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2007 CRF450R Compression Ratio

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I am in the middle of rebuilding my '07 CRF450R's engine.  It had over 110 hrs on the piston so it was time.  The crankshaft also needed a rebuild as the stock connecting rod was scuffed up in the small end pin bore.  Luckily, it didn't blow up.

 

I am doing a basic blueprint job on this engine while I am at it so I am doing what needs to be done to calculate the actual compression ratio.  I CC'd the head, measured the piston-to-deck clearance, and CC'd the piston dome on the OE replacement piston.  Got all done with that and figured the compression ratio out.  Honda specs the stock compression ratio at 12:1, my engine's actual compression ratio is 9.9:1.  I was thinking "What?" but I double checked everything and sure enough, 9.9:1 is my engine's actual compression ratio.  The cylinder head CC'd at 31.8cc and the deck clearance was 0.075".  The OE piston as a small dome on it that basically makes up for the lost compression of the valve notches, which are about 0.22 CC's a piece.  So I am treating it like a true flattop piston in the compression calculations.

 

Has anybody else out there checked their actual compression ratio on their '07 CRF450R and ended up with about 9.9:1 compression?

 

When I put it back together it will be 12:1 like it's supposed to, I am going drop the head down closer to the piston and then mill the head to make up the rest.  I have plenty of piston-to-valve clearance to do this.

 

Thanks for looking.

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I'm not trying to cut down your build.. But you just made it a hell of a lot more complicated than needed. Not to mention that milling on a piston is a bad idea. Those thing put up with way too much as it is to be changing its design.

You start milling the head and you start messing with the timing and cam chain length.

I would of just ordered a 13.5 cp piston and be done with it

Edited by Diggla117

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I'm not trying to cut down your build.. But you just made it a hell of a lot more complicated than needed. Not to mention that milling on a piston is a bad idea. Those thing put up with way too much as it is to be changing its design.

You start milling the head and you start messing with the timing and cam chain length.

I would of just ordered a 13.5 cp piston and be done with it

Nowhere did I say that I was milling the piston, I'm am certainly not doing that.  That would be counterproductive.

 

I thought about a 13.5 piston, but with the way that this engine was built up in it's stock condition, I think the compression ratio would only end up at about 11.5:1.  The head is way too far from the top of the piston, don't know why, but it just is.  Ron Hamp said that it is OK to mill the cylinder down until there is about 0.045" of piston to head clearance.  This is all fine as long as I have enough piston to valve clearance, I have 90 on the exhaust and 110 on the intake.  Plenty of room to lower the head down.  I wanted to stay with the OE piston, they wear better than anything else I have used and the lower the dome the better the burn is.  I also installed a MCCT, so the chain length change shouldn't be a problem.  I will definitely pay attention to the cam timing when I put it back together and make sure that it's all good.

 

You are correct, I am making this more complicated than it has to be.  You should have seen what I went through with the 447 big-block Chevrolet I built a couple of years ago, to get 10.1 compression on that.  These engines are easy compared to that. 

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I thought about a 13.5 piston, but with the way that this engine was built up in it's stock condition, I think the compression ratio would only end up at about 11.5:1. The head is way too far from the top of the piston, don't know why, but it just is.

Oh So you have a special 07 crf450r? How many of these were made and why does yours have such a low compression ratio? Also you can watch the timing all you want but you will not be able to change it. I'm just sayinn.

I just don't understand how so many company's have studied this motor to come up with the best Product they can for it and somehow messed up the compression ratio or missed your particular model.

I'm not trying to start some arguement but let's be real, what do you have a special crank or a special limited edition longer cylinder? Was your head ported on the wrong side? Because if it's all factory then whys your special? Ever think maybe your going about getting your compression ratio wrong? Leaving something out?

Like I said in not trying to argue. I'm always up to learn so if you have a minute please explain.

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Oh So you have a special 07 crf450r? How many of these were made and why does yours have such a low compression ratio? Also you can watch the timing all you want but you will not be able to change it. I'm just sayinn.

I just don't understand how so many company's have studied this motor to come up with the best Product they can for it and somehow messed up the compression ratio or missed your particular model.

