Boring out the VF750?

[buster]

Great question was asked on another forum, "And if you could do one thing to your bike what would it be and why?"  I started thinking about a '71 CB750 chopper that I bought 'back in the day', already bored to 900cc and started researching.  There are still numerous bore kits available for CB750 and CB900 and others, and after a while I located a shop that offered to make me a VF750 kit for $400 - 600.  That conversation is just moving into the details and options and wanted to see what thoughts or experience ya'll had on the subject.  Is there a less expensive way to get more performance out of a Magna without getting to risky?  I've seen a lot of discussion about going to what seems like a lot more trouble to fit a VFR750 motor, and I guess the advantage is that the VFR750 would be closer to being 'bullet-proof" than boring cylinders out 20 - 25%.  Fuel injection would also be nice, but not as much as more hp and torque

[hootmon]

I'm not an expert in this area, but...
Honda does produce .020 over pistons for the Magna, but that's not going to make much of a difference.

Would they be able to bore that much out of the existing cylinders (Sounds like A LOT).. If not, what would they use for cylinders?
IF you were able to go 25% larger bore, what would that do to your compression, or would you have to effect the Stroke as well?
IF they go larger, wouldn't you also need to play with Valve sizes and Duration time of the valves (Cams)
IF you could go larger, then would you need to go with an aftermarket computer to re-adjust timing curves etc..
IF you are REALLY interested... Why not go to the expert and contact Dave Dodge.. He would probably have some ideas...

My concerns would be with the crank and the transmission handling the extra power... It's like a Harley where you can exchange out the Transmission to a heavier duty unit, or buy and aftermarket crank

Not trying to be a kill joy.. Just going thru the thought progressions..

Sorry.. Had to go back and look for the dyno on Magna vs VFR.. Attached below
Ever considered a V65??? V-Max?? 'Busa???

You mentioned the VFR750. I've looked at the comparisons between same year VF750 and VFR750's and there doesn't seem to be THAT much difference in Horsepower and Torque, if I remember right..

OK.. Enough for the brain right now..

Sorry - I had to go look up the Magna vs VFR chart again.. Attached

[buster]

...
IF you are REALLY interested... Why not go to the expert and contact Dave Dodge.. He would probably have some ideas...
...
Would they be able to bore that much out of the existing cylinders (Sounds like A LOT).. If not, what would they use for cylinders?

Sorry - I had to go look up the Magna vs VFR chart again.. Attached

Excellent ideas! I'll ask Dave Dodge to take a look at this. 
I'm not an expert either, but here's what I understand from a lot of reading and very little practical experience:
How much can they bore out is the biggest question where a track record of kits doesn't already exist.  Most motors have a volume of metal surrounding each cylinder allowing pistons to be replaced with 1st oversize, 2nd oversized, etc.   There is risk compared to other motors where there have been hundreds of bore kits sold and tested over 40 years (like for the CB750, where 836cc seems to be a fairly common option on the conservative side).  There are, of course obvious major differences in 750 motors, especially in the bore and stroke (stock CB750  62 x 62mm  vs. 70 x 48.6mm for the VF750).   Each 1 mm of bore increase yields 21.5cc across 4 cylinders stroked at 48.6 mm and ignoring any additional changes in the combustion chamber.   
Formula for displacement = Pi/4 x bore squared x stoke x [# Cylinders]/1000
                                  = 0.7854 x bore x  bore x stroke x [# Cylinders]/1000

My understanding is that (most?) motorcycle engines us a lighter weight alloy for the block and then stronger steel alloy cylinder sleeves for longer wear, so the process is to remove the sleeve, bore larger into the relatively softer block, and then install larger sleeves. I think the larger sleeves might also need to be bored to a very small extent.

IF you were able to go 25% larger bore, what would that do to your compression, or would you have to effect the Stroke as well?

