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Methods for increasing compression

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Nunyas

Nunyas

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Just curious. Like a good number of other people on these forums, I'd like to squeeze a bit more juice outa the B. I figger that improving the flow through the head is essential, but what can be done to increase compression?

Shaving the head down a bit, or is that just the easiest way to increase compression?
 
Flowed head and dual carbs will make a gazillion times more difference than raising compression alone. And it won't require uber-octane gasoline to run properly, either.

Then again, you're going to spend a lot of money rebuilding the engine to raise compression, so shy not leave it low and use it to your advantage with... a supercharger! /bcforum/images/%%GRAEMLIN_URL%%/laugh.gif
 
So many options, so little time... /bcforum/images/%%GRAEMLIN_URL%%/smirk.gif
 
Like my grandpa used to say, "Speed costs money, son...how fast do you want to go?"
 
Steve_S said:
Flowed head and dual carbs will make a gazillion times more difference than raising compression alone. And it won't require uber-octane gasoline to run properly, either.

Then again, you're going to spend a lot of money rebuilding the engine to raise compression, so shy not leave it low and use it to your advantage with... a supercharger! /bcforum/images/%%GRAEMLIN_URL%%/laugh.gif
Click to expand...
hmmmm... a Super is a load of money, and the price keeps going up!

I'm seriously considering having a flowed head built for my car, and while it's being built possibly have it skimmed to bump the compression ratio up from 8.0:1 to 9.0:1 or maybe even 10:1 if feasible/possible.

Incidentally, are there any repercussions (aside from possibly needing to run higher octane gas) for upping the compression ratio?
 
Getting the fuel in and out. Whole new induction scheme, 9.5 for a street engine would be a good "upper limit" IMO. profiled, flowed chambers, port matching and real induction devices too. Re-jetted twin HS4's come to mind. 40 DCOE as a "braggin' rights" unit....
 
tony barnhill said:
Like my grandpa used to say, "Speed costs money, son...how fast do you want to go?"
Click to expand...
Still very true in this day and age. However, some things will get ya more speed for the money spent than others.
 
A performance flowed head will cost about a grand. Add a set of rebuilt dual carbs and performance air filters for another $500. Include various small parts, fluids, paint, and whatever else you find needing attention while it's open and you're up to nearly 2 grand and a couple weeks down time. And after skimming the head, you still might have issues with the valves hitting the block, which means the block needs to be machined. Full engine rebuild, machine the block, big $$$. At least another $1.5K if you go cheap, and several weeks downtime. Add a cam while you're in there, high lift rockers, lightened flywheel, etc. Now you're up to $3-4K.

Congratulations, you just added 25 horsepower to your car!

Or, you call Moss and order a supercharger kit for $3K, open the bonnet on Friday evening, close it on Sunday afternoon and go for a drive.

Congratulations, you just added 25 horsepower to your car!
 
<div class="ubbcode-block"><div class="ubbcode-header">Quote:]Or, you call Moss and order a supercharger kit for $3K[/QUOTE]

STEVE!!!
 
Sorry. Habit. I live a short drive away from them. /bcforum/images/%%GRAEMLIN_URL%%/shocked.gif

(ducks under desk and pretends he isn't here)
 
Can do the same thing with a 948 but you get about 30 HP.

Head
carbs
exhaust
cam
flywheel
balance
over bore to .60 or 998cc

However, 30 HP on top of 44 stock is like a 68 percent improvement.

Beep beep, Bugeye comming through.
 
true, and 25 on top of my 50 is like 50% improvement /bcforum/images/%%GRAEMLIN_URL%%/smile.gif

Steve_S said:
Sorry. Habit. I live a short drive away from them. /bcforum/images/%%GRAEMLIN_URL%%/shocked.gif

(ducks under desk and pretends he isn't here)
Click to expand...

Additional incentive for ordering through Tony: no sales tax /bcforum/images/%%GRAEMLIN_URL%%/smirk.gif, and even after you add in the shipping from Tony it's STILL less than getting directly from Moss. /bcforum/images/%%GRAEMLIN_URL%%/smile.gif
 
Nunyas said:
I'm seriously considering having a flowed head built for my car, and while it's being built possibly have it skimmed to bump the compression ratio up from 8.0:1 to 9.0:1 or maybe even 10:1 if feasible/possible.

Incidentally, are there any repercussions (aside from possibly needing to run higher octane gas) for upping the compression ratio?
Click to expand...

If you go real crazy, you can likely screw up the valvetrain geometry. There's only so much slack the in the pushrod adjusters. Probably not a big concern. I'd be more worried about pre-ignition and finding fuel. At the extreme end, I've seen pockets/cheeks cut into the piston crowns and/or block.

As others have said, an engine is nothing more than a big air pump, I think there are easier/cheaper/more efficient ways to stuff more air into (and extract from) the pump.

If it were me, I'd start with the head (port/polish/73 big valve head), move to the exhaust then address the induction.
 
Get a book by Peter Burgess called "How to power tune MGB 4 cylinder engines". I read while we were camping. I sat by the fire every morning, cup a tea and a nice La Gloria Cubana. I read it cover to cover & then started it over again. There is a chart in there that lists many possible combinations to increase horsepower. He states that just instgalling the PECO header/exhaust combo & K&N filters will do wonders fo the car.
 
Scott_Hower said:
If it were me, I'd start with the head (port/polish/73 big valve head), move to the exhaust then address the induction.
Click to expand...

Don't the '74s have the big valve head too?
 
74 yes. The 18V's I believe.
 
vping said:
74 yes. The 18V's I believe.
Click to expand...

Correct, but more a function of 72-mid 74 than the 18V block designation... stolen from a Google search:

"Starting in 1972 and lasting until about 1974 and a half, casting number 12H-2923 was used, this being the only production "big valve" head. We have also seen this same head with casting number 12H-2709 as it was specified in Europe. These heads had two major changes from the 2389, one being the new 1.625" inlet valves and the other being a newly revised "open style" combustion chamber. These heads were still equipped with 1.343" exhaust valves. The rocker pedestal oil feed hole is in-line with the bolt holes. The combustion chamber depth is roughly .375" and has a volume of around 38cc's. These heads do require exhaust valve cut-outs in the block and provide 8.0:1 compression with the deeper dished pistons used in the new, later style engines. Because of the larger valve sizes and shallower chamber, these heads are commonly sought after items for racing and high performance use. These heads were fitted with "Type B" valves and spring hardware, but only used single valve springs"
 
Steve_S said:
74 heads are known to crack. Given the choice, I'd use something else.
Click to expand...

Seriously? I'd asumed this is the head of choice for most racers. Even better if the smog ports are not drilled.

Not a challenge, I'm just surprised as this head has more meat than the earlier variants.
 
Scott_Hower said:
"Starting in 1972 and lasting until about 1974 and a half, casting number 12H-2923 was used...
Click to expand...

What constitutes "1974 and a half" in this case? Where can I find the casting number?
 
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