Topic: Hot Street Motor on a Budget

[stikman]

I haven't built a modified CB750 since my 836 auto a couple years ago, but now I'm prepping to build another semi-hot motor for the street that won't break the bank.  Looking for your thoughts and input on this.  So far I'm considering the flat top cast 836 pistons paired with HD head studs, stock roundtops, some port and polish work, and a cam.  Since it's going in a chopper, there will be a measure of form over function.  Just so you're aware.  I'll be running 4-4 MAC drag pipes, since it's what I've already got, and velocity stacks.

Now I realize this won't be the most efficient bike, but that's choppers and it's what I'm into.  That said, I'm also about some low end torque and horsepower.  I'll be doing the port work myself and I've got a few cams in my stock (Webcam 41, K0, and K1) that I could use to replace what is likely the stock K3 cam.

I'd prefer to keep as much low to mid range power as possible.  I realize the flat top pistons limit the cam size, which in turn has an effect on the movement of the power range with porting.  Not to mention the drag pipes.  But this is what I'm working with.  I want to avoid the slippery slope, but if there's something I haven't considered I might be willing to drop a little extra cash.

And finally, while looking into cam options (which are limited when it comes to short duration and higher lift), I came across the MC 125-N-T3 turbo profile and wanted to get some thoughts on running that with this combo to keep some of the power down low.  Here's the specs on it:
125-N-T3
IN Lift:  .342    Duration @ .040": 215   Lobe Ctr: 109.5  IO   2* BTDC / IC 37* ABDC
EX Lift:  .312    Duration @ .040": 222   Lobe Ctr: 110    EO 41* BBDC / EC  1* ATDC

[madmtnmotors]

You have described my engine to a "T". Had a manufacturer's defect in my Webcam 41a and had to use an F2 cam to meet race schedule while the Webcam was being exchanged. Mild porting, HD valve springs with HD valve spring retainer caps (the K8/F2 had weaker retainer design than early models), simply lapped the valves, HD studs, 65mm flat tops, Mac 4into1, stock jetting, Dyna S w/3ohm coils, and new clutch friction plates with Hondaman oil mod to clutch hub. The bottom cases remained intact and were untouched.


I'm enjoying mine:



even though they are flat top pistons in a K8 motor...

[Old Scrambler]

My K3 has 1st-over flat-top pistons with new rings after 70xxx miles........AND an F2 cam. All else is stock. Noticeably more low and mid-range power with slightly rich jetting at 115. Will change to 110s this winter. 18 x 48 sprockets and stock pipes.

[Jwexperience]

I've got a f2 cam that I bought for my k6 which is now sold if you want it. It's got R1 stamped near the sprocket.


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[stikman]

I'm enjoying mine

No doubt.  I saw that pic recently.  Good stuff, buddy.

My K3 has 1st-over flat-top pistons with new rings after 70xxx miles........AND an F2 cam. All else is stock. Noticeably more low and mid-range power with slightly rich jetting at 115. Will change to 110s this winter. 18 x 48 sprockets and stock pipes.

Looking to go a bit further than that, but glad it's workin for you, OS.   

I've got a f2 cam that I bought for my k6 which is now sold if you want it.

Hmm, I've been giving it some thought today, bud.  Never thought about using an F2 cam before, so been trying to find some specs on it.  Haven't been able to find exact specs for some reason. 

[stikman]

Anyone have thoughts on the MC 125-N-T3 cam in a non-turbo motor?  I'd be pleased to have some discussion on this.      It's actually somewhat similar to the custom grind I had WebCam do for my 77 A.  Not sure how the lower EX lift would affect things, but that 3* overlap would keep things clean down low despite some loss at high rpms (which is fine for me, stock rods and bolts).  And those shorter durations should keep the power bad lower as well.  Feel free to share thoughts here, fellahs.

[Jwexperience]

I'm enjoying mine

No doubt.  I saw that pic recently.  Good stuff, buddy.

My K3 has 1st-over flat-top pistons with new rings after 70xxx miles........AND an F2 cam. All else is stock. Noticeably more low and mid-range power with slightly rich jetting at 115. Will change to 110s this winter. 18 x 48 sprockets and stock pipes.

