karl ruiter

Ok, so I bought what seems to be a euro S motor, with the block marked as a non S.
https://rennlist.com/forums/showthre...hlight=pistons

I've been throught the motor enough to clean it up and get it installed, but I did not pull the heads.

What I know so far is that I have:
Non S Motor number (M/03)
S intake
S heads (casting number= 928.104.348.2R)
S cams (casting number=928.105.187.OR)

The trouble now is that I am trying to decide if I want to boost it or not. If it is a 10:1 motor I probably will not, while if it is a 9:1 motor I probably will, but since the number on the block do not match I don't know what it is.

I can think of two ways it could have ended up this way:
S top end was placed on non S bottom end - or -
Motor number was changed to ease federalization.

I bought a boroscope and took a peak down the spark plug hole, trying to ID the pistons. Unfortunatly they do not exactly seem to match anything in the WSM. The WSM shows non S pistons with a depressed dimple, and S pistons with a raised center. There is no dimple, but I do not see a raised center either. They look pretty flat.

So, I got current barometric pressure off the internet 29.9inHg=14.6 PSI. at 10:1 I should get 146 PSI, while at 9:1 I should get 132 PSI, right? Then I ran a quick test, which showed 140 PSI. Its kinda in the middle, but I am thinking that this indicates a 10:1 compression, but I don't know if this is at all a reasonable way to do it. I don't know how accurate the barometric pressure number I got is, and I don't think my compression tester is real real accurate. Anybody have thoughts on this? Or maybe picture of S pistons from the top so I can see how pronounced the raised center..

AO

That doesn't sound right. Most guys doing compression checks are getting 180's and 190's. Not to mention there is a ton of variability in those gages. I'd trust the markings on the motor first

hacker-pschorr

My stock US 81 registered between 140-150psi

928drvr86.5

Same for my 86.5

WICruiser

Compression ratio can not be measured easily the way you are trying to do it. Compression ratio is the ratio of the space with the piston at bottom dead center divided by the space with the piston at top dead center. Compression pressure is the pressure increase in the cylinder and it is affected by not only the compression ratio but the piston sealing, the valve sealing, and the amount of air charge you actually get into the cylinder before the valves close and you start compression.

I know this doesn't help answer your basic question but I don't have any insight into the potential parts available.

docmirror

Go to wikipedia and look up adiabatic effects, or basic Boyles law of partial pressure of gasses. Short answer, your compression after the engine is warm, with good lubrication should be around 170-190PSI with 10:1 CR. PV=NRT is the formula, but counting all those moles gets time consuming.

BrianG

This would be effective if the valves closed at BDC and remained closed through TDC, but they don't. Different cam shaft profiles will provide for differing valve opening timing and overlap.

Pumping pressure is not directly related to compression ratio because of this.

karl ruiter

OK, I kinda thought it was a bad idea, but I did not know why, exactly. Its been a lot of years since I used to play hookey on Thermodynamics. Anybody got a better idea, or know about flat topped 928 pistons?

Ian928

That sounds correct to me. Here is a picture of my 1980 Euro S pistons: http://www.flickr.com/photo_zoom.gne...2999722&size=l

karl ruiter

Thanks Ian. That is what I have, as nearly as I can tell. Is your motor 4.7, 10:1?

Imo000

You can boost a 9:1 or 10:1, just use less boost with the 10:1. Either way it will give you a nice hp gain.

docmirror

Generally, the little smiley face at the end of a sentence indicates a small joke, or perhaps a bit of sarcasm. It's really a shame that the literal discussion of a topic is taken, rather than the inferred.

However, in this case, as with any other closed system, you certainly could use Boyle and the ideal gas law to discover the actual CR of the engine. But like I said, adding(subtracting for valve overlap) all those moles of air would be, well problematic. Also, the cylinder isn't effectively closed, so stuff is gonna leak out, and you need to count that too.

Karl, The best way to actually determine CR is by removing the head, and measuring the volume of the head recess, and the thickness of the head gasket. This will compare to the volume of that area, plus the volume of the area when the piston is at the bottom. Things start to get complicated when you have either domed, dished, or pistons with valve cutouts. This is called 'CC-ing' the head, and is pretty much required when building high performance engines. guessing at the value is not really suitable for SC applications.

CC-ing is done by using a known quanitiy of fluid, like alcohol, or glycerine and an eye dropper to fill the head recess while it is on the bench. This also allows you to surface the heads if you want, and apply a new head gasket.

dr bob

What Doc said, plus...

You can CC the pistons in the block, using a dial indicator to get precise movement in the bore if the piston top is proud of the block surface at TDC. I make a deck plate out of Lexan, and use a thin layer of Vaseline to make the seal at the edges. I drill a small hole in the middle if the engine is on a stand or at the top corner of the bore if the engine is in normal/upright position. Then a burette with ATF is used to fill the bore, and volume measurement taken. I like ATF instead of alcohol because it doesn't evaporate during the process and it's easy to spot leaks. Alcohol has the advantage of being somewat self-cleaning when you are done, though, and air bubbles come out quickly.

At 45 degress before top center on thge compression stroke to no 1 cylinder, both valves on that cylinder are closed. This is the T-belt swap position on 32 engines. You can rotate the engine to that point, remove the belt, and know that you can rotate the engine safely. You can then determine mechanical compression ratio using the volume of liquid at TDC then the volume of liquid at BDC, through the spark plug hole. Same calibrated burette as for CC'ing the head or block, but this time you are filling to the bottom of the spark plug hole. The ratio of the two volumes is your compression ratio. The block needs to be rotated so that the spark plug hole is the highest point in the combustion chamber to do this, and you'll want to fit a piece of clear tubing to the end of the burette to make sure all the fluid goes into the cylinder. Your handy borescope will help you see when the fluid is just touching the plug hole when you fill.

You'll want to get perfect TDC and BDC points marked on the balancer using a dial indicator or even just a dowel in the plug hole, before you start adding any liquid.

karl ruiter

Thanks Doc and Bob. I thought about pulling it all apart to see what I had, but in the end I decided that I'd rather have it in the car a week sooner. So, I just pulled the cam towers, resealed them, checked the cam numbers, and put it back together with a new timing belt and rollers.
Imoo, I have heard of people boosing the 10:1 motor, but I am scared of it without knock sensors.

Jim bailey - 928 International

Putting boost on ANY 25 Year old engine without KNOWING the condition of the head gasket and the amount of corrosion on the outsides of the cylinders is the sign of a true optomist Especially when the engine is out of the car ! Putting an engine in can be fun....the first time, the second time it is much less so ! We just pulled the heads of a 4.7 Euro and one cylinder the corrosion was just about to compromise the head gasket.....so that one is JUNK .