Topic: Big cams for big bore engines.

[gschuld]

Mike’s recent thread (re)sparked my interest in the subject(big cam choices for big bore engines, more specifically, big bore street gas (+/- 10:1 compression) engines.  So for the sake of argument, l suggest Mike’s potential 69-70mm stroke, 72mm engine with (guessing) 10.5:1 compression.  I’m also guessing this would be for a non straight port but well ported Mike head.

I imagine once you get into mid 1100cc territory, the flow potential of the head becomes a dominant factor.  But without resorting to race only type Straight port heads with Huge valves set at altered angles, I guess Mike’s stage 4 head is an example of the gold standard head porting for a big street motor anyway.  Big flowing carbs are readily available (CR, RS, etc) so no worries there.

Mike is currently running the Megacycle 125-30 in his 1005cc ported head street motor(cam details below)
He also mentioned having a CycleX cx-10 cam( the black background image) which could be good for a stroker 72mm street motor.
Another mention for such a motor, by the legendary Rick Stetson, was the vintage Kenny Harmon Super F grind
I’d be curious where the 125-75 measures up.
And because I have an RC fetish of sorts🙄, an RC454 cam

All the specs for all these cams are below:

Thoughts welcome, bench racing is cheap😁

George

[754]

I am thinking I may have an F. Pretty healthy lift on the intake..

[gschuld]

I am thinking I may have an F. Pretty healthy lift on the intake..

Im lucky enough have an original Kenny Harmon F cam myself. 385/.330 advertised for the F

Most of the cams listed are in the .400 range, with the cx-10 cam the biggest of them at .420/.430(yes, taller exhaust lift than intake, according to CycleX)

Depending on the exact head specs and flow numbers, higher lift cams could be argued as a benefit for a proportionally undervalved headed engine trying to let 1150cc or so breathe well up to around 10,000rpm.

Most of these cams were likely not designed with over 1100cc in mind.  And not a chance at all any were expecting an over 1100cc cb750 engine for “modest” street gas compression(10:1 or so)

George

[PeWe]

Cams with good torque from low then?
I have used 3 cams in my engine this year. Changed them late April from DP315 to Megacycle 125-20 to RC295 profile during a week with road tests between.
I like the RC295 best since it gave better response from low. Like the ignition was too retarded and finally got correct advance, same feeling.

I have a CX-7 I have not tested with my JMR billet block 1000cc, ported head 34mm inlet TMR32 carbs, Carillos that will keep it all together.
Maybe more power that wake up after 6000 rpm and less of the good response from 3000 rpm?
It seems that there is always needs for a new cam

[gschuld]

Thanks for your thoughts:

Your engine offers some quality data points you are of the few with a 70mm on up big bore street gas engine that also has chassis dyno results and has been changing cams to see what happens. I appreciate your sharing your experiences so we all may learn from it. I’d be really interested in how your engine reacts to the CX-7 can you have.

George

[PeWe]

One detail that might affect the result. All cams ran with Lotus Root 4-4 without diffusers. Rather loud and open. 295 cam better in that combo.
The cams might have liked the 4-2-1 better.

I have no dyno result of the 295 cam yet.
I have with 315 but with 4-4 pipes. Now Hindle 4-2-1 since a week back that has transformed bike to a street "racer" from a street cruiser
High lift and relatively short duration and overlap is what needed, right?

[bear]

High lift and relatively short duration and overlap is what needed, right?

In my opinion yes.
But it can add more load ie wear to valve train components.

[gschuld]

High lift and relatively short duration and overlap is what needed, right?

In my opinion yes.
But it can add more load ie wear to valve train components.

Bear,

I know you are a firm believer in high lift short duration cams like Mike.  These cams tend to be considered helpful to street compression engines as the cams generally are said to help build cylinder pressure to compensate for less than race gas only compression ratios.  You run pretty high static compression ratios I recall and methanol.  Being a sidecar engine, piles of mid range torque is your friend while pushing around the weight of your rig.

Your mention of increased valve guide wear with high lift short duration cams, is this a good application for beehive valve springs then?  It would make sense I’d think.

