Raising compression - what's it for ? What does it really do?

[rockbottom]

I WOULD LIKE TO INTRODUCE A TOPIC ...... Anything and everything about raising compression
U know the difference between theory and experience is- have you done it ? or are you talking (#+*&) theory ?
1. how to do it ?
2.how much compression ...?
3. how to measure it ?
4. the effects ?
5. why do it ?
6. what else goes with it ?

Im working on my 200 evenrude (1977))(115 psi on all 6 cold ) trying to get more horsepower from it . so how do I raise the compression ?

thank you guys ,steve

[kingsbiship]

remove the heads and bring them to a machine shop... have them shave .075 off, if they're stock!
This will give more low end power, and poss. mid

[Riverman]

That may be too much. To measure the existing clearance feed a piece of wire solder through the spark plug hole over into the squish area. Rotate the flywheel to crush the solder between the piston and the head the measure the thickness. As racer said you need at least 0.04 inch in the squish area.

Also remember is you raise the compression above 135-140 PSI you will have to start using at least a percentage of race fuel.

[twright55]

I have a question:

In a 2 stroke motor, will a higher compression somehow 'force' or 'induce' more fuel/air mixture into the combustion chamber? It might be a stupid question, but I know that the intake and exhaust tuning are more inter-related on 2 stroke motors than 4 strokes, and 2 strikes are far more likely to have runability problems with low compression than 4 strokes (I know quite a bit about 4 strokes, and almost nothing about 2 strokes.)

[twright55]

. As racer said you need at least 0.04 inch in the squish area.


At least, at most, or right about?
I believe that you don't want too much (or you won't get the squish), too little (or you might get piston slap), but in a certain range.

[terry taylor]

At least, at most, or right about?
I believe that you don't want too much (or you won't get the squish), too little (or you might get piston slap), but in a certain range.

You might get piston slapping on what, please explain thanks.

[terry taylor]

I have a question:

In a 2 stroke motor, will a higher compression somehow 'force' or 'induce' more fuel/air mixture into the combustion chamber? It might be a stupid question, but I know that the intake and exhaust tuning are more inter-related on 2 stroke motors than 4 strokes, and 2 strikes are far more likely to have runability problems with low compression than 4 strokes (I know quite a bit about 4 strokes, and almost nothing about 2 strokes.) I don,t think so. My answer is no thanks.

[rockbottom]

I can't even figure out why raising the compression give's it more power????

if someone can explain it that would be cool especially if they have actually done it to thier motor, steve

[twright55]

You might get piston slapping on what, please explain thanks.

The tighter you make your squish (the distance between the flat area of your piston and the flat area of the head), the better the squish works (squish is where the piston comes so close to the head that it 'squishes' all the fuel/air mixture out of that area at a very high speed, which promotes a more thorough mixture by vaporizing any droplets of unmixed fuel, which also has a cooling effect that decreases the likelihood for unintended detonation.) But if you make it too tight, you run the risk of the piston actually making contact with the head.

Anyway, you won't change squish by skimming the head...it doesn't change that particular dimension. You have to skim the block deck or use thinner head gaskets to decrease the squish dimension.

[twright55]

I can't even figure out why raising the compression give's it more power????

if someone can explain it that would be cool especially if they have actually done it to thier motor, steve

Raising the compression increases the thermal efficiency. It's how an internal combustion engine works. It's the most basic feature. More of it means more power AND better mileage. But more compression also requires better quality fuel.

[rockbottom]

remove the heads and bring them to a machine shop... have them shave .075 off, if they're stock!
This will give more low end power, and poss. mid

where did you come up with this number 0.075 ? :confused:

I very curious especially if you have hands on experience because I can't tell how much experience you have by reading your post , at least untill I get to know you ,steve

[rockbottom]

twright55

Ok that I can wrap my head around a litter better , with that in mind there must be a max ratio 11-1 or something recommended for pump gas as I"ll never put racing fuel in my motor ??????????

what is the max compression ratio for my motor and where did that come from ????????

or what is the max psi (when doing a compression check ) for the same motor

1977 evenrude 200 115psi on all 6 throttle plates WFO and cold

STEVE

[Mark75H]

Some right on the money and some sort of right.

Raising compression increases cylinder pressure during combustion and increases the motor's torque, which increases horsepower. There is potential for power gain across the rpm range, but it may be more noticeable at some rpms rather than dramatic at all rpms. Generally, in theory you can estimate how much more torque and power a motor will make by estimating the increase in "Brake Mean Effective Pressure" caused by the compression increase.

Milling the block or head both will affect the "squish" but will not change the angle of the squish band. That's because squish clearance and squish angle are not the same thing. The pressure and temperature in the combustion chamber vaporize all droplets of fuel long before the turbulence caused by the squish band comes into play. As stated, the cooling and shading of the gases in the squish band directly reduce the chances of detonation beginning in the squish band area and the turbulence helps the main combustion pressure raise less explosively and more smoothly.

