Topic: Web Cam #41 in a hi-comp 849cc, advantages?

[PeWe]

- Your pistons are made for F2? F2 pistons have higher dome for the F2 deeper chamber

CB750 high compression pistons have often a compression ratio of 10.25:1 10.5:1 or 12,5:1. Cycle X 849 kit has 10.5:1 that is very common. Most of these pistons are made for standard combustion chambers. Big pistons look funny when the domes start to ríse a few mm in to fit std chamber.

12.5:1 need stronger connecting rods and a cam with big duration so the dynamic compression will  be not too high.

I have attached photos of my 3 different 836cc pistons. All 10.5:1
- Russ Collins forged that have wider crowns, my head is adapted to those with opened chambers, same diameter as the pistons 65mm.
- Action Fours cast. Together with my head lower compression when they are designed for std.
- NOS RC cast pistons. I have them in  now and have got increased CR compared with Action Fours due to the higher piston crown.

Name of the photo show the order of the pistons

[HondaMan]

- Your pistons are made for F2? F2 pistons have higher dome for the F2 deeper chamber

CB750 hi compression pistons have often a compression ratio of 10.25:1 10.5:1 or 12,5:1. Cycle X 849 kit has 10.5:1 that is very common. Most of these pistons are made for standard combustion chambers. Big pistons look funny when the domes start to ríse a few mm in to fit std chamber.

12.5:1 need stronger connection rods and a cam with big duration so the dynamic compression will  be not too high.

I have attached photos of my 3 different 836cc pistons. All 10.5:1
- Russ Collins forged that have wider crowns, my head is adapted to those with opened chambers, same diameter as the pistons 65mm.
- Action Fours cast. Together with my head lower compression when they are designed for std.
- NOS RC cast pistons. I have them in  now and have got increased CR compared with Action Fours. These RC are wider and need opened chambers to not hit the head.

Name of the photo show the order of the pisons

Yeah, PeWe, I sure do miss the old Action Fours parts. They had some real good common-sense upgrades for these bikes, and good quality to boot. Except their head gaskets: those leaked oil a lot.

[banzaibob]

As an aside; I was rummaging through my storage unit and found another F2 engine.  It is seized up but a little snip, snip to the camchain and off comes the head.  Maybe this head is even better?   

[banzaibob]

Pictures of the seized F2 motor.  Valves are shot.  Notice the green #$%* growing in #4. 

[HondaMan]

Pictures of the seized F2 motor.  Valves are shot.  Notice the green #$%* growing in #4. 

Wow, was that one sitting upside-down, collecting water? That's really rusty!

[banzaibob]

The other F2 head had a broken fin.  Using magnifiers, I was able to see what looks like an extremely small crack in one of the valve seats of the other head.  Even thought this thing is rusty the engine itself looks like it has fewer miles.  My biggest concern is that the valve seats will be too pitted with rust. 

[HondaMan]

The other F2 head had a broken fin.  Using magnifiers, I was able to see what looks like an extremely small crack in one of the valve seats of the other head.  Even thought this thing is rusty the engine itself looks like it has fewer miles.  My biggest concern is that the valve seats will be too pitted with rust. 

I've had the machine shop cut new seats pretty deeply before to get to good material, it worked out OK. I also had the head milled accordingly (I think it was 2mm?), which helped with the CR in the end, too. The valves ended up pretty shrouded from being further up in the head, so I had to cut away the ridges in the chambers, but all was done evenly and in the end it CC'd out to less than .2cc difference between chambers. Honda called for .5cc max.

The only thing that came of it: the valve adjusters had enough range left to accept a stock cam, but I don't think a high-lift cam will ever fit that head unless the rocker towers are shimmed up at least 1mm and new seals made for the oil feed ports. Or, the valve stems could be trimmed back (all by 1+mm to make the difference, which would also have worked.

