Compression psi and ratio and fuel

Jim McVay

Member
Is there a rule of thumb telling you what psi you have doing compression check as to what compression ratio you have and for what ratio do you need what octane fuel?
 
only way to know correctly is know the head chamber(combustion cc's) and use the formula. A gauge only shows that the rings are sealing,and all gauges vary a lot.if you hear pinging,lower timing and add fuel with higher octane level,too high robs power too,but preignition and detonation are not your friend.If you have a quality gauge,check it on an engine of known compression ratio,compare !! A close rule of thumb is numbers around 260 - 300 psi are generally in the ten plus range,where you would want 110 octane
 

Not usually an issue on a tractor..but..as the valve timing increases 9 lift/DURATION..THE Lower the cranking pressure, while the operating pressure ( At Peak Torque/ & Horse Power rpm range will be ( or could be) higher..
 
Most of the old shop manuals for a lot of different makes and models of tractors, say 5:1 is 100 psi. Ad to what modern day technology brings, its still being recorded and tested.
 
(quoted from post at 15:30:52 02/26/17) Is there a rule of thumb telling you what psi you have doing compression check as to what compression ratio you have and for what ratio do you need what octane fuel?
An interesting question. When I left racing and took up restoring/pulling antique tractors, it was the first time I heard anyone talk about cylinder pressure as though it could be considered an indication of compression ratio. For racing engines or any high performance engine, cylinder pressure is only a measure of the relative health of an engine when comparing cylinder to cylinder numbers and has little to do with compression ratio.

Looking at old tractor service manuals, I found it is not uncommon for a listing that shows compression ratios along with a specified minimum cranking cylinder pressure. For Waukesha engines in an 88, 4.75:1 is 90 psi, for 6.75:1, 120 psi, and 15:1, 350 psi. Cylinder pressure used for these old tractors is valid with stock cam timing with their very mild timing events.

When the cam timing is changed to more radicaL specifications, cylinder pressure is no longer a true indication of mechanical compression ratio. Cylinder pressures now becomes an indication of the mechanical conditions such as ring seal, valve seat integrity, and gasket condition.

As far as when high octane gas is required, that is subject to actual testing
and is a result of mechanical compression ratio, combust chamber efficiency, cam timing, ignition timing, carburetor efficiency, flow velocity, and to some extent, rpm range being used under load.
 

Always ask what the Piston Rings are made from....If CAST IRON do NOT walk...RUN from them..they WILL shatter at the 1st sign of Pre-ignition..

NODULAR IRON ( Ductile ) Rings are what you want..they are not brittle and will not break/shatter with detonation...but that

does not mean damage won't happen ( Ring Seal, Bearings, etc)..
 
I would like to add a question I don't know the answer to. I hear a lot of talk about having cranking pressure a lot of times of 200 to 250psi. What advantage is there in high pressure over stock of say around 160? I run a bored c301 at 10.7 to 1 ratio but still have a pressure of about stock? Thanks for the education, good topic. Lee
 
All other factors being optimum: higher compression produces higher horsepower (at least to a point. It is possible to have to much compression and start losing hp ) case in point: a GB Moline gas with 10A4209 heads and 140-160# compression produces about 60 hp. Put 10A4210 heads on that engine with 200-220# compression and it will dyno 80 plus.
 
Cam timing and duration has as much to do with compression pressure as the mechanical compression ratio does.
There comes a point of diminishing returns with increased compression ratio and compression pressure. vs. keeping the valves open farther and longer to flow more air/fuel mixture.
 
As a cam becomes more radical in timing, max cylinder pressure is quite a bit less than a typical stock tractor cam because pressure is continually being lost until the intake fully closes after BDC. So if everything is in mechanically excellent shape, a lower cranking pressure is nothing to be concerned with. The real test of cylinder condition is a leak-down test which is quite simple to perform. Using the Twin gages, it's a measure of leakage and is easy to determine if there are any problems with either valve seat, piston seal, or cracks. Instructions say to use about 30 psi air pressure, but we prefer to use 80 or more psi because a leak is easier to hear in the pipes or intake.

About 10 yrs ago I put together a racing engine just to determine the effect of mechanical compression ratio on our 360 Late Model motors for Saturday night racing. Started at 16.5:1 and worked down to 12.5:1. Down to 16:1 the motor made good power, but also generated quite a bit of added water temperature. At 15.8:1 motor made the same power with much less water temperature. With the cylinder heads I used, there was a significant drop in HP below 14:1. Interestingly, when we worked on the later style Bowtie Vortec heads, there was more measureed HP at 13.7:1 using .030 piston to head, and 1.85 rockers. The lessons from racing engines are not a direct correlation to these antique tractors. It's hard to create an actual quench and the stock ports ae just too much of a right angle, so more flow is not necessarily good quality. However, in spite of the drawbacks these old tractors make impressive power.
 
On your 360, I would be extremely
curious as to what the cyl psi would
be with a bone stock camshaft in it.

In antiques, in this topic, I belive
we agree upon the fact, that a
tractor cam could be a holy grail to
mech ratio and psi relationship in
its stock designed form.
There is one true test tractor known
that could confirm the data, but I
doubt the museum is gonna let us in
with a toolbox and a grin.
 
How are you performing the test?
Fully charged battery,
All spark plugs out,
Throttle open,
Crank till gauge peaks.
Cold, hot, dry or wet.
A bored out 301 has many definitions lol.
A worn out 301 is 160. Lp 301 were 215.
 

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