Calculating compression ratio

[Tom Colitt]

Has anyone else ever calculated the compression ratio on a late 280SL? I was curious to see how much things change when you re-build an engine and so I measured the volume of the combustion chamber (fairly precisely).

I came up with a compression ratio of 9.19 to 1 at the stock bore size of 86.5mm and a stroke of 78.8mm. Has anyone else come closer to the 9.5:1 that MB quotes? My measurements were done with a spark plug installed and the small volume around the electrode was considered part of the combustion chamber (without that, the compression ratio would be even lower)

If you re-bored the cylinders to 87.0mm, you'd increase the ratio to 9.28 to one. If went to an 87.5mm bore, you'd get a ratio of 9.36 to one. If the head was machined down close to the wear limit, you would easily approach a compression ration of 10 to one. I am surprised that MB doesn't offer thicker "repair" head gaskets to compensate for this.

Also, a 9.5:1 compression ratio is somewhat modest for other higher performance engines using premium fuel. Does anyone have any thoughts on the subject? Is this partially do to the rather inefficient shape of the combustion chamber and the cross-flow head?

Regards,

Tom Colitt

[Shvegel]

Tom,
One thing to consider is that usually the replacement pistons have some form of relief in the dome area to keep the compression ratio down in the event of an overbore.

As for the head gasket I think that normally the concern is losing cam timing due to the cam being closer to the crank centerline which causes the cam timing to retard. The thicker gasket restores the center to center distance between the cam and crank and corrects the cam timing.

It's true that 10 to 1 isn't as high as modern performance engines but the trick is that modern engines normally incorporate a knock sensor into their ignition and fuel management systems. With a knock sensor the system will retard timing or increase fuel to compensate for load or even bad fuel.

Of course the 113 has a knock sensor as well. I wish I knew the term in German for a ventilated piston.

[glennard]

Couple questions re 113s:  1, cross flow on 113 engines? 2, knock sensor on 113?  3, ventilated piston?  Inquiring minds want to know.  Tom, good question on compression ratio.  
did you do it mathematically by pencil, etc. - or by volume of fluid actually in the cylinder, etc?

[Paddy_Crow]

Cross flow just means the air flow inside the cylinder with all valves open is roughly perpendicular to the axis of the head. In a four valve head, intake valves are on one side, exhaust valves on the other. I guess on a two valve head the valves would be situated axially at the centerline of the cylinder. It isn't a very significant detail.

I believe a ventilated piston is a sarcastic term for one with a hole burned through the dome. Further, the 113 knock sensor would be the driver. If it's knocking, buy premium fuel!

Democracy is two wolves and a lamb voting on what to have for lunch...

[glennard]

Is 'cross flow' like the 190E engine-the intake manifold is on the opposite side of the head from the exhaust manifold?  Gives better aspiration.

[Tom Colitt]

You're right. I meant to say "non cross flow head". I measured the volume with a syringe to 53ccm using a clear plexiglas plate and grease on both sides of a used cylinder head gasket, so I feel I can't be too far off with my result. I don't think the over-sized pistons have any reliefs in them to compensate. The thicker head gasket would be exactly to compensate for the problems you mention (to avoid changing the distance between the timing gears and to keep the combustion chamber volume constant.

P.S. Too bad the location of my post was changed. Now the number of people that read it will certainly be much lower :-(

Thanks,

Tom Colitt

[Paddy_Crow]

quote:Originally posted by glennard

Is 'cross flow' like the 190E engine-the intake manifold is on the opposite side of the head from the exhaust manifold?  Gives better aspiration.

From my experience, it has more to do with the placement of the valves. It is difficult to execute with a single cam, push rod engine because the rocker arms for the intake and exhaust valves would have to be in the same place. So push rod engines tend to be parallel flow or diagonal flow. DOHC makes it more possible to do a true cross flow, but it still requires some engineering problems to be solved.

Democracy is two wolves and a lamb voting on what to have for lunch...

[Shvegel]

Glenard,
Indeed a ventilated piston is a piston that has overheated to the point that a hole has been melted in the dome or top of the piston.
 The odd thing about the knocking of an engine is that when an engine "knocks" or preignites the boundary layer of air that sticks to the piston is disturbed What all this means is that the piston crown temperature can be 5 times higher due to the knock.

In order to fit a knock sensor to a 113 you would have to retrofit an entire electronic fuel injection and ignition management system.

[ja17]

Hello Tom,

I believe the stock postons actually extend out of the block a bit. This would raise compression as would a compressed or torqued head gasket.

I have seen these engines with very high compression due to over cutting the head. They have real difficulty running correctly.

Mercedes replaced the M130 engine with a M110 straight six with double overhead cam, cross flow head with basically the same bottom end. It was a heavy, and  more complicated and only  developed about ten or fifeteen more horsepower.
The extra weight of the additional camshaft, and the separate cam box made the accelleration  about the same as a good running W113 car.

