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High vs Low Compression Engines

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GlennMGB Avatar
GlennMGB Silver Member Glenn G
Fort Worth, TX, USA   USA
1967 MG MGB GT "Rose"
Quote: Right now, I need to figure out what I have. Does that sound about right?

Yes, of course that's a good idea. But if you just want to regain hero status, tell us what the problems are. Does it start? Does it miss? Backfire? Does it hesitate on acceleration? Runs, but no power? Can you even drive it?

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Jim Klaas Jim K
Hood River, OR, USA   USA
I'm either going to pull the head or get a camera and find out what I have.



Edited 4 time(s). Last edit at 2018-05-15 06:52 PM by Jim Klaas.

riley1489 Avatar
riley1489 Gold Member Bruce H
Great White North, QC, Canada   CAN
1953 Jaguar XK120
1959 Riley 1.5 "King George"
1973 MG MGB
Jim,
I have no idea anymore what this thread is all about. Forget about the spare engine! forget about the last 15 years!

A bit of tough love;
Hero? who cares. Let your ignorance, (learning curve expected) and ego aside and read and learn. The answers to your queries have been provided but you have provided segue every time.confused smiley

First thing to do is answer Glenn G questions he posted earlier, (below), for the current engine installed now, as is, instrumented and all.
"
1) Does it start?
2) Does it miss?
3) Backfire?
4) Does it hesitate on acceleration?
5) Runs, but no power?
6) Can you even drive it? "


we can help but of you need to focus to allow us.

B



Check your ego Amigo!

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lgorg Avatar
lgorg Larry Gorg
Renton, WA, USA   USA
1966 MG MGB "Robbie"
In reply to a post by riley1489 Jim,
I have no idea anymore what this thread is all about. Forget about the spare engine! forget about the last 15 years!

A bit of tough love;
Hero? who cares. Let your ignorance, (learning curve expected) and ego aside and read and learn. The answers to your queries have been provided but you have provided segue every time.confused smiley

First thing to do is answer Glenn G questions he posted earlier, (below), for the current engine installed now, as is, instrumented and all.
"
1) Does it start?
2) Does it miss?
3) Backfire?
4) Does it hesitate on acceleration?
5) Runs, but no power?
6) Can you even drive it? "

we can help but of you need to focus to allow us.

B

Before this takes off in another direction, most of the information that is requested, is already known from his previous...mission continues... post. The OP's posts seem to wander which is ok as he is learning, and this thread originally started off, with the OP asking about what he had, a low or high compression engine. Without an engine tag, its hard to figure out. That is the reason for looking under the head to find out what he's got. The spare engine is what really should be under his hood, but it needs a rebuild. Up until this point, it has been the consensus that he has a mismatch between the block and head. Late low compression block with deep dish pistons and an early high cc combustion chamber. But the only way to really find out, is to pull the head....

Jim Klaas Jim K
Hood River, OR, USA   USA
1) Does it start? yes....with about half choke, after it warms up it will idle at about 1000 rpm
2) Does it miss? ...with new plugs it starts and idles well after warming up for a bit
3) Backfire?it almost feels like it is starved for fuel if I accerate or if I rev the engine...but usually I can slowly increase rpms..it will sometimes during a quick rev backfire through the carbs..which I think means it is starving for fuel.
4) Does it hesitate on acceleration? yes, and might even drop off and die.
5) Runs, but no power? runs...lack of power until about 2000 rpms at 2000 rpms it seems to behave better but not great..plugs foul...
6) Can you even drive it? " maybe a short distance...but the plugs will foul with dry carbon and it wouldn't run that well.


I have also tried what I would consider extreme adjustments to the mixture with not a lot of difference. I have started at 1.75mm jet deth and tried down to 1mm and all the way down to 5mm. I am sure there was a difference but not much.

Timing- seems ok ....I have tried above and below about 12-15 degrees...with new plugs I would say it ran the smoothest in that range.

When someone asked what engine, I realizedI had assumed it was stock. Which is terrible thing to assume. I also don't know if it ever ran well or ran at all after the block was changed.

I agree the mission has wandered off on in a new direction.....a side trip to figure out what engine block and head I have. At some point somebody asked me and I didn't know. It seemed like a good thing to figure out. Removing the head to see what I have is a fairly new plan. I was considering getting a mentioned boroscope for $32 and avoid takng the head off and buying a new gasket. For whatever reason, I thought the numbers on the block would tell me even if the ID tag was missing. Again, a pretty lame assumption on my part.

Jim



Edited 5 time(s). Last edit at 2018-05-15 05:47 PM by Jim Klaas.

dickmoritz Avatar
dickmoritz Platinum Member Dick Moritz
Philly 'burbs, PA, USA   USA
Jim,

The high/low compression deal is not as complex as you might think.

It's an over-simplification, but this should help you understand. In general, the smaller the combustion area in our engines, the higher the compression ratio. And higher compression ratio translates loosely into more power.

MGBs were built with two different piston designs -- early MGBs, pre-18V, were built with shallow-dish pistons, these pistons had a dish that's 3/16 deep in the center. The capacity of this dish is about 6cc's.

