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Anyone Ever Rebored/overbored A 5/6Th Gen


Mohawk

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On 6/14/2016 at 7:02 AM, Veefer46 said:

Hi Mohawk,

 

due to the fact that I keep my standard cams in first place I am caluculating static CR. I'd just like to compare the values respectively ask what you think could be possible with 95 Octane (98 would be possible as well).

 

Thanks,

Julian

 

Just dropping in over bore, higher compression pistons and tuning for fuel alone on only euro 95 RON octane fuel (only 91 US ron+mon / 2) Is not going to let you go up much on the compression ratio. 12.15 static may be possible, but like mohawk was saying, it all comes down to the dynamic compression ratio and some other tuning and factors. The VFR800 comes with relative mild cams with very little overlap, which keep the dynamic compression relatively high, compared to the static compression ratio. If you were able to degree your factory cams and add some more over lap (this might not be cost effective on a gear driven valvetrain), you could bleed off some dynamic compression and raise your static compression much higher.

 

There are many more factors to address for knock resistance than just fuel octane rating and compression though.

First, obviously using 98 RON or 93 (RON+MON)/2 will allow much higher compression or much safer operation on a given compression, versus 95 RON or 91 (RON+MON)/2.

 

Also as mentioned earlier, dialing in more valve over lap with factory (slotting the cam gear/sprocket bolt holes and rotating the cams relative to the gears a few degrees) or bigger custom cams will bring dynamic compression down to safer levels while adding more top end power and not losing too much low end, if dynamic compression is kept at the same level.

 

Secondly, retarding your ignition timing will reduce combustion pressures by igniting the air/fuel mixture later in the power stroke so peak pressure is applied later on, as the piston is accelerating downward at a faster velocity, causing less chance of detonation.

 

Third, reducing temperatures in the combustion chamber can also make your engine less prone to detonation on higher compression levels. This can be done with a manual fan switch and spark plugs that are a temperature range colder.

 

Another way to reduce temperatures is a full high flowing exhaust system (I am not talking slip ons and ebay headers...) That is catless and ceramic coated. Also a slightly richer air fuel ratio will help lower temps too.

 

There are other things like deburring and smoothing the piston tops/valve reliefs/heads and ceramic coating the piston tops and combustion chambers, but that is probably out of the scope of this discussion.

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Thanks for your detailed explanations.

 

I just didn't get the concept of DCR right and thought it would increase with more overlap due to the better delivery degree in higher rpm-ranges. Now I know better.

 

This gives me the answer that I cannot go as high in static CR as Phil because of my std. cams.

 

98 RON, Ignition timing and avoidance of hot spots on the piston should do the trick for 12.1:1 with standard cams.

I guess the standard VFR runs fine on even less than 95 RON because of the bad fuel quality in some countries. I don't think they used different maps for e.g. CN-versions like Porsche does.

 

My target is not to gain the highest power output possible but perhaps a little bit more low- and midrange-torque. This is my first engine build, so I'll start small :blush:

 

Decatted and ceramic coated exhausts were used in our formula student cars, hence I know the prices and you are fully right - this is out of scope of the discussion :goofy:

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Yup the VFR was built to run on 90 RON and 87 (RON+MON/2) from the factory and there are no higher state of tune variants of VFR800's for different country's. Also there is no knock sensor, so there is no way for the bike to adjust for different fuels. But, yeah, I would imagine 12.1:1 would be very safe with optimized ignition timing and 98 RON and some other heat management tricks.

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Back in the 80's, I think it was Lamborghini that was having financial difficulties and didn't have the money to design, build, and test an all new engine. They bored out the cylinders as much as possible (for additional displacement) but realized it wasn't enough. They then put spacers in between the block and heads to increase the stroke for even more displacement. Wow.

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On 6/14/2016 at 6:26 PM, 2FAST4U said:

Back in the 80's, I think it was Lamborghini that was having financial difficulties and didn't have the money to design, build, and test an all new engine. They bored out the cylinders as much as possible (for additional displacement) but realized it wasn't enough. They then put spacers in between the block and heads to increase the stroke for even more displacement. Wow.

 

Deck plate spacers, with longer sleeves, have been used by hot rodders for a while, but it's hard to believe a manufacturer would do that lol. Man I'm sure the reliability and maintenance on those engines must be even worse than the already bad usual super car stuff.

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17 hours ago, Veefer46 said:

I just didn't get the concept of DCR right and thought it would increase with more overlap due to the better delivery degree in higher rpm-ranges. Now I know better.

 

 

I think you were on track with your train of thought, because I believe cylinder pressures could be higher, at higher RPMs with bigger cams, due to better cylinder filling (Ram air effect), but I think most engines are less prone to detonation at higher RPMs, being that the piston is moving away from top dead center, at a faster rate of speed.

 

I think detonation is more of a concern at lower RPMs, when the engine is lugging, as the piston is moving away from TDC at a much lower rate of speed, so when the fuel is ignited, it doesn't have as much time to expand, therefore running the risk of detonating if it expands at a faster rate than the piston will allow.