George

[Tracksnblades1]

Great Thread George.
I agree, like to see the megacycle 125-75 dynoed too.
I think the CX10 will be similar to to Megacycle 125-X12, maybe slight smoother idle.
I think I'm going to have to check if Frank's  kits work on F1 frames too.

[Jerry Rxman Griffin aka MuthaF'er]

No problem with franks kit on F1

[PeWe]

CX-10 seems to be a hot one
I have to get a dyno time before winter to get data of the RC295 profile before I change to CX-7 and dyno again next spring. I plan to ride the bike to the shop. I have to see (and show) the torque curves from 2500 rpm, not only top hp at 8500-9500 rpm.

[cbr954]

I am running a CX-7 in my 970.  I have no dyno info but it pulls smooth from idle clear through to where ever you decide to shift.  Idle is very smooth.

[MRieck]

I am running a CX-7 in my 970.  I have no dyno info but it pulls smooth from idle clear through to where ever you decide to shift.  Idle is very smooth.

Good cam profile for the 750.  Good for the 915 to 970. Keeps compression up and idles well as stated. One of my all time favorites...Megacycle 125-70 or the CX7

[bear]

I use the mega cycle cam in my two remaining race motors.

[PeWe]

I am running a CX-7 in my 970.  I have no dyno info but it pulls smooth from idle clear through to where ever you decide to shift.  Idle is very smooth.

Good cam profile for the 750.  Good for the 915 to 970. Keeps compression up and idles well as stated. One of my all time favorites...Megacycle 125-70 or the CX7

Beyond 970 (70 mm pistons)?  1004.73 (71.25mm pistons) is not that much more  We have bikes to measure and publish the result of, dyno time is one issue here.

[gschuld]

I am running a CX-7 in my 970.  I have no dyno info but it pulls smooth from idle clear through to where ever you decide to shift.  Idle is very smooth.

Good cam profile for the 750.  Good for the 915 to 970. Keeps compression up and idles well as stated. One of my all time favorites...Megacycle 125-70 or the CX7

Mike, that’s an interesting statement regarding the 125-75 and near identical cx7(advertised as the same profile but with slightly extended duration) as being good for 915-970.

My assumption(meaning guess🙄) is that as the displacement increases from there, a well ported 33.5 or 34/28 head starts to lose breathing potential in the upper rpm range with these cams.  The street gas compression(below around 10.5:1) is assumed to benefit from the pressure boosting effect of the short duration profiles and the extra airflow comes from the high lift.

The 125-30 cam you chose on your current 1005cc engine ads a bit of lift and duration over the 125-75/cx7.

125-75.  .400/.375 262/257 105/104.5. 26/56 53/24
125-30.  .407/.407 266/268 105/106.    28/58 60/28

I’m assuming the 125-75 cam in your engine would be expected to run out of steam a bit lower in the rpm range than the 125-30 offered you(reducing peak HP)

Am I on the right track here?

George

[Jerry Rxman Griffin aka MuthaF'er]

Do the larger exhaust numbers help a larger engine using stock size 28mm exhaust valves breathe better?

[cbr954]

Guess I should also clarify I am running a ported F2 head and cr31 carbs.  On a K bottom end.  Had the head milled to get the compression back up using 970 K pistons.  Doesnt seem to run out of steam at high rpm feels like a steady pull from bottom on up.  But again this is all seat of the pants as I dont have access to a dyno around here.

[gschuld]

Guess I should also clarify I am running a ported F2 head and cr31 carbs.  On a K bottom end.  Had the head milled to get the compression back up using 970 K pistons.  Doesnt seem to run out of steam at high rpm feels like a steady pull from bottom on up.  But again this is all seat of the pants as I dont have access to a dyno around here.

Good info, what did you end up with for static compression?  Did you stick with factory 34/31 valves in the ported f2 head?

George

[bear]

We use the 410 castings also.
Our motors are around the 1100cc mark these days.
That seems to be saturation point re how much the head can flow.
We reduce exhaust valve diameter to allow for the largest possible inlet.
Have used many different piston options and now use RC45 pistons after we machine about 5mm off the shoulder and hand cut the valve pockets for minimum clearance to achieve the highest cylinder pressure possible.
That 125-75 is a perfect fit for our motors.
Many thanks to Mike for putting me onto it some years ago.

