translating compression ratio into psi

[jay dezman]

compression test on 200 hp suzuki is 110 psi at whatever compression ratio they have. what/how much does the c.r. need to be raised to bring the psi to about 145?

 

[jim dozier]

Re: translating compression ratio into psi

If you are worried about your engines compression you don't need to be worried about its compression ratio. The compression ratio is just the ratio of cylinder volume plus head volume to head volume alone. The actual psi readings on a compression test are related not only to the compression ratio but to the ability of the piston and rings and head gasket to seal. An engine with a high compression ratio but bad piston rings will not have a very high psi cylinder pressure. Conversely a low compression engine with a good seal may have good psi numbers.<br /><br />To calculate theoretical (assuming perfect seals) static pressures for a compression ratio of 10:1, if ambient air pressure =14.7 psi at sea level, 14.7 X 10 = 147. Since the gauge reads zero at ambient pressure you need to subtract 14.7 from 147 which equals ~132 psi for a 10:1 compression ratio. This also doesn't take into account the effects of temperature on pressure. This theoretical exercise is probably useless.

 

[livebait]

Re: translating compression ratio into psi

Man, goes to show you; don't ask if you don't want to know. Good job jimd.

 

[jay dezman]

Re: translating compression ratio into psi

its what i was after though!<br /><br />i'm trying to figure out how much i can mill the heads on my v6 outboard and still keep control over any preignition.<br /><br />suzuki doesn't advertise a cr for their 87+ engines. i do know my current psi is 110-120 and want to raise it to around 140-145. i have spare heads that i want to have milled. i've calculated .053" off will bump the ratio 1/2 point. just have to cut one open to see how thick the casting is so i can figure how much it can be cut and not break.

 

[jim dozier]

Re: translating compression ratio into psi

Where did you get the target psi from? And why do you want to raise it? Is this a performance issue? Increasing the horsepower of a 2-stroke is way more complicated than just arbitrarily shaving 50 thousandths off the head. I would be very leary of increasing the compression ratio in a 2-stroke very much for the very reason you mention - preignition, which is a very bad thing. If your piston and piston rings and cylinder wall aren't in tip top shape your compression may not go up as much as you think. If you want to go this route you should measure the volume of your cylinder head (one chamber) with a burette and add the volume of the cylinder (Stroke X Pi X bore X bore/4). Stroke should be increased by the thickness of the compressed head gasket. Then you can calculate the compression ratio before and after the shaving. You might want to reevaluate the carb jetting afterwards to make sure the engine isn't running too lean which will aggravate pre-igntion. Also you may want to reevaluate the stock ignition advance for the same reason. Don't forget you will have to use high octane also.<br /><br />There are others on this forum with racing experience that may offer better advice on how far you can go with this.

 

[jay dezman]

Re: translating compression ratio into psi

yes i am looking for a performance increase. my target is 8-10%.<br /><br />the target number is based on a couple pwc's i've worked on that are 2 cycles and both have 145-150 psi, burn regular gas and have no preignition. by shooting for something less than the it should allow for steady cruising vs on/off throttle operation typical of a ski.<br /><br />i'm familiar with cc-ing heads and making tuning adjustments after the fact.<br /><br />i will convert from the standard single plug heads typical of a 200 and swap in the dual plug ones from the 225 efi. yes, i know there are different ignition components required, too - i've got those also ready to go.

 

[JB]

Re: translating compression ratio into psi

"Compression ratio" is a myth in 2 stroke engines, Jay. <br /><br />Because there are openings in the cylinder wall, cylinder volume at BDC is irrelevant, and any modification of port timing or profile further complicates the issue.<br /><br />If you want an 8-10% improvement in performance (torque) in a 2 stroke, look to breathing and scavenging. You are not going to find much by increasing cylinder compression.<br /><br />Good luck.

 

[jay dezman]

Re: translating compression ratio into psi

so then what are the differences between the suzi 225 and 200? the 225 has fuel injection (which everyone says does nothing for hp in a 4 cycle engine) AND the dual plug heads. the dual plug head engines are the only ones that they mentioned a c.r. for which leads me to believe that c.r.IS a factor in the increased output.

 

[JB]

Re: translating compression ratio into psi

Not everyone says that EFI does nothing for HP in a 4 stroke engine, Jay. It depends on what you are comparing it to. It certainly improves the area under the torque curve over multiple carbs.<br /><br />Because EFI engines are computer managed overall performance is significantly improved, along with "drivability" and economy.