 

This same concept is why retarding the ignition timing (igniting the air fuel mixture later) will allow more safety (possibly at the cost of power) as when the combustion starts expanding, the piston is already moving away from TDC at a high rate of speed, so it is allowed to expand at a safer, more controlled rate.

 

It is when the flame is not allowed to expand outwards fast enough, that combustion pressures spike and the remaining unburnt air/fuel mixture detonates all at once and you get knock or worse, a smashed bearing or new window in your block.

 

Tuning is always a compromise, even more so on a daily driven street engine. You want to make as much power as you can, but not at the cost of reliability. That said, I do believe that there is a lot of room for safely making more power on the VFR engine, when using higher octane fuel, as Honda has tuned it to last forever on very low grade fuel and very mild cams.

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21 hours ago, CandyRedRC46 said:

because I believe cylinder pressures could be higher, at higher RPMs with bigger cams, due to better cylinder filling (Ram air effect)

 

This is exactly what I meant.

 

Your explanation regarding higher piston speeds sounds plausible, though.

 

I guess retarding ignition is only useful to the point when ignition is really triggered by the spark plug and not by hot spots on the piston/chamber.

But as you already pointed out, egalizing all sharp edges should help a lot.

 

Another question:

I'm about to choose new crankshaft bearing. Via Plastigauge I measured clearances of ~0,05mm. Therefore I want to replace with the original bearings according to the code-stamps on the weights of the crankshaft and on the housing.

 

Upper crankcase reads A-A-A

Crankshaft reads 3-2-1

 

This gives me colour codes brown-green-yellow according to the table in the service book.

 

Is there any way to countercheck with the codes on the bearings itself?

 

Example brown bearing:

According to the spare-parts catalogue this is "13315-ML7-691 BEARING C", the bearing reads "MZ5 817L".

 

All three bearings have different numbers stamped in so I guess it must have something to do with the colour codes. I just don't know how to transcode it.

 

I want to order the OEM parts in the US so sending them back won't be an option. Hence I'd like to double-check if possible.

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You are beter off using plasti gauge to work out the shell bearing clearances. The factory codes are a good start & excellent for a quick build at the factory, but less accurate than your own measurements.

 

The cylinder filling efficiency can be better than 100% but ONLY at the peak torque RPM.  Cylinder filling can reach +10% or 110% in total, but that is already the case, so for the current dynamic compression ratio which according to my calculations is around 10.86/1 which +10% wold be 11.95/1 which is within tolerance at higher rpms. Normal 95RON fuel can be used at 12/1, so if you increase static compression then you MUST increase the valve overlap to maintain a safe DCR, or as CR46 says pull the ignition back in the peak efficiency rpm range.

 

Regarding Knock which is caused by the piston location after spark plug ignition as described by CR46. I feel you may be confusing Knock with Pre-Ignition.

Pre-ignition is caused by the heat or hot spots in the combustion chamber & is unplanned & often happens before the piston reaches TDC & can destroy an engine in short order.

This is normally caused by a combination of over heating & too high a compression ratio, take your pick !

 

YMMV

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2 hours ago, Veefer46 said:

I guess retarding ignition is only useful to the point when ignition is really triggered by the spark plug and not by hot spots on the piston/chamber.

But as you already pointed out, egalizing all sharp edges should help a lot.

 

 

The thing is, advancing ignition timing will raise combustion pressures and in turn temperatures, while retarding timing will lower pressure and temperature. So yes pulling timing will be useful while spark is triggering the combustion, but also it will lower temperatures and in turn further lower hot spot ignition/combustion/detonation as well.

 

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1 hour ago, Mohawk said:

You are beter off using plasti gauge to work out the shell bearing clearances. The factory codes are a good start & excellent for a quick build at the factory, but less accurate than your own measurements.

 

The cylinder filling efficiency can be better than 100% but ONLY at the peak torque RPM.  Cylinder filling can reach +10% or 110% in total, but that is already the case, so for the current dynamic compression ratio which according to my calculations is around 10.86/1 which +10% wold be 11.95/1 which is within tolerance at higher rpms. Normal 95RON fuel can be used at 12/1, so if you increase static compression then you MUST increase the valve overlap to maintain a safe DCR, or as CR46 says pull the ignition back in the peak efficiency rpm range.

 

Regarding Knock which is caused by the piston location after spark plug ignition as described by CR46. I feel you may be confusing Knock with Pre-Ignition.

Pre-ignition is caused by the heat or hot spots in the combustion chamber & is unplanned & often happens before the piston reaches TDC & can destroy an engine in short order.

This is normally caused by a combination of over heating & too high a compression ratio, take your pick !

 

YMMV

 

My bad, you're right. :)

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In all of these situations the best approach is to make sure there are no points in the combustion chamber that can give rise to hot spots.

Retarding the ignition should really only be used as a last resort to curing detonation as it is very hungry on power production.

 

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  • 1 year later...

Bringing back a great topic,, Wow, I'm impressed with the dedication here. even though it's a couple years old,  it's still relevant.  And to think if Honda wanted too,  they could have done this as it's been proven doable,  add in the fact god knows what else can be had "if".   Hmm,  cams and 929 pistons  :goofy: 

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