Bill Eastman wrote:
> HP Liter HP/Liter % change hp/liter
> 68 1.489 45.7
> 80 1.588 50.4 10.3
> 93 1.622 57.3 13.7
> 94 1.789 52.5 -8.4 (yuck!)
The specifications on the MG Web Site list the 1622 bhp at 93 ...
I thought this was a bit high, but it was the only reference that I had
at the time ... taking a look at the MGA Workshop manual we can
revise the numbers as follows ...
Liters CR HP HP/Liter %change Torque Torque/Liter
1.489 8.3:1 68 45.67 78 52.38
1.588 8.3:1 80 50.38 10.31% 87 54.79 4.58%
1.622 8.3:1 85 52.40 4.02% 92 56.72 3.53%
1.622 8.9:1 90 55.49 10.14% 97 59.80 9.16%
1.798 8.8:1 94 52.28 -5.78% 110 61.18 2.3%
> HP / liter has increased both times although, as you say, the 1622 had
> a greater increase than the 1588. I believe that the same cam was
> used for all MGA's as well as most early MGB's so something else
> must be at work.
Exactly ... the 2% increase in capacity, from 1588 to 1622, resulted in a
12.5% increase in bhp and a 11.5% increase in torque ... the only significant
changes where bore, CR, and cylinder head.
> In general, I believe that compression ration is overemphasized in
> horsepower changes. A change from 8.3 to 8.9 would not make a lot
> of difference. I would say that the increased engine efficiency is could
> almost all be attributed to better breathing.
Nope ... if the CR is held constant, at 8.3:1, then the increase in bhp,
attributed to better breathing/bore, is 6.25% and the increase in torque is
5.75%. IOW, 50% of the increased engine efficiency can be attributed to
the increase in CR.
It should be noted that the 1489 68 bhp figure is for the original 1489 head
... during the production run there were two modifications to improve porting
so that by the end of the run the head was identical to that used on the
1588 and bhp was up to approximately 73 ... so the results of the increase
in capacity from 1489 to 1588 are much less than outlined above ...
Liters CR HP HP/Liter %change
1.489 8.3:1 73 49.03
1.588 8.3:1 80 50.38 2.76%
1.622 8.9:1 90 55.49 10.14%
1.798 8.8:1 94 52.28 -5.78%
... therefore, the increase in power for the 1588 to the 1622 is even more
pronounced.
Per my previous post ... IMHO, fitting the late 'A'/early 'B' head (provided
that the CR is maintained) and/or increasing the CR would substaintially
increase the efficiency of 1489/1588 engines.
> Other books that I have read mention valve shrouding as the main
> limitation of B series engines although, I would expect that sooner or
> later port geometry limitations would take over. As Kirk mentioned, this
> is probably why the MGB engine doesn't make much more horsepower
> than the 1622.
Agree ....
> Horsepower is only part of the performane equation. I expect that the
> 18 series engine produces significantly more low end torque that either
> A engine and I am also sure that it runs a lighter flywheel.
Agree ... the 1798, given its larger capacity, does produce a good deal more
torque and the peak torque is reached at 3000 rpm versus 4000 rpm for the
1622. However, comparison is not that straight forward as the 1798's carbs,
intake/exhaust manifold, dizzy, and flywheel aren't the same as those on its
predecessors.
> Low end torque is what make a car feel responsive in day to day driving
> and a lighter flywheel will also make the car feel more responsive. When
> street engine builders talk, they talk about increasing the "area under the
> torque curve" as the ultimate goal of performance modification. This means
> increasing the torque output of the engine over a wide operating range rather
> than going for peak horsepower.
Hmmm ... the 1489 reaches peak torque at 3500 rpm, the 1588 at 3800 rpm,
the 1622 at 4000 rpm, and the 1798 at 3000 rpm ... so, do the 1489/1588/1622
engines have a greater "area under the torque curve" than the 1798? ... ;^)
Safety Fast! .... larry.g.unger@lmco.com
'61 MGA 1600 MkII
|