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New S1B owner
GavinT:
OK, I think I've figured it out, so correct me if wrong.
I believe the error is in column E where the "Raised piston volume (cc)" is already accounted for in the "Head Volume (cc)".
Also, "Head Gasket Volume (cc)" wasn't included in the calculations for columns B & C, and when you do, the compression ratio seems to come out wrong.
Columns B & C also appear to use different formulae to calculate "Combustion volume (cc)" so I didn't go further down that path.
As a cross check, here's my effort.
My assumptions are contained in column B where I just tweak the "Head Volume (cc)" until I get the "Calculated compression ratio" right.
This establishes the "Head Volume (cc)" which is carried across as a fixed value.
Column C is tweaked similarly via "Raised piston volume (cc)" until "Calculated compression ratio" looks right.
This establishes the "Raised piston volume (cc)" as a fixed value.
Columns D and E are then extrapolated and I've included the "Head Skim allowance (cc)" mentioned in a previous post. I took a stab in the dark and assumed 3.5cc as an estimate of the material removed.
Does that look right?
Hoping this link works.
https://docs.google.com/spreadsheets/d/1c3XQoc8EcwDdMsnPR-BS5--745BCabZZrsURfPMtvwI/edit?usp=sharing
314159td:
--- Quote from: GavinT on Friday,February 16, 2024, 01:16:44 PM ---
Also, "Head Gasket Volume (cc)" wasn't included in the calculations for columns B & C, and when you do, the compression ratio seems to come out wrong.
Columns B & C also appear to use different formulae to calculate "Combustion volume (cc)" so I didn't go further down that path.
https://docs.google.com/spreadsheets/d/1c3XQoc8EcwDdMsnPR-BS5--745BCabZZrsURfPMtvwI/edit?usp=sharing
--- End quote ---
This was the main issue, somehow it's always the fancy formulas that are right but the simple addition gets messed up. I should have caught that head volume and combustion volume were equal in column B sooner. When I fix that, I get a stock head volume of 49.5cc and the raised piston volume stays the same at ~16cc, as you've reached manually by iteration. Other values of the sheet remain unaffected, as that stock head volume is just for sanity checking once the raised piston volume is calculated . I added a tab to my sheet with the original head volume formula corrected.
The issue is that my head chambers are ~39cc, as I've measured directly. That is now a removal of 11cc from the calculated factory value, which is closer to the 4.54cc estimated value you proposed earlier (in the incorrect sheet, it was 17cc off). When I make a copy of your sheet and adjust the head volume to be what I've measured, we have the same values.
Another possibility to explain that misalignment is that the published stock head thickness may be incorrect? Or at least for my head? That would tidy all of this up nicely if that were the case.
Let me know if I've misinterpreted something there, but the 38.5cc is a fixed value. The sanity checking is much appreciated!
Sidenote as it didn't create any issues here: the difference in formulas for combustion volume between B and C was a remnant of working backwards a different way, but it happened to not matter as the subtracted value was 0. That's just the inverted static compression ratio.
Further sidenote: I'm basically ignoring dynamic CR here, as I intend to keep the stock camshaft. Matching static CR should keep everything in order for this particular rebuild.
If anybody has a stock 697-04 head laying around and could cc it that would be great! I'd be willing to send you the supplies for it. Might make a separate thread for that for visibility.
GavinT:
Alrighty, let's accept the 38.50cc chamber volume because it's an empirical measurement.
Without argument to the contrary, we also need to provisionally consider "Head Volume (cc)" of 49.50cc because it results in the published compression ratio.
Obviously those two numbers are at odds.
There's 11.00cc missing.
If we accept the head skim accounts for ~3.50cc, we still need to find ~7.50cc thus:
38.50 + 3.50 = 42.00cc
Initially, I couldn't convince myself the space above the piston would account for 7.50cc because the raised plateau of the Lotus pistons is quite broad.
For that to make sense, the top of the piston plateau would need to be ~2.00mm below the liner height, but perhaps that is the case.
Need more data.
I reckon the next step is to discover the total volume via JB's method.
Alternatively, use the Lotus piston/liner kit from R-16Shop and get a 2.7mm custom head gasket. In saying that, I'm not sure how the rubber perimeter seal around the camshaft might handle a thicker gasket.
Lots of basic caveats to be aware of around all this, too.
If the head has been skimmed, I'd assume it was overheated at some point. Given it also has low compression pistons, I'd wonder why. Was that part of the overheating issue or something else entirely? What if they also took a lick off the block and decked the liners to suit?
As I say, lots of caveats with an unknown engine - measure everything.
314159td:
More measurements on the way! As I don't care about liner seals I'm going to rotate the engine (by rolling the car, not turning the cam pulley) and verify/measure that the pistons reach the top of the liner. That's one of the larger assumptions I haven't seen confirmed yet. Most engines do that...not all.
The low compression pistons are still puzzling; it means someone tore into this engine pretty early in its life, or it left the factory like that for some reason. The person that had the head skimmed said it was only about 4-5 thou removed, so maybe another person before that removed the bulk of the material? Perhaps they could only get low compression pistons and chose to raise the CR via a head skim?
I'm pretty set on just getting the Lotus pistons and liners, and machining some off the top to keep CRs in line. I hate sending stuff to the machine shop but that's an easy job on the lathe at work. Cheaper and easier than figuring out a custom gasket.
Richard48Y:
I would be looking for a standard head.
Skimmed that heavily brings the entire valve train into question as well.
I think I have a "scrap" head here, that might actually be repairable with some welding.
Had a blown gasket between two cylinders and got notched.
I will look and see if it is still around.
Yours for shipping cost if I find it and you want it.
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