but doesn't necessarily work that way. An old engine with 10.5 compression ratio may have 150psi while a new engine with an 8:1 ratio may have 190 psi. Slide the scale down and you can get the same results in tractor type compression ratios.

In general, assuming all other things are equal -- ceteris paribus, as I remember from years ago in academia -- the higher the compression ratio, the higher the cranking pressure you'll read. But outside academia, in the real world --Engines get old and tired, worn rings and valves that don't seat so well anymore and your cranking pressure goes down even though the compression ratio (which is the relationship between the swept and unswept volume of a cylinder) stays the same. By the same token, take a stock engine, hot hone it, install killer dykes rings, do a trick 3 angle valve cut and you can double the cranking pressure while not changing the compression ratio one iota. Or, you could deck the head or block, thereby increasing the compression ratio and install crappy rings and get lower cranking pressure.

Steve

What you say about engine condition is true.
But an 8.1 engine will never produce 190 psi with
out boost from some source.

Psi is related to compression ratio. The higher
the compression ratio - the higher the psi.
It is psi that heats the air to allow combustion
in diesel engines.

Most any diesel mechanic should be able to tell
us what psi is need for a diesel statically.
It is 14 to 1 compression ratio-wise.

best,
bill koob

Agreed. In general, you're right about the gas engines. But as they say: "never say never". I just posted a chart above that says the normal 8:1 engine you see trundling down the street or plowing out on the back 40 should crank 120 to 150 psi.

At the far reaches though, if you follow the link below; you can buy a set of rings that will help get you far better than 200 psi on an 8:1 engine. Granted, you've got to get a really good hot cylinder hone, deck the block and head(s) nice and level, use a really good head gasket and have a good seat cut and valve fit - but it can be done. Question is: why would anybody want to throw that much money at an 8:1 engine. Generally too much C/R to turbo or supercharge, too little to give performance normally aspirated. Those rings cost over $300 per set per piston.

The NASCAR "stock car" guys run a nominal 12.5:1 compression ratio, and according to my Wadell Wilson book, they crank at over 400psi. Used to build Ford Cosworth motors for formula cars, and use those C&A rings on nominal 10.5:1 motors. They'd crank around 350psi when they were fresh.

Don't mean to be argumentative, but went and checked my old engine building notebooks -- and was once again astounded at how much different the "specs" were for a breathed on motor vs. one the assembly line kind of specs you're quoting. Got to remember to also change your engine life horizons from 5 years / 50,000 miles to 5 months / 5,000 miles -- or whatever that translates to in terms of hours on a tractor. Those C&A rings will wear down a cylinder fairly rapidly.

take care,

Steve

• Good, expensive piston rings.

I'll just have to defer to your superior
knowledge - but I do know several things
- such an engine could not run on pump gas
and carburation would also be an obstacle.
Then - I wonder how you gain volumemetric
overcharge of air. When I test my pressure
I look for how long it takes to draw down
the pressure - Some engines hold the pressure
very - very well. If they do not hold pressure
there is some mechanical problem - valves or
compression rings. I never waited longer than
a minute - but most well built tractors seem
to hold psi well. Now I have to assume from
what you have told me - that most rings and
valves lose air until reaching the seal limit
of the rings and valves and thereafter hold
their psi limit. Well we can see the result of
ring failure out the the crankcase vent
system on tractors - I suppose if we measured
the volume of air escaping a normal engine -
we would find the missing psi potential.

Thanks for the information
best,

bill koob

Good thinking. You can buy that gizmo you're talking about that measures the air escaping at the C&A link.

What you're doing with the compression gauge by waiting and what you're talking about with measuring the amount of air escaping is basically called leakdown testing. Like you say, it lets you know where the weak link is in holding pressure. You get to TDC, or better yet - pull the rocker arms or OHC and go to BDC and lock the flywheel in place, then pump air into the cylinder thru the spark plug hole, then seal off the compressor side at say 100psi and watch how much escapes via a pressure meter on the air line. A good race engine will hold 98 to 99% A good street engine, or good tractor engine will do nearly 95%. Anything below 90% and there's real trouble in that cylinder. The nifty thing is that it tells you where the problem is. Hear hissing thru the carb and you know there's an intake valve problem. More often you'll hear it thru the exhaust; a burnt exhaust valve. Pressure from the crankcase vent means rings aren't sealing. Get a leakdown tester and you'll throw rocks at your old compression gauge -- and only use it for quick checks. But leakdowns are a little pricey -- 80 to 150 bucks at most places. There was a web page I saw on how to build your own -- you need one or two air gauges, a coupler, a adjustable pressure valve, and an air fitting to seal into spark plug hole -- and of course you need an air compressor capable of 80 to 100 psi. I'll see if I can dig out the info if you're interested.

Other things: you're right, those 10.5:1 engines cranking 300+ psi don't like pump gas. Something around 104 to 106 octane is more to their taste. The carbs don't have cute things like chokes, and there's a lot of work done getting the air to flow in and out well too -- otherwise all that cylinder/ring/valve work would go to nought. I remember reading that somewhere around 11:1 to 11.5:1 is where you in effect get volume metric overcharge on most stock gas engines. Those 12.5:1 gas engines need steel O rings machined in the tops of cast iron blocks and all kinds of other tricks to hold together.

Keeping those kinds of engines is sort of like dating a large chested, good looking woman. They're both generally very high maintenance. My time of doing valve jobs and cleaning ports once a week for fear of losing a HP or two at the track next weekend is past. Give me something like my 50 year old popper that's built to run forever with minimal maintenance. And this woman who has a few years on the popper, but has aged well and is easy to get along with is just fine -- just don't make her green.

good talking with you, let me know if you want more info on the leakdown tester, take care...

Steve