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  1. #11
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    Re: cyl head squish in relation to shape of piston

    Quote Originally Posted by niteridder64 View Post
    I had a rotax 670 built for me for use in and exp aircraft. I am lower on power than I thought and low on compression-120- w/ a good known gauge. I checked the squish and it measured .120 w/ a piece of twisted solder parallel w/ the pin up against the cyl wall. After removing the head I saw the squish area is beveled up yet the piston top is flat. the squish is .063 deep into the head and the deck height is .010. So .073 squish but only at the outermost point and gets bigger really fast toward center. My question is, should I have domed pistons in this eng? What is a good squish and cranking compression for a safe stock set up? Thanks.
    ================================

    Who Built your Engine?

    There were (2) Skidoo 670's made. The Standard with a Dome Piston that was 11.5cr made 1992 to 1999 and the 670HO with a somewhat Flat Top Piston was 12.5cr was only made 1998-1999.

    Your Dual Plug Head is for the Standard 670 Dome Piston. So if you have 670HO Pistons lower end and used your Dual Plug Head, the CR would be lower than 11.5cr, how much exactly I can't tell you at the moment. There are Head CC/CR Calc out there.

    IF you just figure Atmospheric Pressure is 14.7 at Sea Level (NASA Standard) and it looks like your in Tampa, FL. Your 120psi/14.7 = 8.2cr. So to get back your CR, your Head would need to be Milled. What Thickness of Head Gasket was used? Was (1) or (2) Gaskets used?

    Numbers at Sea Level. For each +1000 ft you start out at minus 3% for Density Altitude, or 1.5% per +500 ft!
    12.5cr x 14.7 = 183.75 psi
    11.5cr x 14.7 = 169.05 psi
    10.5cr x 14.7 = 152.25 psi
    9.5cr x 14.7 = 139.65 psi
    8.5cr x 14.7 = 124.95‬ psi

    Are you maybe using a Rotax 582UL or 618UL Muffler on your 670? Your Motor will make less HP also.

    670 with an R&D Aero 618 Tuned Pipe. 106.1hp@7000rpm, 97hp@6600rpm, 90.1hp@6400rpm!
    http://www.rotaxservices.com/dyno.html#89

    670 with Rotax's 618UL Muffler. 85.1hp@6600rpm!
    http://www.rotaxservices.com/dyno.html#90

    A 670 with a 582UL Muffler will make even less HP than with the 618UL Muffler.

    Rotax Rick in Naples, FL makes a 670 and with his Tuned Pipe it Dynoed 92hp@6350rpm!
    Attached Images Attached Images
    Last edited by ARMILITE; 10-04-2019 at 02:01 PM.

  2. #12
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    Re: cyl head squish in relation to shape of piston

    Engine Compression Ratio (CR) Calculator.
    http://www.csgnetwork.com/compcalc.html

  3. #13
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    Re: cyl head squish in relation to shape of piston

    If you look at this Calc: https://www.mk5cortinaestate.co.uk/calculator3.php

    A 582UL is 580cc and if making 65hp@6500rpm per Rotax = 73% VE with Rotax Muffler!

    A 618UL is 617cc and if making 73.4hp@6750rpm per Rotax = 79% VE with Rotax Muffler!

    A 1993 670 (669cc) Dynoed 115.7hp@7750rpm in a Sled, = 94% VE with Skidoo's Tuned Pipe.

    So your 670 (669cc) at 94% at 6500rpm with a Good Tuned Pipe Designed for 6500rpm should make 96hp! Key Words: Designed for your 6500rpm, not 7750rpm!

    Rotax Ricks 670 with his Tuned Pipe Dynoed 92hp@6350rpm = 92% VE. You can see by his Dyno Print his Tuned Pipe dropped out after 6350rpm. It looks to be fairly Smooth Power Band with no Dead Spots. Could a better Tuned Pipe be made for 6500rpm sure, it just takes Time & Money.
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    Last edited by ARMILITE; 10-04-2019 at 02:54 PM.

