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MG MGA - Increasing Compression Ratios

I'm having my machinist cc the combustion chamber to verify numbers but I thought I'd start thinking ahead.

The new CAM I'm putting in recommends a Compression Ratio (CR) of 8.7 to 9.5.

I will be shaving the head due to warpage and planing the deck which will obviously increase the CR but I will also be adding eyebrows which offset that.

The question then is if I need to get a smaller combustion chamber after all that can I just keep skimming the head to get there? or is there a point at which I have to give that up and have to go with new pistons?
T McCarthy

The point has already past when you should have bought flat top pistons. Shaving the deck and head for compression is a very bad idea.

Having done so, at least invest in some tubular pushrods so you can have them shortened and restore more or less normal valve train geometry. If you don't, you'll be looking at pulling that head off again much sooner than you'd like to deal with worn guides/valves.
Bill Spohn

Bill,

Unfortunately the deck planing is to address a small rill burned in the deck between #3 and #4. The head shaving is for a small bit of warpage so the increased compression is just a side "benefit".

Does anybody know the area over the standard dished piston.

T McCarthy

No problem with that sort of shaving - thought you might be one of the twits that carves great slices off a perfectly good head an block for no good reason except that they are too cheap to buy pistons. Glad to hear that you had a much better reason! :-)
Bill Spohn

Most motors run very similar "real" or "effective" compression ratios- around 6 to 1 from memory. This number is the volume above the piston when the intake valve closes divedid by the volume at TDC. Bigger cams close the intake later so you need to bump the "mechanical" compression- the ratio of volumes above the piston at BDC and TDC.

If you add duration and can't change compression, installing the cam at the most advanced suggested cam timing may help. This can help stock engines too since most cams are installed timed a bit late due to production tolerances.

Modern performance engines run very high mechanical compression ratios. Although computer control is often given the credit, emissions requirements are the root cause. Overlap is bad for emissions so they need to keep the intake open longer to get decent filling. The measurement describing this is the lobe center angle. Older engines ran 108 to 110 degrees between the exhaust and intake while modern performance cars can run 112 degrees or more.

Normally, this would cause intake flow reversion. "Early modern" cars (think Corvette "Crossfire") managed this with really long intake tracks. This is less critical now that manifolds are "dry" due to sequential fuel injection.

The theoretical "correct" valve open duration and lobe center angle is 90 degrees at the cam (180 at the crank).

Bill
Bill Eastman

Rather than cutting the pushrods, shim the rocker stands to correct the valve train geometry. We machined the combustion chambers (we had gotten pictures of the Siebring Sprite head) to unshroud the valves. To compensate for the materiel removed I had the head milled on my A 0.110 and shimmed the rocker stands 0.050 to correct the rocker angle. I have seen these shims for sale in Mini parts catalogs, probably available for the B series. Remember also to verify that there is adequate valve to block clearance (I had to pocket the block for clearance). As I said, it was a long time ago, hopefully some one has more up to date info on this.
Fuzz
Russ Carnes

This thread was discussed between 29/08/2008 and 06/09/2008

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