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Vfr848Rr Anyone ?


Mohawk

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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 :)

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Weren't the bigbore VFR800's 870cc's?

If so, then 848cc's should not be a problem.

As for the lighter 954 pistons, one could always lighten the crank to suit the pistons, which would have further benefit beyond ensuring proper balance and lack of vibration.

Also if you did all the necessary work to your spare engine, then fit it to your bike, you would not waste any riding time, and have your standard engine in reserve in case (god forbid) anything happened to the modified engine.

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Yeah the racing ones were 870cc but I don't know if they used steel liners, but I know they regularly went off like hand grenages. 870cc would be a 76mm bore & 12.82/1 compression ! I don't want to push to the outer limits :)

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I seem to remember the old Dynamo Humm kits being 870, not sure what you are referring to as being a "racing" kit though. With the VFR being a big soft couch, I don't think anyone ever raced them.

www3.sympatico.ca/dyno/wolf.htm

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Well I've never seen an 870 VFR, I assumed you were referring to the RC45 870's they built for the TT, same basic engine, all of them went bang !

They obviously didn't do much work to that 870, approx 120rwhp (unless that was a factory pro dyno, which would equate to 138 dynojet hp !) But the torque is where I'd expect it to be :)

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I realize I am just a babe in the woods . . . the skirts on the OEM pistons seems to be a lot bigger. I assume this is because of the V4, with the other two being I4's. Do you think that would cause a problem running pistons with such small skirts?

Now to address your question. My vote is as follows:

3. Machine the 954 pistons to lower the compression ratio, to acceptable levels & match the VFR valve spacing.

I don't think you'd have to reduce the compression ratio too much, and you get the lightest pistons. This of course assuming you don't mind dealing with a peakier powerplant.

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I seem to remember the old Dynamo Humm kits being 870, not sure what you are referring to as being a "racing" kit though. With the VFR being a big soft couch, I don't think anyone ever raced them.

www3.sympatico.ca/dyno/wolf.htm

Sure they raced VFRs, especially before the RC30 days when great racers like Bubba Shobert raced them in the Superbike series with much success. Although the engines had a lot of tech infusion from HRC, they still had enough VFR DNA in them. Another instance when the VFRs got a lot of racing cred was when Ron Haslam took a bone stok VFR from a dealer showroom and race it into the top places in Atlantic 200 race in England. A British motorcycling magazine also entered a mostly stock 4th gen into a 24 hour endurance race in the 90's and place at something like 7th place out of at least 200 entrants. Never heard of any sort of racing done with 5th to present gen VFRs though...... So there is something to it when older VFR owners say that VFRs had lost a lot of the "sport" it had......

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Sure they raced VFRs, especially before the RC30 days when great racers like Bubba Shobert raced them in the Superbike series with much success. Although the engines had a lot of tech infusion from HRC, they still had enough VFR DNA in them. Another instance when the VFRs got a lot of racing cred was when Ron Haslam took a bone stok VFR from a dealer showroom and race it into the top places in Atlantic 200 race in England. A British motorcycling magazine also entered a mostly stock 4th gen into a 24 hour endurance race in the 90's and place at something like 7th place out of at least 200 entrants. Never heard of any sort of racing done with 5th to present gen VFRs though...... So there is something to it when older VFR owners say that VFRs had lost a lot of the "sport" it had......

I don't think it's so much that the VFR lost it's "sport", because I see no advantage of a 3rd or 4th gen over a 6th gen in terms of performance......but I feel it's more of the "sport" lost the VFR. There was a time when supersport and superbikes were using the same technology as the VFR, then they got newer technology while the VFR didn't.

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I seem to remember the old Dynamo Humm kits being 870, not sure what you are referring to as being a "racing" kit though. With the VFR being a big soft couch, I don't think anyone ever raced them.

www3.sympatico.ca/dyno/wolf.htm

Sure they raced VFRs, especially before the RC30 days when great racers like Bubba Shobert raced them in the Superbike series with much success. Although the engines had a lot of tech infusion from HRC, they still had enough VFR DNA in them. Another instance when the VFRs got a lot of racing cred was when Ron Haslam took a bone stok VFR from a dealer showroom and race it into the top places in Atlantic 200 race in England. A British motorcycling magazine also entered a mostly stock 4th gen into a 24 hour endurance race in the 90's and place at something like 7th place out of at least 200 entrants. Never heard of any sort of racing done with 5th to present gen VFRs though...... So there is something to it when older VFR owners say that VFRs had lost a lot of the "sport" it had......

Well aware of the VFR's racing credentials in the hands of Shobert, Rainey Et al. I owned a 1986 VFR750 after all.

