Oversize pistons and compression ratio

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Oversize pistons and compression ratio

Postby johnmcguire » Thu Aug 01, 2013 10:41 am

I am a newbie having purchased a 1960 bugeye that has been "in storage" in Southern California for 45 years! The body has NO rust but the mechanics need rebuilding. I realized someone had been there before when I encountered a bright red engine. The 948 had been bored out to fit 0.010 oversize 998 pistons.
In order to clean up the cyclinder walls I am boring out to max oversize ( 0.125 inches over the original 948 pistons) and I found flat top racing pistons this size.
According to Vizard's tuning book, the new cylinder volume is 1046/4 or 261.5 cc. To get a CR of 9:1 I need a combustion chamber volume of 32.687 cc and to get 10:1 I'll need 29.055 cc.
Presently the measured volume of the chamber in the head is 26 cc but that will change after flattening the head and replacing the valves. Right now the combustion volume is : ring land 0.78 cc + gasket 1.40 cc + chamber 26.0 cc + whatever the piston deck volume is above the piston. This is my quandary, what volume to have above the piston to get a CR of 9 or 10?
My calculation of the bore ( 2.600 " ) volume is that each 1/1000 of an inch equals 0.087 cc.
So for a head chamber volume of 26 cc I need 0.052" above the piston to the top of the block to have a CR of 9:1. To have a CR of 10:1 I need 0.010" above the piston.
Obliviously I'll have to recalculate after the head and valve work.
HELP! Some of you with experience and expertice please check my amateur engineering and tell me where I might be about to really screw up, please.
Thanks, John McGuire. 828-230-9562
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Re: Oversize pistons and compression ratio

Postby Charlie Tolman » Fri Aug 02, 2013 12:12 am

John,

I went through your calculations and all the values look very good.

A stock head gasket, when compressed, might have a chamber volume close to 2.0 cc.
The stock deck height is usually 0.006", corresponding to a volume of 0.5 cc for your bore.
Your racing pistons might not have a stock wrist pin height of 1.342" ?

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Re: Oversize pistons and compression ratio

Postby johnmcguire » Fri Aug 02, 2013 3:44 pm

Charlie, Thanks for checking my math and your expertice. I have standard gaskets so will use 2.0 cc for that and will have to remeasure my head volume after new valves and flattening the head.
I misspoke in that my pistons are not full race, they are Russell Engineering flat pistons for the Mini Cooper 998 engine but 60 thousandths over. The stroke and pin height is supposed to be the same as the standard 948 and 998 pistons and I plan to use my 948 clamp on connecting rods.
My big concern is the deck height to get a CR of 9:1. Does anyone have problems with higher CR than this and 93 octane fuel?
I am not planning to run thIs engine hard and suppose 9:1 is low enough. Or do I need to get an even lower CR?
Thanks again, John
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Re: Oversize pistons and compression ratio

Postby Charlie Tolman » Sat Aug 03, 2013 12:33 am

John,

A friend has Hepolite pistons with a 2 cc pop-up in his 998 Mini with a 12G295 head. The combustion chamber volume probably is 28 cc. The compression ratio is near 10:1, and the engine has no problems with 93 octane non-oxygenated gasoline. Mild upgrade cam.

An important factor for higher compression engines is the closing angle of the intake value, which strongly affects the compression PRESSURE in the cylinder. Longer duration cams cause the compression pressure to be lower than with standard cams, and thereby reduce the occurrence of pre-ignition or pinging.

I posted the results of compression pressure calculations in the Engine section of this forum a few years ago, demonstrating how the pressure is related to the mechanical compression ratio and the closing angle of the intake valve. A "search" (upper right corner of page) should locate the posting.

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Re: Oversize pistons and compression ratio

Postby Charlie Tolman » Sat Aug 03, 2013 12:43 am

Please see my posting in the General section of the forum, on Dec. 23, 2008, regarding compression pressure.

CT
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Re: Oversize pistons and compression ratio

Postby Charlie Tolman » Sat Aug 03, 2013 12:50 am

I should have mentioned that the posting is listed under "Compression - - - -",
dated Nov. 22, 2008.