From about 1972 on through the end of production, MGB engines had deep-dish pistons, which are 3/8 deep in the center. The capacity of this dish is larger, at about 16cc's.

OK so far?

For the sake of this discussion, the remainder of the combustion area is the combustion chamber in the cylinder head. In other words, if you turned a cylinder head upside down, with valves and spark plug in place, and measured the capacity of the combustion chamber, this is what is referred to as cc-ing the head -- that is, measuring the volume of the combustion chamber in the cylinder head.

Early MGBs used cylinder heads, including the 12H906, 1326, and 2389, which had a combustion chamber volume of about 43cc's.

The 18V engines used from 1972 through the end of production usually used one of two cylinder heads -- the 12H2923 and the CAM1106. Each of these heads had a combustion chamber volume of about 38cc's.

Still with me?

OK, the early MGBs used shallow dish pistons (6cc's) and cylinder heads with the larger combustion chamber (43cc's). This combination yielded a compression ratio of about 8.8:1, which is fairly favorable.

Starting with the 18V engine around 1972, the switch was made to deep-dish pistons (16cc's) and cylinder heads with a smaller combustion chamber of 38cc's. So while the combustion chamber in the head was five cc's smaller, the deeper dish in the pistons was ten cc's larger. So this was a net loss of about 5cc's, resulting in a reduced compression ratio of 8.0:1 and, coupled with the addition of smog controls and the dreaded Zenith-Stromberg carburetor in 1975, power was greatly reduced from previous levels.

Still there?

Given the above information, we have the opportunity to mix and match. There are two options here.

First, we could use late deep-dish pistons with the early larger chamber cylinder head. This yields a dreadfully low compression ratio of about 7.4:1.

However, on the other hand, if we couple the early shallow-dish pistons with the later cylinder head with the smaller combustion chamber, we end up with a delightful compression ratio of about 9.4:1 without making any other modifications. This a particularly favorable sweet spot for our engines, in that it affords a nice and noticeable increase in power, and will run quite happily on middle grade fuel, and probably will run on regular without a problem.

This last combination is the one that most folks opt for, since it is easy, relatively inexpensive, and affords immediate and noticeable increases in power. It also is the first step in a series of steps that, performed in sequence, continue to increase power output while maintaining complete and comfortable streetability.

Beyond increasing the compression, the next step in search of more grunt should be porting and polishing the cylinder head so the engine can breathe better, after which you can add a street performance camshaft.

After that, assuming you're already running dual SU carbs, the only other thing you might choose to do is to send your distributor off to Jeff Schlemmer at Advanced Distributors to have him custom-rebuild your distributor for your specific engine configuration.


There. Wasn't that easy?

BTW, there'll be a test Monday morning... cool smiley

Dick



Errabundi Saepe, Semper Certi
(Often wrong, but always certain)

Jim Klaas Jim K
Hood River, OR, USA   USA
Yup, I got all that....and it makes perfect sense. What was completly new to me was that there was a chance of a mismatch. I went from adjusting the carbs being the problem to....wait a second, there are different stock compression ratios...until about 12 hours ago, I had no idea there were flat vs dished pistions and I had no idea there was even the possibility of a mismatch. I was blissfully leaving the innards alone. I have changed pistons on my motorcycles but only ever increased the size not the shape of the top. Totally makes sense... It is entirelly possible, I am doing something else wrong but even so, I should know what is in there. The scopes that wifi to a smart phone are so cheap it would be crazy not to get one and look inside. I'm curious how much I can see for $27.

Jeff did the distributor

Jim
Detention



Edited 2 time(s). Last edit at 2018-05-15 06:53 PM by Jim Klaas.

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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
Jim - What ever your head block combination its certain that your carbs are totally UNtuned. There are NO acceptable "extreme adjustments" to SU carbs, period. I'm wondering what you are relying on in the way of a 'manual'. If you are relying on your prior experience, you're likely wasting a lot of time.

SU carbs are the simplest that there are. Following accepted procedures, they can be refurbished (if needed) and tuned to create a 'driveable' B, even with lousy compression. There are plenty of Bs crying out for an engine rebuild, even worn "low compression" late 18Vs, that are driven regularly on the street. You might try to focus your attention to tuning what you have before proceeding with your chosen path, so far.

Jim Klaas Jim K
Hood River, OR, USA   USA
Carbs---When I removed the HS4's I looked them over and besides the expected gunk in the bowls I didn't expect to see the jet depth so deep 3mm on one and over 9mm on the other one. The information I am using are two books. HIFs are mounted now: not leaking, correct float bowl height at 1mm below casting and the starting jet depths I have tried are 1mm to 1.75 mm below bridge. Carb experience comes from a few old motocycles both single and multiple setups. I'm not an expert but I always got them to run pretty well. The path now is to figure out what engine pistons I have but I am not ruling out the carbs. Speaking of carbs and pistons.....the dash pot pistons drop in 4.5-5 seconds but seem to be a matched set. I think Twist said time should be little longer...actually, I am not ruling out anything.

I have taken this out on the road, it ran....I would say it ran better once it got going and hand lousy pickup. It felt like it wasn't getting enough gas.

