compression ratio

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C J Murray
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Re: compression ratio

#16 Post by C J Murray »

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 »

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 »

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 »

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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Bill Oldham
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Re: compression ratio

#20 Post by Bill Oldham »

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 »

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 »

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 »

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 »

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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Re: compression ratio

#25 Post by Harlan Halsey »

On the subject of compression ratio, I use Elgin cams intended for street use or mild vintage racing, 4,000-6000 RPM power, stock displacement. Dema suggested a maximum CR of around 9.6:1,(I shoot for a range of less than .15 across all 4 cylinders.) .040" minimum piston to head clearance, stock valve springs shimmed to S90 specs. On street gas.

I measure the piston to head clearance with modeling clay in several 1/4" wide strips across the relevant places. Measure the flattened places with the tang on a caliper. (I rejected solder long ago because I think it has more chance of cocking the piston on the bore.)

This has always worked well for me on several engines using either Dema's cams or the stock SC cam. On the other hand, I don't plan to run my 356s flat out for hours on end in 100 deg. temperatures in the South Western desert. This applies to stock street engines. If you want to re engineer your engine to accommodate "big bore" cylinders, then you are on your own.

On the subject of measurement, see #3 and #4 above, I leave the rings off and use tape and grease, rather than the rings and grease simply because it is easier to put the piston in that way. And I space the piston 6mm below the top of the cylinder using a spacer and a straight edge because it is easier to get a complete fill that way. My spread sheet subtracts the extra volume from the piston crown volume measurement in the CR calculation.


I think stock street engines are pretty forgiving, but race or modified engines are completely different.

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Re: compression ratio

#26 Post by Wes Bender »

Harlan Halsey wrote: Fri Dec 06, 2019 5:54 am I measure the piston to head clearance with modeling clay in several 1/4" wide strips across the relevant places. Measure the flattened places with the tang on a caliper. (I rejected solder long ago because I think it has more chance of cocking the piston on the bore.)
If you use Vic's suggestion of a piece of solder at the top and the bottom of the piston, and then average the thickness measurements, cocking isn't a problem. I used to use four pieces, but found the ones on the sides made no noticeable difference to the average.
Some days it's just not worth chewing through the restraints.....

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Re: compression ratio

#27 Post by Vic Skirmants »

Thanks, Wes. It's good to have someone agree.

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Re: compression ratio

#28 Post by Bill Oldham »

Plumber’s putty and solder. Light oil on head so they don’t stick. Before and after photos. Valve at zero lash

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Re: compression ratio

#29 Post by Ron LaDow »

Top/bottom clay or solder, the piston will rock in the bore about the pin C/L to the limit of the skirt clearance and the delta between the two compressible elements will be predictable based on the delta between the areas compressed (with limits defined by skirt clearances and some correction for the piston dome weight to cover the pedantry).
But unless there's something really screwy, test above the wrist pin ends; they had better be equal and equal to the top/bottom average if you also checked at those locations.
Depending on the dome design and chamber machining, you should be finding quench (piston/chamber clearance) and deck height dims. Really loose deck height and tight quench tells you the chamber needs machining:
Peripheral chamber.jpg
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Re: compression ratio

#30 Post by Vic Skirmants »

If your piston is rocking enough to affect the measurement, you have a problem. Besides, if you are measuring top and bottom, then there should be equal force on the solder. I only use one piece on top and one on the bottom, in the center.
Bill; thanks for posting the photos. Of course somebody will be confused because the photo is rotated 90 degrees. "Hey; must be a Carrera, valves are on the top and bottom." :D :P

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