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MG MGB Technical - Matching pistons and heads

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"The use of 8.7:1 CR pistons, with cyl/heads designed for use with 8:1 CR pistons, will result in abnormally HIGH CR's unsuitable for street usage with today's low octane fuel unless octane boost is added; otherwise, detonation and engine damage will occur."
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I pulled (and saved) the above comment from a webpage some time ago - can't remember where. I'm doing a rebuild now on a engine out of a '80 B. Cylinder bores were in excellent shape and will be left standard. Instead of using 16.2cc deep dish pistons (OE standard), I'm installing shallow dish 6.2cc pistons (OE for '63-'71 B's). If I use these pistons with the OE head (CAM1106, approx. chamber volume = 39cc), my CR will increase from 8:1 to near 9.4:1. It has been said by some, for trouble free day to day driving, keep CR's close to 9:1.

Just wondering, will I have any problems with this higher CR set-up, or might it be better to change over to an earlier style head (e.g., 12H906 head, approx. chamber volume = 43cc) to give a CR of 8.8 (stock for '63-'71 B's).
Steve Buchina

Steve-
9.5:1 is about as high as you can go without having some rather sophisticated combustion chamber modifications made by Peter Burgess. However, you should be fine if you run the engine on 93 Octane fuel and take the time to keep the engine in a good state of tune.

Both the CAM 1106 and the 12H 906 head castings use intake valves of the same diameter (1.565"), so there would be no performance advantage to switching heads. In fact, preignition problems should be less likely with the CAM 1106 head due to its redesigned coolant passages giving better heat transfer.
Steve S.

Steve-
I forgot to mention that the OE North American Market Hepolite pistons in 8.8:1 Compression ratio use a floating wristpin. If your engine has a CAM 1106 head as Original Equipment, then you have the later connecting rods that use a press-fitted wristpin. Simply have the bores of the small end of the connecting rods enlarged to accept the bushing of the earlier connecting rods and then you should be good to go. If you don't have access to a shop that can do this, simply order the later UK/European Market 8.8:1 pistons for the 18V engines that use the press-fitted wristpin.
Steve S.

I had my engine rebuilt nearly 2 yrs ago - its a 73 18V with 12H2923 head casting. Original CR was 8.0:1. I opted for the shallow dish pistons, which by my calculation resulted in 9.5:1 CR. The cam was modified to a "fast-road" profile. I find that I need to stick with the highest octane fuel (91 is what it is rated here in Canada) that Esso provides to avoid pinging. Engine runs and pulls great - a huge difference over the pre-rebuild state (how much improvement is from CR increase and how much is from renewing a tired engine, is hard to say).
Erick Vesterback

Steve S. and Erick,
Had my machinist checked the wristpins diameters today - they're the same as the ones that were pressed out. The pistons I'm going to use are Hepolite brand, but come with 4 rings instead of three and have grooves for using retaining clips (supplied in box, if required). I don't have a part number for them right now (pistons are at the shop, perhaps PN 17625), but I'm guessing these are replacement pistons made to fit either floating or press-fit rods. Skirt lengths aren't much different from the original pistons (i.e., not anywhere near the length of the older 18GB style pistons).

The 12H906 head I cite was just an example of what I could use to lower the CR back down to around 8.8. I have others in my inventory, both with and without air injector ports and some with the larger intake valves (e.g., 12H2923 head). Would the a larger intake valve offer any more performance being that it will remain stock (i.e., no Peter Burgess work)? Should add, this engine will be totally de-smogged, fitted with a stock cam, early stock exhaust manifold (e.g., '66B), twin SU's (AUD135) and probably a 25D distributor.

The engine is going in my wife's newly acquired '80B along with a new OD tranny via John Esposito. She doesn't want to push it either way - just ride with confidence.
Steve Buchina

Steve-
The larger intake valve of the 12H 2923 head would definitely offer greater tuning potential. The AUD 135 carburetors use a #21 needle, but the bigger valve head used an AAU needle. However, the camshaft timing has an influence on the needle required. It is not commonly understood that the B Series engine underwent changes to its valve timing during the course of its career in the MGB. Originally, the engine used a duplex (double row) roller-type camshaft drive chain and sprockets. The specifications of this camshaft (Original Equipment Part # 88G 303) were:

Opens @ Closes @
Intake Valve 16° BTDC 56° ABDC
Exhaust Valve 51° BBDC 21° ATDC

This camshaft had an intake a lobe center of 110° ATDC and an exhaust lobe center of 105° BTDC. The LSA (Lobe Separation Angle) was 107.5°. The camshaft was retarded @ 2.5° ATDC.