My weak understanding is that compression ratio is mostly a function of stroke, so in the ratio of 10.8 units to 1, the volume of 1 is larger and as the piston travels down, the volume still expands x 10.8.  This is commonly tweaked with different pistons, less often by porting and flowing the head, and for more complete jobs, with different crank shafts cam shafts and carbs.  I'm hoping I can get enough flow through the carbs solely by re-jetting, and perhaps Dave Dodge's expertise can define any threshold(s) of bore size wear more aggressive carb changes are required (bigger carbs) vs rejetting vs only tuning adjustments

IF they go larger, wouldn't you also need to play with Valve sizes and Duration time of the valves (Cams)

Depends on the kit size, generally not mandatory except for the more aggressive kits but certainly improves performance even further.

IF you could go larger, then would you need to go with an aftermarket computer to re-adjust timing curves etc..

I think the answer is the same as the previous question.

My concerns would be with the crank and the transmission handling the extra power... It's like a Harley where you can exchange out the Transmission to a heavier duty unit, or buy and aftermarket crank

Since bikes are usually engineered to handle well over the rated stock horsepower and torque, this would only be an issue when developing max hp/torque of the rebuilt motor, that exceeds the pre-engineered buffer, and then it applies to the entire drive train, but I'd expect it primarily in the wear items, i.e.,  chain, clutch, etc. which many of us typically upgrade at their normal replacement time anyway.   But in reality, I'd expect to more often be applying something like 50 – 70 hp much sooner and easier.

Ever considered a V65??? V-Max?? 'Busa???

Yes, quite seriously.  I don't like the idea of the V65 being another decade+ older vs. having a rebuilt motor.  V-Max – Love it except for the range of under 100 miles is unacceptable, and since boring out the Magna would also reduce its range, you've help me highlight another significant negative I'd not yet thought of.     Might need to counter some of that with gear change up from my 17frontStock rear, which then affects clutch wear, but I'm due for one of those anyway.     

My ideal bike would be a fuel-injected 1300cc 2015 Magna with DOHC, big disk brakes, bigger fuel tank, adjustable windshield.  Perhaps start with the Busa and put it into a cruiser frame....

You mentioned the VFR750. I've looked at the comparisons between same year VF750 and VFR750's and there doesn't seem to be THAT much difference in Horsepower and Torque, if I remember right..

That's my point as well, only about 10-12 more horsepower and only in those brief moments when it is wide open.  More displacement should raise both lines through the full range, more closely resembling the V65 but mounted a lighter frame.  I think increasing displacement is generally the most cost effective way to get big improvements in performance, but happy to hear other ideas from folks who do more than just think and read about it.

[Smoked U]

I'd like to respond to this Buster, but, I want to know first what the end game is here.

Specifically what are you wanting this bike to do and why? Drag race 1/4 mile? Beat every other stock Magna in a five mile Interstate highway race? pull stumps on the farm? Spend that extra cash that is cluttering the floor of your bedroom? What?

[hootmon]

 Might need to counter some of that with gear change up from my 17frontStock rear, which then affects clutch wear, but I'm due for one of those anyway.     

OK.. Now you have successfully confused me...
You want a faster Magna off the line, but you change the gear ratio on the chain to slow it down...

You want it to go faster off the line, put a 15 Tooth front sprocket.. That will make a difference and be a lot cheaper and reversible when you want to go on a long trip (Like MootMag)..
Want it to go even faster, Put a 15 on the front and a 43 on the back... (With each of these choices, you will loose top end speed, but it will be faster off the line)

[buster]

@ Smoked U and Hootmon,
The primary goal would be to make many of the more common, more usable advantages of a bigger motor available in everyday riding.  Think of more torque immediately available at any point in time, which means fewer times to red line, stronger pull before needing to downshift in many (but not all situations).  Versus buying another bike, I've done a lot to make this one mine, and this would take it a big step further.  I guess another goal is making my Magna more distinctive, i.e., a 515 pound V60!

I've only gone from 0 to max a handful of times and I didn't time it when I did, so improving that is not terribly important to me. On the other side, I get near redline more than once a week just getting onto I35.  Putting the 15 tooth sprocket (or for me, going back to a 16 from a 17) just means I'm redlining it more often and getting terrible mileage on the highway (majority of my miles). 

I'm not at all committed to going this, but thinking about it is certainly entertaining while I'm laid up this winter.  Reducing the range on a tank of gas, combined with the risk of a major mod that has no track record, are compelling arguments to do nothing, keep it relatively bullet proof, and save the money for when Honda (or whoever) launches the next rendition of the Magna.