Was referring to the VFR800's specifically. Never saw DuHamel flog one on the track in anger, with numberplates, etc.

There's also this food for thought, but it's in a 750 engine:

http://www.vfrdiscussion.com/forum/index.php/topic/68268-96-vfr750-cbr929-pistons/page-10

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If you watch this clip from 1:20 onwards, you can tell that the HRC RC45 was NOT a 750. They were reportedly 870 or thereabouts and it obviously has some power , because it is pulling past a race prepared R1. (note the dual exhaust on the RC45)

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I love the approach you are taking. I have a set of CBR929 pistons in my garage for the same type project... only I haven't picked up a "spare" 5th gen engine yet.

I would be concerned about two things... the first you already are considering (#1 & #2) and #4 would have to be done regardless of which oversized bore you choose. I would recommend evaluating the stroke to con-rod length con rod to stoke ratio to ensure the short skirt 954 piston will not lead to excessive piston rock at TDC & BDC leading to cylinder wall scoring and possible catastrophic failure (especially at TDC with the thinner wall and increased pressure load). As the connecting rod length gets shorter when compared to stroke the piston rocks more at the top and bottom of it's stroke. On a lot of used 929 pistons you can see scoring where the skirts contacted the cylinder walls. That may be why Honda went with the shorter skirt in the 954.

Looking at the 929 & 954 they have the same stroke (54 mm) just different bores; Ron Ayers list them as the same connecting rod so the stroke/con-rod rod/stroke ratio is the same. What is the length of the VFR con-rod? If it leads to a higher stroke/con-rod length lower rod/stroke ratio then the 929/954 you may suffer excessive piston rock and decreased engine life.

PS: Just ran in the garage and measured the 929/954 con-rod and it is 105mm C-C

PPS: I just googled it and I have the terminology backwards.. the concepts the same. It is the Rod/Stroke ratio.

929/954 R/S ratio = 1.94

The 5th gen R/S ratio may be good for our stock pistons but may be too low for wider pistons that could exaggerate the piston rock. If the 5th gen R/S is greater than 1.94 the 929/954 pistons will be more stable with less rock in your project engine than in their original engines; if it is less they will rock more.

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Riding my VFR back to back with my Tuono I can't help but to think the perfection a 1000 cc version of my 98 VFR would be. I'd buy one in a heart beat. Good luck with this project. Hope it works out.

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Well I've been trying to locate the JE piston numbers for the dynamo humm 870 cc big bore kit on the VFR800's.

So finding this post has me thinking a little bit differently.

And it has me wondering about few things.

first, what do you do about the Nikasil coatings on the VFR800 bores, If you cut the cylinders more than the 3mm aren't you going to have to replate or reline the cylinders?

But then if you are relining the cylinders, why not re-sleeve and go big!. Why go from 72mm bores to only 75mm? compression goes up less than a full point. Cbr1000rr slugs are 76 stock, with 77mm and 78mm available, that would yield the magic 871, 894 and 917 respectively, Rough calculation says the 917 would have CR of just under 13.6, that is assuming you do nothing with the combustion chamber in the head.

R/S ratio has to with the rod length verses the stroke, It is independent of the bore.

Changing the cylinder bore on the VFR will not affect the R/S ratios, so if the piston has the same size or short length skirt, it has roughly the same or less chance of rocking in the bore and scuffing the liners.

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R/S ratio has to with the rod length verses the stroke, It is independent of the bore.

Changing the cylinder bore on the VFR will not affect the R/S ratios, so if the piston has the same size or short length skirt, it has roughly the same or less chance of rocking in the bore and scuffing the liners.

I agree the R/S ratio has nothing to do with the bore. My points are that the larger the piston diameter the more it will exaggerate the rock by moving a greater distance at the edge of the piston for the same degree of piston rock when compared to a smaller diameter piston; second, when the Honda engineers designed these pistons they know way more than I do about piston rock and they specifically design the skirts, rings & landings, and piston geometry for specific applications and using them for alternative applications may have unforeseen complications. If anyone knows the con-rod length of the 5th gen VFR a quick calculation will tell if these pistons will be more or less stable in the cylinder bore than their originally designed application. I'm hoping more stable... that will open up more opportunities!

I believe the whole 8mm cylinder thick sleeve is made of aluminum Powder/Metal composite so boring it out won't remove any coating. It is a mixture of argil, aluminum oxide and graphite. The manual states that as long as heat is controlled they can be "re-bored in the same manner as conventional sleeves".

