Mercury 2.4l with 2.0l heads compression question

[sasquitch]

Hello, I have a 1988 mercury 2.4l v220(converted to carbs). The WH34 carbs are jetted with 62's in the idles and 70's/72's in the mains. I just swapped my stock uncut 37cc-38cc 2.4l heads for a very clean stock uncut set of 34cc-35cc 2.0l heads. I checked the compression with engine cold and warm at wide open throttle. Before the swap I got right at 119-120 psi (2.4l heads) on all 6 holes. After the swap I now get 125-126 psi.(2.0l heads). I was expecting more psi. Do these compression numbers look right. I dont have the heads part numbers in front of me, but I know the 2.0l heads have 3 1/8 stamped on each head. I can get the part numbers of both 2.0l and 2.4l heads if it will help. Thanks again. Brad

[fyremanbil]

Somethings wrong with those comp numbers. The 2L head should put you at about 145-150. The 3 1/8 stamp indicates a 2L head.

[mach351]

Try a different comp gauge

[sasquitch]

Hey fyremanbil, I was thinking the same thing. I now have the part #s. The 2.0l heads are 96442. The 2.4l heads are 15221c. I followed the instructions for checking compression that I found on this site. Engine warm-plugs out- battery full charge- plug wires grounded-throttle at wide open-5-7 turns of motor or until gauge stopped going up. I also cc'd the heads using Jay Smith's instructions on bighammer.com There was only about 4cc difference from the stock 2.4l heads and the 2.0l heads. How much compression is 1cc worth in a setup like mine. Thank for any help. Brad

[sasquitch]

Thanks mach351, I just checked all 6 holes with a different gauge . I got 130 psi on all 6 cylinders. Maybe I am missing something? Thanks again,Brad

[mach351]

Not sure about on 2.4's, but on a stock ported 2.5 when you go to a thinner gasket (from a stock 1.2mm to a 1 mm) usually nets you about 7-10 more psi. Some math could probably figure it out - but I'm not that smart LOL

[sasquitch]

Thanks mach351, I am not sure how thick the gaskets I took off were, but the ones on there now are 1.2mm. So I guess I could have had 1mm gaskets before. So If you add say 8psi for the .2mm difference to what I have now that would give me around 138psi. I dont know? sounds like I need to do a leak down test. If I new it would not mess up the gasket? I would like to bolt on one of the 2.4l heads and see if there is a difference in the readings with new gasket. Thanks for any help.

[fyremanbil]

Well, if you checked them at only 4CCs difference, you might not get as much. I thought most 2L heads were 32CCs, but I've only checked a few that were uncut.

[ChrisCarsonMarine]

I'm not aware of anyone offering 2.4 head gaskets in different thicknesses?If you are ,fill me in,Chris

[wrechin2]

I'm not aware of anyone offering 2.4 head gaskets in different thicknesses?If you are ,fill me in,Chris

Not like the 2.5L.....BUT I have ordered 1 gasket from Pro and 1 from southern for the same application ( 87 2.4 200) and have come in 2 different thickness. There were .1MM difference and both were victor gaskets and had the same OE numbers on them! I had to order another one from southern to get the same thickness for the engine I was building. Then I ordered another from pro for stock.


Well, if you checked them at only 4CCs difference, you might not get as much. I thought most 2L heads were 32CCs, but I've only checked a few that were uncut.

I have cut quite a few and I always measure 34CC~34.5CC in the 2 piece and 1 piece verticle 2L heads. Just my experiences

[sasquitch]

Thanks for the help everyone. I am still very new to outboards. I bought a set of 28cc heads from a member on this site a year or so ago. He gave me a set of new gaskets with the heads. I dont know which company made the gaskets, but I did measure them to be 1.2mm. I have another question . If a 4cc difference from 2.4l (38cc) heads to 2.0l (34cc) heads gave me a 10psi increase in compression. If my math is correct, thats 2.5psi per cc. If I install the 28cc heads, which should have a 10cc difference from the stock 2.4l(38cc) heads. Then at 2.5 psi increase per cc in compression- 2.5psi x 10cc=25psi . If I take the original reading of the 2.4l heads- 120psi + 25psi = 145psi with the 28cc heads. Does this make any sense at all. I thought I would be at 145psi with the 34cc heads? By the way it looks maybe I can run the 28cc heads on 93 octane. Thanks any advice. Brad

