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View Full Version : Compression Ratio - Why is it too much?


spetz
06-09-2006, 03:25 AM
So with my engine rebuild I got the heads decked to raise compression ratio.

Stock was 10:1 and I was initially wanting around 11:1 but according to the engine builder, should not go any more than about 10.8:1

Now, besides the obvious pre-ignition/detonation/pinging, the engine builder said that going too high of a CR will result in too much cylinder pressure. And that if I had upgraded cams this wouldn't be such a problem. But, with stock cams apparently the pressure will not be able to be released quick enough and too much CR will result in bad idle, up to blown pistons from too much pressure (depending on how much "over" the CR is).

So, is this a problem I should be worrying about? Or is it not really likely to happen and I should be more worried about detonation?

The builder is Milton from Harris Engines in Sydney
aaronng
06-09-2006, 03:39 AM
As far as I know, cams for high compression have longer intake durations. Since the intake valve closes later into the compression stroke, there is less cylinder pressure which helps with the seals and detonation.

Stock cams with shorter duration coupled with high compression can result in overly high cylinder pressure which would cause the engine to detonate at low rpm.
BlitZ
06-09-2006, 09:20 AM
works like an air pump..

If you compress too much air you need a head which will open the valve big/long enough to let it all out (more aggressive cams)..

Else the compression will be wasted...

It can also equate to premature head wear..

But wiht that said.. 11.1 cr isnt that high... its proabably a better compression if u intend to upgrade.. even just to ITR cams..
Twincam16
06-09-2006, 05:07 PM
What engine model?
spetz
06-09-2006, 05:56 PM
It's a 6A12 MIVEC from an FTO in a lancer :)

Yes I know it's a Honda forum
spetz
07-09-2006, 01:53 AM
Detonation is not really a worry as I think 11:1 is not that high, and it will be tuned etc (hopefully tuned well too)

More the idleing/low rev issue with cylinder pressure?
spetz
07-09-2006, 09:03 PM
Yeah there is a knock sensor

But the thread I started not because of detonation problems but rather the cylinder pressure before the secondary cam kicks in

You car is 10.4:1 but as you said that is how it is from factory. Mine was 10:1 so upping it to 11 might cause too much cylinder pressure?
todaek9
07-09-2006, 09:49 PM
uhh...u deck the head...nice..
spetz
08-09-2006, 04:06 AM
hmm?
Yeah the head has been shaved and adjustable cam gears were put in to compensate for timing change from the heads sitting closer to the block
ProECU
08-09-2006, 05:06 PM
more compression will AID in preventing detonation, if done correctly.
iamhappy46
08-09-2006, 08:13 PM
Get the cams gears dialled in to suit the high compression and stock cams. It will be a fine balancing act but it can be done. 11:1 CR is nothin on a modern engine, especially with MIVEC. The hi-lobes should safely release that much gas pressure but I would be sticking with the factory rev limiter to prevent valve float.
JohnnyVtec
09-09-2006, 02:46 PM
It's a 6A12 MIVEC from an FTO in a lancer :)

Yes I know it's a Honda forum


Buy a Honda! Patient is cured :wave:
spetz
09-09-2006, 08:35 PM
Is there a magical number where say 1 point is X % more power?
iamhappy46
09-09-2006, 10:39 PM
Hypothetically, if the engine has 10:1CR stock and you raise it to 11:1CR, it theoretically should yield a 10% increase in torque.
As: HP = Torque x RPM / 5252, a 10% increase in torque is available across the entire rev range and therefore, a 10% increase in power across the rev range.
Such that, 12:1CR should yield a 20% increase in torque and therefore power.