Looking to go a bit further than that, but glad it's workin for you, OS.   

I've got a f2 cam that I bought for my k6 which is now sold if you want it.

Hmm, I've been giving it some thought today, bud.  Never thought about using an F2 cam before, so been trying to find some specs on it.  Haven't been able to find exact specs for some reason.

I read somewhere a long time ago( I think hondaman) where it was compared to the k0 cam. The R1 grind was supposed to be best performance. It's not going anywhere, just sitting in a box so let me know if you're interested.


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[754]

The R number is just the batch I think.
You should measure lobes with caliper and note measurent...90 degrees from lobe tip gives base circle.

[seanbarney41]

R1 does not indicate what grind it is, only which casting mold was used to cast the cam blank.  I have seen R1 in a few different motors that had no evidence of a cam swap, K1, K6, K7 or 8.  The only F2/3 motor I have torn down had R8.  I bought an F2 cam from a member here to use in another motor and it has R10.

[PeWe]

Minimum overlap such as Turbo engines use is due to the forced induction system with an over pressure of 1-2 BAR.
With standard carbs I doubt the engine will be fun to drive with such cam without the extra draft where the exhaust will help to suck air/fuel into the chambers thru the carbs during the overlap.
Flat top 836 is the cheap pistons from cruzinimage? They have not much space for higher lifting cams.

If in valves are std 32mm there is one mm extra in clearance piston/valve compared with 34mm.
Webcam 41 is an alternative.

Pistons with deeper pockets and more CR will allow a DP315 cam. Not specific high lift and it has a little bit longer duration which combined will not give an abrupt lift giving the valve train a hard life.
I was surprised that DP315 was so good from idle and all the way up thru the rev range. RC was right when describing the RC315.

[TurboD]

My clear cut choice would be the Web 41, it and the 41a are very good cams. I have used them in a number of builds. Last winter I spent a lot of time running cams through my dyno simulator, all the stock profiles along with a few popular aftermarket grinds. The 41 and 41a both holdup very well against many "popular" cams that guys are running. To some people's dismay, many of the aftermarket cams I see used are simply poor choices.

I have also had good luck with the Cruzin image 836 kits, I just install them in my Turbo build.   

[754]

You could probably trade the turbo cam, for something more suitable.

[Medyo Bastos]

me looking for turbo cam  , and have other suitable cams to trade 

[stikman]

This new motor I'm building is a K4.  Still up in the air whether to go with the cruzinimage cast 836 pistons or forged with deep valve reliefs.  I'd really like to run a bigger cam but it's definitely more costly to go with the forged pistons.  Are they a pain to run on the street, with the heat/cold cycling and piston slap??  Or does it mainly mean you have to wait longer to warm up before riding, and longer to cool down before starting back up?

Anyway, still considering bigger intakes, bigger cam, and pistons that will allow them for the K4.  If anyone has any of these bits they might part with, let me know.

me looking for turbo cam  , and have other suitable cams to trade 

Medyo, I don't have that MC turbo cam but the custom cam I had ground on my auto core by WebCam is very similar to that Megacycle turbo cam, has even less overlap in fact.  I can send you the specs on it if you're interested.  I've been running it in my auto for a couple years with flat top 836 pistons, roundtops, and mild porting.  The combo works, but I might be looking to change the cam out.

[Medyo Bastos]

Sure, when you've got a minute


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[stikman]

Sure, when you've got a minute

PM sent

[TurboD]

Stikman Todays forged pistons are fine, between materials and skirt design you won't know the difference. The old hot/cold thing is a little dated this days. lol

To add to my earlier post on cams, the F2/3 cams were the best production for performance. Somewhere a while back I had a complete thread on here discussing the different factory cams. I ended up using a F2/3 in my turbo motor.

[stikman]

Stikman Todays forged pistons are fine, between materials and skirt design you won't know the difference. The old hot/cold thing is a little dated this days. lol

To add to my earlier post on cams, the F2/3 cams were the best production for performance. Somewhere a while back I had a complete thread on here discussing the different factory cams. I ended up using a F2/3 in my turbo motor.