[mach351]

Sounds good to me - put 'em on and check the comp

[wrechin2]

On a 2.4 the lowest CC you would want to go is 32CC. I run 28CC on 2L with 1.595 porting and they are around 145-148. You really need to use another gauge to verify the readings. Another thing...This isn't a fresh engine is it??? Have seen times when people rebuild engines and expect high compression from the get go. just my .02

[sasquitch]

I bought my boat a 1985 allison xst about 3yrs ago. The motor was supposed to be a fresh rebuild with less than 2hrs on it ? I have put around 50 to 60 hrs on the motor . I thought a new rebuild would yield the best compression numbers. I will buy a leak down tester and check the health of the motor before I change anything. What % leak down is acceptable ? How many hrs. on a motor is considered a fresh rebuild? Sorry for so many questions. Thanks again, Brad

[ChrisCarsonMarine]

A leakdown test is next,we've seen many motors folks have tried to hop up and found that no matter how much they mill the heads they cant get the compression their looking for,...worn out motor....and the milled heads will just help finish it off.After the rebuild the heads will be too tight,chris

[wrechin2]

I agree with Chris. The "common" cc on 2.4's is 32 CC. A broken in engine will have around 4-8% at about 10hrs. I like to see 10% or less at 1/2" to 3/4" ATDC on fishing engines with a lot of time on them. I have found many engines with issues that you measure at TDC and will pass but move them off of TDC and they will fail badly. Engines will produce a carbon ring at the top of the cylinder and act as a seal when the top ring is against it. If the cylinder is true and seated, it does not matter where the test is taken as long as the rings are not at a port or idle relief. I do them quite often like this. I use a screwdriver and rotate the cylinder till it quits moving and place a piece of tape on it at 1/2-3/4 from the plug threads. I then rotate it to where the tape is even with the threads and use a pair of vise grips on the flywheel at the starter (the vise grips press against the starter and keeps it from rotating). Then I run my test making it a 1 person job. Just my experiences.

I use a OTC 5609 and it can be had for fairly cheap (around $70) if you do a seach on it. Here is a thread with multiple choices.


http://www.screamandfly.com/showthread.php?205976-Leak-down-tester&highlight=leakage (http://www.screamandfly.com/showthread.php?205976-Leak-down-tester&highlight=leakage)


Here is a thread that a customer started after we found a issue on his engine. At TDC it had 6% and at 3/4" it had 70%. The other 5 cylinders had 6% at 3/4". We tore it down and found the bottom ring was coked in so bad had to use pliers to remove it. This is a prime example why I measure them like I do. Just my experiences!

http://www.chattanoogafishingforum.com/forums/thread-view.asp?tid=27877&posts=27&start=1 (http://www.chattanoogafishingforum.com/forums/thread-view.asp?tid=27877&posts=27&start=1)


Just my .02

[sasquitch]

Thanks chris and Wrechin2 I am going to do a leak down test tomorrow. Thanks Brad

[wrechin2]

No problem. Hope it is a bad compression gauge. Have you CC'd the heads to verify them??

[sasquitch]

Yes, I cc'd the heads they the 2.0l heads were very close to 34cc- 35cc. I have borrowed another gauge I will check the compression today as well as the leak down %. When I bought this motor it was supposedly a freash rebuild, but you know how that goes. Thanks Brad

[wrechin2]

Yes, When I bought this motor it was supposedly a freash rebuild, but you know how that goes. Thanks Brad

Yeah, I had a guy who brought down a 88 2L 150 that was "JUST REBUILT" with 5hrs on it with issues. I found #5 had 80%:eek: and the rest were 20-40% He asked if that was normal for a "fresh" engine. I said no and then took him in the shop and done a leakage test on a engine I had just built that was still on the engine stand that had never been started. It had 12% with unseated rings. I then asked what he gave for the boat and he told me. I told him if the engine was just rebuilt, he would not have sold the complete rig for $1000:rolleyes:. Hard lesson learned!!!