[buster]

I spoke with Dave Dodge today, he might update this thread when he has more time.  There is a bit of a track record with 71 and 73 mm bores, and he typically increases the compression ratio at the same time, I think to 12:1.

Dave was just thinking off the top of his head that it was more like 836cc for 73 mm bore, while I calculate 73 mm bore as yielding only 814cc and 74mm for 836, but maybe whatever he does to increase compression ratio accounts for the difference....  still well short of what is possible with the popular CB750 900 kits.  To get that kind of performance improvement, it would be three changes that I could spread out over say, 2 or 3 winters.  There may be options for changing the sequence, and combining say, steps 1 & 2 while they're already off the engine.  The website also mentions availability of package pricing – sure would be cool if 3 or more of us went in on a group discount for this winter or next winter....

1)   DRP (Stage 1?) Cams -  web site looks like $650 for the set, yields 8hp and can be done while leaving the engine in the frame, and the cams can be turned around in about 3 weeks, so much simpler than step 3.  Although this increases the distance and time the valves are open, I'm guessing only minimal hp gains at low and mid-range rpm compared to option 3?
2)   $800 to port the heads.    VERY tempting to do while already torn down for step 1.  An alternative is using VFR heads which have much more aggressive porting.   Hell, one might be able to pick up the entire VFR motor and carbs for that price?
3)   73mm bore kit with compression increase to 12:1  (price is probably a little upwards of the $650 he gets for the more standard 71 mm kit, and I think price not include the required boring (and sleeving?) of the top case???  I'll have a longer down time this winter, so still thinking about doing this more complex option first, but I think Dave mentioned I could have pre-detonation issues if this option is done without the above options?   It's also tempting to just do nothing and relax more this winter...

A fourth step to unleash some of the potential created by the above steps, would be putting larger carburetors (VFR's are 1 mm larger and sometimes available on ebay), but that strikes me as the least bang for the buck, especially since I just had mine rebuilt last year.  I'm still unclear on where I'd be if I did nothing to the carbs (think I'd be running a little lean but OK), versus re-jetting them (again) versus upsizing the carbs for best performance.

Looking forward to Dave making whatever corrections are needed for this page. 

Dave Dodge Racing Performance
http://www.magnaownersoftexas.com/drp3rdgen.html 


I heard there's a Magna with significant engine mods that rides with the group out of the Grapevine DFW Honda dealership, and I'd love to pick his brain over lunch if I ever cross paths with him.

[buster]

Numerous VFR heads available on ebay for about $20 each, now that sounds like a cheap performance upgrade!  VFR Carb sets are about $200, $345 for rebuilt http://www.ebay.com/itm/Primo-Carb-Service-Rebuild-90-97-VFR750-Interceptor-/251499440222?pt=Motorcycles_Parts_Accessories&hash=item3a8e88e85e&vxp=mtr

Dang, is the cheapest way to do it one part at a time?!  Sure would've been nice if Honda would've done it that way for the Magna in the first place...

[Smoked U]

Dang, is the cheapest way to do it one part at a time?  Sure would've been nice if Honda would've done it that way for the Magna in the first place...
[/quote]

The 3rd gen was a downgrade from the previous generations. I'm sure this was so Honda could compete cost wise with the competition. They played the looks over performance/engineering gain. Back to non hydraulic clutch controls, chain in lieu of shaft drive, big single disc brake up front, drum brake in the back, plastic chrome parts, etc. It is what it is.

I like to think about the long run and like everyone else, I know that big money spent on upgrades can never be recovered. If you are going to keep the bike for ever and ever and the thought of not being able to recover a bunch of money down the road when you sell it doesn't bother you, then, push on ahead.

I know everyone here understands that there are direct and indirect effects of modifying a machine like this from the base line.

Gear down the rearend (poor man's supercharger), lose MPG, gain launching speed and higher acceleration, lose top end, engine runs at a higher RPM for any given speed vs stock setup.