Good to know about the pistons, thanks for that.  I swear I've read a bunch of info on the forged ones being an issue, but good to hear some feedback based on experience.

Are you able to link that thread, Turbo?  Or maybe post the F2/3 specs here?  I can't seem to find them anywhere.  Can't decided either way without the info.  But you got me thinkin about the forged pistons now, which would likely cause me to go the bigger cam route.  Not too big, I still want to hit the highway with it.  Suggestions in that regard are also very appreciated.

[dragracer]

Have the valve reliefs fly cut on the cruzin images pistons to accommodate the bigger cam. You shouldn't need much.

[stikman]

Have the valve reliefs fly cut on the cruzin images pistons to accommodate the bigger cam. You shouldn't need much.

Wondered about that, if there is enough meat on them for that.  Does anyone have any actual experience doing it on those pistons?

[754]

If you know how much , it helps..usually done on fixture in a mill.
But if its a very slight amount, some have done them with a dremel

[TurboD]

Stikman Unfortunately bikes are no different than cars when It comes to people hanging on to old outdated info/wive's tales. Now with the web and reading different forums, my opinion is the vintage bike guys are the worst, they really hang onto the past. 

Posted below is a copy of a post of mine from the old cam thread, it pretty much sums ups the 3 pages of discussion.

*** Here are the cam specs taken from Honda service manuals. I have come up with 4 different cams used. I cannot speak on a early sandcast cam, but one of the manuals is dated 1970.

I have listed these in order by power potential, not only by specs but also by running them on my dyno simulation software. They fall in order just as one would expect by reading the specs. This is only to help rate the stock cams, all of these are very mild by any performance standard and should be replaced if more power than stock is wanted.

Hondamatic
Intake open 5 AFTER tdc 
Intake closes 30 after bdc 
duration 205 
intake centerline 97.5

Exhaust open 40 before bbc
exhaust closes 5 before tdc
duration 215
exhaust centerline 107.5
--------------------------------
K model
intake open 5 before tdc
intake close 30 after bbc
duration 215
intake centerline 102.5

Exhaust open 35 before bbc
exhaust close 5 after tdc
duration 220
exhaust centerline 105
-------------------------
K model 77-78
intake open 0 tdc
intake close 40 after bbc
duration 220
intake centerline 110

exhaust open 40 before bbc
exhaust close 0 tdc
duration 220
exhaust centerline 110
---------------------
77 78 F
intake open 5 before tdc
intake close 40 after bbc
duration 225
intake centerline 107.5

exhaust open 40 before bdc
exhaust close 5 after tdc
duration 225
exhaust centerline 107.5

I hope this helps to answer as to which "stock" cam will make the most power.***

[PeWe]

Higher CR need more accurate ignition timing, fuel with high octane and not too lean jetting. My first 836 was RC forged pistons. 2.3 got hole due to pinging l guess. Pistons after (Action Fours 836 cast) got lower CR since head was opened for RC wider crowns. 24.4cc chambers instead of common 22,5cc.
The bike ran really well with same top speed. No more pinging and could take worse fuel. A really good touring bike. Head was ported, 34mm in valves F2 in k head. Cam was Action Fours SS-1. Same head today that has got 5mm conv kit. A little bit more porting to adjust for new guides/ valves. + more mods $$$$

[MRieck]

Have the valve reliefs fly cut on the cruzin images pistons to accommodate the bigger cam. You shouldn't need much.

Wondered about that, if there is enough meat on them for that.  Does anyone have any actual experience doing it on those pistons?

Did it for OS intake valves. I think the cam was a 41 or 41A.....it was 3 or 4 years ago.

[TurboD]

I currently have the CI pistons in my auto along with the CX-2 (41A -341A ) cam, I do not have any work done on the pistons. EDITED.

Now Stikman I would like to refer back to your original post/question. You are wanting a *budget* semi hot motor for the *street*, that is going in a *chopper*. Those are key words I focus on when giving my opinion. In the last 35 years of building racing engines for people, the first thing anyone giving advice needs to understand when putting a combination together is what the person wants and to know what is needed to get there.