[sasquitch]

Hey wrechin2, I just checked the compression with a third gauge I got around 127psi to 130psi on all 6. I just checked the #1 cly. with the leak down tester I bought at Harbor Freight. I know Harbor Freight has some very cheap tools. I wanted to check it today, so I bought the one thay had. I have a question about how to use it correctly. I have #1 cly. at tdc . I have my air compressor at 100psi. The leak down gauge regulator knob fully counterclockwise - I connect to air supply and connect the to adapter hose to spark plug hole. I then adjust reg. until % gauge on tester reads zero or in the yellow band that reads set. Then connect tester to adapter hose.It only takes about 15psi adj clockwise to achieve the set position or zero. AT these setting the #1 cly only leaks down 5% or less. Seems like its ok. I just took another reading on cly #1 at about 1/2" ATDC. An got a reading of 6%-7% leak down. I just completed a clyinders all were 5%-7% TDC and between 6% and 8% at 1/2" ATDC . This is making no sense? I just had a thought. When I cc'd the heads I used water is that ok? Thanks again, Brad

[sasquitch]

Ok here it goes. I did a little research and found that the leak down tester from Harbor Freight has a working pressure of 15 psi.I did a little modification to my gauge. I removed the % gauge and replaced it with a 100psi gauge. I tested all cylinders at 15 psi. and at 70 psi. Here are the results. Thanks again for any help. Brad




--------------Modifided Gauge--------------------

CYL--psi----%leak down @ tdc--% leak down @ .50 ATDC
1----70--------6%-7%---------------4%-5%
2----70--------5%-6%---------------6% 7%
3----70--------6%-7%---------------6%-7%
4----70--------7%-8%---------------7%-8%
5----70--------8%-9%---------------6%-7%
6----70--------4%-5%---------------4%-5%


----Original Harbor Freight Gauge--------

CYL.---PSI---% leak down @ TDC---% leak down @ .50 ATDC
1-----15------5%-6%----------------6%-7%
2-----15------5%-6%----------------7%-8%
3-----15------5%-6%----------------7%-8%
4-----15------5%-6%----------------7%-8%
5-----15------7%-8%----------------6%-7%
6-----15------5%-6%----------------7%-8%

[wrechin2]

That is why I don't like most gauges is because they test at low pressure. The OTC uses 100 PSI and has 2 100 PSI gauges. If the second gauge reads 95 PSI that is 5%. If it reads 60 PSI that is 40%. A lot easier.

What you need to use to CC the heads are Isoproplyl (rubbing) alcohol with just about 2-3 drops of food coloring. It makes it easier to read. What tool are you using to measure it with??? 32CC will give you what compression you are looking for 140-145 PSI. You are about 2.5CC off of the mark of what it normally takes to get around 145 psi. I just looked at my "DOOR OF KNOWLEDGE" and the 2.4 2 peice heads I have measured are 37CC. I write all kinds of information on my shop door. I have measured several blocks deck to piston heights and found that they vary about as much as the porting. You may have a exceptionaly high deck causing the lower compression numbers or high porting (unlikely). Most 2.4 200's have around 125 PSI from the get go and I have never measured a 220 porting. You had about a 10 PSI compression change with a drop of 2.5CC and a additional drop of 2.5CC will give you about 140 PSI if the leakage numbers and compression numbers are true. Just my .02

[sasquitch]

Thanks alot for your help Wrechin2, I feel a whole lot better after checking the leak down . I must have tripple check every cylinder. I know the gauge is probably not the most accurate one. Both gauges were at 70psi before each cyl. test. I am sure that my mearsurement (cc) of the heads was not perfect. I think I will just run it like it is for a little while. When I do have the heads cut to 32cc. Which heads would you cut the stock 2.4l 37cc one piece heads. Or the 2.0l 34.5cc 2 piece ones. Thanks everyone for all the help. Brad

[Capt.Insane-o]

Cut the one piece heads, They cool much better.

[wrechin2]

The 1 piece are suppose to be better but I have ran 2 piece with no issues on several modified 2L and 2.4L engines. Either will get you to 32CC. It just requires more material to be removed on the 2.4 heads. The 1 piece are easier to cut and look cooler when cut. I had to make tools to hold the 2 peice down on the mill but are a peice of cake now that I have taken the time to make my life easier. Just my .02

[sasquitch]

Thanks capt.Insane-o and Wrechin2. Brad

[wrechin2]

You PM replied to.......

[Magcat 62]

Cant agreee common cc on 2.4 is 32cc. 2.0 yeah but 2.4 36-38 imo is common. guy putting 2.0 heads on 2.4 getting about 145-150 psi. my 2.4 jumped 10# w/ 2.0 heads.?