Anyone ever rebored/overbored a 5/6th gen ?

Mohawk - my apologies, I just caught up with your past postings on VFRD, I see you have done your homework ! Now I'm even more interested!!
Cheers and good luck!

 

Well, I finally have some progress to report. Having had problems finding somewhere to rebore my block, I spoke to TTS who informed me that they have never destruction tested a block to find out the MMC liner limits, but they are prepared to test bore a cylinder for me to test Honda's claims that they can be bored as per normal steel if kept cool.

To that end I acquired a spare upper crankcase block today & have sectioned cylinder #1 as per the attached pics. The upshot is this.
The total cylinder wall is 8mm thick, the MMC liner is 2.5mm thick on its narrowest section. It does NOT amalgamate with the cast alloy as per Honda's claims. This was evident as I hack-sawed through the section & should have had 1 piece of outer water jacket & 1 piece of cylinder wall, but got 3 pieces ! 1 x MMC piece, 1 x cast Cylinder wall & 1 x outer water jacket !

So assuming the MMC can be bored as per Honda's manual, then a 2mm over bore (74mm giving 825cc) would reduce the MMC thickness to 1.5mm, I believe this is doable. A 2.5mm overbore (75mm giving 848cc) may be possible, as it would reduce the MMC to 1mm thickness, I would expect this to require Nikasil plating to reinforce the MMC.

The other pictures show the bore offsets & the inside of the upper crankcase/block.

 

Lots of info here on HOW to do this semi-unique mod ( and some smart engineering), but nothing about WHY, expected power improvement for all that dough. There are lots of faster (and better) bikes than the VFR; seems more sensible to buy one than to grind on into your VFR.

(Or maybe i'm gettin more wimpier as i age cuz i think my VFR's power is MORE than adequate.:loyal: When i wished for a smidgen more, i re-geared my sprockets, that's all. Faster than scarey-fast is crazy-fast, and that's too fast to think about using almost anywhere! Never felt i need something like the ZRX1200 with big cams i rode once. VFRs don't want amateur surgeons digging in with new ideas just for kicks around the ole Ace cafe, dear lord !! )*

BUT WAIT !! Maybe you just needed a big project suited to your more-advanced skills rather than any sizable hp gain.......and have the time/money to persue it. Heckeroo, it's your rocket ride, Bunky !

Talk to NCB for advice before you go near any machine tools. One thing worried me was "i'll just lighten up the pistons". Better get that right or BANGERS & MASH !





*Admittedly irrelevant

 

He he, it's all fun and games to see what can be done. Don't want another bike, my Viffer is my project of choice.
No good asking NorCal anything, he's NOT the sharing kind.
This is all good fun to me & currently not costing much at all. It would if you paid someone to do it for you. And then I agree the HP gain would not be worth it.

 

Ladies & gentlemen, I give you one x Honda VFR800 MMC liner for your deliberation

Having made a second cut to find the horizonal locking tabs, it appears Honda did away with that design as it was probably difficult to manufacture & replaced it with a far simpler system that turned some horizontal grooves into the MMC & let the casting lock into those.

This is good news for you Big Bore fans, as it means the liner is thiner than previously expected
This liner I removed is between 2.41-2.55mm thick at its thinest section & 2.69-2.91mm at its thickest, these measurements were taken at 6 points around the circumference for both thin & thick points.
It is evident that the casting process does NOT produce the finished bore allignment & that the liners are bored or honed insitu as part of the upper case machining process.

So if we say a nominal 72mm bore & a nominal 8mm combined MMC & casting liner thickness, giving a measured 88mm outer diameter on 8 cylinders from 2 blocks, then;-
1. A 6mm overbore is required to totally remove the MMC liner, giving a nominal 78mm bore.
2. A 78mm bore would leave only 5mm wall thickness, this may NOT be sufficient even if Nikasil plated.
3. There is what appears to be evidence of water ingress between the liner & the casting, a thinner liner my increase this effect due to capillary action & increased flexation.
4. A thinner MMC liner may flex more & may crack of D chunk off causing engine damage, this is common in some newer Porches.
5. The last big bores for these engines were 76mm, TTS did Nikasil, which means they were running the liner at .5mm thick or slightly less allowing for a .1mm nikasil thickness !
6. A 78mm bore would yeild a 916cc VFR !!!!!!
7. Wiseco do a 78mm piston for the CBR1000RR with the correct 17mm wrist pin diameter !

Would this work I don't know (yet), more research required. But food for thought A quick calculation based on HP per CC says that my state of tune engine with a 916 displacement would deliver 125rwhp !

 

Before anyone says it, no I have not looked at compression ratios for that huge bore increase !

 

Doing some dreaming looking around at crazy engine mod stuff and came across your threads, any progress?


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Check out the latest updates on VFRD, Phill in New Zealand has got ahead of me ! http://www.vfrdiscussion.com/forum/index.php/topic/75067-anyone-ever-reboredoverbored-a-56th-gen/

 

As Mohawk outlined above, I'm keeping the thread alive with updates over on VFRD.
Currently just finished the piston mods, bores complete and I'm waiting on my cams to come back.
Check the thread at http://www.vfrdiscussion.com/forum/index.php/topic/75067-anyone-ever-reboredoverbored-a-56th-gen/
Cheers
Phil