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High vs Low Compression Engines

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Jim Klaas Jim K
Hood River, OR, USA   USA
I apparently have a low compression engine bottom end with the heads off a high compression engine. Does this work? What things do I need to be aware of to make sure everything plays well together.

Jim

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RAY 67 TOURER Avatar
RAY 67 TOURER Ray Marloff
Fort Bragg, CA, USA   USA
1967 MG MGB "My Girl"
Do you have a 38 cc or a 43 cc head? The block itself can have dished, low compression, or flat high compression pistons. RAY

benhutcherson Avatar
benhutcherson Gold Member Ben Hutcherson
Louisville/Frankfort, KY, USA   USA
1970 MG MGB
Deep dish pistons(low compression engine)+a 42cc(high compression) head makes for a really low C/R in the 7.5:1 range or so. The reverse-shallow dish pistons(high compression) plus a 38cc head(low compression, 12H2923 or CAM1106) makes for a really nice C/R in the 9.5:1 range.

If indeed you have an 18V block with original pistons and a stock 12H906, 12H1326, or 12H2389 head(large chamber) you likely have a less than ideal combination. What are your absolute compression numbers?

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Jim Klaas Jim K
Hood River, OR, USA   USA
Ok, I will take some pictures of as many engine numbers as I can find and report back to you. This is somebody else's can of worms and despite my best efforts I can not get it to run well. I feel as if I am making a basic "will never admit it" mistake or I have a bigger problem. Something is not playing well. The history is a little murky as well. It is time to put the tools away and start checking a few numbers.



Thanks guys.....car is defn cool, but it needs to be a runner! At this point, the wife is growing skeptical and my 17 year old son has threatened to downgrade me from hero to sidekick....

Jim

ohlord Avatar
ohlord Gold Member Rob C
North of Seattle, N.W., USA   USA
1957 Land Rover Series I "EYEYIYI"
1971 MG MGB
1971 MG MGB "Bedouin 2"
Build for compression
Right now you current build as spec'd is a dog missing hind legs.



"I'm a long way gone down this wild road I'm on
It's gonna take me where I'm bound
It's a long way around"



"These are the days that must happen to you"

RD2 Radar/ Electronic Warfare Technician
Vietnam 1969-1972

tigger00 Avatar
tigger00 Rick O
Creedmoor, NC, USA   USA
1974 MG MGB "Queenie"
If you can, do a compression test before tearing anything apart
numbers in the 120-130 range low compression. 160 to 175 high compression
You can also see health of the engine with the numbers you get, at least top end seal , all the cylinders should be within 10%

barry s Avatar
barry s Silver Member Barry Stoll
Alexandria, VA, USA   USA
1972 MG MGB GT
1974 MG MGB
1976 Triumph TR6
1980 MG MGB
Remove the valve cover and look at the head for one of the numbers given by Ben in post #3. As to the block, a plate (1/2" by 3"winking smiley near the spark plugs will identify the block, if present. Also, please identify the year. A plate in the drivers door opening will provide the VIN and build date.

While you're at it pics of the engine bay would be good.



Edited 2 time(s). Last edit at 2018-05-14 05:22 PM by barry s.

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Jim Klaas Jim K
Hood River, OR, USA   USA
ok guys...lets see what I've got.

Bad News:
1. No Engine Plate
2. No Drivers Door ID

Good News: I took a lot of pictures of all the numbers I could find on the engine. As I am sure you all know, getting the number from behind the starter is ...ummm difficult.

FYI stuff: Full disclosure: I am pretty sure you know most of this ...HIFs are not stock. I am currently rebuilding the HS4s but these were a spare that I rebuilt. Distributor was rebuilt by Jeff Schlemmer. Vacuum advance on the HS4 is on the carb, vacuum advance on the HIFs is on the manifold. I may have to get a different vaccum advance unit. I did not repaint this car black and the guy that did should stick to the cross walks...they are much safer! I really don't know the most of the story. The car is a 1969 but under the Hood has been changed. I checked compression a month or so ago and the numbers were all within 10% of of about 125....if I remember. I concluded it was low compression and healthy...

UGH....ok two posts to get all the pictures. Sorry...apparently I reached my max.

Ok...look over the pictures and "be gentle"

Jim



Edited 1 time(s). Last edit at 2018-05-14 06:52 PM by Jim Klaas.


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Jim Klaas Jim K
Hood River, OR, USA   USA
More pics to figure out what the heck I am up against. I hope this helps....all part of the journey....

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Jim Klaas Jim K
Hood River, OR, USA   USA
Build for compression
Right now you current build as spec'd is a dog missing hind legs.


NO Scooby Snacks for THAT GUY! ....I am about to be downgraded to Sidekick...waaaaa I wanna be a hero in my 17 year old kids head....

dog missing hind legs....LOL! that is really funny in a tragic sort of way.....

The MGB looked EASY compared to the other rig currenlty on the "misfit toy" list. 1968 Nissan Patrol....

Jim



Edited 1 time(s). Last edit at 2018-05-14 06:55 PM by Jim Klaas.

Jim Klaas Jim K
Hood River, OR, USA   USA
A couple more pictures under the bonnet....and one of the kid that is about to downgrade me from hero to sidekick...WAAAAAAAAA! I don't wanna be a sidekick, I wannna be a hero!!!


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Steve S. Stephen Strange
Harrisonburg, VA, USA   USA
1972 MG MGB MkII "The Mouse Trap"
Well, your car number plate identifies it as a very late 1969 model. However, the 12H 3508 engine casting number is almost certainly an 18V engine, unless its a very late (1971) 18GK engine (Engine # 18GK RWe-H/14,365 and later) which used that engine block castingas well (although with studs to retain the main bearing caps, like all pre-18V engines). The 12H 2389 cylinder head casting was used on late 18GK engines. If it is a late-18GK engine, then the cylinder head has been replaced since it was cast in 1970. The SU HIF4 carburetors weren't introduced until the advent of the 18V engines, however.

Jim Klaas Jim K
Hood River, OR, USA   USA
(drum roll) ......and that means.....

a.) I am a happy camper

b.) I am toast and will need to do some major changes.... (where or where is that bottle of scotch)

Jim



Edited 1 time(s). Last edit at 2018-05-14 07:42 PM by Jim Klaas.

Steve S. Stephen Strange
Harrisonburg, VA, USA   USA
1972 MG MGB MkII "The Mouse Trap"
Jim-
Don'y worry, be happy! the 12H 3508 engine block casting is the stoutest of them all, using the stronger main bearing caps, and the 12H 2389 Cylinder head cast is the most resistant to warpage and cracking. The SU HIF4 carburetors are also the better carburetors, having distinct advantages while eliminating the worst problems of its predecessor, the SU HS4.

Jim Klaas Jim K
Hood River, OR, USA   USA
checking numbers....



Edited 2 time(s). Last edit at 2018-05-15 01:56 AM by Jim Klaas.

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