[dave500]

high compression pistons or shaved heads or any other means to increase compression ratios are used to offset the results of hotter cams with a longer overlap duration(both the inlet and exhaust valves are open together),the actual dynamic compression ratio is decreased with a longer overlap so it needs to be brought back up in this case,if a hotter wild cam is installed without increasing your compression ratio youll have poor results as your dynamic compression ratio will go down(the static stays the same),the same as increasing the compression ratio say with high dome pistons or a shaved head but not swapping in a hotter cam(read longer overlap)youll have way too much compression,12.5 to 1 sounds high when measured statically but with a hot cam the dynamic might be say 10 to 1?or less?

[dave500]

static compression ratio is the swept volume of the piston in the cylinder divided by the combustion chamber volume,dynamic is the same combustion chamber volume but divided by whats left of the swept cylinder volume once the inlet valve has closed,this might be where the piston is almost halfway up its stroke?its where actual compression starts though,its dynamic,the static ratio stays the same,the dynamic ratio can be altered by the camshaft.

[banzaibob]

What I think I'm going to do:

I think I am getting ahead of myself.  Installing the F2 cam by itself will probably lower the dynamic compression ratio enough to solve the problem of low rpm/high load spark knock.  Also installing the .040" base gasket by itself will lower the static compression ratio and give me the same results.  I should probably do one at a time in order to keep from getting cumulative error (and screwed up engines). 

With all the extra F2 parts (and wealth of knowledge, thanks) I ought to simply build a hot rod F2 engine and use that as a spare.   

[rlvitt]

I've been following all these posts and re-read them today. My brain got a little mushy with all the talk about dynamic and static CR's and that. But let me see if I get the basics right........ Your opinions and input here are important to me as I am already planning a build on my second K0.

If you want to gain a more effective powerband starting at a lower RPM and gain HP along with that, using the F2 heads and the stock F2 cam is more effective than dropping in a hotter cam like my Web Cam 41a grind. And that the F2 head breaths better due to larger valves and allows the engine to breath easier both on intake and exhaust strokes. AND that this works well with an 849cc kit or other big bore kit (836/811) dropped in. Also, that head work seems to be a better avenue to HP gain than a new cam while keeping the same stock benchmarks for HP increase along the powerband.

As someone pointed out, a power increase that starts at 8K is useless unless you're running around at the upper end of the tach. Personally I'd find it more to my liking to have the power come on about 4K to 5K at a higher level than the stock numbers.

I bought the 41a with the understanding that the cam did just that. That it was not a cam that only dumped its HP at high RPM's only. The opinion seems to be split on that. Initially my 41a had some problems with running rich. My timing should be on the money. I tore the carbs back off the bike and re checked everything, putting in 110 mains, dropping the needle to the top notch and putting in #35 idle jets with the old style drilled air screws. I reset my floats to a lower fuel using a clear tube observation. I re-synched the carbs also. The results were a leaner fuel ratio and less black exhaust out the tail pipe. The bike would also idle pretty well at 1K RPM's. The throttle is more responsive and there is little lag if it is thrown WFO from an idle though there seems to be a small delay in spin down from a high RPM.

That's enough for now. I'm sure I'll get enough food for thought to wade through. Thanks.

[HondaMan]

static compression ratio is the swept volume of the piston in the cylinder divided by the combustion chamber volume,dynamic is the same combustion chamber volume but divided by whats left of the swept cylinder volume once the inlet valve has closed,this might be where the piston is almost halfway up its stroke?its where actual compression starts though,its dynamic,the static ratio stays the same,the dynamic ratio can be altered by the camshaft.

Yep, and if the inlet opens BTC, it improves this ratio. Honda elected (stock) to open at 0 TDC on the post-1975 engines, not so much for performance as for meeting the 1974 emissions rules at 2500 RPM. They left the inlet open later to try to charge the cylinder more at higher RPM (where the emissions were not "counted") and recover some of the lost power. You can get the power back yourself by either advancing the F cam those missing 5 degrees, or just use a K0-K5 cam instead. Either way will raise the CR over the stock F cam.

BTW: this was also why Honda raised the CR in these later bikes: with the later-opening inlet valve, it lost compression, so they did it mechanically with the little domes.

[rlvitt]

Since we're talking about heads and porting here and there, what's your opinion on these Cycle X valves. I'll post the ad in quotes.....

