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MG MGB Technical - Uber-compression..beware?

I recently replaced pistons on an 18V engine. Being cheap, I opted for the best deal which happened to be 8.8:1's from the lovely folks at BPNorthwest.

My compression readings now are -evenly- at 180psi. Since that's about 15% over 160 I'm feeling a bit queasy.

I've read that some people have been doing this on purpose with no major issues when using 92octane pump gas.

I'm fine with that, but worried how this will affect my engine. Should I consider trading in my head for a pre 18V version or just lean back and enjoy the power?

Also, should I get new needles with my HS-4's and modify timing differently than before? I'm seeming to have problems getting the engine to run rich enough (may be unrelated to pistons/CR)

thoughts?
philip
Philip

Don't worry about the readings, they are strictly relative and may say more about your battery and starter than anything else. What year is it? US cars were 8.8 though 71 anyway, which covers the ones that came with HS carbs. If you haven't done anything else like cut large amounts off the head or block, you have what most of the world got outside the US. If it has emission stuff on it, use 70 -71 settings, otherwise use pre 68 or set it up with UK spec needles and distributor settings, depending on whether the carbs are fixed or bias needle type. Have you changed anything else, like the cam? You don't want the cam from the single ZS engines, it's a pig. But you should not have trouble getting it rich enough, check for air leaks or wrongly assembled carbs.
FRM
http://www.usachoice.net/gofanu
FR Millmore

The early (HC) heads have 43cc chambers, the later (LC) heads have 38cc chambers. When mixing a late head with early pistons you'll get a fairly high compression ratio (9.7:1 IIRC). Conversely, if you mix an early head with late pistons your CR will drop to around 7:1.

I accidentaly ran into this issue years ago (LC head, HC pistons). The car would detonate badly on acceleration running up steep hills even though I was using 91-92 octane gas. Retarding the timing helped, but it was then down on power.

You may want to direct your question(s) to Peter Burgess or Mike Brown. I suspect they'll have the experience you need to sort this out satisfactorily.

Good luck,
Tom
Tom Sotomayor

Tom,
Interesting. At what point are you making the distinction between early and late heads? Is it between the "beaked" chamber of the 18G* and the open chamber 18V. or at the 8.8 to 8.0 change on the 18V? All a good illustration of why it is necessary to measure instead of guessing or relying on rumors.
FRM
http://www.usachoice.net/gofanu
FR Millmore

The car is a 73B with, indeed a slightly skimmed LC head and now HC (8.75:1) pistons.

i also changed the HiF carbs to HSU-4's as on the earlier engines and put a slightly hotter cam on it a while back. (nothing very dramatic.) Idle was never a problem with the LC pistons before.

i think i can handle the retarding/power loss as well as purchasing higher octane gas.

My main issue is engine life. Will this affect it badly? Should I just not worry, if i don't have pinking?

p
philip

FRM,
I've seen the difference in chambers at the start of the LC (18V) engines. The earlier heart shaped or "beaked" chamber in the 18G series have the larger chamber.

Side note:
One way of approximating the vintage of the 18G heads is to look in the rocker area at the casting "roundrel" - it has a date code built in. I don't normally pay attention to the month and day, but the year is easy to establish. Blocks have this too.

Philip,
As long as you don't have detonation, I don't see any issue with the higher compression. However, the engine I had was retired for a while then redone with the "proper" head. It's now done many thousands of reliable miles in a club member's GT. For the last year or so with the new Moss SC.

Tom
Tom Sotomayor

Tom, thanks, as I expected.
Phillip, You want some version of a 71 or earlier distributor with this. The pre 72 distributors have approximately 30 deg mechanical advance, 72 on are near 40 deg, and it comes in at lower RPM, which is what is causing your trouble. Assuming you have the biased needle HS, I would try the 70 needles and distributor/timing. If you have fixed needles, go back to the 65-67 setup. It would certainly be best to set it up on a rolling road, if you could find one with a competent operator.
None of this will hurt the engine if it's right, but too much advance (eats pistons) or retard (eats valves and causes overheating inside the engine) will do evil and kill power/economy.
FRM
http://www.usachoice.net/gofanu
FR Millmore


thanks for the info. i feel more comfortable now.

Do i need a new distributor (currently running a stock 25d4) or do you know if weights/springs are available to change the curve?

As for needles I do have the biased needles, but not sure if they're 70. Is there a way to tell?


pf
Philip

Distributor parts are very much a problem, but there are people who either have the bits or know how to make do. I believe that John Twist can recurve them, and folks on this board can no doubt suggest others. BTW, the weights are the same, it is the breaker cam and springs and vacuum advance unit that change.
I would recommend either getting someone who has a lot of actual experience to recurve it, or find a good used unit from an earlier HC car. There is a fair amount of contradictory information around, the aftermarket parts sellers are pretty cavalier about substituting pieces, and I have found a number of US "rebuilt" units that have completely wrong parts in them compared to the specification number on the distributor. That is the 5 digit number stamped on the side, not the 25D4 which is the model of all these units.
The needles are marked, but you must remove them to read the marks on the needle base. If your carbs still have the ID tags on them, it will tell you what needles they had in them, which could of course have been changed.
In all, it is necessary to either get someone knowledgeable to set it all up, or to arrive at a factory setup that is for an engine as close to yours as possible. For starters you can study catalogs to try to decide what you have, then post that along with any other changes like air filters, whether the vacuum advance is off the manifold or carb, what cam you are using, etc.
FRM
http://www.usachoice.net/gofanu
FR Millmore

This thread was discussed between 01/10/2004 and 02/10/2004

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