Help with (semi) Hotrod Engine 2.4S

[Allen Henderson]

Motor geeks,

Here are the main specs of the engine I am building:
70.4mm crank
circa '72 S Heads.
'67 Alu case
Solex Cams
'71 911E P/Cs, 84mm (slightly higher compression, stock, than 72/73 ones)

Questions/Observations:
1. This engine SHOULD BE a bolt together correct?
2. When we got the long block mostly together the chain box was about 1.7mm too high and so the seal/plate around the cam didn't fit. Then we discovered the shoulders of all the Cs had been cut down about the same 1.7mm, same as we were off on alignment. This was done by PO on a 66mm crank engine with stock "normal" heads, I guess to increase the compression with those heads.
3. We then checked the old chain boxes (the ones the Cs came off off) and sure enough they had been cut on the bottom the same 1.7mm.
4. We have checked and for combination we are building the Ps do clear the heads.
5. What would be the estimated resulting compression ratio using the stock combo above WITH a\THIS further reduction of 1.7mm (liner travel of P) up into the head, any estimates? It is already going to be higher than the stock 2.2E (9.3:1 I think it is) due to longer stroke. I've forgotten at the moment what factor one applies but as I recall it was, like, up to say 9.5:1.
6. Finally, this is elementary but just want to check. IF a P is at top dead center and the intake valve was so mistimed as to be at its maximium extension into the cylinder there would be a collison. Correct? Put another way, if a chain tensioner were to fail and we had a skip time, a collison could occur, correct? I ask this because I never really realized that the cam timing was such that the intake valve was not all the way out when at TDC. Of course now that I think about "overlap at TDC" and look at the amount of lift we are measuring on the valve, we are still a ways away from maximium excursion into the cyclinder.
7. No to belabor the point but #6 above was "discovered" in trying to figure out why the chain box was not aligned with the cam correctly. Started checking more things. Never thought to actually measure the Cylinder before that; but then..we said..what could it be. But when we took one off, sure enought we could see the machining marks and it measured 1.7mm short.
8. In summary then, the question is: do I make some spacers to bring the C's back to stock. Or do I use the modified chain boxes I have and go for even higher compression?

All input appreciated! Thanks.

-Allen-

[edmayo]

Allen, since you don't actually know what your compression is (or will be) I would opt for the safe way and shim up rather than bring everything down. You could end up with too much compression. Anytime you build an engine with mix and match parts you have to have an accurate way of measuring your piston dome volume and cylinder head volume, otherwise you just don't know what you have!! A lesson I learned a long time ago is to never believe the C/R written on the piston box, there are simply too many variables.

By the way, one method we use is to make spacers to bring the cam housing back up the amount machined off the cylinders.

[sithot]

Anytime you build an engine with mix and match parts you have to have an accurate way of measuring your piston dome volume and cylinder head volume, otherwise you just don't know what you have!!

+++++1

Tom

[Scott Clarke]

As stated above, ALWAYS measure the compression ratio. I was the recipient of a box of pistons mis-marked just as Ed describes. Given the huge number of variables you are dealing with, this seems essential. Also, I don't know how far you can move the cams towards the crank without compromising the function of the chain tensioners, but I'd sure want to find out before putting it all together.

[Allen Henderson]

Engine geeks,

My question was too vague. OK, so I measured the deck height using solder and it is 0.90mm. This is about the minimium I believe but in spec. Agree/disagree? I also checked the intake valve clearance at specified overlap at TDC and I had an addtional (measured with dial indicaton) clearance of ~2.08mm which should be more than enough, agree/disagree?

So, we started with 9.1:1 compression ratio stock 84mm Pistons from 2.2E. We have two "adders" (my heads are "spec." 911S heads which have the same volume as other big valve heads) So we have 1. increased from 66mm to 70.4mm stroke plus 2. a reduction of, say 0.5mm, from nominal Porsche factory nominal (assuming that nominal is 1.5mm) deck height. From your experience or past calculation (no I don't have CC values), where will my compression end up? I am VERY tempted to go with this with out putting in any more spacers under the C's to increase the deck height! Thanks.

-Allen-

[tom1394racing]

Allen

The increase in stroke should add about 0.5 CR points putting you at around 9.6. This is getting close to the limit for single plug on today's pump gas. All the more reason to do the complete compression volume checks on your dry build.