Raising compression sometimes DOES give a increase in volumetric efficiency, even with the mechanical loss of compressing to a higher pressure; any extra net gain from a volumetric pumping efficiency from compression increase is one of the hardest things to predict in engine building. Basically don't count on it, but if you get unexplained power increase, take the gift.

Cranking compression on my motor ... 275# (two hundred seventy five, not a typo) ... good squish bands and some other tricks

[twright55]

twright55

Ok that I can wrap my head around a litter better , with that in mind there must be a max ratio 11-1 or something recommended for pump gas as I"ll never put racing fuel in my motor ??????????

what is the max compression ratio for my motor and where did that come from ????????

or what is the max psi (when doing a compression check ) for the same motor

1977 evenrude 200 115psi on all 6 throttle plates WFO and cold

STEVE

Every engine is different and has it's own characteristics that determine what compression ratio it can run at. And considering my knowledge of 2 strokes is sketchy, I wouldn't want to say. That's where somebody with experience with that particular motor can hopefully help you.

The only thing I've picked up from reading various threads here is that most of these outboards tend to top out at about 150psi (except for maybe very highly tuned motors.) I don't know if that would apply to an OMC crossflow.

[twright55]

Cranking compression on my motor ... 275# (two hundred seventy five, not a typo) ... good squish bands and some other tricks

Well, that throws my 150psi estimation out the window..lol!

But that seems to be the highest number I've seen mentioned for 'typical' outboards...let's put it that way ;)

[150aintenuff]

mark 75's motor is far from stock and far from a pump gas motor.... 150 psi is a 92 octain threshold where the fuel becomes un stable.... 275 is more like a C 16 motor... or even alcohol only..

[Mark75H]

Or AV gas :)

150 is a good number for a hot rod motor ... I just put my numbers in there to show I kinda know what's going on with raising compression. In its original form from the factory my motor would have had 120-130 or so. I actually lowered the compression on this motor to run on gas ... its original state as a racer was an alcohol motor with cranking compression over 300. My motor also has dramatically smaller bore (less than 2 1/2 inch) which makes it able to handle higher compression on lower octane fuel. That's one of the reasons we're not all driving huge one cylinder motors.

[rockbottom]

What motor are you talking about or does it matter ?

Sounds like I should start with 150 psi as a focal point ...

Anybody got any idea how much to cut the head for starters ? I'll check the clearance

Does anyone make a copper or reusable head gasket ?

keep talking guys Im learning a lot

THANK YOU ,STEVE
Or AV gas :)

150 is a good number for a hot rod motor ... I just put my numbers in there to show I kinda know what's going on with raising compression. In its original form from the factory my motor would have had 120-130 or so. I actually lowered the compression on this motor to run on gas ... its original state as a racer was an alcohol motor with cranking compression over 300. My motor also has dramatically smaller bore (less than 2 1/2 inch) which makes it able to handle higher compression on lower octane fuel. That's one of the reasons we're not all driving huge one cylinder motors.

[jerry1865]

my race bike was born a 96 air cooled 250 w/ 130 lbs of sqeeze. after going as far as was possible a 250 we found another brand bike from some time back that had a "big bore kit". we had to shave the case to allow for the bigger skirt. and some other tricks were involved in making it work. the end result was a big time over-sq bore. i settled on 155 lbs of compression.
that is the the sweetest machine ive ever had the pleasure to run the guts out-of. the only draw-back was that if you were not real serious when you went to kick it over, it would kick you back. (once, it nearly broke my foot)

[terry taylor]

Sorry but squish will be changed by milling the head, thats basis or reason for doing so. A thinner head gasket reduces combustion chamber area volume along with increased comp. and squish. Milling the block surface is a complete different area to get into , doing that theoraticly raises the ports thanks.

[Mark75H]

How does milling the block raise the ports?

[terry taylor]

The tighter you make your squish (the distance between the flat area of your piston and the flat area of the head), the better the squish works (squish is where the piston comes so close to the head that it 'squishes' all the fuel/air mixture out of that area at a very high speed, which promotes a more thorough mixture by vaporizing any droplets of unmixed fuel, which also has a cooling effect that decreases the likelihood for unintended detonation.) But if you make it too tight, you run the risk of the piston actually making contact with the head.

Anyway, you won't change squish by skimming the head...it doesn't change that particular dimension. You have to skim the block deck or use thinner head gaskets to decrease the squish dimension.
How does one increase comp. to 275 lbs. without the piston crown hitting the had

Well, that throws my 150psi estimation out the window..lol!