They soon replaced the design with the highly successful M104 engine in the 300E. It was an inline six, once again with a single overhead cam, cross flow head, and fuel injection. It was the least complex, lightest and produced the most horsepower (215 hp).

Weight is the biggest performance reducer on these cars. Once the weight is reduced to around 2500lbs, things begin to come to life. Handling, acceleration and braking improve a good deal.
However it is the nature of the Mercedes SL to be a bitover-weight, over-built and engineered to outlast most, the best of both worlds I suspect!


Joe Alexander
Blacklick, Ohio

[Benz Dr.]

You won't get good compression unless the piston comes all the way to the top of the cylinder. Some replacement pistons are made about .050mm lees in height than the original pistons which will result in a lot of preformance loss. The only way to correct this is to deck the block until the pistons are at least level with the top of the block. I had this problem on my engine and gained about 25-30 PSI per cylinder with a .030'' cut. In rough terms you can figure about 1 PSI per .001'' of cut - it makes that much difference.

Something that most people aren't arware of is that the engine runs at less compression while driving down the road. An engine with 175 PSI will develope about 90 - 130 PSI at speed.  Most of this lowered compression relates to vave overlap.

Diesel engines don't have valve overlap and produce much higher compression ( 375 - 400 PSI ) even at higher engine speeds. Cooling of pistons in turbo charged engines is always a factor.

Dan Caron's
 SL Barn
benzbarn@ebtech.net
 slbarn.mbz.org
  1 877 661 6061

[Tom Colitt]

Thanks for the recent responses. I think I wasn't quite clear with what I did in my measurements and calculations. I checked the compression ratio on a stock and standard size piston with the standard flat top piston (as supplied from the factory in 1970). At TDC the piston comes to about 0.6mm below the top of the block. This also has not been changed on this stock engine. The stroke I measured on this engine is exactly as specified 78.8mm (i.e 79.4mm below deck height). I am quite certain that the first repair size pistons from MB are flat as well although it would make sense if they offered pistons that were slightly lower in height as Dan pointed out, in order to control the undesirable increase in compression that Joe and I mentioned. That way you would not need a thicker head gasket as I suggested, in order to maintain the same compression ratio.

I'm still curious though. Joe, do you have a number in your mind, or from experience or hearsay (i.e. 10 to 1 or 10.5 to 1?) at which you would consider the increase in compression from re-surfacing and increasing bore size to be too high and leading to running problems instead of a performance increase? (raising the compression ratio is, of course, often used within reason to increase performance. I'm guessing MB didn't see a problem with shaving up to 1mm off the head, so whatever compression that results in should certainly be in the allowable tolerance. I will calulate that number and report back.

Regards,

Tom Colitt

[ja17]

Hello Tom,

Consider what the required octane rating for these cars is with the standard 9.5 to 1 compression ratio. So you are very limited to raising the compression unless you want to use racing fuel all the time.

Back then it seems that the engine had 185 lbs to 200 lbs compression and would not run right on any standard high octane premium fuels.

Joe Alexander
Blacklick, Ohio

[Benz Dr.]

Well, when you remove the original piston from the block and replace it with a new one that's .50mm bigger, and that piston is somehow lower in the block, how do you expect to end up with the same compression as before? The answer is you can't. Increasing bore sixe will increase compression ratio by maybe .1 and isn't as big a factor as deck height or or cumbustion chamber size.

NEVER cut the cylinder head to increase compression ratio. Heads are worth twice what a block is worth and good ones can be hard to find. The factory calls for a small amount of protrusion out of the block as far as piston height goes and the stock configuration is at deck height. Regardless of that, you need a minimum of .9 mm ( about .035'') from piston to intake valve clearance at 5 degrees ATDC on the power stroke. this is MUCH closer than what you will find in other engines but it seems to work just fine.

 I really doubt you could get 200 PSI from a new factory engine. Factory new specs call for about 175 and that's where I build to.

Dan Caron's
 SL Barn
benzbarn@ebtech.net
 slbarn.mbz.org
  1 877 661 6061

[rwmastel]

quote:Originally posted by ja17

Back then it seems that the engine had 185 lbs to 200 lbs compression and would not run right on any standard high octane premium fuels.

Joe,
PSI would not be related to compression ratio, right?  Regardless of the ratio, compression is how tight the engine is.  Am I missing something?

Rodd
Pagoda Technical Manual
please contibute: http://www.sl113.org/wiki/pmwiki.php
1966 230SL
2006 C230 Sport Sedan

[Paddy_Crow]

quote:Originally posted by rwmastel

quote:Originally posted by ja17

Back then it seems that the engine had 185 lbs to 200 lbs compression and would not run right on any standard high octane premium fuels.