Shave heads (increase the compression ratio, but watch out for valve to piston clearance issues), port and polish head runners and maybe the barrels of the carbs, hotter cam lobes, design some kind of air ram system. All things that are designed to get a higher air/fuel mixture into the combustion chambers. Again, MPG goes down, may require gearing changes, may require tinkering with carb jetting, engine will require higher octane fuel to eliminate detonation issues, so pump gas may not even be good enough and the fuel may require aftermarket octane booster at each fill up. Engine will probably run hotter which may require additional cooling system modifications especially in Texas summer heat. Engine ignition timing could be effected. Increasing the flow on the front end of the cylinders will require serious modifications to the exhaust system to open it up.

Buy a nitrous oxide system and modify it to work on the engine. Play around for months re-jetting carbs and adjust nitrous flow. Be prepared for massive acceleration gains until the nitrous bottle is empty, expect the engine to run like crap when the nitrous isn't on since it is way over fueled in that scenario. Be prepared for the bike to do a back flip every time you upshift gears while under full nitrous control. expect to change clutches more often. Pray your pistons don't melt and your valves don't burn.

Totally different engine swap? Do you own a machine shop? Do you own a welder and know how to use it? Do you love to solve electronic/electrical puzzles. How about a mechanical engineering degree? I cannot begin to fathom the engineering nightmare this could turn into in short order.

Lighten and streamline the system. Strip every piece of non-essential equipment off the bike, go on a diet and install a bullet faring and learn to ride it hunched over the gas tank like a crotch rocket. Paint flames on the tank. It may not go faster, but it will have that illusion and look cool besides.

Honda spends a lot of time engineering and testing these systems to meet a certain market demand at any given time in history. I have always felt that the 3rd gen engineering was designed to sell more bikes due to lower cost and appearance than trying to sell a higher cost machine under the guise of higher performance than the competitors ala Vmax vs V65 wars.

Considering safety and strength design factors that are typical in any design (or should be), I am sure a person could make a lot of mods boring, drilling, polishing, changing and adding without blowing up their powerplant, however, it could turn into a engineering/financial nightmare very quickly as well.

Regarding any modification to this bike, I would want to find someone who has been there, done that. I would without a doubt want to verify that the mod actually did something positive that I wanted that was worth the investment in time and money that was required.

I would start by going over to the V4 forum and find some guys that have actually done some mods to the 3rd gen. Find out what the result was and balance that against the cost. My guess is some of this stuff can only be measured with a dyno. Seat of the pants feel can be compromised by how much money a person puts into one of these endeavors, i.e., I spent all this money and time therefore it must be better. Yeah, maybe. http://www.v4hondabbs.com/

It's your bike, time and money. What ever you decide, Buster, just do your homework and get something positive out of the experience.

"Sometimes the wanting is more desirable than the having. It is not logical, but it is often true"  -  Spock

[hootmon]

Numerous VFR heads available on ebay for about $20 each, now that sounds like a cheap performance upgrade!  VFR Carb sets are about $200, $345 for rebuilt http://www.ebay.com/itm/Primo-Carb-Service-Rebuild-90-97-VFR750-Interceptor-/251499440222?pt=Motorcycles_Parts_Accessories&hash=item3a8e88e85e&vxp=mtr

Dang, is the cheapest way to do it one part at a time?!  Sure would've been nice if Honda would've done it that way for the Magna in the first place...

Question that I don't know the answer to..
There is a difference in the head between the Magna and the VFR.. The Magna Cams are chain driven and the VFR heads are gear driven..
How do you adapt this difference when you swap the heads?

[Smoked U]

Numerous VFR heads available on ebay for about $20 each, now that sounds like a cheap performance upgrade!  VFR Carb sets are about $200, $345 for rebuilt http://www.ebay.com/itm/Primo-Carb-Service-Rebuild-90-97-VFR750-Interceptor-/251499440222?pt=Motorcycles_Parts_Accessories&hash=item3a8e88e85e&vxp=mtr

Dang, is the cheapest way to do it one part at a time?!  Sure would've been nice if Honda would've done it that way for the Magna in the first place...

Question that I don't know the answer to..
There is a difference in the head between the Magna and the VFR.. The Magna Cams are chain driven and the VFR heads are gear driven..
How do you adapt this difference when you swap the heads?

Rube Goldberg.com???