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Re: Oversize pistons and compression ratio

Postby johnmcguire » Thu Aug 08, 2013 6:36 pm

Charlie, once again thanks for the expertice. I think I might be getting a handle on things now. I went back and followed all those trails and learned a lot. I plan to use the original 948 head but have it flattened and the valves replaced. Then I will need to measure the chamber volume. How best do I do that without having to buy or borrow graduated glassware from a lab? As a retired surgeon I do have access to sringes and needles. Kerosene, water with alcohol or just plain water and a spirit level seem awfully primitive to me. I am a glass blower so can drill a hole in a piece of flat glass if required.
How do you do it? John
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Re: Oversize pistons and compression ratio

Postby Charlie Tolman » Fri Aug 09, 2013 1:57 am

John,

To measure head chamber volume, you can use a graduated syringe or burette, calibrated in cubic centimeters or milliliters.
Use a piece of glass or plexiglass to cover the head chamber, with a small hole in the cover for adding liquid and allowing air to exit the chamber. A very thin layer of grease around the head surface next to the chamber edge will aid in sealing.
I have used water to fill the chamber.
Other readers might suggest a different technique.

Typical 948 engine heads, stock, have a chamber volume near 24.5 cc.

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Re: Oversize pistons and compression ratio

Postby Sprite33 » Fri Aug 09, 2013 1:00 pm

I use that same technique to measure the head chamber volume. I always add a little ethyl alcohol to the water as a wetting agent. If you don't have an accurate graduated syringe and have a good gram balance or scale, you can use the weight to determine volume. Water is 1 g/cc at room temperature. The alcohol is about 0.82 g/cc but you need less than 1% and should not change the overall density very much. You can measure the syringe/water mass before and after to determine how much went into the chamber. Just make sure you don't lose any liquid other than what goes into the chamber.
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Re: Oversize pistons and compression ratio

Postby johnmcguire » Tue Aug 13, 2013 9:05 am

A really accurate determination that way. I'm not use how accurate the marks on a plastic syringe are unless you use the really small ones. Luckily I have a digital micro scale.
I'll let you know how these two methods compare.
Again thanks for the help, John
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Re: Oversize pistons and compression ratio

Postby johnmcguire » Sun Sep 22, 2013 7:25 pm

Charlie and Sprite33,
I finally got my heads back. I measured the chamber volume using both volume and weight methods. The volume markings on a 60 cc syringe are not very precise but I got 25.25, 25.75, 25.75 and 25.50.
By weight I got 25.62, 25.45, 25.66 and 25.74 so I might grind a tad off of cyclinder #2 to get them equal.
Depending on what the head gasket adds ( either 1.7 or 2.0 cc ) the present CR is 10.39 or 10.19. If I want a CR of 9 then I will need the piston about 50 thousands below the deck. If we take the block down to flat with the piston then to get to a CR of 9 we could enlarge the chamber volume in the head to 32.687. I could modify the shape as suggested by Vizard and be able to remove 4.237 on average from each chamber. As compulsive as I am I can see my getting them all to exactly 32.687 with improved flow and a low enough CR to use filling station gas. For a CR of 10 we could still take each chamber up to a volume of 29.055 but only a half a cc will not help much.
What do you think?
Thanks, John
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Re: Oversize pistons and compression ratio

Postby Charlie Tolman » Sun Sep 29, 2013 12:08 am

John,

Before you attempt to micro-manage the piston height, deck height, and/or combustion chamber volume to the nearest 0.001 cc, you need to decide what camshaft will be installed in the engine.
As I mentioned earlier, the camshaft, and especially the intake valve closing angle, will suggest what mechanical compression ratio should be employed to obtain the optimal compression pressure. Gasoline octane is also an important consideration, as you know.

Most of David Vizard's recommendations are for competition engines, where large air-flow numbers through the head are desirable. Profession engine builders of the A-series engine went too far, a few years ago, with large head runners to achieve flow, and thereby reduced port velocity to where engine efficiency and net power were reduced. Depending on your application, it might be prudent not to duplicate Vizard's designs that are based on very high rpm competition engines. Vizard has done some very good work, but one must remember the application at hand.

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Re: Oversize pistons and compression ratio

Postby johnmcguire » Mon Sep 30, 2013 9:40 pm

Charlie,
Once again thanks for your expertise. The cam is at the engine shop but I assume it to be stock. Can I easily tell what it is and how?
I will have a 948 bored out to 1046 with standard 948 valves and head. I would like to use unleaded high octane fuel and can find it without the ethanol. So I am assuming a CR of about 9 or 10. I will not be racing and it will not be my everyday car.
Do I need to change the cam for a little better performance and if so to what? Then I think I can adjust the CR for the cam.
Thanks, John
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