Which compresssion ratio should I pick for re-build?

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Hello All and Happy new Year.

I am in the process of rebuilding my MK3 850. I swapped out the stock cam for a Megacycle 560NR cam, and went with .040" over JCC pistons in the cylinder barrels, so it's a true 850 CC's. After carefully measuring (and matching) the combustion volumes in the RH4 cylinder head I came up with 51CC for both sides. I am now trying to decide on the thickness of my copper head gasket. I have both a .030", and a .040" thick gasket to choose from. When I do the arithmetic with the .030" thick gasket I get a theoretical 8.79 compression ratio, and if I use a .040" thick gasket the ratio drops to 8.62.
Conventional wisdom says to up the compression ratio with a hotter cam to make up for the increased open valve duration, but how much do you increase compression? Megacycle's 560NR cam is their mildest grind with only .030" added lift and 10 degrees more duration. I am worried that the higher compression ratio may be too high and lead to detonation. I will be running 32MM twin carbs.
Any thoughts from users in the know would be greatly appreciated.
Rick
 
Go online to dynamic CR calculators and plug in the data points, ie: crank degrees where intake closes to get the actaul effective CR your octane needs to tolerate in lower rpms till cam gets into inertial mixture ram effect then may need even less octane as piston speed out runs time to heat to detonate. Ping cam sellers for range of CR they suggest - per octane expected to use. If ya keep digging into online calculator you should be able to find minimal octane per effective CR to allow full pressure igniton timing w/o pingadinging. My 10.5 CR only gives ~7.5 CR with Norris D cam for example for the 30-40 hp pressure needed for normal C'do operation well legal speeds and higher outputs/rpms only needs 112 octane to stay on full spark timing up over 16:1 effective CR when rpm ram effect and blower comes into their power band. By only asking simple questions can only expect simpleton general responses. Special coatings can lower octane needs by a few points down too. Type of ignition system comes into play too but that's another subject area.
 
rmaten4 said:
kommando said:
You can go for .003 thick gaskets and get a higher compression option.

http://www.jsmotorsport.com/technical_compression.asp

So with .003" gasket I calculated a 9.28:1 compression ratio. Wouldn't this high ratio cause detonation problems? I should also mention that I live in California where we are stuck with 91 octane pump gas.

With 91 octane fuel and most use down near sea level I would not want to go over 9-1 compression ration. Jim
 
Wouldn't you guys retard the ignition slightly to cope with the low octane fuel ? I use methanol which is very slow burning, and I run an extra 4 degrees advance over that recommended for petrol. I started riding motorcycles with a 1942 Indian Scout. It had the advance and retard on the RH twist grip, so I found out what happens as you advance and retard the ignition timing . There is an optimum for every fuel ? If you are changing cam timings etc. and comp ratio why do you expect to be able to use the manufacturers' recommended ignition timings ? A lot of things are a matter of balance. Perhaps what you need is the manual advance and retard lever on the handle bar as the bikes had in the 50s ? Set it so that when it is fully advanced the ignition is a bit retarded to cope with your crappy fuel, then you can pull on a bit more advance if you get some decent stuff. You would need to make the points plate rotate by cable control.
 
If you speak to the laboratory people in the petrol companies, they will have done knock rating tests at various comp ratios, and should know how much you have to retard the ignition at the various ratios to stop detonation. All you really need is two points and draw a straight line graph between them and read off what your ignition advance should be for the comp. ratio you are using. The optimum is just behind detonation and your mixture affects that too.
 
acotrel said:
Wouldn't you guys retard the ignition slightly to cope with the low octane fuel ? I use methanol which is very slow burning, and I run an extra 4 degrees advance over that recommended for petrol. I started riding motorcycles with a 1942 Indian Scout. It had the advance and retard on the RH twist grip, so I found out what happens as you advance and retard the ignition timing . There is an optimum for every fuel ? If you are changing cam timings etc. and comp ratio why do you expect to be able to use the manufacturers' recommended ignition timings ? A lot of things are a matter of balance. Perhaps what you need is the manual advance and retard lever on the handle bar as the bikes had in the 50s ? Set it so that when it is fully advanced the ignition is a bit retarded to cope with your crappy fuel, then you can pull on a bit more advance if you get some decent stuff. You would need to make the points plate rotate by cable control.

