| Hayden74SB |
Mon Dec 15, 2025 4:03 pm |
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I just tore down the engine from a 74 SB that I just bought, and I cannot make sense of the measurements I am finding.
The pistons are 88mm. I do not have a micrometer, so the following dimensions are from my digital calipers so are only good to about 0.1mm.
The head bore measurement comes out at 96.5 but the OD at the top of the cylinders is only 95.4 so there is over 1mm gap between the head and the sleeve. FWIW, this engine had 145psi compression before I pulled it apart.
The case bore is 92.25mm and the case bore is 92.57mm and it slips in with just a bit of drag.
It seems that the head bore is about right for a thick top wall 88mm, but the actual top of cylinder is less than expected.
The bottom of these jugs fit the case bore but I cannot find any piston/cylinder kits that have this measurement.
I measured the stroke and it is 69mm so stock crank.
Also, when I back calc the compression ratio from the measured compression, it seems like it is close to 10:1. Should I add cylinder shims to lower that or just use 92 octane fuel? |
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| runamoc |
Tue Dec 16, 2025 10:01 am |
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Quote: close to 10:1
that's static CR. With some cams that's the correct CR because the dynamic CR is lower because on those cams both intake and exhaust valves are open at the same time. |
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| Hayden74SB |
Tue Dec 16, 2025 2:12 pm |
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runamoc wrote: Quote: close to 10:1
that's static CR. With some cams that's the correct CR because the dynamic CR is lower because on those cams both intake and exhaust valves are open at the same time.
What I calculated would be a dynamic CR of 9.83:1 based on the 145psi compression with an atmospheric pressure of 14.75psi. SC = gauge compression / AtmosP = 145-psi / 14.75-psi = 9.83. my static CR would be higher than that. |
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| Slow 1200 |
Tue Dec 16, 2025 3:13 pm |
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It sound like you have the old style (as in really old machine-in 88mm barrels and the heads machined for the not so old thicker kind
If you have the heads out measure the chamber volume and deck height and you will know the actual compression ratio for sure |
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| runamoc |
Tue Dec 16, 2025 5:31 pm |
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Quote: I calculated would be a dynamic CR of 9.83:1 based on the 145psi compression with an atmospheric pressure of 14.75psi. SC = gauge compression / AtmosP = 145-psi / 14.75-psi = 9.83. my static CR would be higher than that.
Impressive...yet all of the dynamic calculators shown online factor in inlet valve closing ABDC...based on the cam profile. |
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| BFB |
Tue Dec 16, 2025 6:33 pm |
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| Why’d you start a new thread on the same subject as your old one? Hoping a new thread would provide new and different answers? |
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| Glenn |
Tue Dec 16, 2025 6:48 pm |
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BFB wrote: Why’d you start a new thread on the same subject as your old one? Hoping a new thread would provide new and different answers?
Do you mean this one?
https://www.thesamba.com/vw/forum/viewtopic.php?t=808176&highlight=
Or this one?
https://www.thesamba.com/vw/forum/viewtopic.php?t=808148&highlight= |
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