xu10 8v with 88mm cracnk
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Thread: xu10 8v with 88mm cracnk

  1. #1
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    Default xu10 8v with 88mm crank

    Hello I am new here I just bought a 306 XSI 2.0 8v (rfx) and I want to get a little more power I thought about changing the crank to a 88mm off a diesel 1.9 and bring a performance camshaft
    will the crankshaft bolt straight up?
    do I need to change the ecu or remap it?

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    Thanks
    Last edited by oror494; 23rd September 2017 at 09:37 PM.

  2. #2
    1000+ Posts PeterT's Avatar
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    It fits, if you use XU9 thrust bearings but no idea about the resultant compression height with RFX rods & pistons.

    '92 205 Mi16
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    I don't think the compression will go too high and from the search I've done it looks like I need to take 1mm off the piston
    the compression of the RFX engine is not very high to begin with [9.5:1]
    do you thing it will work with the original eco? [8p-20]

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    8 valve engines don't like high compression at all. The 205 1.9 (XU9Ja) at 9.6 to 1 is right on the limit with 98 octane although these engines have a mechanical distributor, the RFX management may allow a bit more.
    I have a 91 mm special racing crank that I'd like to try in an RFX.

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    Quote Originally Posted by GRAHAM WALLIS View Post
    8 valve engines don't like high compression at all. The 205 1.9 (XU9Ja) at 9.6 to 1 is right on the limit with 98 octane although these engines have a mechanical distributor, the RFX management may allow a bit more.
    I have a 91 mm special racing crank that I'd like to try in an RFX.
    so you think i should stay with stock crank? and just do a rebuild?
    even if it wont start or make problem with the management I don't have a problem taking the engine out and put the old crank back

  6. #6
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    Quote Originally Posted by GRAHAM WALLIS View Post
    8 valve engines don't like high compression at all. The 205 1.9 (XU9Ja) at 9.6 to 1 is right on the limit with 98 octane although these engines have a mechanical distributor, .
    That's a bizarre thing to say. Dynamic compression is based static compression, inlet closing time, rod length and stroke. If you get the recipe correct it's rewarding.
    ducksonetime likes this.

    '92 205 Mi16
    '90 Mi16x4

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    Just saying that I've problems with many 8 valve engines, even with modified cams. The three XU9Ja (130 hp leaded) engined cars that I've bought were all retarded in the timing by the previous owners , because of this problem. I drove from Sydney to Melbourne with noisy interior exhaust boom (even bought ear muffs) and poor performance and economy, turned out to be just retarded ignition. My present 16 valve is perfectly happy with 10.5 to 1 as was Bens engine with 12 to 1 albeit with big cams.
    The old pushrod 504 engines were worse, I had problems way back with leaded fuel in an 1800 504 at 8.3 to 1, had to use non projecting plugs to cure the problem. These engines never had any more than 8.8 to 1. I'm sure they would have used more than 9.5 to 1 in the RFX if they could.

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    As Peter says it is more than just fitting the crankshaft, you need to calculate where the pistons will sit with the different crankshaft, then work out compression ratio and clearances. How many km has your engine done? I have one at close to 400k which is getting a bit tired but less than 200k should still be OK.

    Quote Originally Posted by oror494 View Post
    so you think i should stay with stock crank? and just do a rebuild?
    even if it wont start or make problem with the management I don't have a problem taking the engine out and put the old crank back
    Last edited by GRAHAM WALLIS; 25th September 2017 at 09:10 AM.

  9. #9
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    Quote Originally Posted by oror494 View Post
    Hello I am new here I just bought a 306 XSI 2.0 8v (rfx) and I want to get a little more power I thought about changing the crank to a 88mm off a diesel 1.9 and bring a performance camshaft
    will the crankshaft bolt straight up?
    do I need to change the ecu or remap it?



    Thanks
    I am building a XU10J4 with 88mm crank. If the 8v short block and rotating assembly is similar to the 16v you do not need to shave the pistons as they sit a bit over 1mm below deck with the 86mm crank. The 88mm crank will bring it 1mm higher at TDC to almost flush with the deck giving you that nice squish distance.

    You will need to be extra careful with your valve clearances and will probably need bigger flycuts cut into your pistons. Don't forget to compensate for thermal expansion. Static compression will be quite high though, 11.5:1 or so depending on the combustion chamber volume and head gasket size (again, I don't know what these values are for the 8v), so not sure how nicely it will play with the standard cam on pump fuel (as others have mentioned there are other factors you need to consider to find dynamic compression which will ultimately determine if you've built a hand grenade or a not). Don't compensate for compression with a thicker head gasket. Also remember it is this increased squish effect that will make your motor more responsive, not the extra 45 odd cc. You can achieve a similar thing by shaving the block down rather than installing a larger crank.

    I don't think it will run too well on standard management, it will probably not be able to richen up the AFR enough for the additional air coming in. Also having flexibility in timing will help a lot. Also worth checking how much the 8v head can actually flow -- might be easier and cheaper just buying a 306 S16 or GTi6 if you can stretch your budget. I think the S16's are better value at the moment but they are both a much better base for this kind of project IMHO. Or maybe consider shaving the block instead increasing stroke, might save you the headache of management at least.

    Take all advice with a grain of salt though, I do not know too much about the 8v motors. Sorry about the stream-of-consciousness style post.
    Last edited by ducksonetime; 25th September 2017 at 11:39 AM.
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    can i fit an ew10 or dw8 crankshaft in the engine? what kind of bearings do i need?
    i'm having a bit of a problem finding a good xud9 crank....

  11. #11
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    I think DW10 was the XUD9 replacement and I think those cranks fit. The XU9J4 cranks fit too if you can get one.

    Off the top of my head you need diesel thrust bearings and XU10 journal and big end bearings.

    Don't take my word for any of this though, make sure you fact check. The info is pretty easy to find though.

    Sent from my BBB100-2 using aussiefrogs mobile app
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  12. #12
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    I may have an XU9J4 crank for sale in a couple of weeks if you're interested. I just need to get this other crank crack tested first.
    Current rides:
    '99 Honda VFR800 (daily)
    '02 Ducati 998 (weekender)

    Current Project:
    405 Mi16 2.045L
    http://goo.gl/ha5jLL

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    156 GTA
    https://youtu.be/HmKwwCwGrVg
    https://youtu.be/y-kgE4HyWzk
    306 GTi6
    405 Mi16
    Series 5 RX-7 TurboII
    E30 325i

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