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Old 2 Mar 2021, 11:18 (Ref:4037869)   #1201
Paul D
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There's a lot more to understanding compression ratios than simple bore x stroke, as has been said.

Combustion chamber volume has to be known and factored in, but also, if you want to know the actual CR at cranking speeds, compared to the theoretical one, then cam timing also has to be factored in.

For example, if your inlet valve closes at, say, 70 degrees ABDC, then at cranking speeds, where the engine is turning slowly, you're not compressing anything (much) prior to that point. So you would then need to calculate how far up the bore the piston has travelled by that point, then subtract that figure from the actual stroke to find the effective stroke (for compression ratio purposes) at cranking speeds. Using that figure for the stroke in your calculations will give you a better idea of your actual CR at cranking speeds.

Wilder cams designed to deliver power at higher revs will, naturally, produce a lower (shorter) effective stroke at cranking, and therefore a correspondingly lower CR.

So from this, you can see that, with wild cams, and even with a high theoretical CR, your actual CR for starting may not be so high - may even be lower than the standard engine with standard cam.

So why, then, are high compression engines slow to turn over when cold starting you may ask? Well, as has been mentioned, it's more likely to be related to advanced ignition timing than compression ratios: tuned engines generally run more advance than standard engines, so at cranking speeds the spark is arriving effectively a little too early for what the engine wants at cranking speeds, and this early spark (and so early combustion) slows the piston travel as it approaches the top of the cylinder.

If you want to know for sure, then simply crank your engine with the ignition disconnected - if it spins faster than with the ignition on, then it's advanced timing that's slowing it down.

Last edited by Paul D; 2 Mar 2021 at 11:40.
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Old 2 Mar 2021, 11:34 (Ref:4037875)   #1202
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This is all fascinating stuff guys, thank you & keep it coming
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Old 2 Mar 2021, 11:58 (Ref:4037883)   #1203
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Lancsbreaker should be qualifying in the top 3 on the gridLancsbreaker should be qualifying in the top 3 on the gridLancsbreaker should be qualifying in the top 3 on the grid
Trying to remember last time I had to think about points, condensers and ignition timing...... I'd better do a crash-course retraining before I start on my Imp-based Husky My diesels don't seem to have these issues
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Old 2 Mar 2021, 12:05 (Ref:4037887)   #1204
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Good "points" Paul. Thats why on Yank tanks they have a separate switch for the ignition. If they hadn't, very difficult to start with a fix advance timing settled at c 39°.
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Old 2 Mar 2021, 12:25 (Ref:4037898)   #1205
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Really enjoying the info from these stories. Interesting about the Yank tanks, not being able to start the car due to the temperatures makes sense
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Old 2 Mar 2021, 13:12 (Ref:4037908)   #1206
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What we call a… hot topic!
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Old 2 Mar 2021, 15:17 (Ref:4037955)   #1207
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I think we ruined a couple of heads trying to find the optimum CR for my engines, the thing is that the combustion chamber shape makes an awful lot of difference to what you can get away with and you have to strike a compromise with ignition timing.
If you look into engine specs you will find that the most efficient petrol engines like hemi head designs don't run as much ignition advance as "conventional" ones like the old Ford Kent/ BMC heads. The old A series BMC engines in road trim were not the best at out and out performance and would take loads of advance without "pinking", and would still drive almost the same.
Modern multi valved engines with computer controlled fuel/ignition/valve timing are a whole new ball game though.
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Old 2 Mar 2021, 16:12 (Ref:4037978)   #1208
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Fully agreed. We learned on the hard way that any TR engine and may be MG B can't achieve great figures at the dyno if you dont reshape the combustion chambers.
Yes again there are huge dissimilarities between engines, even though they have the same cylindre capacity, when it comes to set the ignition advance.
Another detail not to neglect is if you have, or not, double ignition. A good example is the old Porsche 2.0 litre found under the bonnet of the Carrera 6 too. More or less the same. But the Carrera one has two plugs by head which makes an important difference in terms of pistons head shape and SCR ratio too.
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Old 2 Mar 2021, 16:12 (Ref:4037979)   #1209
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And to go on a bit from what Gordon said about head design .
Combustion chamber shape can make a big difference in the power produced by making the flame spread more evenly through the fuel/air mix & also allowing a more optimum spark timing .

Taking that to the extreme , Fiat/Lancia had a Group B engine with the 4 valve head with an inlet & an exhaust each side .[ Unlike a normal one which has inlets one side & exhausts the other .]
The idea was that the 2 inlets at 180 degrees to each other would create a large " Swirl " effect in the combustion chamber & give a much better performance .
These engines are quite rare & not many people have seen one .
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Old 2 Mar 2021, 16:33 (Ref:4037988)   #1210
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And to go on a bit from what Gordon said about head design .
Combustion chamber shape can make a big difference in the power produced by making the flame spread more evenly through the fuel/air mix & also allowing a more optimum spark timing .

Taking that to the extreme , Fiat/Lancia had a Group B engine with the 4 valve head with an inlet & an exhaust each side .[ Unlike a normal one which has inlets one side & exhausts the other .]
The idea was that the 2 inlets at 180 degrees to each other would create a large " Swirl " effect in the combustion chamber & give a much better performance .
These engines are quite rare & not many people have seen one .
By some miracle I have managed to find a picture of one .
Plumbers nightmare with exhausts out from both sides of the engine

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Old 2 Mar 2021, 16:40 (Ref:4037992)   #1211
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Gerard C should be qualifying in the top 5 on the gridGerard C should be qualifying in the top 5 on the gridGerard C should be qualifying in the top 5 on the grid
Thanks for the pic!
The Italian had really good ideas, I suspect Ferrari giving help or advice to Lancia. I used to drive a Delta S4, compressed and turbocharged engine, what a wonderful car it is.
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Old 2 Mar 2021, 17:18 (Ref:4038004)   #1212
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By some miracle I have managed to find a picture of one .
Plumbers nightmare with exhausts out from both sides of the engine

I was going to say that engine m ust have been a plumbing nightmare. Looking at the (small) picture the whole thing looks highly complicated and therefore so typical of the way things were going in Group B!
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Old 2 Mar 2021, 17:39 (Ref:4038014)   #1213
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I wouldn't say the picture is small but the engine is…*Imagine the heat in the engine compartment.
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Old 2 Mar 2021, 17:49 (Ref:4038019)   #1214
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Dont want to spoil the calendar thread just to say that when PA has to delay or, worse, cancel any event its really bad news. Those are very serious people standing always close to the governing body. As to ACO, just oblige to get rid of fifty people, I couldn't imagine that this will happen one day. Very serious people too, big money involved and close to the gov' too. I'm not sure they can stand another year of 24 H or LMC without spectators without any financial issue. Between nightmare and tragedy.
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Old 2 Mar 2021, 21:07 (Ref:4038083)   #1215
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I agree Gerard, I don't expect many organisations will be able to stand another year of this.
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