No compression at all, apparantley

[frostyadonis]

when i got my bigport head overhauled, i assumed the notches on the cams meant
(top) but i was wrong. the exhaust cam notch should be at the bottom (180deg)

the cam gears can be installed both ways on the cam

check this before you remove the head in fustration

[af300e]

leaving throttle closed will lead to virtually no compression, as there is virtually no air.

That'd make idling a problem...........

[oldcorollas]

intake changed to ITB's, no idle bypass, no PCV, and throttle stops not set properly can lead to very low compression...

the actual compression at idle is very low... the cylinder is nowhere near filled... so if you did a compression test when idling, it would still be well down.
http://www.rbracing-rsr.com/calcinjpulse.html
no idea on accuracy, but they suggest 10-15% VE at idle..
that means the cylinder is 10-15% full.

assuming 10:1 CR, and cylinder is 10% full..... that means the compression at idle is actually.... standard air pressure (roughly) = no movement on gauge at all...

edit, more mathematically..
assume cylinder has 9X volume, and CCV is 1X. total volume = 10X and so CR = 10:1
10% VE = 0.9X air going in.
0.9X/1X = 0.9 compression.. the pressure in the cylinder is LESS than atmospheric at TDC!! you will get negative reading on gauge.

with 15% VE
1.35X compressed to 1X size = 1.35 compression
0.35x14.7 = 5psi reading on gauge.

when that amount of air and fuel is burned, it will increase in volume maybe 3 times, and still exter pressure on piston.... just enough to overcome friction and keep enging spinning at idle speed...

or something like that....
so.... what compression will you get with throttle closed?

[af300e]

intake changed to ITB's, no idle bypass, no PCV, and throttle stops not set properly can lead to very low compression...

the actual compression at idle is very low... the cylinder is nowhere near filled... so if you did a compression test when idling, it would still be well down.
http://www.rbracing-rsr.com/calcinjpulse.html
no idea on accuracy, but they suggest 10-15% VE at idle..
that means the cylinder is 10-15% full.

assuming 10:1 CR, and cylinder is 10% full..... that means the compression at idle is actually.... standard air pressure (roughly) = no movement on gauge at all...

edit, more mathematically..
assume cylinder has 9X volume, and CCV is 1X. total volume = 10X and so CR = 10:1
10% VE = 0.9X air going in.
0.9X/1X = 0.9 compression.. the pressure in the cylinder is LESS than atmospheric at TDC!! you will get negative reading on gauge.

with 15% VE
1.35X compressed to 1X size = 1.35 compression
0.35x14.7 = 5psi reading on gauge.

when that amount of air and fuel is burned, it will increase in volume maybe 3 times, and still exter pressure on piston.... just enough to overcome friction and keep enging spinning at idle speed...

or something like that....
so.... what compression will you get with throttle closed?

Perhaps, in theory.
I've seen a friend (incorrectly I know) test comp on closed throttles. 2t/3t hybrid, approx 10:1 static cr, 280 and 300 advertised duration cams, no idle bypass and idle openings already set, 225psi reading.

If there was just 1atm pressure in the cyl at cranking, why is it so hard to turn a motor over tdc with the plugs in?

EDIT: Ok, negative pressure, so vacuum, the engine is trying to crank on it's own??

[oldcorollas]

because when you turn the motor over slowly, there is time for the air to fill the cylinder.

those numbers before are idle speed.. cranking speed will have higher VE tho
how much higher i don7t know, but VE at hand speed = 100% minus the overlap amount (how much air goes out before intake closes)

edit: IF you couls spin up the motor, and measur the amount of power used.. it shoudl be hard initially, as lots of air gets into cylinder... then it should decrease as the VE goes down with less time for air to get into cylinder....that make sense?

225psi with large overlap cams and closed throttle? there is something seriously wrong with it cams set wrong?

edit: bu tyeah, IF you have a PCV valve which allows air into the manifold, and a large brake reservoir, then you can get some compression on gauge, maybe up to 100psi or so... more if you haev lots of air getting past the throttle...

but original poster has swapped to ITB's, no mention of setting up PCV or brake res, or setting throttle stops or throttle bypass for idle... so i assumed worst case scenario (well, worst case before suggesting something mechanical was borked )

[af300e]

because when you turn the motor over slowly, there is time for the ir to fill the cylinder.
225psi with large overlap cams and closed throttle? there is something seriously wrong with it cams set wrong?

