Compression test vs. compression ratio

SoCar72

SoCar72

Well-known member
I think I'm missing something...

According to Motors Manual specs for the '63 170, the CR should be 8.7:1. Given that average atmospheric pressure is 14.7psi then a compression test should yield 142.59psi ((14.7 x 8.7) + 14.7).

Testing my still original engine, my average was about 150psi (overall range of 7psi). This suggests that the CR is more like 9.5:1.

Furthermore, Motors Manual states that a fresh build should test at 170psi (+/- 20 psi :shock: ) for the 8.7:1 which computes like a 10.5:1 - 12:1 CR.

Are the "build" CR's spec'd as dynamic ratios whereas the static is considerably higher? Or is this some anomaly generated by the inline configuration? I did notice, again according the Motors Manual, that the 221 - 260 V8's of the same or comparable CR were spec'd to have a significantly lower compression test pressures, figures closer to what the math predicts.

What am I missing?
 
Your camshaft events have a large influnce on cranking compression. Mild camshaft higher #'s more camshaft lower #'s with the same compression ratio.

If you don't mind using 93 octane try to get your cranking compression close to 190-195#.

For example if you use flat top pistons that will raise your static compression close to 1 point.

But you will need a camshaft with a later closing intake to bleed off some of the compression.

150# is fine for an engine with regular fuel. Bill
 
There is undeniably a difference between dynamic and static compressions which brings me to my question. Static assumes 100% volumetric efficiency of the compression stroke : in the simplest expression - BDC volume / TDC volume. Dynamic factors in the "real world" of fluid, rotational, and thermal dynamics. The dynamic result can vary greatly, usually lower, from the static.

If the static compression is supposed to be 8.7:1 then a compression test should yield about 145psi. But I'm getting 150psi which suggests the static compression ratio is more like 9.5:1. Furthermore, Motors suggests that I could measure as much as 170psi, translating to about 10.5:1 static CR.

If end pressure = (CR x atmospheric pressure) + atmospheric pressure, then the the compression tests suggests a higher CR than what the specs indicate. Unless the spec'd CR's are dynamic CR's and not static CR's.

What I find odd is that the V8 compression test specs are more indicative of the pressure formula above. If you run their CR's through the formula, the results are numbers close to what their respective compression tests should deliver.

However, with the L6's, the compression test specs are considerably higher than what the pressure formula predicts. Even if you compare a L6 to a V8 of the same or comparable CR, you'd expect the respectful compression tests to register similar numbers, but they do not. The L6 pressures are consistently higher, even with the same CR's. Why is that?

I can only explain this as some funky anomaly within the L6 that seemingly defies physics, or the L6 CR's are spec'd as dynamic whereas the V8's are static. Can the cam duration, lift and LSA's be so different to allow such a difference in the pressure tests?

I'm sure the answer is simple, I just don't know what it is.
 
You also have to take into consideration any error that your compression gauge may have in it. Im sure that when all the OEM testing was done they used laboratory quality instrumentation to develop the standards for measure we use today. Our tools and methods of testing in no way could ever be as accurate as theirs.
 
Wrong wrong wrong,
The compression pressure is dependant on many factors, and your right in assuming that the ratio is most important, however the calulations your using and simple ratios which DONT take into account any heating effects contributed from other reasons than the compression itself. The simple gas laws use adiabatic compression which it isnt in an engine, leakage etc wont let that rule work.
Ive tried to figure out what pressure you will get from the CR but it always comes out wrong. Ill give an example, we have CNG engines wich have a CR of 12.5:1, they regularly give a compression pressure of 350psi, which is way higher than you would expect?
Overall, Id simply use your compression figure as a testing tool only.
A7M
 
A7M - Now that's interesting.

Compression is work, work creates heat, heat expands air, thus compression can exceed what the CR predicts. That I know, but evidently chamber and piston shape can more greatly influence how much heat is generated by the work than I expected. Chamber shape is different between the L6's and V8's and the L6's have a higher quenching effect which creates more chamber turbulence, hence more work, heat, expansion = higher pressure. That's the variable I wasn't wrapping my head around, turbulence during compression.

I suddenly remember an article I read many years ago about some mechanic in India that was experimenting with grooving combustion chambers (on small engines like scooters, mostly 2 stroke) with the intent of creating higher turbulence during the compression stroke. His experiments were backed with higher hp/tq and fuel economy findings. He postulated that is was due to better mixing in the chamber and faster flame travel. It seems to reason that the higher turbulence could have yielded extra chamber heat and pressures as long as the heat generated created greater thermal expansion than the net loss of chamber volume due to the grooving. I don't recall any discussion in the article about thermal dynamics, but then again, this was a shadetree mechanic with no formal training, a Dremel tool and a lot of time. Though he did state that he had to adjust timings, mixtures and sometimes increase the octane of the fuel; which are common "high-compression" solutions.

I wonder how the chamber and pistons are shaped in that CNG engine...
 
I think the Indian mechanic's name is Singh; I've read a few articles which mention him but they're all bland overviews and interviews that really don't address the "why" behind his "miracle work". I think one was in Newsweek so that might be a good place to start searching; I'd like to find out more about his ideas and correlate them to current CC theory.

The articles I've read haven't shown much in the way of pictures of his work but one of them did show a combustion chamber with a series of grooves, in a "star shape" centered near the top of the combustion chamber. Interesting, definitely enough material removed to drop compression ratio a point or two (if he didn't do anything else in there).
 
