| Author | Topic |
|---|
Location:
Lost in the K hole
Registered:
May 2002
|
|
7MGE Compression Ratio - How High ??
|
Mon, 28 July 2003 00:43
|
 |
Its descision time...
Ive done the dummy build, checked all my clearances, and have caculated how much material can
come off here and there in order to get my static compression up.
Stock compression ratio is 9.2:1 and it looks like 11:1 is very achieveable (even a little higher), but
im getting a tad worried about how smart an idea 11:1 is...
Its running STOCK cams (~256deg flat ), and as such, im a little worried about what kind of
cranking pressures im going to get.
fwiw: its a pent roof 4v chamber with 2 decent sized quench pads. piston head squish clearance will
be 0.035". the combustion chambers and piston domes have been polished, all the machined edges
have been nicely radiused, and ill only ever be running Mobil 8000 or BP ultimate.
I'd like to run a decent wad of timing (+20?) and there may be a water injection system runnig on it
(which im yet to decide how to trigger...)
how high a static compression ratio do you think i should go here? is 11:1 with the stock cam going
to push it into obscene dynamic cranking pressures??
cheers
ed
|
|
|
Location:
Canada
Registered:
June 2002
|
|
Re: 7MGE Compression Ratio - How High ??
|
Mon, 28 July 2003 00:57
|
|
Ed,
Do you have the gas to support that combination in Aus?
I've been told that the gasoline isn't very high octane as opposed to here (canada)
how much timing do you run already?
|
|
|
I supported Toymods
Location:
Perth
Registered:
May 2002
|
|
Re: 7MGE Compression Ratio - How High ??
|
Mon, 28 July 2003 00:59
|
|
|
You can buy pump fuel that is 98RON, there are plenty of cars over here that run 11:1+ CR's.
|
|
|
Location:
Canada
Registered:
June 2002
|
|
Re: 7MGE Compression Ratio - How High ??
|
Mon, 28 July 2003 01:06
|
|
then balance the extra timing w/ water/meth
might work... i like 
|
|
|
Location:
c'town, NSW
Registered:
May 2002
|
|
Re: 7MGE Compression Ratio - How High ??
|
Mon, 28 July 2003 01:13
|
|
time to break out the 'Ed gauge' again 
|
|
|
Location:
Lost in the K hole
Registered:
May 2002
|
|
Re: 7MGE Compression Ratio - How High ??
|
Mon, 28 July 2003 02:41
|
|
| justcallmefrank wrote on Mon, 28 July 2003 10:59 |
Ythere are plenty of cars over here that run 11:1+ CR's.
|
yeah, but franky, you can combine v.high static CR with a lumpy cam, and itll give you lower cranking (dynamic) compression.
im concerned that 11:1 with the stock cam might produce a dynamic compression through the roof (which is bad, hmmkhay..)
yeah, 98RON is available at the pump.
*im scared*
cheers
ed
|
|
|
I supported Toymods
Location:
Perth
Registered:
May 2002
|
|
Re: 7MGE Compression Ratio - How High ??
|
Mon, 28 July 2003 02:47
|
|
|
How lumpy do you think some of these cars have though dude? I was talking factory cams, not modified ones.
|
|
|
Location:
Lost in the K hole
Registered:
May 2002
|
|
Re: 7MGE Compression Ratio - How High ??
|
Mon, 28 July 2003 03:00
|
|
fair enough i spose 
im just getting a little edgy about the compression ratio as there aint no going back once i machine everything off  (which is this friday btw  )
i really really dont want a detonation monster on my hands.
at the moment, with current machining limits in mind, im looking at something in the region of 10.75 - 10.85:1
h20/methanol injection is most certainly a goer, i just have to figure out how to trigger it.
and rob - back off the 'ed gauge' buddy - you know damn well its been calibrated and demonstrated to 0.001" tolerance...
|
|
|
Location:
Lost in the K hole
Registered:
May 2002
|
|
Re: 7MGE Compression Ratio - How High ??
|
Mon, 28 July 2003 04:01
|
|
crazy OT post that i cant be bothered making a new thread for
WHAT IF...
...i made a 50% (15% Nitromethane + methanol) 50% water mix for the injectoion system  ??
hehehe
ed
|
|
|
Location:
Kita-Ku, Sapporo, Japan
Registered:
January 2003
|
|
Re: 7MGE Compression Ratio - How High ??
|
Mon, 28 July 2003 04:10
|
|
although it's a pain, you could always go half way and then get more machined off later. much more work, but as ya say, aint no going back (without a thick copper gasket).
well, bikes with high overlap run up to abut 13:1, some of the euro cars are running around 11:1.
in my rolla (4K) i have 275/285 cams (solid lifter) which have 65 deg overlap and high lift rates. i have 10.2:1 CR only run mobil 8000 (or occ. optimax if i have to) and currently have stupid amounts of advance. something like 20-25 deg static and 35-40 all in. i have not been able to get it to ping yet even with 35 deg static advance. just makes it harder to start and loses much power. the combustion chambers have been seriously smoothed compared to stock, but it's still a crappy chamber (tho it is about 1/3rd squish area with 0.8mm squish (32 thou) i'm sure the bigger than stock overlap gives me lower dynamic compression at low revs (135-140psi cranking speed) but at high rpm, the dynamic compression should be as high if not higher than stock??
i'm thinking of at least 10.5:1 next time, with a smaller overlap cam, but i guess our motors are pretty different as i have bore of 76.5mm....
at worst if you go maybe 0.5 points too high you could always go a 1.2-1.5mm copper gasket.