I think the ball is in my court....and I will pull the head or use a camera to see what is inside. I will go over the information I have to see if I missed something.

Jim



Edited 1 time(s). Last edit at 2018-05-15 10:57 PM by Jim Klaas.

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78 B PaceCar Avatar
78 B PaceCar George Funderburke
Kennesaw, GA, USA   USA
Jim, A lot of good information has been shared on this post. I would like to add that I had an opportunity to actually tune one of these engines with this configuration. In the end, the engine idled very well and performed okay. It didn't have the power of the typical 18V motor, but actually ran okay. If you compared it to 69 with the 18GH motor, you could tell the difference under heavy acceleration. If you had a new B driver that didn't know the difference, he would probably thinks it drives okay. The difference in your engine and the engine that I tuned, it had the HS4 carbs and for me it was easier to deal with. Running the HIF's would probably work, but I prefer the HS4 because of the mixture control on the HS4 carbs and the current engine configuration. Also, the HS4's are simple carbs to work on. The HIF's can perform very well, but if not properly cleaned and adjusted can give you a real headache. Don't give on what have. It sound like some tuning is in order.

I hope this post doesn't confuse you. I just wanted to point out that the 7.4:1 compression will run okay. Your compression test tells me that all cylinders are close, therefore you should be able to get it run okay.

Down the road maybe consider rebuilding the 69 motor and reinstalling it. I have a 69 B that runs very well with the original engine (rebuilt). You will definitely notice the difference in performance with that engine versus the engine currently installed.

Good luck.

Rick Fawthrop Avatar
Rick Fawthrop Gold Member Richard Fawthrop
Seattle, WA, USA   USA
The 125 lbs compression is normal for low compression.
Higher compression pistons would yield about 140 lbs.
If it were me I would set the jet tube down .060 down from the top of the bridge.
Set the total timing at 32 degrees at 3,000 rpm.
And then take a wrench to the drain plug on the tank and take a fuel sample.
But probably just drain the tank and filter and check for rust.
I think you are close, hang in there.

benhutcherson Avatar
benhutcherson Gold Member Ben Hutcherson
Louisville/Frankfort, KY, USA   USA
1970 MG MGB
I agree with Rick that the problems you're describing seem exactly like what I've experience both with the timing off and with bad gas.

Straighten out your timing, and THEN go to the carburetors.

albeegreen1 Avatar
albeegreen1 Gold Member bob tresch
bordentown, NJ, USA   USA
1972 MG MGB MkIII "ALBERT"
I got an interesting dig from this. Straight line block letters for 12H3503 were produced after 75. 72 to late 74 18V's had no block clock. Also, I think also a good LED flashlight can see the pistons. Use a feeler. Jim I think your becoming consumed with it too fast. Get a Bentley shop manual and sleep with it. Your in for a real lesson like us all had at some point. Cheers!



"Be yourself, because everybody else is already taken". - Oscar Wilde.

"If wishes were horses, beggars would ride" - Phineas Quimby.

GlennMGB Avatar
GlennMGB Silver Member Glenn G
Fort Worth, TX, USA   USA
1967 MG MGB GT "Rose"
Quote: I apparently have a low compression engine bottom end with the heads off a high compression engine. Does this work?

Yes. Now, moving on . . .

Quote: 1) Does it start? yes....with about half choke, after it warms up it will idle at about 1000 rpm
2) Does it miss? ...with new plugs it starts and idles well after warming up for a bit
3) Backfire?it almost feels like it is starved for fuel if I accerate or if I rev the engine...but usually I can slowly increase rpms..it will sometimes during a quick rev backfire through the carbs..which I think means it is starving for fuel.
4) Does it hesitate on acceleration? yes, and might even drop off and die.
5) Runs, but no power? runs...lack of power until about 2000 rpms at 2000 rpms it seems to behave better but not great..plugs foul...
6) Can you even drive it? " maybe a short distance...but the plugs will foul with dry carbon and it wouldn't run that well.

I agree with Ben. Double-check the ignition timing at idle and at 3,000 rpm. At idle, it should be around 10-15°BTDC. At 3,000 rpm, it should be around 30-34°BTDC. But the real problem seems to be that your carburetors are out of adjustment. I'd start by checking the metering needles. Make sure they are installed correctly and check the marking to be sure they are stock needles, not extra rich. Dry carbon fouling of the plugs means the mixture is too rich. If all of the above checks out, then adjust the mixture leaner. You will have to use the choke to get the engine started, but that's what it is for.

Other things that could produce a rich mixture:

- Overadjusting to compensate for an air leak (as Barry suggests).
- Fuel pump providing too much pressure.
- Float level too high.
- Float sunk (lost buoyancy).
- Needle upside down in the fuel inlet needle-and-seat assembly.



Edited 3 time(s). Last edit at 2018-05-16 04:05 PM by GlennMGB.

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
I promise (at least to myself) that I'll stay quiet after one more point. When properly tuned, the mixture needle settings should be the same or darn close. One carb set at about 0.050" (1 mm) and the other set at about 0.085 (1.75 mm) should not be possibly correct. Best guess is the richer carb is drawing excess air from somewhere.

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