Guest Anonymous Posted August 9, 2001 Share Posted August 9, 2001 This may be a bit off the subject, but I'm sure some of you guys know the answer to this. I'm looking at a used boat. It has a mercruser 454 and all cylinders check out at 125 to 130. They are all close so I know that is good but should it be higher? Test was done with closed throttle. Thanks for any info. Mark Quote Link to comment Share on other sites More sharing options...
blueovalz Posted August 9, 2001 Share Posted August 9, 2001 It all depends on the compression ratio, and cam dynamics, as well as cranking rpm and if a little oil is used in each cylinder to help seal the rings. All in all, these readings sound fine. My 10.5:1 with a nice came runs about 120 to 125. Quote Link to comment Share on other sites More sharing options...
Kevin Shasteen Posted August 9, 2001 Share Posted August 9, 2001 I agree, cyl.pressures will depend on the same dynamics "What kind of power should my engine make". The very same answers to that question will answer your question. There's a formula I've put into Excel that I thought was pretty cool; incoperates the P-L-A-N / 33,000 = Horspower. I've seen this formula in a few auto math books...& finally decided to [figure it out]! What does that do for your question(?); good question (tongue & cheek). Think of an air tank that has a tire valve on it...as you apply compressor air on it you can see the PSI gauge climb...higher & higher & higher till you remove the air line. What ever reading you stop at equals the amount of air you have installed in the tank. This is similar to your engine's cylinders. We all know the engine has intake/exhaust valves that allow air to enter & exit at predetermined times (the camshaft). Go back to your air tank...now depress your bleeder valve & what does the needle do; of course it decendes towards the "0" indicator; the needle will continue to loose pressure till you release the bleeder valve. However much air you release will determine how much air remains in the tank. Understand this is exactly what is taking place on your engine & is why its hard to say; your pressures are good/bad w/out knowing the camshaft's profile & the power band of the engine. The cam opens/closes & allows the air to enter/exit; while the compression determines how much air will be compressed & at what speed the air enters/exists will determine how much air is actually compressed...this will determine your cyl's psi figures. Again that all depends on how well your engine cyl's are sealing. I plugged the 454 bore/stroke into my P-L-A-N / 33,000 equation & it gave your psi ratings for (street engine) of 350hp & 386ft.lbs at 5000rpms. The "P" in P-L-A-N stands for MEP (Indicated) Mean Effective Pressure, the "L" stands for Length of the stroke, the "A" stands for top surface area of one piston in sq.in's while the "N" stands for the number of power strokes per minute. Its pretty cool when you alter one/or all of the perameters (Bore, Stroke, RPM's, HP, Torque) & watch the numbers change...really helps you understand what changes occur when rpm builds! Still this equation doesnt take into account specific cam timing (that which controls the filling/bleeding of the cylinders)...it picks up where the camshaft leaves off...so there is still some additional math to do afterward; isnt there always(?)! Kevin, (Yea,Still an Inliner) [ August 08, 2001: Message edited by: Kevin Shasteen ] Quote Link to comment Share on other sites More sharing options...
Guest Anonymous Posted August 9, 2001 Share Posted August 9, 2001 You guys are great!! Thanks for the reply. By the way Kevin the engine is rated at 330 hp. Should be fun on the ocean. Thanks Guys Mark Quote Link to comment Share on other sites More sharing options...
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