But that seems to be the highest number I've seen mentioned for 'typical' outboards...let's put it that way ;)

[Mark75H]

How does one increase comp. to 275 lbs. without the piston crown hitting the head? With a custom made head with different shape from the factory head.


How does milling the block change the porting?

[terry taylor]

With a custom made head with different shape from the factory head.


How does milling the block change the porting?
HI. it,s my understanding measuring from the top block surface to the important top of the ports you get port heigth. Milling the block surface will give a reduced measurement , this means the ports are higher . DO you agree? thanks.

[jphii]

HI. it,s my understanding measuring from the top block surface to the important top of the ports you get port heigth. Milling the block surface will give a reduced measurement , this means the ports are higher . DO you agree? thanks.

No. It changes the port timing, but not the porting.

[Mark75H]

That's correct, it does not change the port timing, the porting or the function of the motor. It changes the measurement.

Measuring down from the head is just a matter of convenience; port timing/porting is relative to the crankshaft, not the head.

[terry taylor]

No. It changes the port timing, but not the porting.

Sorry but your answer should have been yes. Raising the ports is what I said, and that means changing the port timing wich is what you said. My point was to keep this comp.thread on course thanks jpii.

[Mark75H]

it does not change the port timing, the porting or the function of the motor. It changes the measurement.

Measuring down from the head is just a matter of convenience; port timing/porting is relative to the crankshaft, not the head.

[TheRickster]

HI. it,s my understanding measuring from the top block surface to the important top of the ports you get port heigth. Milling the block surface will give a reduced measurement , this means the ports are higher . DO you agree? thanks. .

Sorry but by milling the deck surface you have NOT raised the ports all you have done is changed the measurement of the deck height of the block. The measurement from the deck surface is only a convenient way to port map a cylinder. You cannot take a 200hp fishing motor with 1.610" port height and deck the block .045" and now the motor is a 1.565" 300 drag..

Rick

[terry taylor]

What motor are you talking about or does it matter ?

Sounds like I should start with 150 psi as a focal point ...

Anybody got any idea how much to cut the head for starters ? I'll check the clearance

Does anyone make a copper or reusable head gasket ?

keep talking guys Im learning a lot

THANK YOU ,STEVE Rockbottom in regards to building an older cross-fow v6 for dependable performance and fun i,d keep the comp. around 135-140 psi. with premium 91 octane pump gas have the heads shaved 30 thou. and use OMC factory head gaskets dry . Most will advise never reuse a head gasket thats the best I can advise knowing this is just one aspect of a performance build thanks.

[Mr. Demeanor]

Can you resuse the old gaskets just to bolt the head up after milling to check and see where your compression is at?

[Mark Poole ModVP]

If your motor has a flat squish like a 49 cu in OMC then shaving the head will not change squish height. Shaving the head will only reduce the bowl volume. Decking the block has nothing to do with where the ports are in relation to the crankshaft rotation.

Another thing to think about.....anytime you raise the exhast ports you are adding time to the duration, thus "punch" will be lost unless compression is raised back up by shaving the head.

[racer]

You need to understand a omc crossflow is not a flat piston engine so milling the head WILL change the squish. Decking the block does not change port timing as mentioned that is an effect of crank rotation

[Forkin' Crazy]

150 psi is a 92 octain threshold where the fuel becomes un stable..

Not on a crossflow. I'm not going over 130 on premium.

As I understand the small bore 235s had the best heads.

[terry taylor]

Can you resuse the old gaskets just to bolt the head up after milling to check and see where your compression is at?

HI. the factory thick cross-flow with the heavy crush ring [ might] allow that if any gasket will. Be sure to retorque afterward and maybe add a couple of pounds over spec. [shoud I duck my head now]

[terry taylor]

Sorry but by milling the deck surface you have NOT raised the ports all you have done is changed the measurement of the deck height of the block. The measurement from the deck surface is only a convenient way to port map a cylinder. You cannot take a 200hp fishing motor with 1.610" port height and deck the block .045" and now the motor is a 1.565" 300 drag..

Rick I recognize apparently my understanding was a misunderstanding in that regard thanks. Could anyone explain with the piston at top dead centre as designed .What will result after removing 40 thou. from the block deck thanks.

[Riverman]

Not on a crossflow. I'm not going over 130 on premium.:iagree:

With stock port timing the best stock uncut heads on the V4 will get you about 135 PSI. Safe only if you back off the timing and run fat.

[Mr.Rude]

I recognize apparently my understanding was a misunderstanding in that regard thanks. Could anyone explain with the piston at top dead centre as designed .What will result after removing 40 thou. from the block deck thanks.

Think of it this way, the crank, rod, and piston dimensions have not changed. At any give point (lets say 40 degrees of rotation past tdc) the crank has pulled the piston down "X amount. Unless the rod length, pin height, crank throw or crank offset this number will never change. whether its in a short deck block, tall deck block or no block at all. This means that the 1.5" that it traveled from TDC at the give 40 deg, the top of the port will always be at that same location regardless of what is above it.