 "Cycle X ....... Super Flow Valves:
Cycle X super flow valves outperform any valve on the market today. The Nitrite coating is perfect for high temperature, high RPM and run cooler.
The reduced stem diameter (.033) in the port area gives our valves greater flow for noticeable horsepower gains.
Cycle X valves are one piece forged, high nickel stainless steel alloy, swirl polished, strict tolerances and Nitride coated. Making our valves the obvious choice.
The first photo shows an aftermarket valve ,OEM and Cycle X valve, notice the reduced stem diameter in the port area.
Light weight?
A complete set of Cycle X valves weighed approx. 30 grams less than the competitions racing valves.
Designed for the preferred 69-76 spring retainers and keepers."

[Nic]

I'm using them, they work

[HondaMan]

Since we're talking about heads and porting here and there, what's your opinion on these Cycle X valves. I'll post the ad in quotes.....

 "Cycle X ....... Super Flow Valves:
Cycle X super flow valves outperform any valve on the market today. The Nitrite coating is perfect for high temperature, high RPM and run cooler.
The reduced stem diameter (.033) in the port area gives our valves greater flow for noticeable horsepower gains.
Cycle X valves are one piece forged, high nickel stainless steel alloy, swirl polished, strict tolerances and Nitride coated. Making our valves the obvious choice.
The first photo shows an aftermarket valve ,OEM and Cycle X valve, notice the reduced stem diameter in the port area.
Light weight?
A complete set of Cycle X valves weighed approx. 30 grams less than the competitions racing valves.
Designed for the preferred 69-76 spring retainers and keepers."

I will never use them again.

I built an F2 head for one of our members, he sent me those valves to use. In less than 300 miles, the "nitride coating" ground off and ruined the brand-new guides, destroyed the seats, and damaged the chambers. It was like pouring sand into the engine, only worse, as it stuck well to the valve faces.

I had to re-replace all the guides, re-cut the seats deeply, then mill the head another 1.2mm to get the compression back. Then I polished all that 'coating' off the valves and found what appears to be stainless valves underneath. I had them re-faced, and put it all back together. It ran fine, then.

Other members here have suffered a similar fate from them.

[Nic]

Oh that sounds great. Oh well, I'll be pulling the engine down soon so I'll see what's what, sounds like I should polish them at least. I have 1800Ks on mine with no dramas so far but I plan to fit the flat top 836 kit when it get's cold around here so I'll have a good look at the head.

[rlvitt]

Mark, or any one else, is there a set of valves and springs that are highly recommended to use along with a porting and polishing of the head? Are there combinations of pistons and valves and springs recommended to use with the 849 kit? My understanding is that lightened valves and strong springs allow an honest HP gain, however big or small, and protect the engine from floating the valves in a high rev situation. Is my concept wrong?

[HondaMan]

Mark, or any one else, is there a set of valves and springs that are highly recommended to use along with a porting and polishing of the head? Are there combinations of pistons and valves and springs recommended to use with the 849 kit? My understanding is that lightened valves and strong springs allow an honest HP gain, however big or small, and protect the engine from floating the valves in a high rev situation. Is my concept wrong?

Well, the stock valves are actually pretty good up to 836cc with 8000 RPM limits. They get a little restrictive above that RPM, but the cost of the rods to get there usually limits how many folks are doing it. The 849cc is similar: you can install larger diameter valves because there is room in the head area to widen the chamber, just watch out for the pistons. Maybe some flycuts are needed? The 849 engine at 9500 RPM is likely to be more torque than at least the stock rod bolts can manage, and you might wish to think about at least shot-peening the [stock] rods if you're heading "up there". Or, do like RXman did, get some Carillos?

[rlvitt]

Are you speaking of making a stock K head with larger valves or using an F head. which I understand. has larger valves to begin with?

[banzaibob]

Well crap....I probably shouldn't mention that I just received my new Cycle X F2 valves today (along with my new .040" base gasket).  Actually my intention is to (slowly) build a hod rod F2 motor.  That still leaves the 849 to figure out for now.

The advice here has been stunning and educating (thanks).  However I think that I am going to take things one step at a time.  Rather than pile solutions on top of solutions to make a problem, it appears to me that I only have two choices.  Keeping in mind that my problem was the low rpm spark knock, I need to reduce the compression ratio without having a super rich pilot jet wetting the combustion chamber all the time. 

