MG MGB and MGB GT Tech Talk
My engine is in the machine shop and its looking like we will have to go to 0.060" oversize pistons. It was already bored out to 0.030" and there has been quite a bit of wear such that 0.040" might be a bit too close for comfort.
My question is - hsould i be getting high comp or low comp pistons?
My car is a 76 (Early RB) 18v 5 bearing engine.
Also, are the ones that moss europe list ok for the job?
Greg,
I'll let the engine experts chime in but my first thought is the answer depends on what else you're doing or not doing to the rest of the engine. Rebuilding an engine is a bit of a balancing act in that things need to match up. For example, the exhaust ports in the head are the weak link in the B's breathing. Absent having the head ported properly, there's not a lot to be gained in doing other engine mods. Hap, Kelvin, Dick and others are much more knowledgable.
Steve
Greg, here in the US, for good long while, you could't find the 8.0 to 1 piston, they started to resurface now with people doing supercharging, the HC pistons, are not really that high, at 8.8 to 1, and here in the US, thats doesn't even regulate them to run fuel octanes above regular unleaded, many folls will alos shave the block and/or head with the 8.8 to 1 piston go even higher. Long story short, with your later car, the 8.8 to 1 HC pistons would be a good choice for you, and restore some needed horsepower. Hope this helps.
Thanks Steve and Hap!
Hap - the head i have has an unleaded conversion done, but everythign else in the engine will be pretty standard (apart from a very small skim of the head just to make sure its straight).
Im not going to be doing anything too major on the car engine other than put those pistons in, so if having the high comp pistons means that car can benefit from a wee bit of extra power, then that sounds good!
I presume it doesnt mean i can or cant run particular petrol grades here in the uk ? (i know, this is most likely a stupid question!)
One more question (sorry) : Which of the items on this page is it i need?
http://www.moss-europe.co.uk/Shop/ViewProducts.aspx?PlateIndexID=188
Would it be 12H5163H/60
and how would i be sure that that would be compitable with the con rods i have in the engine?
Hi, Greg. I agree, there's pretty much no down side to using the 8.8 pistons, since, as Hap correctly points out, that's still pretty low on the compression scale and should run just fine on 87/89 octane (R+M/2...) fuel. I'd also suggest switching to the double roller timing set, as that retards cam timing about 4 degrees, moving the torque curve up just a little higher in the rpm range, making for a more pleasant and rewarding driving experience...
Dick
Thanks Dick - there are actually two timing chains on my engine already - im guessing someone has most likely done that upgrade already?
Greg,
If you have two timing chains on your engine, then it's gotta be a Twin Cam... :D Actually it's one chain, but a double roller design. With the chain off, hold it extended in a horizontal position and look for sagging. If there's any noticeable sagging replace it. Practically never necessary to replace sprockets unless you can see visible wear. Regardless, replace the timing chain tensioner assembly, and be sure to use a gasket between the tensioner and the front engine plate.
Good luck and keep us posted...
Dick
I'm not an expert but wht about the head? Don't some MGB heads have a smaller combustion chamber thus effectively raising compression? Maybe you already took that into account.
Greg, if your car is a euro RHD MGB then you require HC pistons as you have a HC head with 42cc combustion chambers.
Only our US counterparts suffered from Low compression engines, all uk engines are HC unless they have come back from the US.
Just make sure you calculate your volumes (and compression ratios) with your head combustion chamber volume in mind.
As odd as it may seem, the low compression (18V) engine had a smaller combustion chamber than the high compression engine.
So - whether a piston is HC or LC depends on what head it is mated to. It is easy to get higher or lower compression than you anticipate.
Terry is correct.
Also, the cam is the achilles heel of the engine. And they aren't that expensive. Even the stock cam will restore a lot of performance if the existing cam has worn lobes.
warmly,
dave
Just to be clear - I'm not sure what the status of the Euro-spec cars were - the North American 18V engines were LC. Make sure you know what head was used on the European engines.
I have a head that is 37cc and no air injection ports. Unfortunately, someone ground the numbers off of it and so it is a mystery what it was on. That, on a .060 over block gave me almost 10:1 compression. I didn't know that all of the Euro heads are 42cc like the US market ones before 1972. That would give you a little higher than the 8.8to 1 because of the larger bore but only a few tenths of a point.
Greg,
some years ago, I had the same question and decided to use County flat top pistons with .060 over. It boosts the CR to 9.5:1 on an EC specs engine. The result was perfect and I love it so much, I would do it again with this same pistons on the next engine.
More crisp, less fuel consumption and very noticeable increase of power.
Ralph
Allan, I got a local buddy that I'm helping with engine build, he has #4735, just exactly how you describe, air pump bosses are there, but not drilled.
Ralph, I can get the flat top County piston right now for the MGB engine, but they cost more than 3x the cost of the 8.8 to 1 pistons, so less expensive to just resurface the head to get to 9.5-10.0 to 1 compression ratio, also if you use the flat top piston you have to try to mantain stock piston to bock deck height, I never ran the CR numbers on a flat top at stock deck height (aprox. .035-.040") but a flat top at near zero deck height with a 38cc head, with only a clean cut of less than .010" would put you at 11.0 to 1, that one I do know.