Compression ratio (2021)

It might surprise you, however I don't think you will ever try it
My Rapide was shipped to Malta when new. The factory built it at 5.9 to one CR because of poor fuel in Malta.
Most of the twins that went to South America were also at 5.9.
According to the Police Force that used them in great numbers, the bikes easily loped thru the Ton.
Doesn't sound like much power loss does it?
When I rebuilt the top end on my Rapide I bumped the CR from 7.3 to 8.
Going from worn out rings and bores on the 7.3 ( probably more like 6 to 1 effectively)
netted me just 2 mph on Dyno hill.
I was expecting a lot more gain in performance.
2 mph= just over 3kmh. Each KMH in higher speed on that hill correlates to a 1 HP gain +-.
So Dyno hill was telling me that the rebuilt bike had an additional 3 horsepower.
The Wallace calculator agrees . It tells me that going from 6.2 ( worn out 7.3s) to one up to 8 to one will net 3 HP.


Glen

Fast Eddie said:

Glen, as I’ve said before, I don’t think that calculator works for our small engines.
I dyno’d my 850 at 64rwhp.
According to the calculator it’ll give 51 even with 5:1 CR. I somehow doubt that…

Your old Compression Ratio of 10.5:1 and HP of 64 is now calculated​

as a Compression Ratio of 5:1 and 51.01 Horsepower.​

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Somehow doubting it isn’t a good test.

I’m still not convinced!

Given the heat and stress caused by easing CR are clear ‘downsides’ then I struggle to I understand why modern OEMs would aim for such high CRs.

Even your boring petrol hatchback grocery getter these days is probably around 12:1…

These things are not decided by “how come…?” debate.

I wonder what sort of boost in torque you would get, moving from say 9:1 to 10:1. I suppose the Combat figures can be compared to a standard Commando but the cams are different.....

Atlas Commando said:

I wonder what sort of boost in torque you would get, moving from say 9:1 to 10:1. I suppose the Combat figures can be compared to a standard Commando but the cams are different.....

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Yes. And its actually often the case that raising the CR is not actually done in order to ‘raise’ the CR. It’s often done to compensate for the effective CR loss of a hotter cam. Hence the old argument that these things have to be taken into consideration as a whole package of changes, not a cherrypicking excersise.

High CR often gets slated as offering no performance increase, as do twin carbs. Until recently a mate of mine had a very nice, low mileage, almost stock late ‘Harris‘ Triumph Tiger. My T140 has exactly the same engine spec, but with a higher CR and twin carbs. Mine left his for dead. Everywhere! That was proof enough for me. But that’s ‘confirmation bias’ for you !

Triton Thrasher said:

Somehow doubting it isn’t a good test.

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True. But if you’re expecting me to rebuild my engine with a 5:1 CR just to test something I don’t believe, then I’m afraid I’m going to disappoint you.

Fast Eddie said:

True. But if you’re expecting me to rebuild my engine with a 5:1 CR just to test something I don’t believe, then I’m afraid I’m going to disappoint you.

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I’m not. You can bet “Wallace Racing” didn’t synonym test down to 5:1 either.

What do you make of their expertise here:

My sense is that twin carbs make a lot more difference than a point or even 2 points of compression.
Comnoz has posted the numbers from his dyno tests. I believe the single Mikuni made mid to high 30s rwhp and the twin carb stock 850 mid to high 40s.
So ten rwhp for a carb.

The reason that modern cars run high compression is that it does add power and efficiency. Perhaps not as much as we imagine though.
The great thing about a compression bump is that it adds some power everywhere.
One down side is the possible problem of pre ignition in some situations. Another is more wear and tear on an engine that will be seeing high mileage. Finally, with the weak Commando head attachment, it's finding a way to hold it all in for the long haul.



Glen

Triton Thrasher said:

I’m not. You can bet “Wallace Racing” didn’t synonym test down to 5:1 either.

What do you make of their expertise here:

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That's because your new engine won't run at 2 to one but they are recommending a 137.45 bhp stationary engine to be connected to your new air compressor!

worntorn said:

That's because your new engine won't run at 2 to one but they are recommending a 137.45 bhp stationary engine to be connected to your new air compressor!

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“Bowling” reckons he can get it running! http://www.bgsoflex.com/cgi-bin/crchange.cgi?hp=100&oldcr=8.5&newcr=2

We should employ Bowling on this build. A known great 2 to one tuner!

Glen

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. 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.

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I believe the hot cam actually has a higher dynamic compression than the mild cam when the hot cam is in its zone. It makes more power by allowing more mixture into the engine, which has to mean a higher dynamic compression vs the milder cam, but only at that high rpm. Down lower the mild cam has a higher dynamic compression and makes more power than the hot cam.
With Nortons those numbers are -6 bhp @ 4000 +- for a PW3 vs stock and +3 bhp vs stock at 6000 or so for a PW3 , from Jim's dyno info.