In my own experience playing on dyno's with old bikes like ours, there is little or nothing to be gained by deviating from manufacturers settings for timing, or by playing with advance curves.
They do however, respond greatly to improve the size and power of the spark.
My own approach therefore is always to start with manufacturers settings, and if it ain't pinking, leave it be!
 
acotrel said:
If you speak to the laboratory people in the petrol companies, they will have done knock rating tests at various comp ratios, and should know how much you have to retard the ignition at the various ratios to stop detonation. All you really need is two points and draw a straight line graph between them and read off what your ignition advance should be for the comp. ratio you are using. The optimum is just behind detonation and your mixture affects that too.


There is SOOOO much more to it than that... Combustion chamber shape, first and foremost. Things like vehicle size/weight/usage is hugely important.

The OP is doing well to ask what others have done, and what results were observed.
I used the .023" gasket on a '74 850, guestimated 9:1. Timing is snappy hot, poor fuel quality is audible at low lug (starting off, for instance)
 
'My own approach therefore is always to start with manufacturers settings, and if it ain't pinking, leave it be!'

Mine too, however what do you do if you build the motor and it detonates ? One thing we never do with our racing two strokes is change from the manufacturers' spec for petrol advance even when using methanol. We always jet with the same ignition advance, and we don't change the comp. ratio much when reshaping the heads . It is easy to introduce too many variables to make sense when tuning a motor. With the crappy petrol, there is probably a comp.ratio and advance at which the motor is happy without richening the mixture.
 
' I used the .023" gasket on a '74 850, guestimated 9:1. Timing is snappy hot, poor fuel quality is audible at low lug (starting off, for instance)'

That is obviously a good starting point, have you tried richening the mixture at low throttle openings to stop the pinging ?

In what I've said previously there are a few assumptions i.e. the head has been decoked, and its chamber shape has not altered much. If you have simply raised the comp. ratio and have crappy fuel and get pinging, retarding the ignition slightly should allow you to avoid rejetting which is not so cheap.
 
acotrel said:
' I used the .023" gasket on a '74 850, guestimated 9:1. Timing is snappy hot, poor fuel quality is audible at low lug (starting off, for instance)'

That is obviously a good starting point, have you tried richening the mixture at low throttle openings to stop the pinging ?
No, runs great on E10 pump premium, 92 octane. Just lets you know when some sleazy retailer is selling regular at premium prices.
 
Many years ago I used to have hotted up 650 Triumphs with 10 to one comp. If I ran avgas they felt beautifully crisp and clean running. Sometimes I could only get super grade petrol and they always felt out of tune.
It would be interesting to know what the max. comp. E10 Premium can be run at in a fairly standard commando without pinging. I have the feeling that using hotter cams actually helps the situation.
 
From the JSMotorsport site:
"
Copper cylinder head and base gaskets in various thicknesses are now available to adjust compression. Standard thickness is approx .040". Adding .040" to a head or base gasket will lower the CR approx one point. Going to a thinner .020" head gasket will raise it 1/2 point etc. Never use a paper base gasket because they can loosen and cause damage - use Permatex Motoseal #29132 (base gasket only) or "the right stuff" by permatex). Note that Hi compression pistons generally require racing fuels. If you want to use high compression pistons on a street bike then you must use a thicker head or base gasket to reduce the compression to avoid detonation ("pinging") or risk failure. 9:1 compression is average for carefree street riding."


http://www.jsmotorsport.com/technical_compression.asp
 
How confident you won't get killed/maimed riding vs how efficient a power plant to create. Here ya go for a sense of risk vs pay back I'm still pensive on... we know comnoz is super sensitized conservative after this years octane let down but look how long he got away with it. How many week ends ya got before it just don't matter a whitworth...

https://www.google.com/#q=dynamic+compr ... calculator

https://www.google.com/#q=octane+vs+cr+calculator

https://www.google.com/#q=camshaft+vs+compression+ratio
 
dude - seriously do you ever imagine what you endless jibber jabber does to anyone with more than 2-brain cells ?

