MGB & GT Forum

I found a formula in a Bosch publication but I don't know how to do the math. Can anyone calculate the compression ratio from this?

Compression = (Compression Ratio minus one) to the 1.1 to 1.2 power multiplied by the atmospheric pressure

I have an average compression of 180 psi and I live at sea level so the pressure is 14.7 psi, and use a power of 1.15. I had calculus in 1964, but that is one of the reasons I had to drop out and I don't remember any of it.

(Compression / Atmos Pressure)^1/1.15 + 1 = CR

(180/14.7)^.87 + 1 = CR

9.84 = CR



Edit: the 1.1 to 1.2 is a fudge factor for valve overlap and cranking speed.

Using 1.1 the CR is 10.8; using 1.2 the CR is 9.1 Pretty big range, so not a real useful formula.



Edited 1 time(s). Last edit at 2014-04-23 04:21 PM by Dave Braun.

Or for the folks with better things to do, here ya go. I've used this many times.

http://www.rbracing-rsr.com/compstaticcalc.html

The last time I did this with "long math" I still had hair, and it was blonde too

For reference, if you want to play with the calculator

MGB Specs, I know them by memory

Standard bore: 3.160"
Stroke: 3.500"
Payen black composite head gasket compressed thickness : .040"
MGB stock piston to block deck height on a virgin block aprox. .035" (the factory could vary on this a few thousands)
8.8 to 1 piston dish" 6cc
MGB cylinder combustion chamber volume:
Early MGB heads
12H906
12H1326
12H2389
these above heads are stock 42ccs
Later heads
12H2923 (factory big intake valve 1.625"
CAM1106 (US head used on RB MGBs)
12H4743 (euro head)
12H4736 (euro head)
These above heads are stock 38ccs

Hope this helps.

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Hap Waldrop
Acme Speed Shop
864-370-3000
Website: www.acmespeedshop.com
hapwaldrop@acmespeedshop.com



Edited 3 time(s). Last edit at 2014-04-23 05:43 PM by Speedracer.

CR= Total volume divided by compressed volume. Get a 4" square of clear lexan or glass with a hole in it. Put the piece over a cylinder and turn the engine to TDC. Pour mineral oil through the hole into the cylinder from a graduated cylinder. When it's full, note how much oil you poured (call that "A". Do the same thing with the combustion chamber of the head and measure the volume of the chamber (call that B).. Then measure the depth of the chamber and and divide that into the volume of the chamber. You now have a factor per thousandth of depth for the heart shaped chamber 9call that C.). Then measure the compressed thickness of your head gasket in thousandths and multiply that by C to get the volume of tor head gasket (call that D.) This is slightly inexact because the hole in the head gasket is larger than the face of the combustion chamber but you're close. Last you know that the swept area of the cylinder is pi x r^2 x H. Where H is the stroke (3.5" or 8.89cm) and r is 1.58" for a standard piston ( 4.0132cm), 1.59 for a .020 piston 94.0386cm)... So for a standard engine the swept area is:
3.14159 x 4.0132^2 x 8.89 or 449.81cc. Then add A +B+C +D +449.81 and divide by (A+B+C+D) That's the compression ration

The formula you presented is incorrect because the 1.1-1.2 coefficients it uses are inexact And it measures effective compression which accounts for compression lost through valves hat aren't fully closed.

If you're looking for anecdotal means to estimate your compression, a newly rebuild MGB with 8.75:1 compression reads about 155psi on a compression tester. My race motor has 14.7:1 compression and reads about 245psi on the compression tester. A simple linear interpolation says that seeing 180 psi, the compression would be 245-155=90. 90/(14.7-8.75)= 15.1 psi per point of compression between 8.75 and 14.7. 180-155 = 25psi. 25 divided by 15.1 = 1.655 points of compression in addition to 8.75 or an estimated (interpolated) compression of 10.4:1
Basil 707.762.0974 basiladams@yahoo.com

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Basil C. Adams
1956 MGA Coupe (Show Car)
1957 MGA Roadster (Driver)
1958 MGA Coupe (Racecar)
1959 MGA Coupe (unrestored)
1960 MGA Coupe (unrestored)
1960 MGA Roadster (Driver)
MKIII Elva Courier (E1056)
1967 427 Cobra
1972 Alfa Romeo Montreal
A coupla late MGBs
1960 Austin Healy BN7
More Cars than Brains

14.7, impressive. Mazda claims to have the highest compression production motor in world: 14.0.
http://www.mazda.com/technology/skyactiv/engine/skyactiv-g.html

guys like Basil can do this in their head without even a pencil in hand!<G>

I'm pretty good at this, but Bas humbles me!

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1973 Pale Primrose Roadster. A nice 10-footer!
SUs, Datsun 5-speed

High CRs were the norm in SCCA, my FP Midget was about 15.5 to 1, as well as my domed MGB engines, now the HP 948s were the king of high CR SCCA LBC motors, ranging from 16-0 to 18.0 to 1.

These days in vintage racing we see a regression from this high CR numbers, a more casual, and reliable approach towards racing most vintage guys have just one race engine, I had 2-3 when I was racing SCCA nationals and going to the runoffs, so we see number in vintage more in the 12.0 -13.0 range, and in SCCA racing we tended to use higher rpms, in the MGB about 8000 rpms, in the 1275, 8100 with the occasional romp to 9000 , on the 948s I set the rev limiter at 9100 rpms. Now most vintage racers rev to 7000-7500 rpms.

I've seen SCCA 948 race engines pump 300 psi on a compression gauge

---

Hap Waldrop
Acme Speed Shop
864-370-3000
Website: www.acmespeedshop.com
hapwaldrop@acmespeedshop.com



Edited 1 time(s). Last edit at 2014-04-24 05:23 AM by Speedracer.

Thank you all for the input. My engine was rebuilt before I got it and was wandering what the c/r might be. All I do know is that the engine was re sleeved with the standard bore,the head was replaced as the original was cracked and that a Crane cam F222/280, lifters and longer push rods were installed. The engine runs strong so I have no complaints regardless of c/r.

Consider the wide range of cams available for the MGB and you'll realize this formula won't even get you in the ballpark. There are plenty of free online calculators to figure it out the right way, if you have the head off to CC it. The formula for the volume of a cylinder isn't rocket science... or is it????

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jeff@advanceddistributors.com

Volume of a cylinder is simple -- area of the bore times stroke.

Area of a circle = pi x radius²

So for a stock bore of 3.160, radius is half of that, 1.580
1.580² = 2.4964
pi ~ 3.14
So pi x r² = 7.84 (approx) This is the area of a standard bore.
The stroke of our engines is 3.5 inches, so 7.84 x 3.5 = 27.44 cubic inches is the volume of one standard bore cylinder, which is what you need to calculate compression ratio...

When using on-line calculators or any other procedure to calculate compression ratio, be sure all units of measure are inches or metric; don't mix and match...

Dick

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Errabundi Saepe, Semper Certi
(Often wrong, but always certain)

NASA did that with two of their suppliers when building an explorer that was supposed to land on Mars several years ago. One supplier did their math using the Metric system, while the other supplier didn't. Guess what happened. The explorer missed the planet by over a thousand miles and cost us a billion dollars in the process. Always double check your measurements and specify which system that you used. RAY