8000 rpm/300 bhp 3.2 rebuild

[RobFrost]

I'm sure I speak for everyone in saying I think you're doing a brilliant job of it. This is my favourite thread on here right now and want to be really pleased for you when the engine's running sweetly. It's just a case of a few people not wanting you to spoil the ship for a ha'porth of tar.

[Gary71]

You can measure the volume of the top of the piston by lowering it into a measuring jug with a known volume of water in it and check the difference.
Good old Archimedes principle or something

[911hillclimber]

Yes Gary, that was my next move, but the numbers could be rather approximate, but a good check.
Rob, thank you for taking so much trouble on my behalf and good that your conclusion is in the right ball park.

The details on the JE piston sheet that came with the pistons do not give the units, poor for an engineering company, but the dimensions on the same sheet show the wrist pin as 0.905 and the pin is 23mm, the dome height of 0.530 and it is that in inches, so in the same section, Dome volume is 46.46 and is the TOTAL volume of the piston, so 46.46 cuin is probably right, not sure why you would need that value.

Piston weight is given in grams!

A dunk in water will sort it.::: just done the test, 100cc when piston immersed in water, dome first, to the first ring grove. Volume of the dome above the deck is about 50 to 60 cc
The to
Going to do the same with the head so I can get a rough idea to put this to bed.
Head volume is 75cc

So, in summary

Volume of the piston into the head is about 60cc, the head volume is 100cc.
Top land volume is stated as 0.67 cu in or 10.9cc (top of top ring up to the start of the dome contour).

I take on board you all would like this to work out for me, guess I'm getting a bit down over the whole thing and is it all worth it.

[911hillclimber]

Used a USA CR calculator, and this gave 11.4:1
Not exactly sure what Piston Deck Clearance Negative is mind which will affect the CR calc.


Enter Bore/Stroke Designation Type
1 = Inches 2 = Millimeters
2

Enter Cylinder Bore Size
95

Enter Piston Stroke Length
74.4

Enter Head Gasket Bore Diameter
95
Enter Compressed Head Gasket Thickness
0.1
Enter Combustion Chamber Volume In CCs
100
Enter Piston Dome Volume In CCs Negative For Dished Pistons (Use '-')
50
Enter Piston Deck Clearance Negative If ABOVE Deck (Use '-') :




Calculated Engine Compression Ratio
11.399835399717483
:1
Total Displacement Volume
50.70882177387352
cc

[RobFrost]

Rob, thank you for taking so much trouble on my behalf and good that your conclusion is in the right ball park.

It was no trouble, literally 5 minutes and I was curious to test my estimation skills - more a sense check in case it was way off, which might have made the case stronger for taking a precise measurement.



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[hot66]

That’ll be deck height … distance of top of piston to top of cylinder at top of stroke

[hot66]

It’s all in Dempsey book

[Cortina]

PS ............ don't use "an on line calculator" (not that they are wrong) ... do the sums - it's very easy , that way you know and understand better.

True story : I had a visit quite a few years back from a (MSA) scrutineer - he brought with him the regional tech scrutineer ..... we want to talk to you about Compression ratio on an engine that we have stripped for inspection bla bla.

Well , to cut a long story short (very awkward conversation) - they did NOT know how to work out the CR for the engine ... had had a go, and got it wrong !!

Yes , we are trying to help you , and know from experience (and education and experiment) that this is key to a build , and only want to see you do it right ............ have fun , and if in doubt , ask.

[911hillclimber]

Dry build summary:
1
the 5mm timing on the cams is good, checked it all again this morning and holes all marked so on the final build I can be there or a very good first stab on both sides.
Yes, 5mm is the correct timing for a GE60 cam.
2
The valve to piston clearance is more than enough for inlet and the exhaust. Those tests were done without a copper gasket under the barrel, so with 0.25mm gasket, even better.(approx. 10%)
3
Compression ratio will be recalculated now I have triple checked the deck height, but I'm confident all is well. Somewhere in the 11's. My 'measurements' today were crude.
4
Deck height.
Thought I had covered this earlier in the build by crushing the plasticine on the edges of the pistons with a result of 1.71mm.
However, since James had re-directed me to the Dempsey section I followed it through.
I am not sure I agree with his method taken from a Porsche engine pro, but followed it.
I made by own crush solder tube as the solder I had was 3mm dia solid, too solid for this test, so drilled a 2mm hole down 2 short lengths of it.

Glued/stuck an 8mm long piece as per the Dempsey instructions, 0.5mm copper gaskets (2 x 0.25mm) to either side of the piston up against the barrel side.
Put the head on and torqued it all down.
Rotated over TDC and felt the crush.
Stripped and cleaned, the lead crushed tubes were 1.77mm, Dempsey says 1.25mm is really a minimum crush.

