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Picking new cylinder wall material need help


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81mm bore with 2mm dry sleeves give only 1.5mm of original cylinder left. That's not enough to hold liners and deal with lateral piston forces. I tried as thin as this on 6500rpm engine and liner & cylinders cracked.

 

You'll want at least 3mm of each if going with dry-sleeves.

 

Going with offset crank or piston-pins would help reduce lateral forces on cylinder walls....

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@Mohawk Thank you for the reply as I forgot to measure the piston height from the center of the wrist pin hole! I remember looking at them side to side and I think even though the new cbr piston has a compression ratio of 13:1 the cbr uses a longer rod by a few mm. If I get custom rods with the same rod length as the vfr, compression ratios should be similar from the old piston to the new one.

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16 hours ago, Vfr800witdawaffle said:

I also weighed the pins and the cbr pin is only a couple grams heavier. It’s also beefier looking. The interceptor pin is 17mm in diameter and longer while the cbr is 18mm. I’m going to have to use a different rod. I’m going to look at oem steel offerings from other manufacturers including Honda. Trying to keep costs down and I will not be using titanium. Titanium rods don’t last as long as steel rods when it comes to putting down hundreds of thousands of miles.

As mohawk mentioned, CBR pin is much stronger than needed for your application. It's 25% stronger than VFR pin. You can taper-bore inside to get identical weight and it would still be more than enough.

 

Then again, you're still looking for rods...

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On 3/21/2021 at 11:00 AM, Vfr800witdawaffle said:

@Mohawk Thank you for the reply as I forgot to measure the piston height from the center of the wrist pin hole! I remember looking at them side to side and I think even though the new cbr piston has a compression ratio of 13:1 the cbr uses a longer rod by a few mm. If I get custom rods with the same rod length as the vfr, compression ratios should be similar from the old piston to the new one.

it's just not compression-height of piston that matters, but combustion-chamber volume of head. Larger displacement for same head volume would result in higher compression. I seem to remember 14.0-14.4cc somewhere. Subtract volume of piston above deck to arrive at actual head cc and use new displacement to calculate static compression.

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So distance from the hole to the top of the pistons measure 14.6mm for the vfr and 15.8 for the cbr. Respectively the holes are 17 vs 18mm. That means piston height is 23.1 for the vfr and 24.8 for the cbr. So my guesstimations are off I’ll need a shorter rod going forward to lower compression. My target is going to be 12:1. Worst comes to worst I’ll back off on timing when tuning.

46E9B9E5-8B70-47B2-9F95-D30202F1434C.jpeg

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@DannoXYZ Thank you that makes a lot of sense. I’m going to be asking a lot of questions to all types of people going forward with this as this is my first custom engine build. On another note I notice the valve recesses on the cbr piston are much more prominent where as the vfr piston is flatter. I think the volumes above deck height might be similar even though the new piston is wider.

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there are at least 2 dozen ways this project could go BANG !

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I'm finding this a very interesting project - looking forward to seeing how it works out.  :fing02: 

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You need to compare valve spacing & angles. The CBR probably uses larger valves, and they are likely at a smaller included angle, so the VFR valves may contact on their lower lips. 
 

IIRC the VFR rod is 4" or 101.6mm c2c. I have the data somewhere but from memory IIRC the RC45 rods are significantly shorter. I dug out the numbers below. 

>>>

here the conrod weights
 
rc45 rod     259/260 grams
rc30 rod     256/257 grams
 rc36          312/311 grams
 
complete pistons with piston pins and rings and circlips
 
rc30 220g
rc45 233 g
vfr 750 rc36 ( no piston pin and no circlipls but with  rings ) 190g
 
rods interaxle c2c
 
rc45 93.8 mm
rc30 107.4 mm
rc36 104,0 mm
<<<
 
Custom rids may be your only answer. If going with the CBR1K pistons. The older 1K had 78mm bore which would give you more space to play with, plus they use the same piston pin diameter as the VFR. You might look at the CRF250 big bore pistons too.
 
Only the top half of the VFR cylinder is unsupported with the lower half passing through the crankcase. A 2mm walled iron liner would be strong enough if fitted with a closed deck as only approx 22mm is above the crankcase base & below the deck lip, assuming 3mm deck lip thickness. I'd completely mill away the open deck portion of the remaining cylinder walks once they have been centrally bored for the liners. This will improve cooling, having the coolant in direct contact with the liner. 
 
Have fun. 

 

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Yes, included valve-angle is 23-degrees on VFR and 9-degrees on CBR1000RR-R, so piston & combustion-chambers are configured differently. 

 

1. First and most important point-of-reference are squish-zones on piston-tops between each set of intake & exhaust valves (blue). As piston rises, it squeezes mixture towards centre of chamber and spark-plug. At higher-revs, this squish action is extremely turbulent and helps fully atomize and blend air-fuel mixture. This removes lean & rich spots and promotes efficient and fast combustion with resistance to knock/detonation. Clearance to head at this squish-zone needs to be around 1mm for optimum effect. Which happens to be thickness of headgasket, so you'll want to get rod-length to place piston's squish area flush with top of deck.

 

uc?export=download&id=1pAF0eC1nnOX9YXO47

 

 

2. Starting around 2000, Toyota's 1ZZ engine showed additional angled squish-zones for more focused squirting of mixture towards spark-plug (green). This created even better mixture swirl and improved combustion on high-compression engines. Also made less crescent-shaped combustion-chamber and more centralized mixture (similar effect as hemi-heads in '60s). You'll need to mill head to provide clearance for this angled squish-zone where there is currently flat metal outside of current bore. I recommend CNC process so you can make same cuts on all combustion-chambers on all cylinders.

 

uc?export=download&id=1iWqwmIGZmXW6Ci2SE

 

 

3. you'll want to clay piston-tops and spin engine 720-degrees. Slice through clay and measure clearance between piston-top and valve. Angle of valve won't match pistons and will be tightest at outside edges of each valve. You'll want 2mm minimum clearance here.

 

uc?export=download&id=1EoTI67VBTrGdGTo0R

uc?export=download&id=1xcvosEFTNslHPLuXU

 

 

 

4. with modern machinery and cutters, there's A LOT more that can be done to improve flow through head. Used to be that 5 or 7-angle valve-seats were better than factory 3-angles. But today, we can use fully-radiused smooth seats for best flow with back-cut valves. And install copper-beryllium seats for much better heat-transfer than OEM seats. You've got extra space with larger bore, so might as well install slightly larger Inconel exhaust and stainless intake valves.  Removing ledge at valve-pockets and unshrouding valve at outsides really improves flow.

 

uc?export=download&id=1bSojGCWJXcWSh1Hor

 

 

 

Final combustion-chamber volume should be higher than CBR1000RR-R and give you less than 13;1. You can measure final head+piston CC using pipette.

 

 

 

 

 

 

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