[hootmon]

Numerous VFR heads available on ebay for about $20 each, now that sounds like a cheap performance upgrade!  VFR Carb sets are about $200, $345 for rebuilt http://www.ebay.com/itm/Primo-Carb-Service-Rebuild-90-97-VFR750-Interceptor-/251499440222?pt=Motorcycles_Parts_Accessories&hash=item3a8e88e85e&vxp=mtr

Dang, is the cheapest way to do it one part at a time?!  Sure would've been nice if Honda would've done it that way for the Magna in the first place...

Question that I don't know the answer to..
There is a difference in the head between the Magna and the VFR.. The Magna Cams are chain driven and the VFR heads are gear driven..
How do you adapt this difference when you swap the heads?

Rube Goldberg.com???

CobbJob.com?

[buster]

Question that I don't know the answer to..
There is a difference in the head between the Magna and the VFR.. The Magna Cams are chain driven and the VFR heads are gear driven..
How do you adapt this difference when you swap the heads?

Dave Dodge could best answer that for us, as he's the one who mentioned it to me on the phone, saying the port tunnels are much larger on the VFR, so that is an alternative to spending $800 to open up those on the VF head.  But if I remember correctly, didn't VFRs not to go chain driven until 2002 5th generation 800 with VTEC?

...
I have always felt that the 3rd gen engineering was designed to sell more bikes due to lower cost and appearance than trying to sell a higher cost machine under the guise of higher performance than the competitors ala Vmax vs V65 wars.

Agreed, as evidenced by the further trend to v-twins.   But I'm surprised at how many Magna owners have gone with exhaust upgrades, re-jetting the carbs , K & N air filter and then stopping right there in terms of performance.

I would start by going over to the V4 forum and find some guys that have actually done some mods to the 3rd  gen. Find out what the result was and balance that against the cost. My guess is some of this stuff can only be measured with a dyno. .... http://www.v4hondabbs.com/

Dodge claims 8 whp on the dyno from the cam upgrade alone - 10.5% must be noticeable without the dyno, and better value than upgraded exhaust in terms of retail $ (perhaps not better in terms of ebay prices).   Smoked U raises great points about possible tradeoffs in terms of fuel requirements and as a result of TX heat, which is why I'm tracking down a Magna rider I've heard of in the DFW area who has done a lot of engine performance work.

I'd already been to V4Musclebike.com but hadn't heard about v4hondabb, thanks!  Lot's of good info and very well organized.  Spent some time there and found a lot of info but not much in terms of "I've done it and here's what I thought of it", but there is a strong sentiment and here's an example in response to suggestions that unlike car engines, there is less opportunity to soop up motorcycle engine:

From http://v4hondabbs.com/index.php?topic=40075.0   
2008
Posted by Sr Member Jamie Daugherty
Aside from compression and cams, the V4's love head porting.  You might consider doing that as it's dollar-for-dollar the best money you can spend. 

[other discussions mention significant benefits from simply swapping out flat Magna pistons for high-domed pistons to get for an instant reversal of Honda's downgrading of Magna compression in recent versions.  Although again to Smoked U's point about Texas heat, higher compression = more heat to carry away.]

[buster]

I just dug up some scary facts from a deeper dive... wow.  So volatile that I don't know what to share here... perhaps ya'll were just being subtle with your warnings, but I'll definitely be looking for someone with positive experiences from a local shop.

[Smoked U]

Numerous VFR heads available on ebay for about $20 each, now that sounds like a cheap performance upgrade!  VFR Carb sets are about $200, $345 for rebuilt http://www.ebay.com/itm/Primo-Carb-Service-Rebuild-90-97-VFR750-Interceptor-/251499440222?pt=Motorcycles_Parts_Accessories&hash=item3a8e88e85e&vxp=mtr

Dang, is the cheapest way to do it one part at a time?!  Sure would've been nice if Honda would've done it that way for the Magna in the first place...

Question that I don't know the answer to..
There is a difference in the head between the Magna and the VFR.. The Magna Cams are chain driven and the VFR heads are gear driven..
How do you adapt this difference when you swap the heads?

Rube Goldberg.com???

CobbJob.com?

I think that's the engineering solution for lack of toilet paper isn't it?