  4. #14
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    Re: cyl head squish in relation to shape of piston

    Thanks for all the info. This was a Rotax Rick's670 eng w/ the pipe modified my Mike's custom pipes, supposedly giving me 90 hp, it was nothing close to it. I decided to take matters into my own hands and build this eng the way it should be. I sent the cyl out to be bored .010 over, according to the the machine shop one cyl was .005 larger, so got clean cyl back and had mike mill a 670 interchangable domed head w/ the right squish cut into it. I removed the .047 base gasket rick installed and repl w/ a .020. The squish angle and height were much better at creating msv, can't remember all the #'s now but put it all back together w/out making any changes to the prop and gained 900 rpm, not a typo, I only increased my static psi by 10- went from 120 to 130. I jumped from a 175 main to a 180 and may go to 185 for a bit of extra safety. I also need to go w/ a higher % of water and wetting agent in the rad as my cyl temp has increased by about 10 deg. I am very happy w/ the way she runs no more burbling at the lower midrange, very smooth/ clean power up to 6600 rpm. I like Rick but shoving a .047 base gasket in and using the 582 head is not the right combination, the squish was way to large which gave a weak charge in the dome and wasted fuel.

  5. #15
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    Re: cyl head squish in relation to shape of piston

    Quote Originally Posted by niteridder64 View Post
    Thanks for all the info. This was a Rotax Rick's670 eng w/ the pipe modified my Mike's custom pipes, supposedly giving me 90 hp, it was nothing close to it. I decided to take matters into my own hands and build this eng the way it should be. I sent the cyl out to be bored .010 over, according to the the machine shop one cyl was .005 larger, so got clean cyl back and had mike mill a 670 interchangable domed head w/ the right squish cut into it. I removed the .047 base gasket rick installed and repl w/ a .020. The squish angle and height were much better at creating msv, can't remember all the #'s now but put it all back together w/out making any changes to the prop and gained 900 rpm, not a typo, I only increased my static psi by 10- went from 120 to 130. I jumped from a 175 main to a 180 and may go to 185 for a bit of extra safety. I also need to go w/ a higher % of water and wetting agent in the rad as my cyl temp has increased by about 10 deg. I am very happy w/ the way she runs no more burbling at the lower midrange, very smooth/ clean power up to 6600 rpm. I like Rick but shoving a .047 base gasket in and using the 582 head is not the right combination, the squish was way to large which gave a weak charge in the dome and wasted fuel.
    =========================================

    Did you CC the 582UL/618UL Head? Nothing wrong with using it as long as it's Machined for the larger 78mm Bore. Usually 1mm over, so 79mm. I think he had the idea of lowering the CR to make the engine last longer. The Flat Top 670HO Piston already lowers the CR. The 670HO Flat Top Piston and the 670HO Head give you 12.5cr, the Standard 670 Dome Piston and Head is 11.5cr. There are about (5) different Base Gaskets to use. I agree the .047" was probably too Thick, which lowered your compression even more.

    If you look at Hirth, Simonini, etc., most modern 2 Strokes are running 9.5cr in all there Engines. Detonation is the #1 Killer of Rotax's. A +1.0cr = 1hp on Avg. If you burn 100LL Aviation Fuel you can run about any CR, but for Airplane use I wouldn't go over 12.0. The 277UL used 11.8cr which Fresh 91 Octane is good for 175 psi. 11.8cr x 14.7 = 173.46 psi at Sea Level. For every 1000ft above Sea Level, you subtract off 3%. Like I'm at 940ft above Sea Level so 173.41 psi - 3% = 168.2562 psi. Most 2 Strokes Cylinders aren't Bored & Hone right with a Torque Plate, so not perfectly Round once put together so they experience some blow-by.

