.971 crank and rods with .960 pistons - Page 3 - PeachParts Mercedes-Benz Forum

Stretch · #31 08-31-2011, 03:30 AM

Quote:

Originally Posted by sixto

Those diagrams make it pretty clear to me.

Sixto
87 300D

They make it pretty clear to me too - those pictorials are a good description of the equation for static compression ratio given by wikipedia

http://en.wikipedia.org/wiki/Compression_ratio#Formula

These bits of "evidence" look to me like:-

If you keep the clearance volume constant but change the swept volume then you change the compression ratio.

Deep breath

I thought ForcedInduction was suggesting that whilst the static compression ratio is different in practical terms it makes very little difference to the dynamic compression ratio...

...I don't NOW think that is what he meant though.

It is a pity he has just been banned - I'd like to know what the f%^&k is going on.

sixto - do you know what he meant? Can you explain it differently?

sixto · #32 08-31-2011, 03:50 AM

I'm as lost as you are. Seems to me every reason he shared that CR is not affected by changes in stroke only proved that it is. As I look back, it appears that his point was as long as the TDC position of the piston doesn't change with a longer or shorter stroke, the combustion chamber volume doesn't change. I agree with that.

Going back to the OP, if someone's going to drop a 3.5 crank and rods into a 3.0 block, the same someone should drop the leftover 3.0 crank and rods into a 3.5 block and see if that permutation causes the longer 3.0 rods to bend.

Sixto
87 300D

Stretch · #33 08-31-2011, 04:12 AM

Quote:

Originally Posted by sixto

I'm as lost as you are. Seems to me every reason he shared that CR is not affected by changes in stroke only proved that it is. As I look back, it appears that his point was as long as the TDC position of the piston doesn't change with a longer or shorter stroke, the combustion chamber volume doesn't change. I agree with that.

Going back to the OP, if someone's going to drop a 3.5 crank and rods into a 3.0 block, the same someone should drop the leftover 3.0 crank and rods into a 3.5 block and see if that permutation causes the longer 3.0 rods to bend.

Sixto
87 300D

If you mean "combustion chamber volume" = unswept volume then yes I agree with that too...

I hope this frantic posting hasn't been the result of people not reading posts / refusing to admit mistakes / not realising that what has been said is misleading.

Still looking on the bright side I've got a new hobby horse to ride => a whole new area of investigation and learning => dynamic compression ratio

47dodge · #34 08-31-2011, 06:39 PM

dynamic compression is effected by when the valves close and open. The intake is open of course for the intake stroke, it stays open slightly after bottom dead center, depending on the cam. This will effect the compression stroke, and effectively lower the compression. I assume you do not plan a cam change so just get the static right.

ajnorris · #35 08-31-2011, 08:53 PM

All right Sixto, Army and VStech have got me convinced. Sounds like I need to get out the pipette and some water. So would 24:1 be a huge deal? seems really high, but can I just go easy on the boost and call it good? Just got a few quotes from a couple machine shops for boring the cylinders and wow, its expensive around here and the machine shops don't seem to know much about mercedes motors.
Winmutt feel free to drop by, the garage is a mess but I have lots of fun om603 stuff.

vstech · #36 08-31-2011, 10:44 PM

most motors with upped boost want lower compression ratio.

I am in contact with FI so I can try to get a explanation from him on what he meant here.

I'm NOT a performance tech, I've worked on lots of 12:1 and 14:1 race motors, but they were gassers, and lots of overlap for scavenging exhaust, exotic ultra lift cams etc...

I know that whenever I had to change the heads on my old 5.7 diesel because of broken bolts I had to adjust the headgasket thickness to maintain the 22.1 CR
I know that whenever I bored or stroked a race motor, the increase in volume of the piston throw would alter the CR requiring hotter gas, or deeper orings for the pressure to keep from blowing out the gaskets... I fondly remember melting a set of venolia gas ported race pistons due to A 14:1 ratio instead of the 12.5 I had originally figured... I forgot to account for the slightly larger tent volume in the larger piston... oops.

Stretch · #37 09-01-2011, 04:18 AM

If you make some measurements of the volume in the head and the prechambers I'd really like to see some pictures of how you do it.

I'm also interested to hear how variations in the positions of valves in the seats influences your measurements... there will be a maximum depth at which you can cut the valve seat from the surface of the head. I'm guessing it will be small compared with the overall unswept volume but surprisingly large if you disregard the volume of the prechamber... it all depends on how you measure it!

