Compression Ratio Ideas

[BruceA]

I would like to hear your ideas for compression ratios for an engine.

I know that too little and the power isnt there, but too much has its dangers.
How much is 'too much' and what would be considered a reasonable amount for a good street/track engine? I realise there are contributing factors like cam selection, fuel type etc. Is there a point at which increasing CR wont help with power increases?
Or am I just asking how long is a peice of string?

 

[DaveM]

My impression is that 10.5-1 is about it for aluminum heads with pump gasoline (91-92 octane in US). Higher octane racing gas allows much higher c.r., but that doesn't fit your premise. Modern engines with sophisticated computer knock control (variable timing, etc.) are now running up in the 11.5-1 range.

 

[Randy V]

It's been my experience that when you get over 13:1 you have an extremely hard time controlling detonation regardless of the octane rating of the fuel and electronics.
Remember that it only takes ~18:1 to make a Diesel - they run on detonation...

 

[Dwight]

I have a couple of buddies with FE motors that Keith Craft build for them.They have 9:1 and 9:5 CR range. They make 601 and 619 flywheel horse power. Very street able and will burn any gas.

You can make hp without high compression. Keith does it with the correct cam and heads.

 

[Cliff Beer]

Ditto what DaveM said. Quality alloy heads on an American V8 with typical configuration - 10.5 is maxed. 10 is probably easier to work with.

Hot rodders do all sorts of stuff to try to raise the compression ratio without causing detonation in a NA engine. It's all good stuff and fun engineering, but the incremental improvements to hp/tq are modest, and often have a down-side. Once you've reached the point of having good flow (good quality heads, port/polish, etc.), it's better to just increase displacement and keep a reasonable CR for a street engine...

 

[Jack Dunn]

At least in the southeast US, there has been a decline in the quality of the "93 octane" premium pump gas available here. A builder of turbo/supercharged engines told me they have seen their pump gas turbo V8 Ford engines max hp decrease roughly 100 hp due to the fuel octane situation(~700 to 600hp). He says that due to fuel price increases, sales of premium have dropped significantly, resulting in premium gas staying in gas station storage long enough to adversely affect octane rating. His advice for high CR/boosted engines(non-catalytic converter equipped, of course!): add a gallon of the highest octane(leaded) race gas available to each tankful of pump premium to assure true 93-94 octane.

Jack

 

[David in Durango]

Static compression is different from running compression. Choosing a cam carefully can help.

Everything should match (cam, heads, CR, carb and so forth).

But we all know this, right?

 

[Jim Cowden]

I had it explained to me that a stock eng may have a static ratio of 9.5:1 but a dynamic comp ratio of about 7-7.5:1.
That is the ratio is worked out at the point of intake valve close and piston is on its way up and you have lost some stroke in the calculation.

When a cam is installed with more lift ,duration you need to get the comp back to the 7-7.5:1 dynamic hence it ends up with a higher static comp because the valve closes later and you are trying to get the same dynamic comp in less piston travel.

Hope that makes sense.

Jim