In October of 1972 the camshaft drive chain and sprockets were changed from a duplex (dual-row) drive chain system to a Simplex (single row) drive chain system on the North American Market 18V-672-Z-L and 18V-673-Z-L engines. While the camshaft lobe profile design was unaltered, the keyway of the sprocket (Original Equipment Part # 12H 4200) was advanced another 2.25° in order to increase midrange torque output. It should be noted that due to the larger 1.625" intake valves of the 12H 2923 head casting which this valve timing was used, the power output at high engine speeds remained unaltered.

Opens @ Closes @
Intake Valve 18.25° BTDC 58.25° ABDC
Exhaust Valve 53.25° BBDC 23.25° ATDC

In December 1974, Rubber Bumper cars for the North American Market with 18V-797-AE-L, 18V-798-AE-L, 18V-801AE, and 18V-802AE engines received a new camshaft (Original Equipment Part # CAM 1156). The specifications of this camshaft were:

Opens @ Closes @
Intake Valve 8° BTDC 42° ABDC
Exhaust Valve 54° BBDC 18° ATDC

This revised, shorter-duration camshaft had an intake lobe center of 107° ATDC and an exhaust lobe center of 108°. The LSA (Lobe Separation Angle) remained 107.5°. The camshaft was advanced to .5° BTDC.

Starting during the month of June of 1977 cars with 18V-846-F-H and 18V-847-F-H engines for the UK Home and Export markets (not North America) received a new camshaft sprocket (Original Equipment Part # ) that further advanced the camshaft timing by another 1°. The specifications of this camshaft were:

Opens @ Closes @
Intake Valve 20° BTDC 52° ABDC
Exhaust Valve 55° BBDC 17° ATDC

The intake lobe center was 106° ATDC, the exhaust lobe center was 109° BTDC. The LSA (Lobe Separation Angle) remained 107.5°. The camshaft was advanced to 1.5° BTDC.

If you choose to install the 12H2923 head onto your wife's engine, be aware that it does not have offset oil feed in the rear rocker shaft pedestal, so you will need to use the earlier rocker shaft pedestal.
Steve S.

Steve S.,
Years ago, I took on classes in classical and quantum mechanics. How I passed is still a puzzle to me. Nearly forty years later, I find myself in a similar boat - translating and comparing cam profiles. What do they really mean? Today, it's mainly just a bunch of numbers - little comprehension on my part as to effect on engine performance.

The cam I mentioned above is stock - at least, I was told such. Stock to what year, I don't know. Unfortunately, I don't have the spec sheet for it (bought at a going out of business sale). It is new in the box, but I don't see any part numbers. I'm hoping my machinist can determine its profile. Another cam, picked up along the way, came with the following description. Would you (anyone) care to interpret?
Would this cam be a good addition to my rebuild?
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Opens @ Closes @
Intake Valve 22° BTDC 54° ABDC
Exhaust Valve 58° BBDC 18° ATDC

This camshaft is described (according to spec sheet):
Intake lobe center 106°
Exhaust lobe center 110°
Lobe Centers 108°
OVLP 40°

Duration at 0.050" Tappet Height:
Intake 211°
Exhaust 211°

Lobe Lift:
Intake 0.257" Exhaust 0.257"

Valve Lift:
Intake 0.360" Exhaust 0.360"

Valve Setting:
Intake 0.014" (Hot) Exhaust 0.016" (Hot)

Check Timing at 0.020" Tappet Height

Application: High Torque

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"The larger intake valve of the 12H 2923 head would definitely offer greater tuning potential."

In what way and how much?
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"The AUD 135 carburetors use a #21 needle, but the bigger valve head used an AAU needle."

All of the AUD135 carbs in my inventory were fitted with either #5 or #6 needles. I do have other needles in my stock - just never used (experimented with) them. The #5's always seemed to work well for me.
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"In October of 1972 the camshaft drive chain and sprockets were changed from a duplex (dual-row) drive chain system to a Simplex (single row) drive chain system ..... While the camshaft lobe profile design was unaltered, the keyway of the sprocket (Original Equipment Part # 12H 4200) was advanced another 2.25° in order to increase midrange torque output. It should be noted that due to the larger 1.625" intake valves of the 12H 2923 head casting which this valve timing was used, the power output at high engine speeds remained unaltered."

Interesting - knew the chain and sprockets were change, but didn't realize keyway was advanced. I was planning on use the double-chain set-up in this rebuild. Should I, if I go with the 12H2923 head? Guess I could use one of those offset cam keys or go with an adjustable sprocket set - keys would be cheaper. But, does one really notice a difference - a couple degrees either way? I wonder if we, sometimes, try too hard for perfection.
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"If you choose to install the 12H2923 head onto your wife's engine, be aware that it does not have offset oil feed in the rear rocker shaft pedestal, so you will need to use the earlier rocker shaft pedestal."