Cya, Stewart
|
|
|
Location:
Kita-Ku, Sapporo, Japan
Registered:
January 2003
|
|
Re: 7MGE Compression Ratio - How High ??
|
Mon, 28 July 2003 04:19
|
|
hmm the nitro has 60% of the O2 it needs to burn already why not just have methanol injection? if you weren't using stock computer, you could run a dual table with petrol injectors on one and small methanol/ethanol (ie metho ) injectors on the other. as you get close to WOT, where you have highest dynamic compression, you can lean out the petrol injectors more than you normally would, and make up for the lost petrol by injecting double that manount of methanol. this bumps up your octane significantly, decreases the chance and severity of pinging etc etc. only prob would be running out of metho .. but if it was only at maybe 90%+ of WOT then a litre of metho should last you a morning maybe
or the other idea would be to get a cam with more overlap....
| ed_ma61 wrote on Mon, 28 July 2003 14:01 |
crazy OT post that i cant be bothered making a new thread for
WHAT IF... ...i made a 50% (15% Nitromethane + methanol) 50% water mix for the injectoion system ??
hehehe
ed
|
|
|
|
Location:
Lost in the K hole
Registered:
May 2002
|
|
Re: 7MGE Compression Ratio - How High ??
|
Mon, 28 July 2003 04:27
|
|
(nitromethane's a fundamentally dumb idea - ignore above post)
old corollas: its interesting to see that youre still only getting a cranking pressure of 130psi...
i get this funny feeling in my bones that the 7m will be cranking 180+psi and that cant be a good thing.
ive just been doing some serious reading on methanol injection, fuel preignition and relative compression ratio's etc, and it 'should' be possible to run almost 11.5:1 with adequate methanol added.
i dont think ill go this far tho
10.75/10.80:1 seems about on the money, and if i can configure (crudely) a methanol/ethanol system to work properly (manifold vacumm and rpm based) then high load, hi rpm situations should be ok.
i just hope fuelling wont be too hard to jerry with the stock ecu (afm and fuel pressure regulator modification)
damn, anyone want to donate a m800?
cheers
ed
|
|
|
Location:
Kita-Ku, Sapporo, Japan
Registered:
January 2003
|
|
Re: 7MGE Compression Ratio - How High ??
|
Mon, 28 July 2003 04:46
|
|
| ed_ma61 wrote on Mon, 28 July 2003 14:27 |
old corollas: its interesting to see that youre still only getting a cranking pressure of 130psi...
i get this funny feeling in my bones that the 7m will be cranking 180+psi and that cant be a good thing.
ive just been doing some serious reading on methanol injection, fuel preignition and relative compression ratio's etc, and it 'should' be possible to run almost 11.5:1 with adequate methanol added.
i dont think ill go this far tho
10.75/10.80:1 seems about on the money, and if i can configure (crudely) a methanol/ethanol system to work properly (manifold vacumm and rpm based) then high load, hi rpm situations should be ok.
|
yeah it's a bit weird. it was getting 135-140 when it was freshly built, but that was 18 months (and a lot of rpm) ago.
a mate had a datto 200B, L20 motor, had somewhere around 11:1 CR, and had 210Psi cranking compression. no idea on cams. it ran very well with either LPG or rich with twin webers.
fwiw, stock cranking compression in a toyota 3K is 170-180Psi for 9:1 and 190Psi for the 10:1 models. both run pretty much similar (stock) cams. i don't hink 180Psi will be that bad.
hmmm, what can i think of that can use only a vacuum hose from the plenum and a wire to the tach? hmmmm 
seriously tho, for $180 and a bit of soldering time, you can have a system where you have 8 MAP points that you can put at any kPa you want around the last 10-15% of the MAP range. that should be enough resolution? (minimum resolution is about 1kPa)
LOL, one day  (after i get mine in)
Cya, Stewart
[Updated on: Mon, 28 July 2003 04:48]
|
|
|
Registered:
August 2002
|
|
Re: 7MGE Compression Ratio - How High ??
|
Mon, 28 July 2003 06:47
|
|
|
Ed- Are you still shooting for 250hp or more?
|
|
|
Location:
Lost in the K hole
Registered:
May 2002
|
|
Re: 7MGE Compression Ratio - How High ??
|
Mon, 28 July 2003 08:20
|
|
ahhh - just checked the toyota manual, stock cranking pressure (@ 250rpm) is only 156psi !! (142psi for the 7mgte)
hmmm, maybe it wont go too pear-shaped afterall...
bloody megasquirt! theres few problems with it for my application:
-im running 2 cold start injectors
-the injectors ARENT pulsed - theyre continuous
-full mapping is a little overkill
to my mind, something simple like this doesnt need a comprehensive hardware setup like a dedicated injection computer
i think im juts going to wing it at 10.75/10.80:1
see what happens 
and Glen, im not a fan of quoting numbers, but youre pretty close
cheers
ed
|
|
|
Location:
Lost in the K hole
Registered:
May 2002
|
|
|
