Compression ratios and Horsepower question??

Moundir

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If one was going with a NA setup and wanted to gain reliable hp, how much compression can one safely run? I'm trying to figure out how much compression I want to run with a heads/cam setup? How much can I safely run, and what is the relative gain of hp per compression increase with everything else being equal? I was going with 10.0:1 but I have seen some go as high as 11.0:1 :eek: what would the difference in hp be from a say 10.0 to 10.6?? Thanx all. :)
 
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You'll get a lot of different opinions on this subject. I'm of the opinion that it is far better to err on the low side a little than on the high side.

What fuel are you talking about running? If it's straight pump gas I would stay between 9.7:1 - 10.0:1. If you run an unleaded racing fuel you can obviously go higher.

Another thing you need to consider is the extra flow from the heads and cam. The higher volumetric efficiency will raise your cylinder pressure. Not your cranking compression, but the amount of cylinder pressure you have going down the track. More air in the cylinder when the intake valve shuts equals higher cylinder pressure. That's why a blower engine needs less compression.
 

Marv S

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Have you seen 11:1 cr on Vipers by just cutting the heads? Or is it getting into custom pistons?

I thought once you get much past about 10:1 when just milling there head and intake don't fit well.
 
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Have you seen 11:1 cr on Vipers by just cutting the heads?

No, I haven't seen it. On a stock cubic inch Gen 2 with 9.54:1 it would take the removal of 14 cc's to get to 11:1. If you did it with milling alone, .098" would have to come off the heads. Can't do it.

The best way to accomplish it would be to order a set of pistons that would have "0" deck, and have a small (about 7 cc) dome. I don't see the need to mill the heads a lot. It screws up the intake fit, reduces piston to valve clearance, and may require a different length pushrod. About the most I'll take off a set of Gen 2's is .025" - .030".

Besides, going with a custom piston and reducing the deck clearance makes the engine less prone to detonation.
 

Torquemonster

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Greg/TNT is correct - cylinder pressure is the real factor not static compression ratio - but is a science in itself and requires good software and expertise to optimise.... therefore most people have to deal with compression as that is easy to deal with.

The more valve overlap you have the more compression you can run - as a general rule of thumb.... ramp speeds/intensity of the cam design also greatly affect the cylinder pressure.

So - to keep it simple - what I would be doing if I did not want to blue print the engine for a tight quench and just bolt on bits to get the best value for money - I'd leave compression stock and go for a cam that has a faster ramp, more lift, and more duration @ 0.050" lift. That will require stronger springs and may shorten the life of hydraulic roller lifters that cannot take the ramp of a solid lifter - whether flat or roller.

By keeping advertised duration under around 280-285 degrees, and getting as much duration @ 0.050" as you can (250 degrees would be intense and run VERY strong) with over 0.600" lift - you'll have a stomper if the heads flow and intake and exhaust are all done. It will lose a little bottom end, but mid range and top end will be huge. That spec is fine on a 0.904" hydraulic flat lifter with good oil and break in procedure - not sure if the roller hydraulic will take it - Comp Cams could tell you.

That cam is worth 600hp on a 440 Mopar with ported heads, or more than that with Indy Heads. It's not at the limit - as Fords can run it on 0.875" lifters in NASCAR, but it is very fast action and requries good springs. It will not lose as much bottom end as most big cams because it builds a lot of cylinder pressure. We'd run it near 108 degrees centreline on a carb, but EFI generally likes wider lobe centers - which reduces peak power/torque, but broadens the rpm range and smoothens it up (reduces overlap).

I''ve not yet played with the V10 - so talk to the guys who have - the above cam may have emission problems. The "bigger" shelf street cams of 290+ advertised degrees get spanked stupid by a cam like that because they are low intensity by comparison and make a lot less power over an operating rpm range (even if they peak higher).

Other things being equal a rise in compression one point is worth around 5%hp, but stock compression and a cam like that with head work will add easily 33%+ (on stock) assuming everything else is up to it. It will be streetable - but it will have a definite rumble! Expect cam and spring changes more often than normal - that's part of the price - get used to it or be content with less power.
 
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Moundir

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thanx Greg and Torquemonster :) Well I plan on running 10.0:1 compression. Now to pick the cam that is right for the job ;)
 
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