Topic: help with cam selection

[nokrome]

hi, this is my first post but i have been lurking in the shadows for a while now, ive gotten loads of great information from you guys and ive got a lot of respect for your opinions so im hoping you can help me out.
im trying to make a decision on a cam for my 750 build, im going to be running a cycle x 850 kit with a ported head, cr carbs, and a 4 into 1 (havnt decided wich one yet). i think that i have it narrowed down to two different cams, one is a .370 lift with 282 duration, and the other one is .370 with 290 duration. i guess my question is really about duration, at what point do you really start losing so much bottom end that it becomes unsreetable? im not going to be spending too much time around red line and im probably going to be doing a fair amount of in town riding. i talked to ken at cycle x (super cool guy) today and he said that i would be really happy with the .370/290 option and i totally trust him but i just wanted some more educated opinions.
                                                                                                                                                                                 thanks!

[Jerry Rxman Griffin aka MuthaF'er]

Tell us more about your ported head (valve size, etc) and CR's (29 or 31).

[nokrome]

i will be running cr 29 carbs with a stage 3 port job and probably cycle x superflow valves which have a reduced stem diameter around the port area for better flow, i was considering going with oversize valves but i have to be realistic about my budget. the .370/290 cam is the same grind as a webcam 63b and the .370/282 is the same as a 63a if that helps

[sparty]

hi, this is my first post but i have been lurking in the shadows for a while now, ive gotten loads of great information from you guys and ive got a lot of respect for your opinions so im hoping you can help me out.
im trying to make a decision on a cam for my 750 build, im going to be running a cycle x 850 kit with a ported head, cr carbs, and a 4 into 1 (havnt decided wich one yet). i think that i have it narrowed down to two different cams, one is a .370 lift with 282 duration, and the other one is .370 with 290 duration. i guess my question is really about duration, at what point do you really start losing so much bottom end that it becomes unsreetable? im not going to be spending too much time around red line and im probably going to be doing a fair amount of in town riding. i talked to ken at cycle x (super cool guy) today and he said that i would be really happy with the .370/290 option and i totally trust him but i just wanted some more educated opinions.
                                                                                                                                                                                 thanks!

A build like that and hotter cam will also require HD valve springs, better retainers(Ti) and cam chain tensioner.  Then you have rods to think about.  Unless you don't want it to last too long.  Trust me, you really can't begin to beef up the top end without changing these things... at least the valve springs and retainers.  I am running a Megacycle 125-75 cam and though I have less of a bottom end, it is still very streetable.  But many things were changed to make it so.  If you will mainly ride in town, go with a mild cam.  It will save the top end and give you better low end response.

Art

[nokrome]

thanks sparty, i definitely got all the other parts covered, its gonna be a stout motor for sure. so, what is the lift and duration on your 125-75 cam? when you say you have less of a bottom end do you mean less than stock or what?
                               i guess it would help me if i could get some examples of other peoples builds and the cams that they used and how they felt about them after they rode the bike for awhile.

[MRieck]

 The Megacycle 125-65 is a great all round cam for an 836. You can check out all the specs on Megacycles website.

[sparty]

thanks sparty, i definitely got all the other parts covered, its gonna be a stout motor for sure. so, what is the lift and duration on your 125-75 cam? when you say you have less of a bottom end do you mean less than stock or what?
                               i guess it would help me if i could get some examples of other peoples builds and the cams that they used and how they felt about them after they rode the bike for awhile.

The megacycle 125-75 has:

intake valve lift of .400" and 262 degrees of duration (open 26 BTC  close 56 ABC)
exhaust valve lift of .375" and 257 degrees of duration (open 53 BBC  close 24 ATC)

however using an adjustable cam sprocket mine is set at:

intake valve lift of .390" (open 23 BTC  close 55 ABC)
exhaust valve lift of .360" (open 52.5 BBC  close 19 ATC)

by less bottom end I mean that the engine doesn't make a lot of power below 5000 RPMs.  The power is there, but REALLY comes on in the upper RPMs.  It is difficult to explain but easy to feel.  A stock cam has better feel down low to about 8K.  Mine has better feel from 5K-6K to 10K.  The power builds from 5K 'ish and keeps going until I shut it down at 10K.  Once past 7500K 'ish it really picks up quickly and you can feel it hit its true power band through 9500 to 10000K.  But I also have a APE race crank which makes a HUGE difference.
Here is an example of what I am talking about: When I am in 2nd or 3rd gear and just about level with 6K, I will open the throttle and it hits hard.  The bike literally jumps up on the rear wheel (in second) and you better hold on.  If I am at say 4K and open the throttle the bike picks up speed (but you don't get the same hard hitting power), but then when it hits the magic number it goes ape again.  It has caught me by surprise many times and I love it.

Here is another example:  Imagine your rear tire is on ice and it is spinning quickly (this is the bottom end of the power, you are moving, but not very fast) then your tire makes contact with dry pavement and it launches you forward (this is how my bike feels once it moves from the low end power into its power band).  It really is that dramatic at times.