Joe,
PSI would not be related to compression ratio, right?  Regardless of the ratio, compression is how tight the engine is.  Am I missing something?

Rodd
Pagoda Technical Manual
please contibute: http://www.sl113.org/wiki/pmwiki.php
1966 230SL
2006 C230 Sport Sedan

Compression ratio determines the upper limit that might be attained. If you apply the ideal gas law (PV=mRT) and assume temperature remains constant, a perfectly sealed cylinder with 9.5 CR and 1 atmosphere of pressure will reach 9.5 atmospheres of pressure at TDC (about 140 PSI). Similarly, with 11.0 CR it could reach 11 atmospheres (162 PSI).

In reality, it's not that simple. Work is done on the gas when it is compressed, which heats it up. And some gas will also leak out. So the actual pressure measured might be slightly higher or lower, depending on how quickly it is compressed.

Democracy is two wolves and a lamb voting on what to have for lunch...

[Tom Colitt]

Ok. I just did a rough calculation and at the factory's recommended wear limit for the cylinder head (machining 1mm off the head), the compression ratio would be raised by about 0.3 or 0.4. In other words, all things being equal (bore, stroke, piston height) the compression ratio would increase from the standard 9.5:1 to about 9.9:1

From this I conclude that using standard pump premium gas, ignition and cam timing, there should not be any running difficulties if one raised the compression ratio to 9.9:1 (either unintentionally as a result of simply increasing the bore-size (as in replacing pistons with repair sizes) or by design to achieve a higher volumetric efficiency.

If anyone disagrees with this conclusion, I would appreciate your input.

Thanks,

Tom Colitt

[Shvegel]

Tom,
You are probably safe at 10 to 1 but keep in mind there is a lot of bad gas out there being sold as 93 or 94 octane. I keep my car tuned to the edge of 94 octane fuel and more than once I have bought fuel along the interstate only to have to pull over and adjust the ignition timing on my engine.

If you do this you might want to consider carrying some octane booster in the trunk.

As a sidebar I found a fair bit of extra power on my car by using an earlier pre-smog distributor and adjusting the timing advance until the engine just pinged then dropped it back a few degrees.

1970 was a big year for cars sold in the US. In order to meet the Nox emissions standards Every US manufacturer dropped compression on their engines. High performance Chevrolet engines went from 10.5 to one to somewhere in the neighborhood of 8.5 to 1.

I am assuming that Mercedes didn't want to bother with lowering the compression of their engines for the US market so instead decided to adjust cam profile and ignition advance curves. A Euro cam and some Ignition work may give you that little extra kick you seem to be seeking.

[Tom Colitt]

quote:Originally posted by Shvegel


I am assuming that Mercedes didn't want to bother with lowering the compression of their engines for the US market so instead decided to adjust cam profile and ignition advance curves. A Euro cam and some Ignition work may give you that little extra kick you seem to be seeking.

I was surprised to hear about your 93 to 94 octane pump gas. In California the most widely available premium pump gas is only 91 octane. That number is calculated as the average between the research octane number (RON) and an experimental octane number (MON). Is that how it is calculated in your state? The numbers found in Europe are also generally higher because, I believe they rely only on the higher laboratory derived figure.

Dan. Your comment about the pistons being flush with the top of the Block is interesting. I am looking at a 100% stock 1970 280SL with standard size pistons. At TDC there is still about 0.6mm to the top of the deck. That's quite little and I don't know if that is what you meant when you said the top of the piston has to come to the top of the deck. BDC on this engine is 79.4mm below deck. That adds up to the correct stroke of 78.8mm
Have other found variations on their stock configurations as well?

Of course, I will only have the head machine by 0.1 to 0.2mm at the most. We have several services here in Los Angeles that straigten the head before the final "skim" is taken off, thereby saving much material and other trouble on warped heads.

I will also have the US cam re-ground to the Euro specs. I believe the Germans originaly used induction hardening, but I've heard that Nitriding is also frequently used after re-grinding a camshaft.
I will still have to do some research in this area.

Regards,

Tom Colitt

[Shvegel]

Ohio uses the same calculations for octane numbers(research+motor/2)
91 is the most common with several stations selling 93 and Sunoco being the only source for 94.

[Tom Colitt]

I have just found further variations to the theme. I am measuring a stock 280SE engine that is also specified to have 9.5:1 and the pistons at TDC are almost 1.1mm "below deck". Also, its cylinder head with the casting number 130 016 3201 has a slightly deeper combustion chamber shape, than the 130 016 0801, likewise a "late" 280SL head... No way they'd ever both have the same compression ratio! I'm also suprised at the variations in piston height at TDC (0.6mm to 1.05mm). I can't be certain the block hasn't been modified, but consider this very unlikely...

