Can anyone tell me how to identify 9 to 1 pistons as opposed to 8
to 1 for a 4.2 engine.–
vacbag
Scarborough N Yorks, United Kingdom
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In reply to a message from vacbag sent Sat 12 Jul 2008:
The easiest way to tell is from the size of the dome. The 8:1 has
a dome which has a diameter of approx 64mm and a height of approx
4.75mm - it is not much of a dome at all. The 9:1 have a dome which
is certainly higher and which may be slightly greater diameter also–
christopher storey
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In reply to a message from vacbag sent Sat 12 Jul 2008:
Vacbag??,
The 9:1 piston has a substantial crowm compared to the 8:1 and the
Haynes manual shows this clearly.–
The original message included these comments:
Can anyone tell me how to identify 9 to 1 pistons as opposed to 8
to 1 for a 4.2 engine.
–
Tony Batten, UK, RHD 1964 4.2 OTS
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In reply to a message from vacbag sent Sat 12 Jul 2008:
Steve,
This should answer your question:
http://www.jag-lovers.org/snaps/snap_view.php3?id=1101569550--
Ray Livingston - '64 OTS Santa Cruz, CA
Santa Cruz, CA, United States
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In reply to a message from vacbag sent Sat 12 Jul 2008:
Thanks Guys, there is so much information out there. next question.
as I now know I have 8 to 1 pistons in my E Type is there much
loss of performance against 9 to 1 pistons.–
vacbag
Scarborough N Yorks, United Kingdom
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In reply to a message from vacbag sent Sat 12 Jul 2008:
You should be much more concerned about the likelihood of pinging
if you go with 9:1. The only safe way to make that decision is the
CC the head and bores, and determine the actual compression ratio,
and make sure it stays down around 9.0:1, and in no case above
about 9.4:1, or you will have pinging. The performance
difference between 8:1 and 9:1 is small.–
Ray Livingston - '64 OTS Santa Cruz, CA
Santa Cruz, CA, United States
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In reply to a message from vacbag sent Sat 12 Jul 2008:
The prior owner of my car, a 4.2 OTS spec. 9:1 CR)rebuilt
the engine with 8:1 CR. It ran very smooth and pulled well
with a 3.54 rear end. I had a 2nd engine professionally
built and spec’d with 9.25:1 pistons or CR. When I pulled
the head and CC’d the chamber vols, I found it more like
11:1 CR. It is now back to about 9;1 stock CR. 8:1’s are
nice IMHO.–
The original message included these comments:
Thanks Guys, there is so much information out there. next question.
as I now know I have 8 to 1 pistons in my E Type is there much
loss of performance against 9 to 1 pistons.
–
Mr. Nice Spokes
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common misconception…
The difference between 8:1 and 9:1 pistons is not power or speed. The difference is efficiency (economy). 9:1 will give you slightly better efficiency (miles per gallon).
LLoyd–
He tried to impress her. He was courteous and surprised her. But he knew the relationship was doomed when she tasted his home made asparagus-beet soup and made the same noise as a cat with a fur ball.
-------------- Original message ----------------------
From: “vacbag” steve.flint@yorkshire.net
In reply to a message from vacbag sent Sat 12 Jul 2008:
Thanks Guys, there is so much information out there. next question.
as I now know I have 8 to 1 pistons in my E Type is there much
loss of performance against 9 to 1 pistons.vacbag
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In reply to a message from soothsayer1@comcast.net sent Sat 12 Jul 2008:
Well I don’t think that’s true.
I think you will find that all things being equal, higher
compression does indeed provide more thermal efficiency and
therefore more horsepower - at least 10bhp and probably more like
20bhp. Jaguar didn’t publish power figures for the lower
compression engines.
As an aside I remember reading ‘‘Jenks’’ review on his 4.2 roadster’s
top speed of 142-143 indicated and at no time mentioning the fact
that he’d specified an 8:1 compression ratio so he could run on low
grade petrol in Europe.–
The original message included these comments:
common misconception…
The difference between 8:1 and 9:1 pistons is not power or speed. The difference is efficiency (economy). 9:1 will give you slightly better efficiency (miles per gallon).
LLoyd
–
graemea
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In reply to a message from graemea sent Mon 14 Jul 2008:
This is correct of course, although in practice people might not
notice a huge difference by the seat-of-the-pants dyno method.
By compressing the charge more to begin with, more of the heat of
combustion is used to push the piston down, leaving less of it as
waste heat lost through the coolant. Depending how you drive you
can/will either use this gain as a power boost and drive quicker
around the place or you could look for a very small economy gain,
but the thermodynamics are mostly win-win. Raising the CR (with
Ray’s caveats about measuring it accurately) is one of the first
and simplest ways of gaining power. 8:1 is lower than I’d want in a
sporting car but perfectly adequate in a saloon.
