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MG MGB Technical - Compression ratio

I have two questions, based on an engine rebuild currently underway. I have reworked the head to Burgess' fast road specs, and reshaped the chamber. It is now 41.3 cc, as the head was milled sometime in the past. I'm installing .060 pistons for the 1868 engine. The shop in the process of truing and squaring the block has decked it .020, so that hte new pistons are .005 from the top. I was planning on a CR of 9.5/1 which I thought was pushing it. Now by my calculations the CR will be more like 9.7/1.
The cam will be Piper BP270.
question 1- what experience has anyone had with this hi ratio? Do you have to sacrifice advance, run octane boost or does it run fine?
question 2-to drop the CR down, what would you do? The pistons are AE/Hepolite are have good amount of metal on top. I could have the dish deepened or mill off the top. Would you go to 9.5 or to 9.3/1 ?
(Steve S. I already know you would choose 9.3)
Thanks, Tom
Tom Lennon

I would not hesitate to go with 9.7 compression ratio. Worst case you may have to recurve your dist a bit and cut back on your high rev advance. But I doubt it at that ratio. Go for it. Bob Thompson/International Auto
Bob Thompson

As you will no doubt be aware C/R is limited by the octane value of the fuel used, now 2 things come to mind. In the US you work out C/R differnet to us in the UK, also I believe that your fuel octane rating figures may also be different. For what its worth in the UK I can run engines at 10.5 to 1 C/R on 95 ron fuel with no pinking whatsoever. Figures higher than that are possible but the engineering needs to be correct, it has been my experience that an engine with the block decked to the height of the pistons and then the C/R reduced by dishing the pistons as you have suggested seems to surpress detonation (pinking) slightly thus alowing a higher C/R to be used before pinking is experienced
Bob

Around wisconsin, we get 92 and 93 octane premium unleaded. fwiw
Fred Horstmeyer

I used the formula given by Burgess in my calculations. CR= stroke volume+compressed volume/compressed volume. IE stroke volume is 1868 or 467 for 1 cylinder and compressed volume to include the head chamber volume + volume due to gasket + voulme piston to deck + dish volume. This was 53.85 cc in my instance.
Tom
Tom Lennon

Tom;
After my earlier entry I looked up my wife's car and it is in fact 9.75 to 1 again using the Peter Burgess book to calculate. I did in fact have to spend a good bit of time setting up the dist. Is bored .060 Crane 342-0010 cam, HIF4 carbs with K&N filters. Stock manifold with Big bore. The dist curve which I ended up with is as follows. This took much time to set up!
15 degrees @1050
19@ 2000
22@ 2500
24@ 3000
25@ 3500
25@ 4000
27@ 4500 max advance
Car must have 93 octane premium. Has A/C and does not ping. Gets 31 mpg on highway "Merican Gallon" In Od at 75/80 going to Florida.. Any more adv and the car pinks under various load/rpm conditions. Bob thompson
Bob Thompson

Yes that is the English method, I thought the US simply measured swept volume against compressed? Not to worry. Tom if you calculated in the same manner I find it very strange that the car can not take more advance than 27 degrees, I wonder if your 93 fuel is on the same scale as our 95, that aside you have done the test, no doubt at great expense so Tom would be well advised to copy your format.
Bob

Bob T-
Thanks for your detailed answer. Your engine looks similar except for the cam. I think the Piper BP has higher lift than the Crane. 400 thou on the piper. The distributor looks to have significantly less advance than stock (I'm going by memory here). Retarding the spark sacrifices power, while higher compression boosts it. While balancing these two, which is more preferable? Lower compression and more advance or higher compressoin and less advance? I won't be loading it with an AC, but I have an OD and a heavy right foot. (Also lightened the flywheel to 16 lbs. I had already expected to use 93 octane.)
Tom
Tom Lennon

Piston to cylinder head clearance can play a large part in the detonation resistance of an engine. Cutting the top of the piston (increasing it's distance from the head at TDC) will in all likelihood, decrease the engines detonation tolerance and make it more likely to do so.