 

[rodbolt]

Re: translating compression ratio into psi

I am with JB. improving one subsystem without the other is a waste of time and beer money.

 

[jay dezman]

Re: translating compression ratio into psi

i've been thinking some more about the above responses. while i agree that the ports in the cylinder wall will affect the "effective compression ratio" there is still the hypothetical design compression ratio (this occurs in 4 cycle engines also when camshaft specs are changed)which is based on the swept area of the cylinder AND the cylinder head and its reduced size at tdc. <br /><br />i'v come across a company called land-n-sea that tweaks v6 and v8 outboards. they sell cnc machined high compression heads for yamaha, mercury and johnson outboards. <br /><br />here is a excerpt from their web site, talking about hign compression dual plug cylinder heads "...the place to start is with higher compression heads. Raising the compression is about the only modification you will make that actually INCREASES MPG and power THROUGHOUT the rpm band. Keep in mind though, the limits imposed by fuel octane."<br /><br />i have already started to improve breathing with the addition of boyesson reeds. i am familiar and capable of rejetting after changes are made.

 

[Ray Neudecker]

Re: translating compression ratio into psi

Land and Sea products are notorious for being a cause of blown engines. If you increase the compression without increasing the octane of the fuel, you are headed for detonation. Land and Sea heads often do not have enough cooling for normal operation.

 

[Dhadley]

Re: translating compression ratio into psi

Jay, can you see that light at the end of the tunnel? Its a train. <br /><br />The good news is, its carring pistons, youll likely need them.<br /><br />It'd be nice to know where youre at now and where you want to be. What kind of rig? Will your boat respond to small changes in torque? What kind of rpm are you looking for? Typically dual plugs (per cylinder) limit the rpm.

 

[jay dezman]

Re: translating compression ratio into psi

motors are 200 hp suzuki's. intended heads are factory suzuki dual plug.<br /><br />looking for a little more "oomph" across the board and top end and will keep rpm within factory limits.<br /><br />basically looking for the difference in cc's between the single and dual plug heads of suzuki's manufacture-looking to increase the cranking pressure to around 140 psi which i feel is still safe for pump gas. <br /><br />the reference to land-n-speed was to point out that others feel a compression increase was beneficial, and not necessarily that that particular product would be purchased, again just the concept!<br /><br />yes factory engineers can do this or that but choose to err on the conservative side, yadda, yadda. <br /><br />i also asked for specific differences between the 225 fuel injected and 200 carbed engines. no responses to that one as yet. from what i've been able to deduce its fuel injection and heads. same midsection, same lower unit. because of the efi the reed cage is different but not necessarily "better" ie higher flowing. cdi unit at least for the 87 vintage single plug vs dual plug is same item! <br /><br />i vaguely recall a magazine series of articles years ago where a 200 hp merc was mildly massaged in stages... higher compression was one step, can't recall the magazine or locate it.<br /><br />jimd did give an excellent definition which is helpful correlating cr to psi. issue then becomes what is the advertised cr? i can't find one for the 200 suzi. can find one for the 225 with dual plugs...

 

[Ray Neudecker]

Re: translating compression ratio into psi

Higher compression will yield more HP. It will also make the engine much more sensitve to detonation. I personally doubt the increase in your situation will be worth the increase in operating cost or the risk of blown power heads. Many people go that route because of the lower initial cost and ease of modification. On a light weight performance hull, it can bring dramatic performance increases worth the higher operating costs to them.

 

[Grant S]

Re: translating compression ratio into psi

Jay, could you explain how changing camshaft specs changes the compression ratio.

 

[jim dozier]

Re: translating compression ratio into psi

True compression ratio is just the ratio of the volume of the cylinder plus the combustion chamber volume to the volume of the combustion chamber alone. However, the reason we talk about compression ratio is it is a measure of how well compressed the inhaled mixture will be. The mass of mixture inhaled varies over the rpm range (one reason gas engines develop differing power at different rpms). In modern engines, both 2-stroke and 4-stroke, the intake and exhaust ports or valves, respectively, are open at the same time during portions of the piston revolution. This is usefull because air/fuel mixtures have mass and interia and you can use the inertia to pack more than a full cylinder volume of mixture into the cylinder at higher rpms. As a result, you would find that the actual pressure in the compressed cylinder would be higher than the compression ratio would indicate at certain rpms. All of this is controlled by the valve or port timing specs.