The Compression Ratio of a gasoline engine determines how efficient it is, how much power is extracted. Modern ratios are around 9:1 ratio. If the engine draws in a liter of air/fuel mixture, when the piston coes up it will compress that to 1/9 the space. This is the compression ratio. The limit to this efficiency depends on the quality of the gasoline. Pre-war cars used low compression in the vicinity of 5:1 to 7:1. But as gasoline blends were perfected, CR could be raised. In the muscle car era most cars were aound 9:1, but factory ratios of some performance models were as high as 12:1. During the 1970s ratios dropped to around 8.25:1 in North America due to the need to control Oxides of Nitrogen. Finally the cars of 2012 are running as high as 14:1 due to computer control of spark timing and variable valve timing.
Going from 9.0:1 to 10.0:1 is worth about 3% power increase. On a stock A12, it will give you about 2 additional HP.
A much better way to increase power is to bore the engine out for larger pistons. Fitting 3mm larger pistons will give the A12 about 8% more average power -- even if you keep the stock compression ratio.
And contrary to popular belief, an engine can make BIG power without high compression. The high compression is like icing on the cake, it's good, but even without it you can have your cake.
With so little gain, why bother? For racing, every little bit counts. For many of us, we go for higher compression because someone said it was good. This article will try to get rid of that reason. When building a custom engine, or your engine needs machining, you might as well choose the best combination that works best. And compression increase will gain power at every RPM and every throttle opening, unlike many other mods.
More power More fuel economy Requires higher octane petrol Exhaust runs hotter Less emissions (HC, hydrocarbons burned more fully)
Requires lower octane petrol Exhaust runs Cooler Less Emissions (less NOx emissions)
Some very general rules of thumb. A stock A12 engine might handle this:
87 RM/2 Octane fuel: 9:01 compression 89 RM/2 Octane fuel: 10:01 compression 92 RM/2 Octane fuel: 10.5:1 compression 92 RON 11.0:1 with long duration camshaft
its all about "cam" & combo, ive run 13;1comp on pump fuel 98(huge cam)ran fine. static comp is a starting point,which is used to work out dynamic comp. dynamic comp is what you need to work out (using cam specs).
You can shave an A-series cylinder head up to 0.090 inch to gain higher compression. But this is the worst method for gaining compression.
don't mill the head (even Nissan Motosports said this is not the way to get compression)
To raise the compression ratio properly, use flat-top pistons -- or if you want more than 10:1, use a slight dome custom piston.
Note that the A12 open combustion chamber is not less compression than the A12 heart-shaped chamber. They both have 29cc chambers.
Determing Compression Ratio
- 1491 cc engine
- un-milled H89 ported out to 32 cc chambers
- flat-top pistons with 3 cc valve reliefs
- 0.5 mm metal head gasket (80 mm bore)
Worked out to 10.9:1 compression ratio. I had to use mid-grade 89 octane gasoline. It would ping (pink) on regular 87 octane.
If I put a standard 1.2 mm gasket on it, it should be 10.1 or slightly higher if it compresses down any. Should be tight enough for a quench effect.
Stock Compression Ratio
Datsun 1200 A12 engine 9.0:1 Japan, Australia 10.0:1 A12GX Hi-gas 9.5:1? A12GX Lo-gas 9.0:1: 1971 North America (69 hp) 8.5:1: 1972 North America (68 hp) 8.5:1: 1973 North America
Combustion Chamber Sizes
Obviously higher compression is desirable for power and fuel economy. What factors influence how much compression ratio and engine can utilize?
- Fuel. The tendency to avoid Pre-detonation varies with the Octane rating. So while "regular" gasoline is good for 8.5:1 in many engine, "premium" allows 10:1 CR.
- Cylinder head material: Alloy cools faster so can take more compression ratio. Cast iron requires about 1/2 point less CR to be safe.
- Bore size - bigger bore (all else being equal) is more likely to see detonation. Smaller bore means the flame quickly and completely burns the fuel without localized hot spots. So the Datsuns (relative to big dinosaur engines) are good on that front.
- Camshaft duration: longer duration and later intake closing bleeds off some compression and means it is safer to run more static compression
- RPM range - believe it or not, the slower it turns, the more chance bad combustion events can happen
- Spark Timing - just how well is the timing suited to the combo.
- Intake air temperature (huge factor) and coolant temp (less of a factor than most realise). If coolant temp soars, you're stuffed, but running it ultra cool won't protect very much
- Combustion chamber and piston design - generally Datsun have decent quench if you're running a closed chamber version of the head, and pistons of either flat top or a dish shape mirroring the chamber shape - both = safer to run higher compression
- Port shape/cross section/entry angle. If it's got good velocity and swirl, it'll suspend the fuel droplets, even mixture distribution, good clean burn, less residual burn going on as the exhaust valve opens, so the exhaust runs cooler, less chance of pre-ignition (detonation).
- Exhaust valve/seat thickness. Sufficient contact area to conduct heat away from ex valve whilst seated. This is why following the Datsun specs for valve seat grinding is critical.