Cheers,
Brian

[cbr954]

Guess I should also clarify I am running a ported F2 head and cr31 carbs.  On a K bottom end.  Had the head milled to get the compression back up using 970 K pistons.  Doesnt seem to run out of steam at high rpm feels like a steady pull from bottom on up.  But again this is all seat of the pants as I dont have access to a dyno around here.

Good info, what did you end up with for static compression?  Did you stick with factory 34/31 valves in the ported f2 head?

George

yes stock F2 valve sizes.  I milled .060" off the head and that got it back to around 10.22:1.  The pistons were 970 10.25 pistons from cycle x.

[gschuld]

We use the 410 castings also.
Our motors are around the 1100cc mark these days.
That seems to be saturation point re how much the head can flow.
We reduce exhaust valve diameter to allow for the largest possible inlet.
Have used many different piston options and now use RC45 pistons after we machine about 5mm off the shoulder and hand cut the valve pockets to minimum clearance to achieve the highest cylinder pressure possible.
That 125-75 is a perfect fit for our motors.
Many thanks to Mike for putting me onto it some years ago.

Cheers,
Brian

Thanks Brian.

So I take it your exhaust valves went down to 30 from 31 and the intakes went up to 35 from 34?  Were you able to retain the original valve seats for both intakes and exhaust valves as well?  If you could magically have a head with a splayed intake(moved guide and seat) that allowed you to go up in intake diameter another 1-2mm, would you expect this to improve your breathing potential for big engines?

I’m curious, where do you expect your motors would be powerwise if your engines ran on gasoline based race fuel rather than methanol(assuming a proper set of carbs jetted to match)?


CBR954, thank for the info.  Please share the results if you ever do run it on the dyno.

George

[bear]

We use the 410 castings also.
Our motors are around the 1100cc mark these days.
That seems to be saturation point re how much the head can flow.
We reduce exhaust valve diameter to allow for the largest possible inlet.
Have used many different piston options and now use RC45 pistons after we machine about 5mm off the shoulder and hand cut the valve pockets to minimum clearance to achieve the highest cylinder pressure possible.
That 125-75 is a perfect fit for our motors.
Many thanks to Mike for putting me onto it some years ago.

Cheers,
Brian

Thanks Brian.

So I take it your exhaust valves went down to 30 from 31 and the intakes went up to 35 from 34?  Were you able to retain the original valve seats for both intakes and exhaust valves as well?  If you could magically have a head with a splayed intake(moved guide and seat) that allowed you to go up in intake diameter another 1-2mm, would you expect this to improve your breathing potential for big engines?

From memory the first head had the exhaust reduced by 1mm and the second by.5mm. The valve seats had to be re cut on both units. Both heads used 35.5mm intakes though ( from memory).
Take into account that we budget for 100 hours of competition on these motor's.
They are not built for toodling down to the shops for a liter of milk.

I’m curious, where do you expect your motors would be powerwise if your engines ran on gasoline based race fuel rather than methanol(assuming a proper set of carbs jetted to match)?

Not sure I could answer that with any gread confidence George.
At a guess if detonation could be controlled (and that's a big if) I don't think power would drop off much at all.
Having said that I would seriously doubt you could get my minimum 100 hrs of competitive use from it and that's my plimsol line for practical usefulness.

[gschuld]

Brian,

Methanol certainly is an advantage regarding its natural cooling effect.  Corey Ford’s green 1204cc road racer(engine built by RPE, factory stroke but huge modifications in the cylinder to accept 78(?)mm bore pistons) is an interesting example.  They run straight port heads and produce a pile of power as well.  They run methanol too correct?

I remember you not recalling your static compression numbers, but I imagine it’s pretty high.😎

George

[PeWe]

I visited a friend today. He had recently got a new old cam. Kenny Harmon Super F ( KH SF). Brand new in a KH marked box. Cam looked beautiful, black, grey on lobes and journal surfaces. Really good condition like made last month except for the box and aging papers.
Instruction followed with the small cam card with big numbers. Instruction that was clear is NOT FOR STREET USE.
I understand those that will have a package like this in the living room to be admired when needed Lets see if this cam will enter an engine.