Combustion chambe on our CNG engines is a bowl in piston "heron" type, we have never been able to experiement with anything else as we are always converting a diesel engine, most of which dont have a chamber in the head. You couldnt have a chamber in the head because you would never be able to get the 18:1 CR they need.
Effectivly there is little difference between six and 8 cylinder engines, your only considering the old log head, which for us Aussies is a curiousity only, way too old and superceded.
The Crossflow six is more like a cleveland and the 2 valve OHC is a twiisted hemi, the DOHC an pent roof.
The turbulence is mostly produced by the squish effect between piston and head in wedge type chambers and to a lesser extent by port shape. if you look at the log head inlet port youll see that it biases the flow to the side of the port. 4 valve heads use port induced tumble.
We find in our cng engines that we can lower emissions and increase torque by changing the squish area, more squish more torque.
Im a bit doubtfull on the term "quench" why would you want to put the fire out? You do need to get combustion temps down to reduce oxides of nitrogen, EGR is often used for this, have you noticed modern diesels using this?
Anyway, I still maintain that compression pressure is only an indictor of the engines health, as so many things affect it.
A7M
 
On-Topic part of reply:
Like A7M and others have said; there's so many things going into what the compression "ought" to be, that I use the compression tester as a rough guide to cylinder condition. If all the cylinders are within 10% then things are probably not too bad (assuming that they aren't all evenly carboned up, or worn out...). A leakdown test and observation of a vacuum gauge can tell you a lot about the inner-cylinder dynamics that a compression gauge doesn't see; for example, WHY one cylinder might have a low compression ratio.

Off-topic part of reply:
Engine design has come a long way. I'm not about to hack up the CI head to test Mr Singh's ideas (although that could be neat!); maybe I'll do it to an iron head. In 10 years, when I get to it. :/
I am rebuilding an ancient lawn mower and the head is currently off. I am debating a few hacks in that; it's a 1950s Clinton Panther 2-stroke that barely ran when I got it and is more of a curiosity than a lawn machine. How much damage can I do? ;)
 
The "quench" was a typo, should have been "squench". I have no idea where the word came from but I came across it on the CI tech pages as well as the Falcon 6 Handbook, referring to the piston area that is under the head surface and not the chamber in which area the mixture is squeezed rapidedly into the chamber proper as the piston comes inducing additional swirl/turbulence beyond what would be established by the intake runner. I haven't seen an Aussie head and only have my '63 log to go on with my rationalization.

A7M : What do you mean by "heron" type. I understand the bowl piston and chamberless head.

Agreed, a compression test is mostly good for comparing the consistency of the cylinders to each other as it only gives you a general (at best) idea of the actual compression.

That's the article. When I read the article, my biggest concern was detonation hot spots. Some of the photos show large grooves, or notches, that left bumps protruding into the chamber. It seemed like some of these could act like glow plugs. The article read like the journalist was an autophile and not a mechanic or engineer, offering little to no explanation to how the mods worked, only illustrations of Singh's theories.

I don't think this is off topic because it is part of the variables that influence final chamber pressure that appears to exceed that what the compression ratio should mathematically generate when applied in its most simple state.
 
As I understand it the Heron design is named after the bloke who dreamt it up, it was used on UK Ford 4 and v6 cyclinders around the late 60s, Jaguar V12s are this design too. It wasnt too succesfull and way over shadowed by the pent roof 4 valve arrangements so common these days.
However most Diesels are this design and I cant tell if this is to get the desired compression ratio or any other reason (perhaps strength of the fire deck).
Oh and Id add that if adding a few grooves in the combustion chamber worked, the factories would have done it, we got to keep in mind the the internal combustion engine has been round about 150 years now, and its a pretty good thing, rest asured the development process has tried just about everything by now.
A7M
 
A7M, I would be willing to accept that Mr Singh's groovy method helps some designs of cylinder head significantly. But I suspect that the improvements one can make to a 1970s lawn mower are very different from what one can do with a modern-design engine.

Look at the difference between the CI head and a log head. Some of that is intake design, but some comes from the changes in the chamber. We've learned a lot about flame propagation over the last 30 years and allowing a path for that, allowing a bit of porting/channeling to unshroud the flame path probably makes a difference in a head that had a poor design to begin with.

I'm not going to doubt Mr Singh in the least; I am going to say that my gut feeling is that his modifications are more appropriate to older designs and his relevance is limited.

But I'm still going to hack up a lawn mower with his design and see what happens. Why not?

I've got a spare Ford Focus head but I suspect that a thinner walled aluminum DOHC casting is more likely to respond badly to grooving. Probably very badly...? It might be fun anyway.
 
Greywolf : Go for it, see what happens. If I had a spare head for my Falcon, especially an earlier and less desireable one, I'd experiment with it myself. Seems like most anything you do could help the early log heads. Ford designed them for low cost production, not stellar performance.
 
We had some pretty heated threads about the Singh grooves here a few years back, dunno if the threads are still viewable or not. Also, over at www.speedtalk.com there has been a fair bit of discussion. I tend to think there is some benefit to be had in relatively primitive engines (such as our beloved Ford sixes) but have not had the time/money to experiment to date.
Joe
 
Squish and quench are the same thing, quench being the Brits' term. The technology goes back to the great pioneering English auto engineer Harry Ricardo in the early part of the last century. His devleopment of the quench combustion chamber design, enabled the ultra low compression ratios of the time to be nearly doubled, so that the British army could get far better fuel economy on the low octane petrol of that era. He was knighted for this service to the realm.
 
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