Ok, we know you can't ride a bike without crashing into a possum or wrecking into a tree, your bike is faster than a alien big rig with rocket jets, and your in personal communications with Elvis on Quaaludes

PLEASE ask your doc to check your your dose
w

hobot said:
How confident you won't get killed/maimed riding vs how efficient a power plant to create. Here ya go for a sense of risk vs pay back I'm still pensive on... we know comnoz is super sensitized conservative after this years octane let down but look how long he got away with it. How many week ends ya got before it just don't matter a whitworth...

https://www.google.com/#q=dynamic+compr ... calculator

https://www.google.com/#q=octane+vs+cr+calculator

https://www.google.com/#q=camshaft+vs+compression+ratio
 
hobot said:
How confident you won't get killed/maimed riding vs how efficient a power plant to create. Here ya go for a sense of risk vs pay back I'm still pensive on... we know comnoz is super sensitized conservative after this years octane let down but look how long he got away with it. How many week ends ya got before it just don't matter a whitworth...

https://www.google.com/#q=dynamic+compr ... calculator

https://www.google.com/#q=octane+vs+cr+calculator

https://www.google.com/#q=camshaft+vs+compression+ratio

Lots of great information. After looking at the above sites I have noticed that our commandos, even with stock cams ( and this "mild grind" Megacycle 560HR) have quite a bit
of overlap on the intake after bottom dead center. We definitely fall into the high performance category when compared to automobile cams. After digesting all the responses I think a good place to be is right around 9:1 for static compression. If I DO experience any detonation under hard throttle openings I can back off the timing on my Pazon electronic ignition.

Thanks for all the responses. You guys are awesome.
Rick
 
rmaten4 said:
hobot said:
How confident you won't get killed/maimed riding vs how efficient a power plant to create. Here ya go for a sense of risk vs pay back I'm still pensive on... we know comnoz is super sensitized conservative after this years octane let down but look how long he got away with it. How many week ends ya got before it just don't matter a whitworth...

https://www.google.com/#q=dynamic+compr ... calculator

https://www.google.com/#q=octane+vs+cr+calculator

https://www.google.com/#q=camshaft+vs+compression+ratio

Lots of great information. After looking at the above sites I have noticed that our commandos, even with stock cams ( and this "mild grind" Megacycle 560HR) have quite a bit
of overlap on the intake after bottom dead center. We definitely fall into the high performance category when compared to automobile cams. After digesting all the responses I think a good place to be is right around 9:1 for static compression. If I DO experience any detonation under hard throttle openings I can back off the timing on my Pazon electronic ignition.

Thanks for all the responses. You guys are awesome.
Rick

I would say you have arrived at a sound conclusion there Rick. Please do keep us all posted as to how it works out "add to the body of knowledge" as they say.
 
Here's MegaCam pdf on some facts and features of offerings. Give em a ping though to get their feedback to help settle mind on final choice.
http://www.megacyclecams.com/catalog/ca ... imized.pdf

506NR seems pretty close to standard but with excessive lash gaps I may call to ask what going on with that. There's is only like 1.5% power-efficiency increase just upping CR a point for 9 to 10, unless other more agressive cams and fuel and rpm used. Regardless any basic well tuned C'do is a pleasing hot rod so low 9's CR is way to go w/o octane concerns - excepting tail wind loaded down long climbs in low-no humidy, so might have to down shift to avoid ping til over the hump. Most riders can only detect 10% power and mass changes and only crazy racers 5% differences. Hope you attempt and enjoy the process of diddling the push rods geometry but likely within tolerance range - like Combats too long p-rods a lot of us get away with a long time w/o optimizing bother.

Mike slapped me back out of fatalistic mood, helped by recent renewed contact of cycle addicted friend that does 1000 mile weekends w/o getting very far from home and leaves me new tires and gear head goodies in my unlocked office i discover, but hadn't heard from since late summer. Last year we did a Polar Bear New Yrs run but this year wear-tear on '72 750 and dogs downing SV650 left me out of the small pack of nut cases riding their life away for fun.
 
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