(Sorry I'm excited anyway)

ajnorris · #38 09-01-2011, 06:13 AM

Measuring the prechamber is what I'm a little unsure about. I was thinking I would put my compression tester in the prechamber (don't have a spare injector other than whats on the car) flip the head upside down, fill with water, including prechamber, and then measure it with some sort of graduated cylinder and pipette and add head gasket volume. Same sort of thing for the piston at TDC, although there is volume taken up by the prechamber tip that would not be accounted for. I'll post some pics once I get some time to tinker with it. Another though I had was to use a spacer to bring the prechamber up a bit to add volume and lower CR. Can you even get a thicker head gasket for one of these? (I see dollar signs adding up quick

)

Stretch · #39 09-01-2011, 06:51 AM

I don't think you can get a thicker head gasket off the shelf - just a standard one... but saying that I've seen prices in the US for custom made head gaskets that weren't too far off the standard price - I'll try and find the links...

I think 47dodge - I think it was him anyway (sorry if it wasn't you!) - was measuring pre-chamber volumes not so long ago in another thread that turned into a long discussion... I'll try and find that link too...

47dodge · #40 09-01-2011, 07:10 AM

It was thread about compression testing, think Funola was the original poster. I don't remember my figures. However they where for the 617, and 616 engines.

Marvelicious · #41 09-02-2011, 12:50 AM

Its amazing how even the right answer sounds wrong when you say it like an asshat! Let me try.

OK, assuming you didn't switch rods, you guys are correct, and CR would increase, because the swept volume has increased, right up into the previously un-swept area. With shorter rods, we keep that same larger swept volume, but it leaves a proportionally larger un-swept area. As for saying chamber volume doesn't matter, I don't know if he was being deliberately opaque, or what, but I can't agree with that statement.

Stretch · #42 09-02-2011, 01:56 AM

Quote:

Originally Posted by Marvelicious

Its amazing how even the right answer sounds wrong when you say it like an asshat! Let me try.

OK, assuming you didn't switch rods, you guys are correct, and CR would increase, because the swept volume has increased, right up into the previously un-swept area. With shorter rods, we keep that same larger swept volume, but it leaves a proportionally larger un-swept area. As for saying chamber volume doesn't matter, I don't know if he was being deliberately opaque, or what, but I can't agree with that statement.

What I've been trying to get across is that so long as the unswept volume is the same in both configuration but the swept volume is increased from 3 litres to 3.3 litres then there can be not other change in the static compression ratio but up - as described by the pictures and the formula...

I unfortunately do not have an engine in which I can do the physical checks necessary to measure the unswept volume. But luckily the OP is doing that - so when he gets back with the data we can finally put this beast of a subject to bed. (And I for one will be metaphorically stamping on the bugger's wind pipe!)

sixto · #43 09-02-2011, 02:31 AM

I'd like to see how one measures unswept volume. Geometrically it's [2996cc / 6cyl / (22CR -1) =] 23.8 cc's. The head surface is flat other than the raised valve faces and prechamber tip. The piston surface is irregular. I suppose you can set a prechamber at the installed angle and with a glow plug in place then seal the holes at the tip and measure how much liquid it holds. Then do the same with the prechamber installed (with unsealed holes) to see how much more liquid it takes to fill the prechamber to the same level. The difference is the unswept volume... if the liquid steady enough to not leak past the rings.

Might be easier and ultimately more relevant to build the engine and do a compression test. Don't worry about it if compression is with spec for a 603.96.

Sixto
87 300D

Marvelicious · #44 09-02-2011, 03:37 AM

Quote:

Originally Posted by Army

What I've been trying to get across is that so long as the unswept volume is the same in both configuration but the swept volume is increased from 3 litres to 3.3 litres then there can be not other change in the static compression ratio but up - as described by the pictures and the formula...

Absolutely. But using a shorter rod you can increase the un-swept volume at the same time. I think thats what he was getting at...

The trouble with Lance is that if you make a perfect square, he'll cock his head sideways and condescendingly berate you for not seeing that its a diamond.

Dude has some excellent ideas, but he can't seem to grasp that not everyone understands everything in precisely the same way, and insults aren't usually effective at getting ones point across.

Sixto, measuring the head volume is easy, the piston tops are a little harder. Maybe cast a piston top in wax or plaster, then measure the liquid it takes to fill the divots.

Stretch · #45 09-02-2011, 06:47 AM

Agreed - we just need to know if the new combination has the same PROTRUSION as that posted by ForcedInduction