PS: A quick calculation based on our 48mm stroke shows that as long as the 5th gen VFR con-rods are longer than 93.4mm C-C than these 929/954 pistons will be more stable in the new cylinder bore than in their engineered application... does anyone have a 5th gen con-rod measurement? Seb... Veefer... Bueller?

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I love the approach you are taking. I have a set of CBR929 pistons in my garage for the same type project... only I haven't picked up a "spare" 5th gen engine yet.

I would be concerned about two things... the first you already are considering (#1 & #2) and #4 would have to be done regardless of which oversized bore you choose. I would recommend evaluating the stroke to con-rod length con rod to stoke ratio to ensure the short skirt 954 piston will not lead to excessive piston rock at TDC & BDC leading to cylinder wall scoring and possible catastrophic failure (especially at TDC with the thinner wall and increased pressure load). As the connecting rod length gets shorter when compared to stroke the piston rocks more at the top and bottom of it's stroke. On a lot of used 929 pistons you can see scoring where the skirts contacted the cylinder walls. That may be why Honda went with the shorter skirt in the 954.

Looking at the 929 & 954 they have the same stroke (54 mm) just different bores; Ron Ayers list them as the same connecting rod so the stroke/con-rod rod/stroke ratio is the same. What is the length of the VFR con-rod? If it leads to a higher stroke/con-rod length lower rod/stroke ratio then the 929/954 you may suffer excessive piston rock and decreased engine life.

PS: Just ran in the garage and measured the 929/954 con-rod and it is 105mm C-C

PPS: I just googled it and I have the terminology backwards.. the concepts the same. It is the Rod/Stroke ratio.

929/954 R/S ratio = 1.94

The 5th gen R/S ratio may be good for our stock pistons but may be too low for wider pistons that could exaggerate the piston rock. If the 5th gen R/S is greater than 1.94 the 929/954 pistons will be more stable with less rock in your project engine than in their original engines; if it is less they will rock more.

The piston rock is what I was initially concerned with., although apparently for the wrong reason. I didn't think the rod/stroke ratio had anything to do with it but I guess I'm wrong. It's happened before.

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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.

Remember that the gear-driven cams don't leave a lot of room to move with different thickness head gaskets.

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If you watch this clip from 1:20 onwards, you can tell that the HRC RC45 was NOT a 750. They were reportedly 870 or thereabouts and it obviously has some power , because it is pulling past a race prepared R1. (note the dual exhaust on the RC45)

that RC45 with the twin pipes is awesome, thanks for the vid !!

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RC45s raced with twin or single pipes depending on the year/team/track.

And those were WSB legal so they had to be 750s. I don't know about the IOM bikes displacement though. Maybe Crazy Larry does.

/threadjack

And the VFR800 has MMC bores (metal matrix composite) not nikasil.

Don't know how thick it is though

https://www.google.ca/url?sa=t&source=web&cd=1&ved=0CC8QFjAA&url=http%3A%2F%2Fen.m.wikipedia.org%2Fwiki%2FMetal_matrix_composite&ei=cR_UUcuEEYjJyAHc0ICIDA&usg=AFQjCNF2IKluXV0SI3kSyTJf6aKvCXSjuA

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RC45s raced with twin or single pipes depending on the year/team/track.

And those were WSB legal so they had to be 750s. I don't know about the IOM bikes displacement though. Maybe Crazy Larry does.

/threadjack

And the VFR800 has MMC bores (metal matrix composite) not nikasil.

Don't know how thick it is though

https://www.google.ca/url?sa=t&source=web&cd=1&ved=0CC8QFjAA&url=http%3A%2F%2Fen.m.wikipedia.org%2Fwiki%2FMetal_matrix_composite&ei=cR_UUcuEEYjJyAHc0ICIDA&usg=AFQjCNF2IKluXV0SI3kSyTJf6aKvCXSjuA

joey was racing against R1's. a bump up to 870cc seems more than fair.

by the way mohawk, this is awesome! keep up the hard work!!!

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a lil research yields:

792cc was the engine size of at least one RC45 raced at the IOM TT. I cant find the article with the alleged 870cc. And I dont know if these numbers help anyway, because it isnt really apples to apples anyway. But interesting nonetheless.

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Since the RC45 and RC46 (non-Vtec) pretty much share engines, it's sorta relevant, but I don't know what trickery Honda employed to increase displacement on IOM bikes.

Was it bore only (easier) or bore + stroke, if they could not go too far with the overbore. And what exactly IS "too far" anyhow?

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In general, I'm not a fan of seeking moar powah out of a bike that isn't first-and-foremost designed for the track. That said, I'm a huge fan of hobbies that aren't 100% logical. I like the amount of research and testing you've done so far. Good luck with the project!

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