What causes high compression?
Hugh Grierson
to do my own engine tuning. For some reason the compression is reading
higher than spec - around 110 psi on the first stroke and 160 max against
a suggested 75/135. What does this mean, and what (if anything) should I
do about it?
[The motor is a 6-cyl Holden 179, late 60's vintage, now powering
a SWB IIA Landrover]
Related question: a "compression ratio" is the ratio of what to what?
--
-Hugh Grierso...@fivegl.co.nz----------Go fast or go home----------------
-Task Management Ltd-Auckland-New Zealand-Ph:+64.9.5206109-Fax:+64.9.5206101-
geo...@mech.seas.upenn.edu
:Related question: a "compression ratio" is the ratio of what to what?
:
it is the ratio of the cylinder volume at bdc to the volume at tdc
--ideally, this is the same as the ratio of the pressure at tdc to
atmospheric ( ideally )
[ why is it ]160 max against a suggested 75/135.
carbon buildup?
are you sure the engine has stock head and pistons?
--
-george geo...@mech.seas.upenn.edu
Mike Drayton
Derek 'Enzo' Small
> to do my own engine tuning. For some reason the compression is reading
> higher than spec - around 110 psi on the first stroke and 160 max against
> a suggested 75/135. What does this mean, and what (if anything) should I
> do about it?
>
> [The motor is a 6-cyl Holden 179, late 60's vintage, now powering
> a SWB IIA Landrover]
>
> Related question: a "compression ratio" is the ratio of what to what?
(combustion chamber, any dish or eyebrows in the piston and the bore) when
measured with the piston all the way down (BDC bottom dead center) over
the volume of the same cylinder with the piston all the way up. With the
piston all the way up about all you have is the volume of the combustion
chamber, plus and indentations in the piston (or minus if the piston has
some kind of dome) plus the amount of the cylinder the piston doesn't fill
when it's all the way up. Add the cylinder diameter squared times the stroke
to get the volume when the piston is down.
Ways to increase compression:
-Allow carbon build up in the face of the piston and the walls of the
combustion chamber to decrease the volume of the cylinder with the
piston at TDC.
-Use a thinner head gasket or have the block deacked (shave some metal off the
surface of the block that the head bolts to or deck the heads
-have the cylinders bored out.
>
> --
> -Hugh Grierso...@fivegl.co.nz----------Go fast or go home----------------
> -Task Management Ltd-Auckland-New Zealand-Ph:+64.9.5206109-Fax:+64.9.5206101-
--
*******************************************************************************
* Derek "Enzo" Small [san.dwsmall]@miavx1.acs.muohio.edu *
* Miami University, Oxford, Ohio 45056 *
*******************************************************************************
* *
* " But if men can't know the rules how are we supposed to avoid those *
* meciless beatings?" *
* Men *
* " You aren't ! " *
* Women *
* " Oh, Gulp !" *
* Men *
*******************************************************************************
rgh...@bb1t.monsanto.com
> I finally spent some money on a compression tester, and other instruments,
> to do my own engine tuning. For some reason the compression is reading
> higher than spec - around 110 psi on the first stroke and 160 max against
> a suggested 75/135. What does this mean, and what (if anything) should I
> do about it?
>
If it's higher, be happy (unless you've only got lower octanes available).
But I noticed you said with the 'first stroke'. My tester said to let
the engine roll over several times (like 6 to 8) to get an acurate
reading. Could be that they had higher compression pistons in your
engine for whatever reason.
All other things being equal, if you lower the compression, you'll lower
the power output.
But I've read in some places not to worry about the actual number, but
rather, the readings of adjacent cylinders. They should be similar.
A low reading on one might indicate a blown head gasket or bad valve, for
example.
Rick Hoffman
Derek 'Enzo' Small
> the power output.
Actually this is a popularly held misconception. It follows from the fact
that higher output engines tend to run higher compression ratios. Although
this ise the higher compression ratiratio is necessitated by the specifics of
the engine. Raising the compression on a typical street engine will have
little if any effect on power output. It's just like ignition timing. There
is a point where all you do is make more heat. According to dyno testing
I've seen raising the compression on a stock or mild street engine by one
point (from 8.5:1 to 9.5:1) was worth only 2 or 3 horsepower, but engine
stress, not to meniton the need for higher octane fuel, increased
significantly.
Chris Warren
: > All other things being equal, if you lower the compression, you'll lower
: > the power output.
In the absence of detonation the above statement is UNEQUIVOCALLY TRUE.
In the range of 8:1 to ~10:1 the improvement in power (or reduction in fuel
consumption) is of the order of ~3-4% per whole ratio (i.e. 9:1 to 10:1).
:
: Actually this is a popularly held misconception. It follows from the fact
: that higher output engines tend to run higher compression ratios. Although
: this ise the higher compression ratiratio is necessitated by the specifics of
: the engine. Raising the compression on a typical street engine will have
: little if any effect on power output. It's just like ignition timing.
WRONG!!!!!!!!!!!!!!
: There is a point where all you do is make more heat.
There is a point of diminishing returns. However, it is beyond 11:1 CR. The
diminishing aspect results from the increase in IMEP being offset by rapidly
increasing engine friction.
: According to dyno testing
: I've seen raising the compression on a stock or mild street engine by one
: point (from 8.5:1 to 9.5:1) was worth only 2 or 3 horsepower, but engine
: stress, not to meniton the need for higher octane fuel, increased
: significantly.
--
*************************************************************************
*My opinions are mine and do not necessarily reflect in any way those *
*of my employer. *
*Chris Warren: c...@slee01.srl.ford.com *