If the hot cam didn't stuff more fuel mixture in at some rpm, generally at a high rpm, there would be no point in using it.
My long winded way of saying " I agree"
Glen

$3.99 at O'reilly. I carry it in my backpack every time I ride. I don't have any soft luggage on my shinny fuel tank otherwise I'd put it in a tank bag. I don't actually tour on a Norton, but I'm sure it would work for 87 octane fuel if that is all there was available. I saw Nigel's note about carrying octane booster, but thought a US source and a pic might be useful. lol

I use the booster all the time to keep the parts in my 750 Norton motor happy. No clue what the CR actually is, but it's not stock for a P11 (7.5:1 stock) since putting a bunch of JSM goodies in it. All I calculated for was not hitting the tops of the pistons with the valves. I knew I could tune it no matter what the CR ended up being. Although I did shoot for lower compression than what I could have gotten away with. My motor does run on the warm side now. Not sure what would happen running across the floor of Death Valley when it's 120 degrees. It does fine in 90 degree weather on the street, so it's all good where I live.



The motor likes 29 degrees all in and a bit more fuel than it did with the higher CR pistons than it did with stock Commando flat top pistons.

Gene, I have no idea what it would take to blow up a 750 Norton engine with whatever CR other than the stuff Ken mentioned. I also skipped ahead on your post. What did you end up doing on the build?

worntorn said:

I believe the hot cam actually has a higher dynamic compression than the mild cam when the hot cam is in its zone. It makes more power by allowing more mixture into the engine, which has to mean a higher dynamic compression vs the milder cam, but only at that high rpm. Down lower the mild cam has a higher dynamic compression and makes more power than the hot cam.
With Nortons those numbers are -6 bhp @ 4000 +- for a PW3 vs stock and +3 bhp vs stock at 6000 or so for a PW3 , from Jim's dyno info.

If the hot cam didn't stuff more fuel mixture in at some rpm, generally at a high rpm, there would be no point in using it.
My long winded way of saying " I agree"
Glen

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I agree with what you have said Glen, but a hot cam behaves differently when you use a 2 into 1 exhaust system. With separate pipes, if you ride off in 1st gear and just let the revs rise, you will usually notice that at about 4000 RPM, there is a surge of power where cam starts to work, You are usually wasting your time if you use revs lower than 4000. So gearing is also important.
With a 2 into 1 exhaust which has zero restriction past the collector, they will usually pull hard from bottom to top of the rev range. - That is providing you advance the cam timing enough to suit the pipe. A bigger gulp of mixture must mean more power. However with a 2 into 1 pipe, there is also a Kadencey effect. Mixture goes down the pipe then gets stuffed back into the cylinder. Any step where the pipe fits to the cylinder head is bad. With my motor, I have stubs in the head - their ID exactly matches the exhaust port ID, and the ID of my pipes matches the ID of the stubs. I have slip joints with springs holding the pipe back towards the head.
With my exhaust system, my inlet valve opens at 65 degrees BTDC and my exhaust valve opens at 92 degrees BBDC I ride the bike using revs from bottom to top, with absolutely no power bump. If the power bump is there, you are always working around it. With megaphone exhausts, the bump can be particularly bad

With a road bike, you are probably committed to jetting a bit too rich. How you tune your bike has to suit it's intended purpose. A motocross bike probably has very quick taper needles in it's carbs. It depends on how you use the throttle.

This might be relevant, but I doubt it.

I took a short 244 mile ride on the Norton yesterday. Passed a few cars in the foothills leading to the Cascades at around 90mph. When I got home, I noticed my base gasket had been weeping. Three reasons come to mind: increased crank case pressure with the higher compression, not using a better sealer on the top of the base gasket, and not an adequate breather solution for the increased pressure in the crank case. Point is make sure you have a good breather solution if you increase the compression up top. By the way, not even a hint of head gasket leakage yet, but I have not been over 100mph with the current gearing, which is a little low.

I do have flat top JSM high compression ratio pistons, a relatively tall JS2 bump stick, and Carrillo rods. Not ultra high compression domed pistons, nor a huge sifton cam. I don't notice any of the negative issues people mention regarding not using stock parts for a better street bike. It seems fine to me riding slow or old man faster.

Narrow squish?

Schwany said:

This might be relevant, but I doubt it.

I took a short 244 mile ride on the Norton yesterday. Passed a few cars in the foothills leading to the Cascades at around 90mph. When I got home, I noticed my base gasket had been weeping. Three reasons come to mind: increased crank case pressure with the higher compression, not using a better sealer on the top of the base gasket, and not an adequate breather solution for the increased pressure in the crank case. Point is make sure you have a good breather solution if you increase the compression up top. By the way, not even a hint of head gasket leakage yet, but I have not been over 100mph with the current gearing, which is a little low.

I do have flat top JSM high compression ratio pistons, a relatively tall JS2 bump stick, and Carrillo rods. Not ultra high compression domed pistons, nor a huge sifton cam. I don't notice any of the negative issues people mention regarding not using stock parts for a better street bike. It seems fine to me riding slow or old man faster.

Narrow squish?

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What is your thinking re the link between high compression pistons and increased crank case pressure ?

Fast Eddie said:

I’m still not convinced!

Given the heat and stress caused by easing CR are clear ‘downsides’ then I struggle to I understand why modern OEMs would aim for such high CRs.

Even your boring petrol hatchback grocery getter these days is probably around 12:1…

Click to expand...

Fuel economy.

Dan1950 said:

Fuel economy.

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Fuel economy is arrived at (in this context) by improving combustion efficiency. Thats the same thing that produces power.