Another 2.8

[cmcfaul]

Related topic and not meant to be a hijack to this thread but here is my question.

While 7k and 9300 RPM's are very high redlines, how do stock,4 cylinder motorcycles, achieve16k redlines. Can't the same principles be applied to a Porsche engine?

Chris

[tadd]

Much smaller displacements with short strokes.

F=Ma

[Trackrash]

Originally Posted by cmcfaul

Related topic and not meant to be a hijack to this thread but here is my question.

While 7k and 9300 RPM's are very high redlines, how do stock,4 cylinder motorcycles, achieve16k redlines. Can't the same principles be applied to a Porsche engine?

Chris

I would like to know how an aircooled Porsche motor can achieve 9K RPM. I'm guessing titanium valves? Would love to see the details of that build.

Lots of factors limiting the max RPM. Reciprocating mass of the rods and pistons, mass of the valves, and air flow in the induction system. There is a big advantage to 4 valves per cylinder.

[faapgar]

I have built 64mm stroke x 98mm bore on Carrera 3.0 930/02 case and spun it up to 10,000 RPM on 46 Webers.Correct head flow & exhaust scavenging is key.Longer rod length improves mid range torque & reduces the angle of the piston skirt as it enters the cylinder.Built this engine in 1995 for my 914-6 which was my test bed for all the different engine types I built.Fred

[tadd]

Question for the pelican brain trust:
What are the opinions on wrist pin locks... I have seen four types:
1. Wire locks (just a ring of wire)
2. snap rings (stamped with the two holes for the tool)
3. spiral locks (PIA helical things you 'wind in')
4. plastic buttons (DUH )

Any idea which is best? I've used 1 and 2. Tried 3 and gave up. Never 4.

About ready to make the pistons 'forever', so would like to make a choice as to what to use.

[racing97]

Well the engines see 9,000 to 10,000 RPM with miss shifts and erant gear selection in the downshift that would be the only logical scenario they certainly don't make power there.

regards

[faapgar]

Hi Tadd,# 1 is the best.# 2 comes with Japanese NPR 1750 P&C FOR A 356.# 3 If you are a masochist.

[tadd]

Update:
Now that the holidays are past, it was time to answer important questions...like what is the CR. I like simple solutions, so my process was to assemble a cylinder stack and use a bit of krytox to seal the gap between the cylinder, piston, and top ring. Bolt on a head, add one spark plug... then rotate to get the other plug upright, add a paint can to elevate one end to get the empty plug hole as 'top' as possible and fill with ATF.

I used a 60 cc polyethylene syringe (no black rubber on the plunger) and also massed it to the nearest 0.01g. The syringe is marked to 2 cc, so best guess is 0.5 cc measurement.

I got 43.5 cc direct read and mass change of 36.44. Looking up ATF density is 0.863. I measured 0.840 g/ml using the scale and a finer marked 10 cc syringe. That gives either 42.22 or 43.38 ml volume at TDC.

Given these are 92 mm pistons and a 70.4 mm stoke, that gives a BDC volume of 467.99 or 468 cc add in the TDC volume gives 510.2 or 511.4 cc...thus a CR of 12:1 or 11.78:1. The direct volume read is really close to the 0.840 density number, so I am inclined to believe that number the most. 11.78:1 is up there when bruce anderson says that 11.5:1 is about right for my 1.1 (0.044") deck height using 2.7 heads with 2.8 RSR pistons.

My guess is that the head chamber is a bit tighter due to the cut down intake taking up some extra volume over a stock valve. Ill need to cc a head to check that theory...

This was all according to plan as it were... the dynamic vs static compression theory... cams are 3.8 RSR on 102 lobe centers. Supposed to be 6.0 to 6.2 mm lift at overlap. This should bleed off a good bit of compression while running. Goal was premium pump gas in three seasons and premium with race gas for the summer...with the long term goal of propane injection.

HOWEVER, for additional CYA, I could switch to a 0.5 mm base gasket to drop the CR a bit more. That would put me at a deck height of 1.35 mm or 0.055". Not so far out of spec. There really isn't a squish band due to the chamfer that had to be cut into the head to clear the larger 2.8 pistons. So I can't see an issue with loss of squish turbulence.

Need to think on this...thoughts from the pelican on the 0.5 base gasket?

[KTL]

What're the thoughts on the base gasket CYA? Meaning are you intentionally trying to back off your compression ratio or would you like a little more headroom for your piston to valve clearance?

Nothing wrong with the 0.5mm base gasket. It works fine