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MG MGA - Compression ratios?
|When I rebuilt my 1622 engine 12 years ago I installed dished pistons on the advise of others. That gave me a 8:1 compression ratio. At this time I have the engine out and partially dissasembled because of cam and lifter issues and am toying with the idea of going back to the original 9:1 compression ratio. What ratio do others run and is there any down side to going to the 9:1 ratio? Also I see several different pistons available. Some 3 ring, some 4 ring, and some with 3 rings at the top and 1 ring near the bottom of the skirt. Any input would be appreciated.|
|Ed since no one has answered yet I'll tell you what I did. At some time in it's life my 1600 had been bored .040 over and had a sleeve put in also. They were 3 rings top and an oil below. Whoever did the job had too great of clearance. I replaced them with flat top .060's made by Aerolite. As you know enlarging the bore and compressing that volume into the same cc head raises the CR even more. I don't now what it's at now,but I have not had any problems.|
|Ed my 1600 was bored by the PO to 1622 + 40 thou making it around 1650 and he installed flat top pistons. He kept the original 15 head which has smaller combustion chambers to the 16 head. I have calculated that my CR is around 10.5:1. The engine runs great but I have to mix in 100 octane racing fuel to raise the octane rating to around 95, otherwise I have to retard the timing to prevent pinging.|
I'd think that your engine with a compression ratio of 9:1 would be about perfect with 91 octane fuel.
|Ed, my 1622 Mk II had dished pistons, as they were standard here for the Australian market with the poor quality fuels we had back then. (8.3:1 compression ratio). However I was unhappy with the sluggish performance of my car, despite having had the head "worked", including with matched 38cc combustion chambers. On the dyno I maxed out at 54 HP at the rear wheels. I wound up fitting a set of flat top pistons. With these my compression shot up to 210 lb/sq in, no doubt due to the 38cc chambers. (43cc is standard for the MGA 1622). We machined 20 thou of the piston crowns (10 thou might have sufficed). It's completely transformed the car. It's now wonderfully responsive and eager to go. I could not be happier. Runs great on 98 octane. Didn't complain when I could only get 95 octane for a top up on one occasion. The car still displays a decent shove at a GPS verified 90 mph! ( Ford T9 0.82 fifth gear by the way. Also I'm +40 oversize, so 1666cc).|
I'd not hesitate, and suggest you go back to the hi compression pistons.
|Ed, I forgot to add that my compression readings were 185lb/sq in on all cylinders when the engine was rechecked after a 600 mile run in period with the new flat top pistons.|
My source for the pistons was New Old Stock Locators, who I believe are the eBay arm of Scarborough Faire.
The pistons are 5 ring (!) and seem of good quality. When measured they were exactly consistent in all dimensions, which was not the case with the pistons which I replaced.
|TA - interesting about your piston dimensions. I bought my 1622 _040 pistons through Moss, County brand. All piston dimensions were within .0002 between pistons, but each piston was oval and tapered, deliberately they call it cam ground. Tighter at the top and looser in the front-back direction. I have not yet run the motor.|
|Why on Earth would you go to low compression? Too cheap to pay for higher octane gas? As was said, these engines wake up with a bit of compression (the small chamber heads with later large valves, suitably relieved around the intake are a great way to get up to around the 9.5 sweet spot without shaving the head).|
|Art, the dimension variation I referred to concerned the piston heights. I'd not understood the compression readings varying by over 10lb/sq in between cylinders despite matched head chamber volumes, and an apparently otherwise healthy engine. Inspection after removing the head revealed that there was a variation in heights between the cylinders of the piston crowns in relation to the deck height. (I wrote the figures down but unfortunately no longer have them). Checking the originally fitted pistons after their removal showed that they were inconsistent in height.|
The new pistons were all identical in height and when fitted were identical in relation to the cylinder deck at TDC. Interestingly they also uniformly sat higher in the bores at TDC than the original pistons, even the tallest original one.
I checked but could find no manufacturer's name on the original pistons; otherwise I could nominate a brand to avoid.
Unfortunately my mechanic's pipette had broken, as I'd have loved to have known accurately what my current compression ratio is. (To further cloud estimates, my block has had the "eyes" for greater exhaust valve clearance cut in).
Also of interest is the variation seen with different (all "screw in") compression gauges when I checked the readings at three different workshops. I saw a 15lb/sq in variation. The dyno mechanic's readings were 110-120, and those at my mechanic's 125-135. My neighbour's gauge read half way between the two! As mentioned earlier, the readings are now exactly the same on all four cylinders, 185 on the gauge that previously read the highest figures.
This thread was discussed between 08/11/2012 and 09/11/2012
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