1) I can take the simplest route; do not install the thicker base gasket just yet, but instead, install the F2 cam in the stock K4 head that I was using.  As was stated in an earlier post, the increased intake duration would lower the DYNAMIC compression ratio.  If I am understanding this correctly it will have good mid range and top end performance yet it will less likely to knock at the lower rpm's because of the compression ratio reduction (right?).  The only problem with this seems to be clearance of the valves in the combustion chamber.  As long as there is 1mm or so I should be ok (right?). 

2) The slightly more complicated route would be to use the .040 base gasket and go back to the cam/head configuration that I had.  The only complexity is that the jugs would have to come off and go back on again (right now they are still on).  This actually may be the simplest route because there are no valve/piston clearance issues.  I assume that power and tuning characteristics would be somewhat "linear" from the way it was (right?) 

[MRieck]

Well crap....I probably shouldn't mention that I just received my new Cycle X F2 valves today (along with my new .040" base gasket).  Actually my intention is to (slowly) build a hod rod F2 motor.  That still leaves the 849 to figure out for now.

  However I think that I am going to take things one step at a time. 

That's a good idea.....not everything you read here is gospel....and remember that because that is gospel. 

[rlvitt]

I was looking at the Cycle X 849cc "coated" pistons today. Has anyone heard any reports on how good they are or if they cause any problems because of the coatings on the piston?

[HondaMan]

Well crap....I probably shouldn't mention that I just received my new Cycle X F2 valves today (along with my new .040" base gasket).  Actually my intention is to (slowly) build a hod rod F2 motor.  That still leaves the 849 to figure out for now.

The advice here has been stunning and educating (thanks).  However I think that I am going to take things one step at a time.  Rather than pile solutions on top of solutions to make a problem, it appears to me that I only have two choices.  Keeping in mind that my problem was the low rpm spark knock, I need to reduce the compression ratio without having a super rich pilot jet wetting the combustion chamber all the time. 

1) I can take the simplest route; do not install the thicker base gasket just yet, but instead, install the F2 cam in the stock K4 head that I was using.  As was stated in an earlier post, the increased intake duration would lower the DYNAMIC compression ratio.  If I am understanding this correctly it will have good mid range and top end performance yet it will less likely to knock at the lower rpm's because of the compression ratio reduction (right?).  The only problem with this seems to be clearance of the valves in the combustion chamber.  As long as there is 1mm or so I should be ok (right?). 

2) The slightly more complicated route would be to use the .040 base gasket and go back to the cam/head configuration that I had.  The only complexity is that the jugs would have to come off and go back on again (right now they are still on).  This actually may be the simplest route because there are no valve/piston clearance issues.  I assume that power and tuning characteristics would be somewhat "linear" from the way it was (right?) 

It sounds like you're on the right track(s) with either choice, here. When the intake tract gets wet and makes spark knock, drying it out is usually my first choice. This happens often with 'pod' air filters, and when the drying out finally occurs, some torque goes with it: drag racers hate this option. Street riders don't like it either, but their tradeoff is fewer spark plug changes, so they usually warm up to it after a while.

1mm piston-valve clearance is a decent number with domed pistons when you're not racing. It's those extended times above 7k RPM, when the oil on the stems is getting pretty thin, that the valves start slowing down in 'retract' mode. This is when a little more is needed. If the valves are wet (intake especially) then this happens sooner with ethanol fuels than with "real" gasoline, as gasoline is essentially 0 weight oil. Before adding ethanol, it is a pretty decent lube: ethanol turns it into a solvent instead.

[banzaibob]

Option #1) going with just the F2 cam......or option #2) going with just the .040" thick base gasket.  Which one sounds better? 

[andrewk]

Cam.  With an adjustable sprocket so you can tune it to your riding style.  Adding .040 more quench with a base gasket should be your last resort, IMO.  Lower the dynamic compression, but keep the power crisp.

You can get some modeling clay and, with an afternoon's worth of messing around, measure piston to valve clearance at several different points in the tuning range you will use.