I'm not trying to start some arguement but let's be real, what do you have a special crank or a special limited edition longer cylinder? Was your head ported on the wrong side? Because if it's all factory then whys your special? Ever think maybe your going about getting your compression ratio wrong? Leaving something out?

Like I said in not trying to argue. I'm always up to learn so if you have a minute please explain.

No, I don't think the bike is in any way special.  I have reviewed my compression ratio calculations several times and I haven't left anything out.  That's why I am asking if anybody else has seen an engine with the compression ratio seemingly wrong like this, it certainly doesn't seem right but the data doesn't lie.

 

Best Product?  My factory Honda crank was running out 6 thou on the left side and 3 thou on the right, when the spec is 2 thou or less.  I wouldn't have believed it except I measured it myself.  Hardly seems like they put out the best product there.  Stuff does get screwed up from the factory, and not just motorcyle parts either.  Tolerances stack up the wrong way sometimes, that may be what I have here.  Just trying to see if anybody else has seen anything similar.

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No I was speaking of after market company's studying the engine...

I just don't get how people(speaking in general) come up with comp ratios in these motors when you can't measure out every nook and cranny.

Hot rods crank, 13.5 piston and see what that end up adding up too maybe? If you feel your crank is off the replace the whole thing right? Boom! Ha Goodluck man

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I am in the middle of rebuilding my '07 CRF450R's engine.  It had over 110 hrs on the piston so it was time.  The crankshaft also needed a rebuild as the stock connecting rod was scuffed up in the small end pin bore.  Luckily, it didn't blow up.

 

I am doing a basic blueprint job on this engine while I am at it so I am doing what needs to be done to calculate the actual compression ratio.  I CC'd the head, measured the piston-to-deck clearance, and CC'd the piston dome on the OE replacement piston.  Got all done with that and figured the compression ratio out.  Honda specs the stock compression ratio at 12:1, my engine's actual compression ratio is 9.9:1.  I was thinking "What?" but I double checked everything and sure enough, 9.9:1 is my engine's actual compression ratio.  The cylinder head CC'd at 31.8cc and the deck clearance was 0.075".  The OE piston as a small dome on it that basically makes up for the lost compression of the valve notches, which are about 0.22 CC's a piece.  So I am treating it like a true flattop piston in the compression calculations.

 

Has anybody else out there checked their actual compression ratio on their '07 CRF450R and ended up with about 9.9:1 compression?

 

When I put it back together it will be 12:1 like it's supposed to, I am going drop the head down closer to the piston and then mill the head to make up the rest.  I have plenty of piston-to-valve clearance to do this.

 

Thanks for looking.

I think you are going about this the wrong way.

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I think you are going about this the wrong way.

Really?  How so?  You need to give more details, your leaving things hanging with that answer.

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CV+CHV(at tdc)/CHV. I do this with piston installed on connecting rod, ccing with head on.

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Sounds like you're leaving out the head cc in the numerator of the equation...

Show us your math and we'll help you figure it out. I hope you didn't use that same math on your Chevy motor build...

Really? How so? You need to give more details, your leaving things hanging with that answer.

Edited by Eddie8v

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Hah!

Disable your decomp system and try to kick start it! It won't take anyone long to realize it's 12+ CR.

Edited by Eddie8v

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Sounds like you're leaving out the head cc in the numerator of the equation...

Show us your math and we'll help you figure it out. I hope you didn't use that same math on your Chevy motor build...

 

 

 

CV+CHV(at tdc)/CHV. I do this with piston installed on connecting rod, ccing with head on.

 

Head CC = 31.8

Clearance Volume CC = 13.8  (0.075" down from deck) 

Piston Dome CC = 0  (The tiny dome on the OE piston essentially makes up for the valve reliefs.)

Cylinder Volume CC = 449.5

 

CR = (449.5 + 31.8 + 13.8)/(31.8 + 13.8) = 10.9

 

Ok, that's a little closer.  Thanks for your input.  Your right, I left the head CC out of the numerator.  It's still kind of low, just not as low as I originally thought.  Luckily for me, when I did the calculations for the big-block Chevy, I did it in my engine analyzer software, no hand calcs on that one.  If I actually did the hand calcs then I might have remembered how to do it correctly this time.

 

bushman45, how do you seal the rings up to keep the measuring fluid from seeping down through the rings? 