    What Redrive Ratio are you using and Prop Spec's Lenght & Pitch to get 6600rpm?

    CR Chart PSI made at Sea Level. 14.7 Atmoshereic Pressure is what NASA uses as a Standard. It could be different in different parts of the World.

    CR RATIO PSI
    8.5 14.7 125.0
    8.6 14.7 126.4
    8.7 14.7 127.9
    8.8 14.7 129.4
    8.9 14.7 130.8
    9.0 14.7 132.3
    9.1 14.7 133.8
    9.2 14.7 135.2
    9.3 14.7 136.7
    9.4 14.7 138.2
    9.5 14.7 139.7
    9.6 14.7 141.1
    9.7 14.7 142.6
    9.8 14.7 144.1
    9.9 14.7 145.5
    10.0 14.7 147.0

  6. #16
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    Re: cyl head squish in relation to shape of piston

    Thanks for the post, both cyl are at 135 psi w/ a warm cyl at sea level w/ an accurate gauge. I run a 4 blade warp drive and to my best recollection am at 11.5 pitch w/ a 2.65 reduction. Right now my pipe is off to weld a crack and sent out my rad and head to be coated w/ a special heat xfer coating. I am approaching 190 in a sustained climb, curious to see if this helps. A +1.0cr = 1hp on Avg. is this true? so from raising the comp from 125 to 139.7 will only yield 1 hp? am I reading this wrong.

  7. #17
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    Re: cyl head squish in relation to shape of piston

    120 thou is crazy big. you really need to tighten that squish up. Tight squish does not cause detonation you can not detonate what is not there.
    Basically if you reduce your squish you then put more in the combustion chamber than in the squish area. as the squish area is cooler you will not make the power you from your fuel. Realistically your piston should hit the head at about 1000 rpm higher than you will ever see and you are about right then.
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  8. #18
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    Re: cyl head squish in relation to shape of piston

    Quote Originally Posted by niteridder64 View Post
    Thanks for the post, both cyl are at 135 psi w/ a warm cyl at sea level w/ an accurate gauge. I run a 4 blade warp drive and to my best recollection am at 11.5 pitch w/ a 2.65 reduction. Right now my pipe is off to weld a crack and sent out my rad and head to be coated w/ a special heat xfer coating. I am approaching 190 in a sustained climb, curious to see if this helps. A +1.0cr = 1hp on Avg. is this true? so from raising the comp from 125 to 139.7 will only yield 1 hp? am I reading this wrong.
    Yes, on these Small CC Engines. Like a Rotax 582UL is 580cc = 35.39377 ci. Hirth, Simonini, MZ, have all went to 9.5cr as a Standard. Even thats pushing the limits of 87 Octane. My New 1979 Chevy with a 350 4 Barrel with 9.5cr Designed for 87 Octane ran fine around town, and out on the highway going 55 mph it also ran fine, but once you went to pass anyone, put the pedal to the metal, it would knock bad. So I switched over to 90/91 10% Ethanol and it never knocked again. I'm at around 940 ft above Sea Level so 9.5cr = 139.7psi - 3% = 135.5psi so 87 Octane is Good for less than that. The Bigger the CC/CI the Engine is the more CR the more HP. On most of these Older 2 Stroke Jet Skies and Ultralight Engines your talking about 268cc/16.5 ci to 800cc/49 ci. A Tuned Pipe will give you the most hp Boost, then Cylinder & Case Porting 10-15%. You still have Big Bores for more CC.

    Example: A Chevy 302 V8 301.6 ci = 4,942.339 cc Hot Rod Engine, same Spec's just changing the CR.
    8.0cr = 419hp@7000rpm
    8.5cr = 432hp@7000rpm +13hp.
    9.0cr = 444hp@7000rpm +25hp.
    9.5cr = 456hp@7000rpm +37hp.
    10.0cr = 466hp@7000rpm +47hp
    10.5cr = 476hp@7000rpm +57hp

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