barnett468
Coast to Coast
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please explain exactly how this is done.
crazypj said:
please explain exactly how this is done.
Big cams and Bernoulli's principle (venturi jetting)
- Thread starter cosworth
- Start date
Going back a couple of steps, how is the cam timed compared to the card? The only cams I ever found to be close to specified timing on all 4 events IO-IC-EO-EC were old Honda race kit cams. The rest are all over the place.
Big cams tend to lose bottom end and improve top end but you mentioned that teh bottom end is OK and top end is weak and that makes me wonder if the cam isn't advanced.
Big cams tend to lose bottom end and improve top end but you mentioned that teh bottom end is OK and top end is weak and that makes me wonder if the cam isn't advanced.
crazypj
Split personality, I fake being smart
Dynamic compression is what you actually get when motor is turning over instead of some BS theoretical CR by measuring combustion chamber volume, ring land volume, head gasket thickness and swept volume.barnett468 said:
The easiest way to do it is with a 'Spintron' driving crank and compression tester in plug hole. (also way too expensive for simple test but many other things can be measured 8) )
To test at different RPM you can tow bike in gear (cheapest but least safe ;D )
teazer said:
Totally agree with this.
Even when you find cam specs, measuring them usually gives something completely different.
If you just used stock markings and not a degree wheel, you have no idea where cam timing really is
barnett468
Coast to Coast
DohcBikes said:
It's not a useless term, a "big" cam is obviously bigger than a "small" cam . . FYI - The terminology goes like this.
small cam
medium cam
big cam
behemoth cam.
I want the behemoth one!
.
barnett468
Coast to Coast
DohcBikes said:
You can buy the extended version on sale now at Amazon.com.
You mean there's even MORE to it?
. .
barnett468
Coast to Coast
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1fasgsxr said:
So is a big cam just bigger in every dimension but with the same lift and timing or just one with a bigger looking lump than a stock looking lump. We used to refer to race cams as bricks on sticks. They are just words and none of it helps us to see if that particular cam has too much lift or dwell for the application.
In this case, going back to a stock cam and getting the bike running well eliminates one variable and would identify if something else was wrong with the bike. I still suspect that the big cam was installed 1 tooth or maybe 2 too advanced.
In this case, going back to a stock cam and getting the bike running well eliminates one variable and would identify if something else was wrong with the bike. I still suspect that the big cam was installed 1 tooth or maybe 2 too advanced.
I've never done it in a motorcycle, but I have in a 350. To get low end, you have to combine a bigger cam with a better intake. I assume with the bike, since the intake is less of a feasible option, then pistons are the methodology. But one thing I notice people doing wrong, as I think Teazer points out, is that with a big cam comes the necessity to advance the timing. My truck has an advance of 12 degrees. Usually, it's within the range of 8 - 12 degrees. Full vacuum advance is 36 degrees. I can't see how a motorcycle would be much different.
Not advising, just pointing out a condition that usually comes with a "big cam"... at least on a V8. Like I said, I haven't done this on a bike. But if I were doing it would be one of several things I'd look at. As well as the cfm of my carbs, compression, and a host of other things.
Sent from my iPhone using Tapatalk
Sent from my iPhone using Tapatalk
I was referring to cam timing with respect top crank position. Advancing teh cam tends to improve bottom end at teh cost of top end and retarding a cam tends to bring peak HP later (higher RPMs)
Deviant raises a good point that with a lumpy cam teh effective compression ratio at lower revs is typically lower and the motr can usually deal with more ignition advance.
A lumpy cam tends to reduce compression at low to mid revs which is why raising compression keeps coming up. In this case, the OP says that bottom end is better than before and top end sucks. That suggests that either it's an unusual cam timing /lift spec or maybe it is retarded by a tooth or two as I mentioned before.
It sure would be nice to see a before and after dyno curve with A:F ratios plotted.
Deviant raises a good point that with a lumpy cam teh effective compression ratio at lower revs is typically lower and the motr can usually deal with more ignition advance.
A lumpy cam tends to reduce compression at low to mid revs which is why raising compression keeps coming up. In this case, the OP says that bottom end is better than before and top end sucks. That suggests that either it's an unusual cam timing /lift spec or maybe it is retarded by a tooth or two as I mentioned before.
It sure would be nice to see a before and after dyno curve with A:F ratios plotted.
barnett468
Coast to Coast
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onev ery simple rule of thumb is that if an engine has less than around 150 psi of cranking compression, the compression should be increased if a "bigger" cam is installed.
in general, a cranking compression of around 170 is safe, but the maximum one would want to run with 91 octane gas.
if you aint got compression, ya better not be puttin a "big" cam in an engine . . do it right and you will only do it once.
i was 16 once too and at that thought that putting an 850 holley on a nearly stock 283 chev would instantly give it another 50 hp and make it haul ass.
onev ery simple rule of thumb is that if an engine has less than around 150 psi of cranking compression, the compression should be increased if a "bigger" cam is installed.
in general, a cranking compression of around 170 is safe, but the maximum one would want to run with 91 octane gas.
if you aint got compression, ya better not be puttin a "big" cam in an engine . . do it right and you will only do it once.
i was 16 once too and at that thought that putting an 850 holley on a nearly stock 283 chev would instantly give it another 50 hp and make it haul ass.
mydlyfkryzis
当有疑问时踢你的敌人在生殖器上，你可以道歉后
Stock cb360 is 170 psi. Recommended fuel is 86 PON (octane).. I run regular at 87 octane. Never have any problem.
Most of the small Honda twins run around the 170 psi mark on 87 octane....
Most of the small Honda twins run around the 170 psi mark on 87 octane....
barnett468
Coast to Coast
Mydlyfkryzis said:
just because an engine might have came with 170 compression 40 years ago, it is no guarantee it will still have 170 compression 40 years later.
also, the cb/cl 250 350 manual i had said 95 octane [for however they rated it back then], not 87, which is more than ron, mon, pon or rm2 rating anyway.