The comp is tested at that slow speed so there should be a reading even if the throttles are almost closed.

So if the starter turned the motor at idle speed there would be close to zero resistance (ignoring frictional losses)?

Yes, 225psi does sound high but I didn't read it myself i might add. Probably cam timing. Still, was making 90kw at the wheels, it can't have been too far out. The point was more that there was a reading, and a significant one even with the throttles "closed" i.e. at idle. Guage was new and tested 175psi on a 4agze the day before.

[oldcorollas]

i edited but yeah, with closed throttle, at some speed (close to idle speed? a bit higher?) there will be little compression able to be measured... not that i have tried it

hmm, second hand knowledge, but a vaguely remember a mates datto which had 11:1 compression and low overlap cams (running on gas), that got 220-230psi when freshly built.. with throttle fully open.

to get the same compression with throttle closed means air is getting in somewhere, and more than should
but then again, for an engine with large overlap cams to be able to idle, it NEEDS throttle open more... so that might explain it? ie, for that cranking speed, it would make no difference if the throttle was open enough to get same air flow as at WOT (ie, the measured compression decreases only if the throttle restricts enough)...

if their battery was low and cranking speed low, then that can also assist with high compression.. due to more time to fill cylinders, larger throttle opening relative to the engine speed etc etc..
but big overlap should reduce that measured amount.. unless engine speed is slow...

then again... thining abotu it....
but 225psi = 15 atmospheres of pressure.
how can a 10:1 static CR motor, with NO overlap, produce 15:1 compression? not technically possible unless there is liquid involved? or heat?

[af300e]

hmm, second hand knowledge, but a vaguely remember a mates datto which had 11:1 compression and low overlap cams (running on gas), that got 220-230psi when freshly built.. with throttle fully open.

then again... thining abotu it....
but 225psi = 15 atmospheres of pressure.
how can a 10:1 static CR motor, with NO overlap, produce 15:1 compression? not technically possible unless there is liquid involved? or heat?

Probably the same way your mates datto achieved 15:1 on a 11:1 static CR lol

There is obviously more to it than that of which these simple calculations are composed. The anecdotal evidence, both mine and yours suggests that a reading of around 225psi is possible on engines with static CR around the 10-11:1 mark. Your calculations suggest this is a CR of 15:1. So obviously there is more to it. You're no doubt better at the theory and the maths than I, I'm sure you'll nut it out

EDIT: Sorry if this has moved from the topic.

[oldcorollas]

for sure there is i think it is heat during the compression stroke, and lower cranking speed also helps with VE... plus any fuel that gets in (carby, or EFI with innjectors still spitting) can raise it by volume or by vapour pressure etc...

perhaps there are some engines which had 2 options for starters, which give different speed and thus compression readings?

225psi IS 15:1.. there is no argument abotu that it is 15 times atmospheric pressure....
i cbf finding articles abotu it, but i am sure there is somethig out there regarding cranking speed vs compression vs intake valve closing timing vs CR etc

[oldcorollas]

this is a good link! considers a few things
hmm they say cylinders will nearly fill at crankign speed...

http://www.pumaracing.co.uk/comp.htm

they mention "If the throttle isn't open then air can't get into the cylinder and the readings will be far too low."
but don't say how low.. (depends how many intentional air leaks there are?)

bugger it.. recopied for those lazy for link

How Much Pressure Does An Engine Generate At TDC ?
At cranking speed there is plenty of time for air to enter the cylinder. We can expect that when the piston reaches BDC the cylinder will be full of air at atmospheric pressure - which is approximately 14.7 psi. As the piston rises, this trapped air will be compressed and the pressure in the cylinder rises. If the engine has a compression ratio of 10 to 1 then all the trapped air will be compressed into one tenth of its initial volume. We might expect that the final pressure at TDC would then be 147 psi. This ignores a number of factors about how both gases and engines work.