Decided to do it ALL again and took one 0.25 base gasket out so 0.25 under the barrel.

New crush was 1.67mm, so that is that, can't get it lower.

The JE pistons give a narrow flat land as a transition from the piston 'round' to the domed head, and you can get a Vernier there and measure to the top of the barrel, but, I did it Dempsey's way.
Measured the flat top stock 3.2 piston for the deck height, simple with the Vernier, 0.45mm, so the stock deck eight is 4 times SMALLER than the snazzy racing pistons. Guess at 8000 rpm things stretch a lot.
According to Dempsey, the stock pistons are not right.

So, with all these clearances I recon my compression ratio will be about 10....

Cleaned up the garage and benches, put all the engine bits back in their Left and right hand boxes and will start the Ring Gapping Game.
5
JE Pistons.
When they set about designing these pistons they must have measured the stock Porsche parts and made the pistons fit those parts. They cannot accommodate the variences of DIY engine builders machined this and that, but stock Porsche parts should be good.
Thus the fit would be right for the stock parts, all my engine parts are stock.

They do however have to consider the cams and lifts too, so that is a critical area to look at, but the GE 60 has been around for a long time so I suspect they have accommodated that, the GE80 too.

So far, the pistons fit the engine parts.

Looking at the calculator used this morning for CR, with the deck height of 1.25mm the CR would be 11.8:1, with 1.67 the CR is 11.2:1

Might get a day or 1.2 a day tomorrow, depends on what Mrs HC wants to do.
If it snows I'm in luck!

Hope tomorrow will be more positive for me.

[Cortina]

One "small" correction - I suspect you have not actually measured your piston to valve clearance. What you have measured is your piston to valve clearance at TDC ? (correct me if I am wrong)
That's fine, since as I said previously - your worst case scenario (with engine running) is NOT at tdc ... so that means you actually have more clearance than you think you have . (I hope that makes sense).

What it does "say" to me is that your CR may be a bit lower than you thought it was (not so good for HP) - why ? Because in my limited experience in the chase for hp (actually in my case it's a chase for torque) - higher lift cams , their timing , and the chase for a higher than average CR , USUALLY means you sail close to the wire, on piston to valve clearance when engine is running. As this is NOT the case on your engine (which you have measured - sort of) that points me to think this. .... make sense ??

[911hillclimber]

I followed the Dempsey book blindly.
Advicewas to time the engine to 5mm in my case.
The Dempsey method is then to set the engine to tdc for the left side inlet, and close the rocker to zero valve.tappet clearance so the rocker is on the cam heel.
Set a DTI to the top of the valve spring top retainer with at least 3mm of plunge.
Zero the DTI.
Turn the engine until the gauge just starts to move, about 40 degrees crank turn, before Z1 comes to the split line.
Stop at this point.
Remove the adjusting lock nut on the rocker and then screw the rocker down until resistance is felt while watching the gauge.
I wound on 2 mm but the inlet went to 2.3mm or so before I felt I had come to a stop.

Repeat for the other inlet and both exhausts, the exhausts you want some more depth, but 2 mm is good.

This engine passed these tests beyond the minimum given in the Dempsey book.

I am presuming I did it right, though I'm loosing confidence rapidly.

[Cortina]

Following blindly ?? (I hope not - exactly)
Mr Dempsey may be good - but he does not know what camshafts you are using or what timing you have them set to ! (apologies if he does ... wee joke)

What he is doing - is getting you to do a rough check for "safety" - as I said earlier , you do NOT have a problem with valve to piston clearance. My comment on CR holds true. .... and you now know how to measure it (long hand hopefully ... it's one simple division, once you decide what is negative and what is positive when adding up what the combustion chamber volume is) don't rush the simple sum ... think about it !

[911hillclimber]

[Cortina]

Hmm ............. don't like it. What on earth is "piston volume" ?. If it's the volume of piston that sticks up (hard to explain) then that figure should be negative , not positive !! If it's a recessed piston like what used to be in some old Fords , then it's positive. One needs to understand the "sums" !!

Just keep it simple : Swept volume , and combustion chamber volume are the only two figures you need ........... BUT !! , you need lots of other data to work out what the actual combustion chamber volume is (it's the volume above the piston where the burning takes place ... sorry about about the eggs and granny bit !)

[Cortina]

The compression ratio is the ratio between the volume of the cylinder and combustion chamber in an internal combustion engine at their maximum and minimum values. Wikipedia
CR= \frac {V_d+V_c}{V_c}
CR = static compression ratio
V_d = displacement volume
V_c = clearance volume

Found and copied this via Mr Google - it's simple , you just need two things Vd and Vc as they have written.
It does not copy well , CR = Vd + Vc divided by Vc !