Compresion v. Boost

bowez

Registered User
Got to wondering and doesn't raising boost have the same effect as raising the compression ratio? If so couldn't you get the same effect of OD with a higher CR without the belt slip.

Am I thinking right, and if so wouldn't this be a better option since we use custom pistons anyway as replacements?
 
Got to wondering and doesn't raising boost have the same effect as raising the compression ratio? If so couldn't you get the same effect of OD with a higher CR without the belt slip.

Am I thinking right, and if so wouldn't this be a better option since we use custom pistons anyway as replacements?

I would say Yes your comparison is accurate. The biggest factors about the comparisons is the amont of money and time that you would have to invest.

Pulley + Belt = Few Hundred Bucks ($X00) + 1~2 hours of Labor
Custom connecting rods and/or pistons = ($X000?) + 2 Days of Labor

:confused:

I would choose Pulley and Belt first.
:p

I am sure there are other factors that would have to be taken into consideration. I am just looking at the two I listed.
Just my .02 cents :D
 
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No, it's not the same. Increasing boost increases displacement also. At 15psi your 3.8L is actually a 7.6L.
 
What is the math behind that? Just wondering

14.6 psi is atmospheric pressure. So when you have 14.6 psi (or 15psi for rounding purposes) of boost you are actually doubling the amount of air that the engine would pump through if it were naturally aspirated. It is however not exactly one to one due to the additional heat that is generated, etc. but Dave is right on. Simply changing pistons to increase compression or changing the dimensions of the combustion chamber will not increase the volumetric efficiency of the engine. Increasing boost will. :)
 
14.6 psi is atmospheric pressure. So when you have 14.6 psi (or 15psi for rounding purposes) of boost you are actually doubling the amount of air that the engine would pump through if it were naturally aspirated. It is however not exactly one to one due to the additional heat that is generated, etc. but Dave is right on. Simply changing pistons to increase compression or changing the dimensions of the combustion chamber will not increase the volumetric efficiency of the engine. Increasing boost will. :)

Actually it's a little more complicated than that. Boost measured at the manifold is just a measurement of restriction. You should be looking at load to dertermine how much air your motor is displacing.
 
Actually it's a little more complicated than that. Boost measured at the manifold is just a measurement of restriction. You should be looking at load to dertermine how much air your motor is displacing.

This should be interesting. How would you measure load?
 
I know its not a direct linear comparison but given the limits of the blower bearings and belts, if doing a a rebuild why not bump up the CR and lower the OD. I'm thinking 9:1 and 10-15%.
 
I know its not a direct linear comparison but given the limits of the blower bearings and belts, if doing a a rebuild why not bump up the CR and lower the OD. I'm thinking 9:1 and 10-15%.

Just remember, it isn't that simple. Careful selection of your cam to match pistons and rods is important to keep your Dynamic Compression Ratio reasonable for your fuel choice. I have seen with higher compression ratios is for Daily Driver cars with positive displacement blowers and just cruising around. End up needing to pull a little more timing at some transition points, especially in manual transmission cars.

The big problem ends up being it is so easy to go back to 20% overdrive on the blower that folks will bump the compression ratio and add back the overdrive. Everyone always wants more.
 
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