Pete–
The original message included these comments:
I think you will find that all things being equal, higher
compression does indeed provide more thermal efficiency and
–
66 2+2, 68 OTS lump, 94 X300 Sovereign, 94 XJR Manual
Cambridge, United Kingdom
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In reply to a message from graemea sent Mon 14 Jul 2008:
I’d like to see the documentation supporting a 20 HP increase from
one added point of compression. I seriously doubt that number; no
books I have support that claim.–
The original message included these comments:
therefore more horsepower - at least 10bhp and probably more like
20bhp. Jaguar didn’t publish power figures for the lower
compression engines.
–
Paul Wigton, steward to a '60 DKW 1000 SP, Tweety, '63 FHC!
Keenesburg, CO, United States
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Yes… we’ve been down this road before.
LLoyd–
He tried to impress her. He was courteous and surprised her. But he knew the relationship was doomed when she tasted his home made asparagus-beet soup and made the same noise as a cat with a fur ball.
-------------- Original message ----------------------
From: “Wiggles” vrooomie@fastmail.fm
In reply to a message from graemea sent Mon 14 Jul 2008:
I’d like to see the documentation supporting a 20 HP increase from
one added point of compression. I seriously doubt that number; no
books I have support that claim.
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I was going to extract selected quotes from this article but it became too cumbersome and I was not able to copy/paste the diagrams/charts: http://www.popularhotrodding.com/tech/0311_phr_compression_ratio_tech/index.html
I will admit to having only scanned this so far and not “getting” all I did read, but it seems on target and relevant.
Paul: I will put my toe in the theoretical pool and comment that expecting a 20HP increase with 1 CR increase depends on the initial (starting – -- base) HP rating. Perhaps a “per cent increase in HP for a given increase in CR” discussion is more appropriate
Craig______________________________________________________
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In reply to a message from Balzer, Craig LTC RET sent Tue 15 Jul 2008:
Using their chart (thanks KC!) the nominal increase in power
resulting from a raise to 9:1 from 8:1 is along the lines of 3.5%,
which is considerably less than the almost 20% a 20HP rise would
suggest.
THAT I’ll buy!–
The original message included these comments:
I was going to extract selected quotes from this article but it became too cumbersome and I was not able to copy/paste the diagrams/charts: http://www.popularhotrodding.com/tech/0311_phr_compression_ratio_tech/index.html
Paul: I will put my toe in the theoretical pool and comment that expecting a 20HP increase with 1 CR increase depends on the initial (starting – -- base) HP rating. Perhaps a ‘‘per cent increase in HP for a given increase in CR’’ discussion is more appropriate
–
Paul Wigton, steward to a '60 DKW 1000 SP, Tweety, '63 FHC!
Keenesburg, CO, United States
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In reply to a message from Wiggles sent Tue 15 Jul 2008:
You’re a geologist then Paulie, not a 'rithmetician?
Unless you’re suggesting our beloved XK could only wheeze to a
measly 100+ bhp? 
Even on thin Colorado air I’d expect them to
approach double that, so 20bph would be nearer 9-10%.
My gut feeling would be 5-7% maybe, going from 8 to 9 CR?–
The original message included these comments:
which is considerably less than the almost 20% a 20HP rise would
suggest.
–
66 2+2, 68 OTS lump, 94 X300 Sovereign, 94 XJR Manual
Cambridge, United Kingdom
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In reply to a message from Wiggles sent Tue 15 Jul 2008:
Paulie,
Time to put a new battery in the ol’ Casio. 20HP would be
roughly 10%, assuming a ‘‘stock’’ engine is around 200HP.–
The original message included these comments:
Using their chart (thanks KC!) the nominal increase in power
resulting from a raise to 9:1 from 8:1 is along the lines of 3.5%,
which is considerably less than the almost 20% a 20HP rise would
suggest.
THAT I’ll buy!Paul Wigton, steward to a '60 DKW 1000 SP, Tweety, '63 FHC!
–
Ray Livingston - '64 OTS Santa Cruz, CA
Santa Cruz, CA, United States
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In reply to a message from Ray Livingston sent Tue 15 Jul 2008:
I did it fast…
and at 6:30 AM!
My point is, a one-point increase in compression is NOT good for as
much as 10%…there? Better?
Sheesh…some people’s geologists!
LOL!!!–
The original message included these comments:
Time to put a new battery in the ol' Casio. 20HP would beroughly 10%, assuming a ‘‘stock’’ engine is around 200HP.
–
Paul Wigton, steward to a '60 DKW 1000 SP, Tweety, '63 FHC!
Keenesburg, CO, United States
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In reply to a message from graemea sent Mon 14 Jul 2008:
Not to be flogging a dead horse but if we start at Jaguars ‘‘GROSS’’
and optimistic 265bhp on 9:1 then 10bhp is 3.5% for 255bhp or 20
bhp is 7.5% for 245bhp.
Don’t know where the talk of 10 or 20% came from??
G–
graemea
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