jkarran
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| posted on 13/9/07 at 05:30 PM |
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5PW pistons for forced induction
Can anyone (perhaps someone with a blown up or stripped one) tell me how thick the piston crown is on the Yamaha 5PW R1 engine? I'm wondering if
I can get away with machining a ~2mm dish to lower the compression rather than fit a decopression plate. I'm guessing they're thicker than
they might otherwise be to accomodate the valve cutouts?
The upsides are the cam chain will remain in the right place and the combustion chamber squish regions should remain largely unaltered helping to
maintain a good quick burn and hopefully reduce detonation problems.
Bad idea?
jk
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jambojeef
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| posted on 13/9/07 at 07:24 PM |
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Hi,
Thought along similar lines myself, in fact still am!
There were a few things that crossed my mind:
Firstly, if they are cast pistons, hopefully they wont be, youd have to check to see if any surface hardening or shot peening was required after
machining the surfaces.
The other thing is that the heat build up in the cylinders will be increased due to the effective compression ratio but if you remove material from
the crowns, a route of heat soak is effectively reduced in effificency potentially leading to hot spots on the piston crowns which could cause the
detonation you're trying to avoid.
If you go on the JE Pistons website have you seen the new 9.0:1 pistons they do?
Think its:
http://www.jepistons.com/cat/je/power/street/yamaha/yzf_r1-fz1-rx1.shtml
Im in the process of stripping my engine down and will measure the thickness of my crowns to give you an idea of thicknesses if thats any use?
I reckon its dependant on the effective compression ratio you are planning to run - any ideas?
Geoff
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jkarran
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| posted on 13/9/07 at 09:22 PM |
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Yeah, there's pros and cons to the piston machining vs plate. As much as the engine is standard and essentially disposable* it is very nearly
new so it'd be nice if it didn't melt down.
*I keep telling myself this, it helps keep the projected power figures up 
I'm planning to run ~9:1 and somewhere between 11 and 12 psi boost depending how much loss there is and exactlty what pully ratios I can source
(I'm thinking toothed pullys). Calculatins sugest 8.5:1 on 98RON but I built the spreadsheet somewhat conservative and am going to run
electronic boost control (proportional bypass valve)
Fingers crossed those forged pistons aren't too too much, it's meant to be a budget/home build, I already have a feeling they're a
good idea... <clicks link>
jk
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garage19
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| posted on 13/9/07 at 11:10 PM |
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I would be very suprised if the crowns on your pistons are thicker than 5mm.
Knock 2mm of that and you will have probs!
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bimbleuk
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| posted on 14/9/07 at 07:52 AM |
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Only car engine experience in my case but I ran 11.5:1 and 7PSI on my previous Celica (same engine as in current factory supercharged Lotus
Elise/Exige) and currently run 10.5:1 with 14 PSI on my 4AGE. Just wondering if you really need to go so low on compression.
If you make a lot of boost low down then yes that will put a lot of force/heat on the pistons. So I always tend to make the power up high so both the
effective compresion is lower and there is les time for detonation to happen. Good mapping and a decent ECU can make a difference too.
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jkarran
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| posted on 14/9/07 at 08:40 AM |
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Interesting, those compressin ratios sound very high but I guess they're good fast burning engines and as you say, you keep low end boost down.
I assume both of those used centrifugal compressors of one sort or another?
I have a roots blower on mine, part of the appeal of it is to fill out the torque curve at lower RPM. That said I have built an electronic bypass
valve so I should have pretty effective and repeatable boost control. Maybe a compromise around 10:1 might be worth trying. I'll work out how
much that requires machining out.
I'm running Megasquirt for fuel and sparks.
jk
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bimbleuk
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| posted on 14/9/07 at 12:57 PM |
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Might change the situation slightly if you are using a roots blower. My Celica used a horrible straight rotor roots blower which would have been less
efficient than the twisted rotor Eaton blowers. So the charge temps after the supercharger were very high requiring a rather large intercooler.
I certainly ran 9:1 compression again with a straight rotor roots blower and 10 PSI on a 1800cc 4AGE hybrid. That used an MBE ECU about a 12 years or
a go! Both fuel and ECUs have improved a lot since then.
Yes my current 4AGE is using a centrifugal Rotrex. So boost doesn't start till 3000RPM but quickly builds to 8 PSI by 5000RPM approx then again
rapidly increase to 14 PSI as I hit the limiter at 7200RPM.
So maybe to try to retain the better response and efficient burn of high compression maybe consider 9.5:1 as a good compromise.
As for running a toothed belt in my experience its fine for drag but isn't such a good idea for a road car. The belt is part of the supercharger
system as it provides a buffer for the blower bearings. Reducing shock loads as you come on and off the throttle. Careful routing of the blower belt
to provide up to 270 degrees of wrap will help reduce slippage problems a lot. Ally pulleys also seem to grip belts better than steel. The worse case
situation for a toothed belt is the teeth stripping off completely!
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jkarran
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| posted on 15/9/07 at 01:11 PM |
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Interesting. The belt I had planned to use is a bit marginal anyway so maybe I'll try cutting a poly-v drive pully, see how I get on with
that.
I'm running a dump valve and the servo controlled bypass so the pressure shouldn't spike too hight when I lift off. I really don't
want the butterflies bending or blowing off!
The heat is a worry, I'm using a core the same size as the CooperS had (its a CooperS M45 blower), I'm sure I can keep inlet temp under
controll on the road as I hardly ever use full throttle or rpm now let alone with a 40% power increase. It'll need watching on the track though.
I also need to test my composite blower outlet in the oven, see how it reacts at 100degC, if it softens the bolts will pull through and it'll
just blow off at 10psi.
There's picture at http://s118.photobucket.com/albums/o118/jkarran/
jk
[Edited on 15/9/07 by jkarran]
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