VFR848RR Anyone ???

Hi All,

I've been toying with the idea of a bigbore VFR800 to carry on where Honda could have gone. I've updated all the cycle parts on my Viffer & now looking toward next winters project. So to that end I've bought a few bits to check the possibilities.

First thought was CBR929 pistons @ 74mm bore verses the VFR's 72mm. That would give 825cc, the other option was the CBR954, but I thought that would be one to big & two to heavy, until I found a site by accident listing the 929 to 954 changes, one of which was the pistons are lighter than the 929's !

So I acquired a piston to test the theory & removed a standard one from my spare engine. So here are the findings;-

All pistons use 17mm wrist pins,

Deck heights & piston crown heights are the same.

As are the valve pocket dimension (but would need to check valve to pocket clearances).
VFR Std Piston 72mm = 238grams (inc wrist pin)
CBR929 Piston 74mm = 246grams (inc wrist pin)
CBR954 Piston 75mm = 227grams (inc wrist pin)

So the 954 piston is lighter than both the standard VFR & 929 one, this means the rod loads would NOT exceed standard specs & the rev range could be maintained Deck heights & piston crown heights are the same, as are the valve pocket dimension, but would need to check valve to pocket clearances.

The next consideration is cylinder wall strength & compression ratios. The cylinders have 8mm walls & Honda supplies .25 & .5mm oversize pistons for the VFR, so max overbore leaves 7.75mm walls.

929 pistons takes that to 7mm walls & is probably OK & gives 12.2/1 compression ratio, which would restore the dynamic compression ratio to the same as standard with my cams (std=10.86, my cams/929 bore=10.91). Which is perfect for standard unleaded (UK=95RON).

954 pistons takes that to 6.5mm walls & may be OK & gives 12.5/1 compression ratio, which would increase the dynamic compression ratio to 11.16. Which is too much for standard unleaded (UK=95RON) & maybe a bit much for Super unleaded (UK=97RON).

Options ?
1. Risk the wall thickness being sufficient. CBR's have thinner walls between the cylinders & use the same MMC liners !
2. Use containment rings to reinforce the cylinders, only need to reinforce the top 15mm to stop flex at peak pressure, which can cause D-chunk failure of the liner.
3. Machine the 954 pistons to lower the compression ratio, to acceptable levels & match the VFR valve spacing.
4. Have made or modify standard head gasket to fit pistons.

So what would this give us ? Well VFR750's taken to 837cc have shown 133rwhp & they used heavier JE pistons ! So a VFR848 should pump out around 135-140rwhp I'd expect around 62-64 ft/lbs torque with the additional compression etc.

Worth the effort ? Well if you do the spannering yourself & use second-hand pistons (they are silly expensive new) Then you should be able to get it all done for less than 1000UKpds (<$1500).

Your views as always are appreciated

 

That will be a kick ass little project.....subscribed!

 

If I read correctly, you have a spare engine? If so, go for it. I would be scared of trying that if I was using the actual engine from my bike. Afraid I can't help with any of the technical stuff, I'm pretty clueless in that dept.

 

:hss: Mohawk must have a lot of free time :wink:
:crazy: project boss, I will follow along to see how this turn out. Good luck :thumbsup:

 

11.5:1 can run 92 octane (US) super unleaded fuel with no detonation. The rwhp math is quite generous with respect to adding 66cc's...or 16.5 cc's per cylinder. Stock is 195.5 cc's per cylinder (782 cc's), which would be a gross increase of 8% of cylinder volume increase.....also, you better start looking at larger tb's and possibly bone up on microsquirt programming logic. I would also take a hard look at the thermodynamic feasibility of 6.5mm cylinder walls.....

 

:bravo:
Good to see you drop by NCB

 

Well working on the VFR837 dyno charts I've seen that show stock 750 at 85rwhp & 837 at 133rwhp, which gives a x1.56 increase, so if a standard 5th gen is 95rwhp, then x1.56=148hp, so 135-140 is NOT an unreasonable expectation. The VFR's are short on compression as standard & the big bores give this back. The 750's have smaller carbs & nothing else done to make this power, so I would expect the extra 11cc to at least be worth a hp or two !
The thermo dynamics should not be a worry, MMC liners are fine if cooled properly & my bike runs cool already due to previous mods.
This is just a paper exercise at the moment, will hopefully become real in the winter, so don't expect any updates soon !

 

op2:
Good stuff in here.

 

Oh & the TB's are good enough to flow 400-500cc more on Bandit1200 & CB1300, both of which have 36mm TB's & consume 50% more air than a VFR at max revs !
Both are NOT short on power

 

I am currently, and imagine you are, chuckling and laughing like an evil genius from some cartoon.

Jealous of your beastie project! I love something unique and that, is certainly going to be unique.

 

Thanks for the giggling support If it works as anticipated it will be brilliant, if not, well then you'll all know not to both and it won't be a huge financial loss. I have another idea, but that's for another day

 

Seems like an awful lot of hard work for a limited return. Plus you have to figure in any decrease in reliability.

personally, I would not try it. however, experimentation is what makes the world go round.....

 

Some enjoy being pioneers. I personally wouldn't do it to my original motor. I'd rather have a spare blow up.
Let's be honest Mohawk, you scored a set of Ducati 848 decals and just want an excuse to put them on the bike

 

I like your thinking, I did a calc for the 75mm bore & it comes out as 847.8cc so 848 came to mind & that Duke competition I was looking at one the other day & thought how much power do they make, could I turn a sport tourer into a Duke beater ? Might just give it a whirl, if it goes pop, well that's life you only live once !

Just had a look, average on a bunch of dyno charts is 118rwhp & 62ft/lb torque. My VFR already makes 107rwhp & 57.5ft/lbs, so the same feed mods to the bigbore 848 should match or exceed the Duke848

 

How about power to weight ratio differences between the two machines? If the Duc has 75 lbs less mass than the VFR, the VFR will need an additional torque increase over and above the Ducati in order to overcome that mass on acceleration.

 

Sounds like a good project. Just a little hot rodding a VFR. Seeing the pistons you chose are lighter, what are you going to do about balancing? I would think you'd want to at least look at it. You might be able to lighten the crank a little. Which should make it spin up faster. Just curious. Keep us posted on the build and good luck.

 

Awesome project- i think Hotrodding that engine is more important than comparing it to another bike. I'm really impressed you came up with this concept!

 

Are the valve relief cut outs on the new pistons the same as on the VFR pistons? Will you run into clearance issues with dome height? Also, is the wrist pin to dome height the same for both sets? If so, this could work, since it seems like you have thought of everything else? And if you have a spare engine, why not do it?

The VFR is really only 781cc, so bringing it up to 848cc and increasing compression could make for a fun bike once you get the fuel injection re-mapped.

 

So, assuming in your photos that the top piston on the scale is the VFR and the last piston on the scale is the donor piston from the CBR, the valve relief cutouts on the CBR piston on the bottom half look smaller than the VFR piston. Or is it just me?

 

i've not fully compared the pistons, but wrist pin, deck height etc are al the same. CBR uses the same size valves as the VFR, si the cut outs should be the right size, just not necessarilly in the correct place. I'll be doing some plasticene molds to check this.