Compression ratio (2021)

NIX said:
In many cases, engine builders raise the static CR when a faster cam is mounted to compensate a loss in dynamic CR.
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But the loss in dynamic CR is normally at low revs when a lumpy cam is installed like the Combat 2S cam, but at high revs the same cam does not have a loss in dynamic CR and could even gain as the air at higher revs behaves differently. Its the same on an ECU where the VE (Volumetric Efficiency) can be over 100% at mid range revs because more air is being pulled in that the cc's of the engine and you have to increase the fuelling to keep the mixture correct.
 
kommando said:
But the loss in dynamic CR is normally at low revs when a lumpy cam is installed like the Combat 2S cam, but at high revs the same cam does not have a loss in dynamic CR and could even gain as the air at higher revs behaves differently..
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In fact, if your late closing inlet valve does not give a gain in cylinder charging at revs that you use, then it is a tuning failure.
 
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mean gene said:
Sooo have read several threads about compression ratio. But what is the limit a standard head, gasket, cylinder barrel with stand. Real life 11:1??? BSA ran it in their Lighting Rockets. Have ran Harley with some high compression pistons And you reach for too much and the studs pull out of case. Looking to hot rod a 750 and if I'm going to bore I might as well up the compression. I run aviation fuel in most of my old stuff so good gas won't be the problem. Kick starting is never a problem, once you learn the little Norton dance.
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All the Commandos I have CCed the head to do the math of Compression ratio were about .3 lower than OEM spec. And that is in heads without deeply recessed valves. Ron Wood told me to limit Compression to 10 to 1 in Nortons. With good head flow, You can get a lot of air into the cylinder and get a real good fill that gets compressed . I like to see about 185 PSI when I do compression checks in a blueprinted engine that has the full or slightly better than OEM spec ratios.

Too high compression ratios cause too much stress to the bottom end . And of course poor assembly and tuning don't help.
 
As noted, the compression ratio is 'fixed" based on the volume of swept area/combustion chamber area. A compression gauge does not measure that; it measures the dynamic compression (at extremely low RPM) which can be dramatically different due to cam profile.

We built 13.5:1 competition engines that had lower compression gauge readings than a 9:1 grocery-getter. This was because the competition camshaft at low RPM had so much overlap that air could almost pass freely in/out of the cylinder at cranking RPM! OK, not really but seeing 110PSI on a gauge was common. An OEM street cam on the same engine would produce a comp gauge reading in the 180+ range.
 
MexicoMike said:
the competition camshaft at low RPM had so much overlap that air could almost pass freely in/out of the cylinder at cranking RPM
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Overlap is the period when both valves are open, around the end of the exhaust Stroke and start of the intake stroke.

It’s inlet “lag” that lets the mixture back out through the carburettor at the start of the compression stroke.
 
I agree that the reversion determined by the length of time the intake valve remains open after BDC. The long duration is common with competition, high overlap cams which is why I phrased it that way. But you are absolutely right - you could make a cam with a long duration intake - high reversion (low gauge compression) - and no overlap at all!

To me - high overlap and (relatively) low dynamic compression (as read by a comp gauge) go together on a conventional ICE engine - one without computer control/variable valve timing, etc.
 
This has been one of the more interesting discussions on the board as of late, always had a sense of the dynamic relationships, but it's nice to have a better understanding of the interplay between the covariates
 
One other thing to consider is that comp ratio increases do not provide as much power increase as is commonly expected. A full point increase with no other changes is usually worth no more than 2-3% percent, if that. Obviously, other changes are commonly made - as discussed, cam, gearing, fueling, ignition timing, etc

OTOH, other than boring an engine to a larger displacement, comp ratio increase is pretty much the ONLY performance mod that will increase power throughout the rev range.
 
Cr gives roughly 1%per full point according to the Wallace Racing Calculator and others.
And it is a diminishing scale.
Going from 9 to 10 does less for you than from 7 to 8.
I'm a bit surprised at the low numbers given by the calculators. I've always thought a Cr raise had quite an effect. But that is seat of the pants and that area is known to be quite inaccurate.

Glen
 
There's also a more psychological element. Or rather 2.
1. Because you 'tuned' your engine, you will feel the difference. Or because you paid for it..
2. It's a matter of balance in parts and even a bit if mistuning. Because low end torque is lost (or at best is kept) in many cases, the kick under de butt when the engine enters it's power range (also in a well tuned and well balanced engine moved to higher rpm's) is experienced as a succes. I would even dare to say that simply a loss in low or mid range will give this sensation. In fact an engine runs rougher, not really better or faster. A bike that performs well in low mid and high range is often not recognised as 'fast'. Until you race someone and the only thing he sees is your rear wheel.
 
YEP...I have personally witnessed the effect of losing low midrange causing a rider/driver to say the bike/car has more power when the timing slip or dyno shows the opposite, The butt feels the "surge" after going through the "hole" in the power curve that wasn't there before and that surge "feels" like more power.