 

Eddie8v, yes, they are pretty much impossible to kick through without working decompression.  I thought my CR500 was bad but these new 4-strokes will break your leg without a decompression system.

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Head CC = 31.8

Clearance Volume CC = 13.8  (0.075" down from deck) 

Piston Dome CC = 0  (The tiny dome on the OE piston essentially makes up for the valve reliefs.)

Cylinder Volume CC = 449.5

 

CR = (449.5 + 31.8 + 13.8)/(31.8 + 13.8) = 10.9

 

Ok, that's a little closer.  Thanks for your input.  Your right, I left the head CC out of the numerator.  It's still kind of low, just not as low as I originally thought.  Luckily for me, when I did the calculations for the big-block Chevy, I did it in my engine analyzer software, no hand calcs on that one.  If I actually did the hand calcs then I might have remembered how to do it correctly this time.

 

bushman45, how do you seal the rings up to keep the measuring fluid from seeping down through the rings? 

 

Eddie8v, yes, they are pretty much impossible to kick through without working decompression.  I thought my CR500 was bad but these new 4-strokes will break your leg without a decompression system.

your measuring methods are sketchy. put the piston atc and seal it with grease. put the head on with a used head gasket, snug nuts. tip engine to where the spark plug hole is about level, fill to bottom of hole using a burret. I like to use atf as a fluid.

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your measuring methods are sketchy. put the piston atc and seal it with grease. put the head on with a used head gasket, snug nuts. tip engine to where the spark plug hole is about level, fill to bottom of hole using a burret. I like to use atf as a fluid.

Thanks for your advice, I think I will try that.  Do you have good success in getting all the air out when using this method?  It's kind of hard to see if there are any bubbles left with the cylinder head on.  The lexan I normally use is pretty easy to see through, but I am not measuring the entire combustion space when I am using to measure the head only.  I have been using a depth micrometer to measure down to the piston deck above the wrist pin at TDC and then calculating the clearance volume.  That leaves the piston top and dome as the only parts that I have not been able to CC completely.

 

Thanks again for your input.

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try this and you should be good. the spark plug is centered in the head so you shouldn't trap any air.

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OK, so I mocked everything up at TDC, sealed the rings with grease, the head studs and bolts torqued down to 25lb-ft to compress the gaskets, and measured with my the head on.  I get 45.2 cc total. 

 

(449.5 + 45.2)/(45.2) = 10.94

 

10.94 versus the 10.86 I got above.  I then took it apart, cleaned things up, and used small pieces of lead shot to measure the distance from the piston top to the head deck after torqueing things back up again.  Piston deck-to-head deck is about 0.070" to 0.071".  The gaskets compress about 5 thou or so if you torque down the head studs and bolts versus just snugging them up.  I guess that I will have to cut the cylinder deck down some to get it back to the stock compression ratio, I have plenty of room.

 

I found out something else that I had forgotten about in going through all of this.  I bought this bike used 4 years ago from my current boss.  The original engine blew up before he sold it to me.  The engine that I am working on here is an OE replacement engine that he put together before selling it to me.  He bought a replacement bottom end from the dealer, new OE piston kit, new OE cylinder, and a reworked head with all stainless valves.  He then assembled it, rode it for a month and I became the new owner.  This may be an engine full of OE parts but it was not assembled by the OE manufacturer.  Hence the tolerance stackup that seems to be pushing the head up high giving me a lower than stock compression ratio.  I guess the lesson learned is that you should always check these kinds of things before you throw an engine together.  The replacement parts may be serviceable, but they haven't been bolted together before so things may not be a good match tolerance wise. That is one thing an OE engine assembler can do, match up the parts, they have plenty to choose from. 

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No way Honda mf'g tolerances could ever be off enough to turn a 12-12.5:1 engine into 11:1. You might have a cylinder from a different year or model bike that's a tad bit longer or something, so the piston is sitting further from the head. Wrong piston, from different year or model? Not sure what it is exactly, but something ain't right.

Edited by Eddie8v

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cylinders are all the same. .070+ squish is not right, should be around .056ish. this is a 12.1 motor and I'd bet it's got the wrong piston and or cylinder on it. X stuff maybe.

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