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Location:
Darwin, N.T
Registered:
March 2005
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whats involved in lowering compression?
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Sat, 26 March 2005 05:32
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i was told that to lower compression, i needed to use a decompression plate.
what is this, and how is it done? are there any other ways to lower compression?
engine in question: 18R-C
thanks
Steve
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Location:
wollongong
Registered:
October 2004
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Re: whats involved in lowering compression?
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Sat, 26 March 2005 05:48
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lower = shave head?
raise = thicker head gasket?
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Location:
Toronto, Downtown
Registered:
September 2004
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Re: whats involved in lowering compression?
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Sat, 26 March 2005 06:17
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you can get aftermarket pistons with different crowm shapes.
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Location:
Castle Hill, Sydney
Registered:
February 2004
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Location:
Darwin, N.T
Registered:
March 2005
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Re: whats involved in lowering compression?
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Sat, 26 March 2005 10:38
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maybe about the 7 - 7.5:1 mark
for turbo
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Location:
Castle Hill, Sydney
Registered:
February 2004
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Re: whats involved in lowering compression?
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Sat, 26 March 2005 11:50
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in that case ide definately want to put in some custom low comp forged pistons
we've got a 18rc here you can buy, rebuild the bottom end, new bearings and a hoan in the bores, new pistons and rings, swop the engines over and itll take heaaaaaps of boost!
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Location:
Somewhere on a dirt bowl ...
Registered:
August 2004
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Re: whats involved in lowering compression?
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Sat, 26 March 2005 12:13
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| jesseT18 wrote on Sat, 26 March 2005 18:33 |
shaving some meat off your head will raise comp
thicker head gasket will lower compression
other ways to lower comp-
different pistons
grinding some meat out of the combustion chambers in the head
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Only if you know what your doing.
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im assuming that by compression plate, he means a super thick head gasket sort of thing, made of steel or something?
never seen this done but dont see why it wouldnt work with a gasket on either side of it
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From what i've i've heard using two gaskets, either by them selves or with a decompression plate between them isn't a good idea.
Using a thicker gasket made from something like copper would be a better idea. But using one thats too thick and i start running the same risk as using two. Blow-outs.
Better to use pistons built for the compression that your after.
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Location:
Brisbane
Registered:
February 2003
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Re: whats involved in lowering compression?
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Sat, 26 March 2005 12:34
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gee... this sounds familiar ...
seriously now: my 18RC was lowered to 7.4:1 by machining out the head only.
i'm guessing that you're sick of people telling you not to turbo an 18RC - but i can tell you that it's an interesting, if regularly frustrating journey 
suggestion? get a stuffed head from the wreckers to practice on - you want to avoid taking out the squish area in the head - it looks like just the thing do to easily lower compression but doing so you make the engine extremely prone to pinging & detonation.
if you had the money, some custom-machined dished pistons would help but you if then result in removing the 'squish' area you're still prone to detonation/pinging.
avoid the de-comp plate, unless you run copper gaskets with o-rings/grooves, you'll blow them whenever you get over about 5-6psi (this is oral history not personal experience).
unless you're running fuel & spark from a computer and can boost-retard the ignition, i'd also look at water injection to control combustion speed at high boost levels. you'll also find that a decent intercooler will help.
cheers,
thechuckster.
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Location:
Darwin, N.T
Registered:
March 2005
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Re: whats involved in lowering compression?
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Sat, 26 March 2005 14:16
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ok, heres the story with out beating around the bush.
i just recently got a RA23. has no motor, or gear box, but has a few bits and pieces which can be used as spares.
i'm in the process of finalising a 18R-c with 5 speed gearbox. motor having been rebuilt, and nice smooth gearbox. i have already made the turbo intake and exhaust manifolds for the 18R-c.
set up will be a draw through carb, using a stromburg. i haven't found a way to incorporate a FMIC in a non-cross flow motor.
may not plan to use excessive amounts of boost, but a usable power band for lap sprints and circuit. may look at a low diff ratio to compensate for low-medium boost levels.
we're trying to NOT blow the motor up, but try to keep it in a fairly reliable form.
what compression have people found suitable for forced induction, at at what boost levels? i still have yet to find out what turbo i will be using.
detonation/pinging. can someone brief me on this, and ways to reduce/prevent it. is this where the mixture ignites before the piston reaches the top of its cycle?
thanks for all this help, it been really useful.
Steve
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Location:
Brisbane
Registered:
February 2003
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Re: whats involved in lowering compression?
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Sat, 26 March 2005 16:21
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mine maxes at 13psi - but anything over 5psi required water injection to control combustion
as noted earlier, compression is at 7.4:1
the detonation/pinging issues comes from:
fuel exploding instead of burning
fuel burning from multiple points in the chamber instead of outwards from the spark
ignition fires to early - for amount of fuel/air being shoved into engine
suggestion: seeing as you're going carbi & stock ignition, you could try the following:
-water injection thru the carbi (mine was setup that way) with a simple pressure switch to turn on a pump (high flowing windscreen sprayer will suffice), will need experimention with nozzle.
-remap dizzi to lower overall mechanical advance and have the curve end before boost starts
-adapt a cordia turbo boost/vacuum dizzi advancing thing (name?) so when under boost, you retard the timing
seeing as you are racing you will want cold air to the carbi - the setup you've got sound very much like my own (which is changing to EFI with FMIC) and the air temps were excessive.
if you're after stromberg (CD175) already setup (e.g. right needlesize i think) to suit a turbo, PM me 
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