Art

[nokrome]

thats what im talkin about sparty, that was a great post! that type of power sounds like fun!
        i take back what i said earlier about doing a "fair amount of in town riding",  i got my stocker for that
         

[paulages]

good to see you on here, phil. take these guys advice for sure, and especially listen to mr. Reick.

as for cam selection- the duration you choose (between those two cams) will probably have the most noticeable effect on your midrange... longer duration and you get more charge in there (not accounting for reversion, etc), but shorter duration will keep you compression ratio up and boost the low and midrange). i'd focus on lift more than duration unless you're really going to be keeping this thing at high RPM's. i run the #$%* out of mine in all ranges (10.6/1 CR, 363/259 lift/duration, stage 2 porting), and even though i didn't focus on top end it still makes plenty of power (70-75RWHP) and the high revs are an absolute blast... i don't think mine's as dramatic as what sparty describes (and it is 118cc less capacity), but it definitely comes on the cam strong in the high RPMs.

you told me you wanted to build a 90+ HP machine...I think we can do it, but the power will mostly be in the breathing (porting, carbs, valves, etc). the other stuff is all about the life span of the engine and it's revving capabilities. head components are important to reduce valve float, but if i remember right you are doing Ti retainers, right? you're headed down the right path for sure.

you're doing the right thing if you decide to go with the oversized valves, as that'll help your breathing as much as the porting will. i haven't seen those cyclex guides, but i can also have dan turn down the exposed section of the guide a bit to help flow around it. check out pictures of mike's head to see what that looks like... 

you should post some pictures of that stocker too- it's a looker.

[nokrome]

thanks paul! i was wondering when you were going to pop up.
       i definitely want to have all the high end super trick parts but as you all know budget is always a factor. ive been looking at cycle x's "git kit #2", seems to have pretty much everything needed at a reasonable price (but comes with standard size valves). take a look and see what you think.cyclexchange.net/Engine Parts Page.htm
        paul, i love the way your bike revs so smooth, is your crank lightened and balanced?........... thanks for the compliment on my green bike, ill get some pictures up shortly.

[MRieck]

thanks paul! i was wondering when you were going to pop up.
       i definitely want to have all the high end super trick parts but as you all know budget is always a factor. ive been looking at cycle x's "git kit #2", seems to have pretty much everything needed at a reasonable price (but comes with standard size valves). take a look and see what you think.cyclexchange.net/Engine Parts Page.htm
        paul, i love the way your bike revs so smooth, is your crank lightened and balanced?........... thanks for the compliment on my green bike, ill get some pictures up shortly.

You will definitely need oversize (33.5mm) intake valves for your project. Stock size exhaust valves will be OK. You will need that 125-75 for 90 HP.

[scondon]

thanks paul! i was wondering when you were going to pop up.
       i definitely want to have all the high end super trick parts but as you all know budget is always a factor. ive been looking at cycle x's "git kit #2", seems to have pretty much everything needed at a reasonable price (but comes with standard size valves). take a look and see what you think.cyclexchange.net/Engine Parts Page.htm
        paul, i love the way your bike revs so smooth, is your crank lightened and balanced?........... thanks for the compliment on my green bike, ill get some pictures up shortly.

You will definitely need oversize (33.5mm) intake valves for your project. Stock size exhaust valves will be OK. You will need that 125-75 for 90 HP.

   Ditto.

    I've got two bikes. Bike one uses the 125/65 cam, cycle-x 850, stage 1 porting w/stock valves/HD springs, 29mm CR's. This bike dyno'd  75hp at the rear wheel and is plenty 'nuff power for town/highway and weekend romps. I am considering putting the stock '78 carbs back on this bike(I like the accel pump) since the CR's are almost overkill without more attention to intake restrictions.

      Bike two used the 63a cam, Wiseco 836, stage 2 porting w/ 33.5mm intake valves and ti retainers/HD springs, APE crank, CR 29mm carbs. This bike dyno'd at 80hp at the rear wheel and the CR's are a very noticeable improvement in the top end.

      Both bikes have Dyna ignitions/coils and 4-1 Kerkers with med. comp baffles.

     For the riding you describe you intend to do bike one is plenty, in my opinion. Power beyond this on a stock cb750 gets a bit scary without a lot of attention paid to improving suspension, handling, and braking unless drag racing on empty streets is ones intention.

   Bike two is(was) my roadracer and as much attention was paid to the chassis as the engine. This was the bike that kept up with some riders on modern bikes in the twisties and outperformed some in the straights. The bike often saw 10,000rpm and finally threw a rod(stock,shotpeened F2) at 18,000 miles. I will rebuild with Crower rods and use a 63b or 63c cam. I intend to use this bike primarily on the track 'cause high speeds on two-lane roads has worn on me a bit.

    This is all just info and my opinion and not intended as a recommendation on what to do. Paul has put some screamin' bikes together and having him right there with ya for help and advice is the best anyone could hope for

[754]

Think of it as a trade off.

You will give up some in the bottom or middle to get more on the top.

Figuring what you really want to use it for most, is important.

I had an overcammed 836 at one time that was really dull and unpleasant to ride at under 5K, a real drag to ride in town..

[paulages]

phil- i'd definitely listen to mike on the cam selection (and everything else, really) if you want to get over 90HP. those cyclex cams are a little milder, and you'll more than likely end up in a similar range to scondon's first bike, but if we do a stage 3 porting and oversized valves, i'd bet we can at least get you in the 80's. i'll call les barker soon for some advice as well... he has built some high HP cb750's, one of which races at PIR currently. i'm pretty loyal to dan hall, but you might consider getting les to do some of the machine work.

[nokrome]

thanks for the great info guys, it definitely helps me get a much better perspective on what im trying to do. im still pretty new to sohc4s but have learned tons over the last year thanks to the interweb and paul of course.
            realistically  im not too concerned with trying to build a "race motor" but i do want a motor that is very fun to ride but is also going to be reliable, something that is a little above my current skill level and gives me plenty of room to grow as a rider, i want to be a little scared when i twist the throttle! !!!!