[edmayo]

Allen, just so you don't think I'm blowing smoke here, here are some pictures of the method I described for accurately determining comp/ratio. We use various plexiglass cylinder heads in conjunction with a trial assembly where the piston is brought to TDC (using a dial indicator), the plexi head is installed and filled using a burette. This not only measures piston dome volume, but also will include deck height volume. Then measure the cylinder head volume, do a little math, and you will now have an accurate measure of compression ratio. If you do all six pistons and heads you can swap the heads around to give you the most even range of compression ratios. As an example of "don't believe the piston box" I recently did a 66 2.0 engine, I used 81mm JE's (not knocking their product at all) with a spec'ed C/R of 9.3. They actually measured out at 9.8 so I was able to shim up to lower the C/R.

1st) plexiglass cylinder head, its volume is written on the head, when you fill it at TDC subtract fill volume from head volume, result is P.D.V.
2nd) selection of heads, 356 to 3.6
3rd) filling head at trial ass'ly
4th) measuring cyl. head vol.

[preS]

Thank you Ed.

Richard

[Allen Henderson]

Ed et al.

No never thought that..that you or any others were blowing smoke. OK, I'm a little lazy but don't want my motor to blow up. I've used the solder method several ways and come up with ~1.0mm deck height. I am sure my heads are stock 2.2S. I am sure my pistons are OEM stock. I am (now) sure my cylinders have had ~1.5mm shaved off, as were the chain boxes of the engine they came off.

As I started, this is a "semi-hotrod." So I asked, "is this a bolt on?" According to Bruce's book and several other sources the motor (IF the cylinders and other stuff are stock) should be a bolt on. So, what else COULD be off.

With ~1.0mm deck height (did I mention I measured the "extra clearance" on the intake valve using a dial indicaton at TDC overlap forSolex cam (4.3mm lift ) and I had an extra 2.0mm clearance for the intake valve to piston?) Anyway, I am not going to go with the ~1.0mm deck height. Because 1. to be safe I am going to shim up the cylinders to stock and 2. because the ~1.0mm is a little tricky to measure (the solder is pretty thin, e.g. 1mm at that point) even though I did it a few times with different solder orientations and several pieces of solder) and 3. because I want to use stock chain boxes and 4. because the compression is likely to be too high.

Using an example in Bruce's book, I put the numbers for a racing engine into the "CBPerformance" engine calculator and it checked perfectly. So, based on Porsche nominal (no doubt conservative) values I plugged in the factory numbers for a 2.2E engine and 9.1 compresson ratio (this then fixes combustion chamber value). Then, I put in the 70.4 stroke and it checked out as adding about .55 of compression (in line with usually quoted adder.) Finally I used my deck height of ~1mm and the compression ratio came out 10.3:1 too high. So I am going to, as I said just bring the cylinders up to stock. Then EVERYTHING will be stock. Plus, it ALL checks out with the measurments (including chain box line up) I made.

Finally, btw, I removed the barrel on on piston and bolted it all up..just FYI there is plenty of room to rock that accessable piston...which is just a subjective measurment of 1.0mm deck height.

So, that's what I'm doing: shimming back up to nominal cylinder height (did I mention mine measure 84mm which should be 85.4-85.45.

Thanks for the input.

-Allen-PS I am a little tempted to use a 1mm spacer (versus .25mm standard copper one) which will give me 1.75mm deck height and a calculated compression of about 9.4:1 (I am a little suspeciouos of my deck height measurement, it might be a little less), if that is the case the compression will go up slightly*. Comments?

Another comment. My pistons are now "poking up" about 1mm above the cylinders. However, as I mentioned I have one barrle off and it is clear that the piston easily clears to head. However again. I'm going with "book" bolt together values. That being said, if I were a real risk player (some are saying this guy has no idea how risky he is ) I'd go with the set up I have right now! It all seems to work but the compression and little to no margin..I don't need that. Was this all too long?
* one of the reasons I think my deck height measurements are a little "suspecious" is the low compression values I 'm getting for a stock "deck height engine." In other words my deck height MIGHT be a little less than I measured with the solder.

[tom1394racing]

Allen

It's a very simple process to do the actual CR volume measurements and calculations. I used a simple plexiglass disc, cut with a hole saw to be larger that the cylinder bore, to get my head volumes. Bruce documented the method in detail in his book.

Even if you simply confirm your theoretical numbers once you've completed it, you'll be glad you did it.