Go to http://www.mgbmga.com/tech/mga3.htm and it's also mandatory to check
out Barney Gaylord's very many pages on this topic. Basically if u have MGB
factory pistons get the 1800 head going asap. Unless the previous owner
removed piston head metal or put in aftermarket dished pistons or double
gasketed the head you are running higher CR and smaller valves with the
current 1500 head on your 1800 block.
In the end I found some of the cc numbers quoted varied with reality on my
50 thou overbored 1600 with 1622 head MGA project. Unless u are absolutely
sure you are stock, get a shop to measure the water cc volume of your 1800
head, (as working from the factory figures and subtracting the machined
thickness did not quite match up for me). Check your bore/overbore
diameter. Measure/calculate the distance from the top of the head gasket to
the top of the piston at TDC. If you have dished pistons you will have to
estimate the dish volume (typically 6 to 7 cc) or pull one for a water cc
measurement. Then accurately calculate your CR range. Having all that and
having fully reviewed the educational web content above, decide on your
options to maintain or upgrade performance.
With my existing aftermarket dished pistons and simply bolting on 1622 head,
my CR would have been way less than 8. With new flat top pistons I could
have gone to 8.9 CR (1622 stock) and using hi-test and fighting pinging. My
etype runs stock 9 CR at 265 HP and I would now be more than happy to trade
off to the optional factory 8 CR pistons that were available back when I did
the restoration.
So with my actual 1660 cc and the larger valves I kept my dished pistons but
still needed to shave 40 thou off the head to get up to 8.3 CR (1500/1600
stock), except that a thermo sensor helicoil got in the way. So I will be at
best wind up with 8.1 CR unless I machine down the block and maybe do the
valve relief notches. In the end between cost and performance relative to
the original small valve 1600 specs, 8.1 CR will give me a bit more HP and
no stress.
If I was going to go the other way I might have gone all the way and built
Barney's hotter cam, lighter VW valve train, free revving version.
Mike
60 MGA, 59 Bugeye, 67 etype
----- Original Message -----
From: "Phil Bates" <jello@ida.net>
> It's really not that question, but I like the subject line. I drive a
> '58 MGA that has a 3 main MGB motor - but it has a 1500 head on it.
> That's the way it came to me when I bought the car. I have a spare 1800
> head, and am concerned that the old 1500 (which I had rebuilt several
> years ago) may be cracked. Anyhow, long story short - too late - I
> think I'm getting a little more power from the 1500 head due to smaller
> combustion chambers (i.e higher compression). Assuming it is still good
> (I do have to get it checked for cracks) I'm considering putting the
> 1800 head on the car. What's the general opinion - will this decrease
> or increase my power?? Bigger valves, but more combustion chamber???
> How's the tradeoff go there?? Anyone have experience with it??
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