Tuesday, May 10, 2016

The Quest: More Engine Power Out a Small Block Chevy

The Quest: More Engine Power!

The Effects on Horsepower of Increasing an Engine’s Compression Ratio

  -Alan Arnell

I am afraid in today’s classic car world, that the shade tree-hot rodder engine builder is a thing of the past.  I have been told and have read that it is much more economical to buy a crate motor and definitely a faster way to get a car restoration towards its completion.  But, when I have my hot rod dreams, I think of back when real hot rodders and drag racers built their own engines.

GMP-10067353-K Pace SBC 350cid 300HP Crate Engine

When I got interested in hot rodding and working on cars as a wee lad, I strongly felt that the ultimate task and car guy proficiency was to build an engine.  Sadly, I have never realized that dream of building my own hot rod motor.  But that does not mean I still do not still think about putting together a rip roaring-mean sound-rubber burning-Ford killing engine.  I can smell the oil and burnt rubber right now!  In those dreams, I think of compitation car parts such as camshafts, rocker arms, push rods, crankshafts, connecting rods and compression ratios.

So, that brings me to the point of what compression ratio I would want for an engine.  My mind wanders to how much of a horsepower increase is there for every point of compression ratio.  My current 1977 stroked and bored 350 classic SBS engine, for example, in my 1957 Chevy 150 2- door sedan has 8:5:1 compression and produces, say. 350 horsepower.   What would happen if I were to bump the compression up to 9:5:1?  How much more horsepower would the engine produce with that increase?

Well, the folks at “Chevy High Performance” have done the thinking for me.

They wrote:
The old-school rule of thumb is that the engine will realize 6 percent more more with one point of compression increase.  Now over the years we have seen a little less and also a little more than that.  There are quite a few things that affect the power increase of compression.

If you raise the compression with thinner head gaskets or by decking the block to bring the pistons up to zero deck, you have reduced the deck clearance when you do this,  you also increase the mixture motion and reduce the effective combustion-chamber size.  The quench areas of the cylinder head force the fuel and air mixture into the combustion chamber, where it is designed to burn efficiently.  When you have excessive deck clearance, the combustion -chamber size effectively becomes the bore size of the engine.  Reducing the compression, via a domed piston, can reduce the efficiency of the gain by shrouding the flame travel across the combustion chamber.  Small, tight chambers and minimal deck clearance is commonly the best combination.

I have tried to explain a very technical question in a very general terms.  6 percent is a safe expectation from a single point of compression if your engine is producing 350hp, you should see the power jump to around 370 hp.  

This answer to the increase in engine compression reminded me that there are so many variables to engine building.  To complete your own engine hop up you really have to know your stuff.  But, that what makes it so lofty of a dream.

I love old school flames!!!!

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