Temperature Rise
When a gas is compressed its temperature rises - this is a basic fact we are told by the Universal Gas Laws. This increase in temperature leads to a further increase in pressure over and above what we would expect from consideration of the compression ratio alone. If the compression ratio is high enough, the temperature generated in the cylinder can ignite the fuel without any need for a separate spark.
This is how Diesel engines work. In an average petrol engine the effect of temperature is to generate about twice the pressure we would otherwise expect. In that case maybe we should expect an engine with a 10:1 compression ratio to generate about 300 psi at TDC instead of 147 psi? There are more factors to consider though. Copyright David Baker and Puma Race Engines

Inlet Valve Closing Point
Inlet valves do not close at BDC but well after this, when the piston is already part of the way up the bore. This is done to help trap more air when the engine is turning at normal running speeds. There is not enough time at running speed for the air already drawn into the cylinder to turn round and go back out of the inlet valve before it shuts so more air gets trapped than if the valve closed at BDC.
At cranking speed of only a few hundred RPMs though, everything is happening very slowly in comparison. There is plenty of time for air drawn in at BDC to get pushed back out of the valve before it shuts. The static compression ratio is therefore not a good measure of how much air is actually being trapped and compressed.
The longer the cam duration and therefore the later after BDC the inlet valve closes the less air gets trapped at cranking speed. For a given compression ratio we can therefore expect engines with race or rally cams to generate lower cranking pressures than engines with road cams. Copyright David Baker and Puma Race Engines

The Compression Gauge Itself
The gauge and the tube leading to it also have some internal volume. In effect by screwing the gauge into the spark plug hole we are increasing the volume of the combustion chamber by several CCs. This reduces the compression ratio below the static measured figure and further reduces the pressure that the engine will generate during the compression test.


So What Cranking Pressure Can We Expect To See Then?
For an average road engine we don't need to calculate the effect of all the above factors because the expected cranking pressure will be quoted by the manufacturer. Haynes manuals and other tuning guides also quote the figures. If the data is not available you can make a good estimate of the expected cranking pressure from a healthy engine just from the compression ratio.
You can expect the cranking pressure in psi for a road engine with a standard cam to be about 17 to 20 times the value of the compression ratio. So our engine with a CR of 10:1 should generate between 170 and 200 psi. The better the condition of the engine the closer the figure will be towards the top end of this range. If the engine has been modified by fitting a different cam then you need to make some allowances.
Fast road cams will reduce the expected figure by 10 psi or so. The best thing to do is take a set of compression readings as soon as the engine has been built so that you know what it generates in good condition.

[af300e]

That is an interesting link OC.
It does seem that the further into it you delve, the less likely that there can be a zero reading for compression at cranking speed. As long even a very small amount of air can make it's way into the cylinder, there should be some reading.

On the 225psi 15:1, no argument here. The issue was that the figures were suggested as proof that the static CR was 15:1 when they are not? Makes sense when it's considered that CR is ratio of volumes rather than ratio of pressures. I.e Static CR doesn't take into account gas behaviour. I really don't know enough about it (gas behaviour and adiabatic compression heat or whichever theory this situation follows). Is there any info on what the temp rise is when air is compressed by particular factors?

Anyway, I accept that the rare situation could exist where the cylinder is so evacuated during intake stroke that the reading might be so small that it can't be read though I've never experienced it.

Is that the situation in this case? Probably not. My money would be on funky cam timing as has been mentioned.

[Cuzzo]

Haha OC and AF have taken it above and beyond the OP and what we all came here for.

Seeing those posts, im not going to bother helping let alone reading those equations.

Lets just help chrisso fix his problem?

[greywolf]

He did mention that he was getting 15psi, not zero.

[Dieseltrain]

Id be guessing 1 cam is 180 out . Seen this happen on a motorbike, similar issues , ex cam was 180 out. changed, bang started 1st go, GL.

[Chrisso]

Thanks for the theoretical discussion of VE at various engine speeds. I think Wilbo might have it at the second post with cam timing.

Just for info, the PVC is open to atmos, there's no idle up mechanism (blocked), and the map sensor, fpr, and brake booster are feeding off an accumulator that's not completely sealed yet. So, even with the throttles closed there should be some air leaking in.

Like I said, I'll get a chance to have a look at it on saturday morning and hopefully it's as simple as the cam timing and I'll be up and running before lunch.