I was aware of this. My plan is to use a rebuilt rocker assembly (e.g., '66 model) that has oil galleys through the rocker arms. Don't know if the extra oil portals are really better, but can't see them as a negative.
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Steve S.,
THANKS again for your comments and added cam info. I'm sure it will assist my machinist who has, up to this point, never worked a 'B engine. His primary experience and work is with Chevy/Ford NASCAR type racing engines.
Steve Buchina

The increased torque output at low engine speeds produced by your custom camshaft would be fine for a heavy vehicle around town or in mountainous areas, but seems an odd choice for an MGB.

The bigger valve does indeed offer greater tuning potential, but just how much depends on what you do with the rest of the engine and how much you're willing to spend. In short, power costs money, how much do you want? If funds are short, smoothing the port walls with a flap sander, and matching the volume of the combustion chambers you can do yourself, and any decent shop can do a three angle grind on the valves and seats for very reasonable expense.

Yes, the difference is enough to be noticed. The factory chose to retard the timing in order to increase midrange torque, which increased appeal to most drivers. How often do most drivers go into the top end of the powerband vs normal low-and-midrange driving?
Steve S.

The chance is great that I'm in over my head here, but isn't the compression ratio just a generic ballpark guess at the safe level of street operation of an engine? Detonation is more a product of cylinder pressures generated by a high compression piston/head combo when used with a cam that had the right duration to produce high cylinder pressures, right? A higher duration cam will generate lower cylinder pressures - by as much as 50 psi - wich will reduce if not eliminate the chance of detonation in an engine that *could* be 9.5:1 compression. Everyone's goal is to chose a cam that works perfectly with their engine, but the cylinder pressures generated never really seem to be considered. Part of that is due to the specific nature of an engine build - no two will likely offer the exact same pressures. A great street engine will produce 160-190 psi cylinder pressure, but that has to do with how the cam is degreed as well as the airflow of the head, intake, and exhaust systems. Or am I just blowing smoke?
Jeff Schlemmer

Jeff-
No, you're not blowing smoke at all. What you're describing is what is called "Effective Compression Ratio", also called "ECR" for short.

Duration certainly does effect the actual compression inside the engine. It should be noted that Geometric Compression Ratio figures are somewhat misleading. To most people they imply that the cylinders are completely filled with fuel-air mixture when compression begins, and that therefore the effective amount of compression is the same as that of the Geometric Compression Ratio. However, this is not the case. Many factors influence the Volumetric Efficiency of an engine, among them the connecting rod/stroke ratio, valve size, valve timing, and flow rate of the ports. This volumetric inefficiency that is inherent to all normally aspirated engines in turn results in an Effective Compression Ratio that is altogether quite different from an engine’s Geometric Compression Ratio. In terms of the effect of duration and timing:

43cc Combustion Chamber with 6.5cc-dish Piston @ Original Equipment Specification .040” Piston Crown-to-Head Clearance Depth:
Total Displacement 1812 cc
Bore Oversize +.020”
Swept Volume 455.5 cc
Clearance Volume @ .040”” Depth 5.2 cc
Head Volume 43 cc
Dish Volume 6.5 cc
Gasket Volume 3.2 cc
Eyebrow Volume .4 cc
Geometric Compression Ratio 8.9 : 1
Effective Compression Ratio (Original Equipment 88G 303 Camshaft) 6.7 : 1
Effective Compression Ratio (Original Equipment CAM 1156 Camshaft) 7.5 : 1
Effective Compression Ratio (Piper BP 255 Camshaft) 6.7 : 1
Effective Compression Ratio (Piper BP 270 Camshaft) 6.1 : 1
Effective Compression Ratio (Piper BP 285 Camshaft) 6.1 : 1

In addition, it is the Effective Compression Ratio that reflects how much fuel-air mixture is actually getting into the engine and being compressed, and thus is the determining factor of the Octane requirements of the engine. To point out a few examples:

A 7.0:1 Effective Compression Ratio requires 92-Octane fuel.

A 7.5:1 Effective Compression Ratio requires frequent attention to ignition timing and carburetion tuning in order to run on 92-Octane fuel.

Over 7.5:1 Effective Compression Ratio requires the above, as well as that the Octane level be boosted with additives.

Over 8.0:1 Effective Compression Ratio requires racing fuel.

You still might run into problems with either preignition, or that great destroyer of engines (God forbid!), detonation. If that should happen, do not become frustrated or fall into a state of despair. Instead, run down to your friendly local paint store and buy a big can of Toluene. If you mix it with 93 Octane Premium gasoline you will get the following results:

10% Toluene + 90% 93-Octane Gasoline = 95.2-Octane

20% Toluene + 80% 93-Octane Gasoline =.97.4-Octane

30% Toluene + 70% 93-Octane Gasoline = 99.6-Octane
Steve S.

This thread was discussed between 20/11/2007 and 01/12/2007

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