2.5 Merc compression ratio?

[stealthperf]

Does anyone know the thickness of a compressed 200 Merc head gasket? Or the compression ratio I would have to run to get 140 lbs of compression. Thanks Dave.

[Jay Smith]

The common Merc head gaskets I use on most all V-6's come in 3/4 , 1 , and even 1 1/2 MM ( but the thick one is special, big $$$ and not common )....The amount of thickness it would be compressed would depend on what it was BEFORE you torqued it ! A 200 HP 2.5 head needs to be cut to 34 CC's with a Mercury Head gasket Part number 27-822844-1 to render 145 lbs cranking at sea level with a motor that has a healthy ring /bore combo... We cut those combos and supply the correct head gaskets to yeild that target compression number everyday!


Hope that helps,

[M.CHAPMAN]

Here's a simple formula to figure your compression ratio on any engine.
Compression Ratio = Cylinder Volume + Combustion Chamber Volume Divided by the Combustion Chamber Volume.
You must first convert your Combustion chamber CC (cubic centimeters) figure to CI. (cubic inches)

Cylinder volume is Bore X Bore X Stroke X .7854. = Cylinder Volume. (cubic inches)
Example 2.5 Merc. 3.5 X 3.5 X 2.65 X .7854= 25.5
25.5 X 6 = Total Cubic inches (153)

Example: If your Cylinder volume is 24.7 cubic inches and your Combustion Chamber Volume is 1.7 CI The formula would be:

24.7+1.7
-------- = 15.529:1
1.7

These are random numbers not exact figures of your particular motor.

[M.CHAPMAN]

If My math is correct this is the ratios different heads would yield on a standard bore 2.5 Merc
21cc heads= 20.92:1 Compression ratio
23cc heads= 19.21:1
34cc heads= 13.31:1
38cc heads= 11.99:1
42cc heads= 10.96:1
Numbers are rounded off to nearest hundreth.

[stealthperf]

Thanks for your response. I do compression ratios on race engines for snowmobiles everyday in my shop. I am building a 150 efi to 200 port specs. My stock set up was 42cc chambers + .040" deck height = about 48.22cc installed. This comes out to 9.67 to 1 c.r. @ 115 psi. cranking compression. When I had the motor apart I decked the block .040" and rechambered the heads .040" in. (o-ring heads so I don't want to surface them). The head now cc's @ 45.2 minus the 6.2cc from decking the block giving me about 39cc installed volume. Which = 11.71 to 1. Given the higher ex. port height of the 200 port spec. I think this may be pretty close. According to Jay's post the spec for a 200 is 34cc + 4.65 cc for a 3/4mm gasket or 6.2cc for a 1mm gasket so installed should be from 38.65 to 40.2cc. Being I am @ 39 I may have it! let me know what you think. 1600ft. elevation. Thanks Dave.

[The Big Al]

Math is correct Michael, but did not add for the head gasket.
Probably change ratio by 1/2 point at most.
But does have a great effect. Your raising the cyc volume the complete size of the bore.

Al

[M.CHAPMAN]

Yeah, the formula does not account for the head gasket thickness.

[Raceman]

Calculating static compression ratio in a four stroke engine is useful tool, not only because it gives a camshaft manufacturer a guideline in requirements for the engine and is also a good yardstick for other requirements. Of course camshaft design as well as other variables will affect actual compression and two longblocks could have a great variation in actual compression depending on these variables.

In a two stroke engine, basically port timing isn't designed around the compression ratio, but obviously factors like port timing and primary compression have effects on the engine that aren't present in a 4 stroke. I think worrying about static compression ratio in a 2 stroke is actually a pretty worthless mathmatical excersize, at least in the case of an existing engine being modified.

[stealthperf]

So does anyone think 39cc installed is close? I could install them and just check it but it would be nice to not have to disassemble everything again and set up the rotary table on the mill twice. Not the end of the world, but at least then I would have some numbers to work with. Dave

[dynobo]

Dave, I'm kinda lazy but, I'm with Raceman all the way here. Compression ratios are just for bench racing and heck most people don't really have a clue what their ACTUAL copression ratio is. A few years back I was building the Mopar RB/ B series engines for NHRA street stock class and every 10.25:1 440 Magnum engine I tore down was about 8.3/8.5:1. And you could definitely design your engine cheaper than you could Blueprint a stock Mopar Bigblock. Never could figure why you wouldn't calculate compression ration after the intake valve closed or in 2 strokes after the piston has closed the exhaust port. On a 200 with a 1.58" port height (deck to top of port) and a stroke of 2.65 you basically don't build any compression for the first inch or so of piston travel.
Getting back to the part about me being lazy, it sounds like you have your engine pretty well thought out but, I would have done all the decking,porting etc, then assembled the motor and checked your new compression with whatever gasket you had and then make whatever adjustments to the heads or gaskets or whatever. Pulling the heads off is usually not a big deal so you can change them whenever you choose. But as for your question: My guess is that you are probably pretty darn close to having your hotrod dialed in.

[stealthperf]

The reason I did a lot of calculating on this engine is because it has o-ring heads. I had to make my compression gain by decking the block. I can always pop the heads off to remove compression, but, if I didn't have enough to begin with I can't cut the heads to get more compression. As far as the compression ratio thing I use it everyday to determine how to set up an engine. I take into consideration the exhaust port timing, rpm, ignition timing, and the type of fuel the customer wants to run. The difference between the compression ratios you are talking about is; Standard Compression Ratio = cylinder volume + installed head volume divided by installed head volume. Corrected Compression Ratio = cylinder volume from the top of the exhaust port + installed head volume divided by installed head volume. A 10 to 1 compression ratio engine for pump gas will have a corrected compression ratio of about 6 to 1. When you make changes to your exhaust port height your c.r. stays the same but your c.c.r. will drop because you have less cylinder volume after the port closes. Then the heads will need to be machined to get your c.c.r. back up. Your c.r. may be 11.5 to 1 to get the same 6 to 1 c.c.r. That's why I use compression ratios when I modify engines. Thanks Dave

[FUJIMO]

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