The 5 main engine is an 18GB, and the 3 main is an 18G. The MGA's head is a
head, while the MGB has an 1800 no emissions head - the 1800 should have
larger valves, and so it should really get better mileage (although I have not
physically checked the valve sizes in the two). I also don't know about the cam
grinds, other than that the 5-main should be stock - I don't know why the 3 main
would be anything but stock. The 3 main got new bearings more recently, but the
oil pressure isn't as high on it (I suspect the pump).
I can't imagine that teh header would be more trouble - except getting it in and
out (which is a pai in the butt on my MGA, when removing the engine) - because
exhaust downpipe has gotten loose about a dozen times from bottoming out - the
loosness occurring at the union between the manifold and the downpipe. A header
doesn't have that union.
Larry Colen wrote:
> On Mon, Dec 27, 1999 at 11:14:47PM -0700, Phil Bates wrote:
> > OK, while I have the engine and trans out to work on the transmission, I
> > am considering putting a header on my '67 MGB. Here is the thought
> > process - I have a 3-main 1800 motor in my MGA with a header, and it
> > seems to have a lot more power. The gearing should be the same for both
> Is that the only difference between the motors? Same cylinder heads?
> Pistons? Cam grinds? Is the 5-main an 18G or an 18V? How many miles are
> on each motor? A well worn in motor will often make more power than a
> tight new one (it's an old racing adage that motors make the most
> power just before they grenade).
> > (I did change the A's rear axle to an early MGB axle). Anyone know of
> > disadvantages of using a header on this car (other than cost) - I have
> > heard the stock manifold is pretty good, but I think a header would be
> > better. I do expect that I'll always have a little less power in the B
> > because of the 2 extra bearings, but it shouldn't be much different.
> You lose a little to friction in the bearings, but I'd guess that one
> would get a lot more safe RPM out of a 5 main than a 3 main. Since power
> is torque * rpm, I'd guess (all things being equal) a 3 main would have
> more torque but a 5 main would have more power.
> I do know that every few years I start itching for "easy horsepower"
> and think of putting a header on my car. I always get several
> knowledgable folks (Lawrie et. al.) saying that it isn't worth it,
> that the headers don't give you anything but more mechanical headaches.
> I've not seen any dyno charts or comparative lap times.
> Sasquatch singing telegrams; for when you care enough to send the furry beast.
> firstname.lastname@example.org http://www.red4est.com/lrc