The planning phase of my stroked engine project is making progress. If successful I hope to one day offer a nice 280SL/ 3.0 engine rebuild with a minimum of modifications.

Tom Colitt

[Benz Dr.]

Would you do this with different pistons, contecting rods or both?
I've also seen wide variations on different engines but generally all the original ones I've had apart were built with the pistons right at or very near the top of the cylinders. I've always considered this to be a stock setting done at the factory. It's the only way you will get real power out of these engines and it usually won't result in overly high compression readings.
My 250SL engined 230SL runs about 175 - 180 PSI  with the pistons right at the parting surface. The head has been planed once with a .010 cut. I imagine it to be a true 9.5 engine and it will run on 91 octaine without pinging. It runs better on 94 when I can find it.

When I installed the new pistons I found they were about .030 below the parting surface. This gave me about 150 PSI and flat performance. I've seen pistons with a raised area on one side of the crown that will do the same thing. These had to be low compression pistons - send those things back unless you want to drive a tank on regular gas.
After I had the block decked about .025 the compression came up to stock specs and it ran like a real car.

Dan Caron's
 SL Barn
benzbarn@ebtech.net
 slbarn.mbz.org
  1 877 661 6061

[Tom Colitt]

Hi Dan

Well, I am trying to do this as a project that won't cost double as much as a regular rebuild, so I have done a lot of research and also gone back to my old school books (I studied engineering for 8 years at the Technical University in Berlin), which have been less valuable than common sense and the practical knowledge that I'm acquiring.

Yes, the pistons are all fairly close to deck height, like I said, 24 thousanths in. below deck on the 9.5 engine (late 280SL) and 40 thou. on the late style 280SE engine (I don't like using inches, but it looks like you did below and most people here relate better to those numbers). I also have calculated how much I'd have to take off the top of the pistons in order to lower the compression ratio back down to acceptable levels after the 5mm/ 0.2in. stroke increase. Using the stock pistons and rods on this trial run, I did not feel comfortable removing as much as I wanted to without making the top (crown thickness) too thin. So, my piston will be travelling all the way to level deck height, whereas stock, TDC was 40 thou. below deck. It turns out the lower compression head off my donor SE engine actually works in my favor with the compression ratio, even with lower compression height of my machined pistons.

I found a reputable company with much experience in "stroking" an engine by welding extra material onto the crank journals that will then be re-ground to give me the increase in stroke. Ultimately, I will probably use custom pistons with 3 instead of the standard and conservative (not to say old-fashioned) 4 ring design used on M130 engines. Of course, custom pistons would be lighter weight and stronger. They also balance the crankshaft with the lighter pistons, flywheel, balancer... I don't feel like the bore should be increased beyond the 87.5mm maximum repair size because of the already thin walls between the water jacket passages.

As far as the head, I am still open to suggestions. Eventhough, I am not adverse to experimenting, I know enough to realize that without much experience and computer aided modelling and bench testing, more harm than good can be done by just increasing everything within the head. The valves are already fairly close to the edges of the combustion chambers although I would love to hear about any 1mm larger, drop in valves with the same valve stem diameter. Obviously, some matching and porting can do no harm, since I will have a 3 liter engine that should require proportionally more air and fuel than a 2.8L. Again, I'd be glad to hear about experiences with extrude honing (how much will that do? ...an increase in port diameters, only to have the restriction be moved to the intake runners?) and other modifications. I'm sure there are some on this list who have experience with this and who don't have a problem with me doing some respectful (hopefully respectable too) work on an MB engine. As I am otherwise a stickler for strict originality, I am only considering modest internal engine changes (no V8s or turbos) and I want to be able to use 91-92 octane (US) gas. (02 cam, modified pump mapping and head work.

I tried to attach pictures of the two different combustion chambers for feedback, but always got a message that I was not signed in....?

Tom Colitt

[Benz Dr.]

I've thought about using 190SL exhaust valves which are slighty larger but I haven't figured out how to do it yet. This valve uses a larger keeper to hold the spring retainers on so that would have to be figured out. Maybe the retainer from a 230SL would work.

I've changed 230SL heads to use 280SL valves so I think you could go back the other way. Some opening in the cumbustion chamber might be needed to increase air flow around the valve seat.
I had a crappy old 250 head tested on a flow bench once once. The guy who did it for me was quite impressed and said it flowed far better than he would of expected. Somewhere up around a 350 small block by comparison so the design is already pretty good. All I usually do is match the ports to the manifold gasket. This adds a small amount of flow which is probably more noticable at higher RPMs if you can tell at all. I do think it can give you better mileage in a well tuned engine.
The bigest place you can make a difference is right at the valve seat. Flow can be increased greatly if you know how to do it and have plenty of spare heads to ruin while trying.


Dan Caron's
 SL Barn
benzbarn@ebtech.net
 slbarn.mbz.org
  1 877 661 6061