Ideally you would want the piston as close to the head at TDC as possible, short of it hitting under running conditions. What this distance is for your particular engine I cannot say, I will say that .040" is generally a safe and effective clearance.

The reason this is so, is because the closer the two come, the greater the "squish" effect at TCD which enhances the burn, thus making that process quicker and more complete. The quicker and more complete the mixture can be made to burn, the less of a chance of detonation occurring.

Also, the CR used will be a factor of the cam in question. The reason Bob can run 10.5:1 is not necessarily due to the possibly better octane of the gas (it may play a factor indeed), but it may in fact be due to him using a more radical camshaft also.

Here is how you can ball-park the CR-for-cam needs of your engine. The formula to use for this is: swept volume from the time of intake valve closing + compression volume/compression volume = 8.5:1.

This is a formula made up by Dema Elgin and I believe it is fairly close. For the Piper 270 timed according to the card, the piston will be .81" up the bore at intake valve closing. Since the stroke is 3.5", you can put your calculator to work to figure the rest. If you come up with 8.5:1 or lower using this technique and you do not have your piston further than about .080" from the head with the gasket installed, you are likely going to be just fine.

Sean

Sean Brown

I run 11-1 compression with no problems on 94 octane gas, I just needed to play around with a few of MSDs addon ignition modules.
Rob

Sounds reasonable Sean, but the cams I like to use are generally 286 degree which I suppose give the engine every chance of breathing well at 4.5K and above. What the effective C/R would be at these revs is a little hard to determine, also what the effective C/R would be a lower revs would appear to be significant. I would suspect a milder cam with 270 degrees should have a higher effective C/R at revs below 4.5K suggesting that a lower initial C/R would need to be used to prevent detonation. I am a great believer in the piston to head clearance being as small as possible to improve squish and detonation suppression, I have found that the thickness of the head gasket is always ample clearance.
Bob

Sean-
Your info greatly appreciated. You have always shown good understanding of the internal combustion process. I was hoping you would leave a comment. Using the formula, my answer is 7.67/1 with the Piper 270 cam. I guess the BP and HR series are timed alike, at least the intake. I seem to remember the exhaust was slightly different and the lift, too. As I mentioned above the piston will sit .005 below the deck surface at max. The gasket, a copper Payen mics out at .040. So, according to your info, I am very much in the safe range. No need to alter the pistons. Will go ahead and have this put together!
Tom
Thomas Lennon

Lots of good info in this thread. Effective CR (as opposed to physical CR) is always a topic for discussion. Several years ago the board had a thread that really was long on this subject. You may want to check the archives.

Any way, my contribution to the discussion is a bit of clarification on the UK vs US octane confusion. Thanks to Bob, I now know that UK octane is RON (Research Octane Number).

There are two ways to measure octane rating, one is by the Research Octane Number (RON) and the other is by the Motor Octane Number (MON). At one time I could tell you the difference but I would have do a little woodshedding with some of my engineering texts to do so now.

Anyway, for the sake of getting rid of the confusion in the U.S. over this very thing, the government decided that everyone and every gas station would do it the same way. The U.S method is to take the average of the two numbers.

That's why you see the littel yellow sign that most folks never pay any attention to on a gas pump in the U.S. This is the one that says R+M/2.

For Bob,

Several years ago I dug into this deeper and after a few approximate calculations came to the conclusion that 93 in the U.S. (R+M/2) roughly equaled 97-98 RON in the U.K. Note that the MON number is drastically lower than the RON number. If I remember correctly the MON for 97-98 RON is somewhere around 87.

Hope this helps.

Richard Smith

I also went through the octane issue after installing a UK replacement engine and being advise to use 97 octane fuel if available. The rule of thumb I saw cited was that adding 5 to North American octane ratings gave a rough equiv to the UK values. The article at the following link made good reading http://www.mgcars.org.uk/mgcc/sf00403.htm

Regards, Barry
B.J. Quartermaine

Correction to the link:
http://www.mgcars.org.uk/mgcc/sf/000403.htm
B.J. Quartermaine

This thread was discussed between 16/08/2002 and 18/08/2002

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