NEVER believe the butt dyno when dealing with engine mods. It's the same as the "louder is faster" phenomena! ;)
 
I have fitted 11.5 to 1 pistons into a 650 Triumph twin using avgas on public roads. I think the bike was faster, but my carb jetting might have been too rich before I raised the comp. ratio. I probably did not change the ignition advance. When you ride a bike on public roads, the corners do nor repeat themselves so often. So it is more difficult to know if you have gained a performance advantage. In the end all you can do is build the motor, then adjust the variables to get the best performance. Some guys probably don't even have the vernier sprocket on the cam drive, so don't vary the position of the cam timing.
When you ride on public roads, you don't have the situation you have on a race circuit, where you know if the corners are arriving faster - you don't repeat what you are doing lap after lap..
When you tune a race bike, it gets expensive when you start adjusting all the variables. You don't usually start changing the cam timing in the pits. So the exercise involves repeated trips to the race circuit.
One of the things Barry Sheene apparently used to do with the RG500, was convince himself he had gained a performance improvement, then lead himself up the garden path
 
In my own case - from experience, I know what happens when you do certain things to a four-stroke motor - so developing the 850 to be fairly quick on methanol was not so difficult. But if you are a beginner, it can be a long hard road.
 
I don’t go above 10:1 on a 750, the cylinder base is the weak spot especially on rebored barrels. Most tuners back in the day would advise you not to go above first rebore on highly tuned 750 engine. It’s why Norton altered the barrel design on the 850s.
 
mean gene said:
I really appreciate all the information. But at what point do Norton parts begin to fail. Are the head and cylinder that good a design that if I run 12:1, head gasket and stud breakage doesn't happen?? If I supercharge what's going to fail? What's the weakest link. This is going to be a short run bike not built for the long haul.
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I know a couple Alky racers, one is running over 13:1, another over 12:1 with a 920 and after many races he broke his long cylinder bolts. The solution was to go to high strength reduced shank bolts and triangular reduced shank head studs so they stretch and keep their tension instead of breaking. Pliobond glue on the copper gasket pretty much takes care of the sealing.

Compression ratio (2021)


Compression ratio (2021)
 
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worntorn said:
Here's the Wallace Cr calculator. I gave the stock 750 45 HP on the tire and then bumped the compression to 10 to one, no other changes.
Hang on!
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That’s for V8 Pontiac engines. I don’t trust it for Norton’s, some of the formulas must not be right for such a small twin cylinder engine.

I used your same start point Glen but went in reverse and the calculator said it’d still produce over 40bhp with a 6:1 comprsession ratio.

Now, I‘ve never tried that, so can’t speak from experience, but I seriously doubt that !
 
MexicoMike said:
As noted, the compression ratio is 'fixed" based on the volume of swept area/combustion chamber area. A compression gauge does not measure that; it measures the dynamic compression (at extremely low RPM) which can be dramatically different due to cam profile.

We built 13.5:1 competition engines that had lower compression gauge readings than a 9:1 grocery-getter. This was because the competition camshaft at low RPM had so much overlap that air could almost pass freely in/out of the cylinder at cranking RPM! OK, not really but seeing 110PSI on a gauge was common. An OEM street cam on the same engine would produce a comp gauge reading in the 180+ range.
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Just to clarify, the two bikes I just did a MK3 with a mild Web cam and a Combat with a original cam had the 180 ish PSI. Both bikes have E starts in them, so that may help the numbers some and both were tested with very a brief time run time on them . All the race motor I have built had similar compression numbers with all models of Web Cams and a Norris Rx cam used in my first racer. My race bikes brought home the goods in USCRA racing and AHRMA events at NHIS and with many famous racers that rode my equipment, they brought home the bacon, after I stopped riding.
 
mean gene said:
I really appreciate all the information. But at what point do Norton parts begin to fail. Are the head and cylinder that good a design that if I run 12:1, head gasket and stud breakage doesn't happen?? If I supercharge what's going to fail? What's the weakest link. This is going to be a short run bike not built for the long haul.
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Norton parts fail just after the point you finish the build. But seriously, I think the Combat 10:1 ( IIRC) stock CR with 2S cam makes great power. I ride mine for short distances around town and love it. Unless you are going to race the bike, there is so much peripheral fragility in the Commando anyway that keeping engine stress down can only be beneficial.
 
You might assume that more horsepower gets you down the straights faster and that is important because in the corners everybody has to do the same maximum speed at the same lean angle. In some respects it is correct and in some other respects, it is a wrong assumption. The gearbox is a torque converter and not all bikes handle the same. I think in MotoGP, because of the extreme power of the bikes, there is a convergence towards similarity. But old bikes are different.
 
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Lots of claim and counter claim of what works and what doesn't, but where are the dyno print outs. If you are confident to put these engines together then no doubt you will be happy to run it on a dyno, it is not expensive and most can be compensated for which method is used.

So come lets see the curves, we all like curves in the right places!!
 
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