[Mr.Rude]

As far as compression simply think of this, light gun power on a table in open air and not much happens. Compress it in a container and there is a drastic difference in the outcome!

[ChrisCarsonMarine]

Port timing is calculated by measuring the degrees of crankshaft rotation between the opening and closing of the intake and exhaust ports,in much the same way the specs for a 4 stroke cam is described or identified...lift would be port width,duration would be port height(measured distance from top to bottom of port),and overlap being the distance between the closeing of the intake port to the closing of the exhaust port.Hope I explained that right,and as you can see,cutting the deck will have no effect on any of it ,Chris

[jphii]

Think of it this way, the crank, rod, and piston dimensions have not changed. At any give point (lets say 40 degrees of rotation past tdc) the crank has pulled the piston down "X amount. Unless the rod length, pin height, crank throw or crank offset this number will never change. whether its in a short deck block, tall deck block or no block at all. This means that the 1.5" that it traveled from TDC at the give 40 deg, the top of the port will always be at that same location regardless of what is above it.

Don't forget piston height. On my decked blocks the pistons are cut down to match it. I always figure the 2 go hand in hand so I always "assume"....

[Mark75H]

The confusion arises from spec sheets and port maps that specify a particular measurement from the top of the cylinder without the other dimensions mentioned. These dimensions assume but generally do not specify factory standard deck height.

[Mr. Demeanor]

Port timing is calculated by measuring the degrees of crankshaft rotation between the opening and closing of the intake and exhaust ports,in much the same way the specs for a 4 stroke cam is described or identified...lift would be port width,duration would be port height(measured distance from top to bottom of port),and overlap being the distance between the closeing of the intake port to the closing of the exhaust port.Hope I explained that right,and as you can see,cutting the deck will have no effect on any of it ,Chris

Unless you cut it too far and run the wrong the fuel. Then it changes all of it into a worthless pile of broken parts.

Good thread. I had considered cutting my heads (yamaha V4) to get some acceleration back now that I am turning a pretty big prop. I get the RPM but its taking me awhile. It would be nice if iy netted a little rpm up top as well but for now I am leaving it alone.

[Mr. Demeanor]

Port timing is calculated by measuring the degrees of crankshaft rotation between the opening and closing of the intake and exhaust ports,in much the same way the specs for a 4 stroke cam is described or identified...lift would be port width,duration would be port height(measured distance from top to bottom of port),and overlap being the distance between the closeing of the intake port to the closing of the exhaust port.Hope I explained that right,and as you can see,cutting the deck will have no effect on any of it ,Chris

Unless you cut it too far and run the wrong the fuel. Then it changes all of it into a worthless pile of broken parts.

Good thread. I had considered cutting my heads (yamaha V4) to get some acceleration back now that I am turning a pretty big prop. I get the RPM but its taking me awhile. It would be nice if iy netted a little rpm up top as well but for now I am leaving it alone.

[Mark75H]

Pistons cut to match a parallel deck height change will change port timing.

The compression will go down and the effective compression will go down even more.

Were the blocks decked to correct head gasket surfaces or were the pistons cut to retain the original squish?

[jphii]

Cutting the heads correctly can bring the compression back up. I run over 200#.

[Mark75H]

So your cut on the pistons is mainly to retain the original squish with the block decked for a dramatic compression increase ... cutting the pistons to retain the squish dimensions increases the port timing as a side benefit. You didn't specify that in the original post and left us to assume one way or the other ... that's why I asked for the clarification.

[Mr.Rude]

Don't forget piston height. On my decked blocks the pistons are cut down to match it. I always figure the 2 go hand in hand so I always "assume"....

I did include piston height, but refered to it as pin height. I should have said distance from center of pin to top of piston or compression hieght to avoid confussion.

[rockbottom]

TERRY : THANK YOU , steve
Rockbottom in regards to building an older cross-fow v6 for dependable performance and fun i,d keep the comp. around 135-140 psi. with premium 91 octane pump gas have the heads shaved 30 thou. and use OMC factory head gaskets dry . Most will advise never reuse a head gasket thats the best I can advise knowing this is just one aspect of a performance build thanks.

[KIRCHNER]

aWHAT do you mean squish angle and clearance not the same? merc heads are milled flat & flat top piston, correct? must be omc/yammy your talking about.

[racer]

Squish angle is the relation from piston top to head. Most with performance Mercs are cutting and angle on the head even with flat pistons, helps with combustion. Clearance is distance from top of piston to head.

[rude tim]

Cutting the head will not change the squish clearance on a looper. Squish clearance is a function of deck height and head gasket. If you are are using “O” rings instead of a gasket it is deck height alone that sets squish clearance.