compression ratio

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C J Murray
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Re: compression ratio

#16 Post by C J Murray » Sun Dec 01, 2019 1:29 pm

You check clearance at the squish first to set the piston to head clearance, like Vic said, the top and bottom of the piston. With that clearance set you can place a ball of clay in the exhaust valve pocket, assemble the rockers with pushrods all set at zero lash at TDC on the compression stroke. Rotate the crankshaft 2 turns and remove the clay to get the valve to piston clearance.
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Bill Oldham
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Re: compression ratio

#17 Post by Bill Oldham » Sun Dec 01, 2019 2:19 pm

Wow! thanks guys! Cliff, your explanation about the cam controlling the effective compression ratio is what I was looking for. Its kinda what Dema Elgin told me when he did the special cam for my S90. (I wanted a torquey cam at low RPM) That was also a 22 degree dome (I cut down an old NPR set) and Dema said CR of 9 was fine, and it was. Lots of miles on that engine. But here, with the stock normal cam I guess I will ease off on the CR.
Ron: Thanks, but I probably do not follow the reasoning correctly. Seems like you say the 1mm quench space near the edge is needed to prevent knock, but adding shims to lower CR kills the effectiveness of the quench space. Sounds like adding shims may not work as well as expected. I wish we had Harry Pellow to resurrect his CR vs engine life data base and comment on role of chamber shape.
Vic: I think the .5mm shims came with the P&C. I have lots of .25mm shims and unfortunately no 1mm at the moment. Looking through odd parts box I found 2 1.75mm shims and 1 of 1.5mm thickness. Strange.

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Re: compression ratio

#18 Post by Ron LaDow » Sun Dec 01, 2019 3:17 pm

Bill,
You can remove material from the head chamber (NOT on the taper) to keep the quench and lower the CR.
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Re: compression ratio

#19 Post by Martin Benade » Sun Dec 01, 2019 3:24 pm

I think L&N sells various thickness base shims, unfortunately at $75/ set retail. I have a couple in my engine cut from shim stock with scissors to take care of an unevenly machined head.

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Re: compression ratio

#20 Post by Bill Oldham » Sun Dec 01, 2019 4:00 pm

Thanks Ron. Why did I not think of that. Lots of meat on the piston and if I shave off 1mm from the 6cm diameter top (~2.8cc) , it will drop the CR to 8.75.

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Re: compression ratio

#21 Post by C J Murray » Sun Dec 01, 2019 4:42 pm

Ron LaDow wrote:
Sun Dec 01, 2019 3:17 pm
Bill,
You can remove material from the head chamber (NOT on the taper) to keep the quench and lower the CR.
True but that would require removal of quite a bit of material. I wouldn't ruin heads that way. The correct way to fix the problem is to get correct pistons. I wouldn't attempt to machine down the AA pistons because I doubt they would be structurally sound once machined. You should be safe from detonation with the cylinders shimmed excessively. Tight is clearly best, loose is fair/ok, in between is more likely to cause a problem due to trapped charge that may burn out of phase.

If you are using the Normal 24mm venturis you will be less prone to detonation due to lesser cylinder filling, aka VE.

The Normal used a 7.5 CR but a C with the same intake closing point used an 8.5 CR. Some of the difference was due to the 30* head angle but you can probably get away with around 8.5 with a Normal cam. I think fuel was better by the time the C was designed. A Normal would be safe in all the oddball remote countries that some owners took their cars back then. Just guessing.
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Re: compression ratio

#22 Post by C J Murray » Sun Dec 01, 2019 4:43 pm

Bill Oldham wrote:
Sun Dec 01, 2019 4:00 pm
Thanks Ron. Why did I not think of that. Lots of meat on the piston and if I shave off 1mm from the 6cm diameter top (~2.8cc) , it will drop the CR to 8.75.
Measure the thickness of the dome first.
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Re: compression ratio

#23 Post by Bill Oldham » Sun Dec 01, 2019 7:48 pm

Cliff, The thickness of the dome for AA is about 6.6 mm. (for comparison I measured the thickness of the NPR cylinders dome as 6.1mm). Errors =/- a few tenths.
Not sure what to do with this but considering the first order bending of a plate goes as the thickness cubed, a 15% thickness loss translates to a lot of lost strength. (Of course this is overly pessimistic because it ignores the large thickness to diameter ratio, making the calculation way harder. And in any case it is something like the surface tension on the bottom of the dome that causes initial crack propagation, and the simple analytic formulas do not give me that calculation.) But if the dependence is on the thickness cubed, then a 15% thickness reduction results in a 40% loss in resistance to stress. So maybe not a good idea to shave thickness.
When I cut the NPR cylinders to make a 22 degree dome I only removed material from the edges, with almost no effect on the dome strength. But here, wanting to preserve the 22 degree taper near the edges, it would have to be from the middle, with significant loss of stiffness, if not strength.
Guess I will go with spacing. (I agree with you about not messing with the heads; pistons and cylinders are consumables, not so much for heads.)

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Re: compression ratio

#24 Post by Martin Benade » Sun Dec 01, 2019 9:14 pm

This doesn’t prove much but I am running an old set of German big bore pistons that I cut to N dimensions for my S90. Nothing has broken in 35 years! I didn’t know any better at the time, probably removed 3-4 mm from them.

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