Cam Overlap

First off, welcome to the insanity.

I am not sure I would use the term cam scavenging. I think you are seeing the increased efficiency of the longer duration cams at the higher speed of the engine. Keep in mind that as the displacement of the engine goes up, the cam lift and duration will need to increase. A cam that is too radical to idle well in a 900 cc engine will be a lot more manageable in an 1172 cc engine.

If you want a clearer definition, read the Bath Breaking thread carefully. Good luck, and send some pictures.

You give lots of lovely detail but then throw in the line.
'The bike isn't running right'
In what way is it 'not right'?
Can you be more specific, or is it just at the 3000rpm you mention in the test?

It's not really my field as I was always a 2stroke guy.
But do you care what it's doing at 3000? Where do you expect the power to be? Somewhat higher than that I'd have guessed?
Is the exhaust right for the set up?
I'd have though the higher pressure was a good thing and indicated good filling of the cylinder...maybe I'll shut up before I sound like an ass.
Anyhow good luck, I shall watch from the sidelines with interest.
Del

Here's a link that might help....

http://www.empirenet.com/pkelley2/DynamicCR.html


Regards Woody

Thanks Woody thats a great site. Very interesting.

Seabeast

Hello Seabeast

I don't know how the valve train is laid out on your bike but if you could see the amount of each valve opening when the piston for that cylinder is at top dead center on the exhaust stroke both valves should be open about the same amount. It sounds like your intake valve is closing to soon.

The intake valve on my single cylinder 1959 road racer closes around 90 degrees before TDC on compression stroke.

Let us know what you find.

Lory Johnson

And tell your uncle to have fun AFTER the tires warm up.

And to you also, welcome to the insanity.

Did it start "not running right" when this cam was installed? I would check to make sure it is degreed in at where this cam should be. Incorrect cam timing could make the apparent compression do weird things.

My gut feeling would be that these cams would not cause the bike to get in its powerband until much higher than 3000 RPM so I am wondering about dynamic pressure affects at 3000 RPM.

What do the cam grinders say? Are you paying your money to an expert like Yoshimura? Their winning experience goes back to even before this motor was first built.

Hi everyone and cheers for all your input.

Some more background:

The previous motor was a high compression 900cc which made about 95hp at the wheel but it melted a piston at the end of the straight after an obvious lean out.

This 1172 motor is a new build up which includes a modified combustion chamber design, lightened crank, HRC grind cams, 33mm CRs, forged rods, dyna ignition to name a few of the bits. The crank, rods, pistons, head and cams havn't been run before in one of our motors.

At the moment our motor pulls fairly strong from 3000 to 5000 rpm then falls off the power between 5000 and 6500 before pulling very strong up to 10500 where it is making maximum power on the dyno. At the flat spot it fluffs and bogs and the gas analyzer says it is off the scale at the rich end which supports the fluffing and bogging. Unfortunately this rev range is very useful for getting out of corners quickly and the flat spot makes it hard and scary to ride as you can imagine...

We are about to try some 35mm CRs which from experience are easier to balance throughout the rev range.

But what really confused me was the pressures we were getting with the different cams.

Thanks again!

And yes the bike is no fun on cold tires and when the power comes back at 6500 to 7000 it will wheelstand in the first 3 gears no worries

That really sounds like a fun beast to ride.

It sounds like you know engine building, or got a good wrench, you'll know if you have this engine close. Did you very carefully degree the cams in, with a good wheel? My Web cams use slotted gears and just a couple degrees ± on the intake makes a very noticeable difference, especially when it 'comes on the cam' around 6200rpm. Two degrees on the intake is the difference from a really hard but smooth pull to the rev limiter to making you feel like you're on a bull with a hot iron shoved up his arse.

I run an '83 GS1100E based 1260cc V&H motor with 40mm carbs from back when Byron Hines was still building high output Suzuki engines down in LA. From what you are describing... flat -- or falling off -- around 5k, I'd first suspect the carbs. Years back, I fought a similar flat spot...chased the damn thing all over. Change main jets and almost eliminate it by 98%. WFO and find I lose top end 300 or 400r's early. Back and forth, still got a box of about 100 jets out there that are all just a teeny bit shaped different. Anyhow... long story shortened... I got a hold of a guy at dynojet here in the states and told him about the exact same thing you've written here. He'd heard it a hundred times. Shot me a modified set of needles and jets with a note that they'd work best if I honed the carbs on the engine side by about .15mm. Tried it before and after... he was right. Anyway, I'd look real close at your fuel delivery. I found you can have perfect looking plugs and still have had jets / needles screwing with your mind.
33mm carbs sound a bit small to breathe enough fire but if it works...

Second place I'd look at is coils and ignition. That can do some strange quirks but they are more intermittent in showing up under load... and never seem to be quite at the same rpm like carbs are. I assume you're running some good Dyna coils and ignition or equivalent.

It does seem like your psi is kinda high and I'd triple-check the cam timing real close. My cams are .352" with 257° duration. I want to say off memory its at around 35° overlap. The motor sits about 225psi but it's been a couple years since I felt the need to check it. Old brute still makes me grin though.

You can bet the psi thing will sort itself out. It'll be one extreme or the other. It's right, and it'll go like a raped ape for a long ways. Or it isn't right... and you'll know that by the hole in one of the pistons or from the rod hanging out the bottom.

Good luck with it and stick around to let us know how you make out.

Intake system resonance can cause the huge 'hole' in power output in the high mid-range.

Yes Keith, I was just going to suggest the same thing.

When you change the cam you must also tune the intake as it has a huge impact. If you can make it "variable" in length then you can improve at all rpm levels. By changing the length of the intake runner you can easily avoid the hole at 5000 to 6500 rpm.

The exhaust also needs to be tuned, its effect is somewhat lesser though.

Experimenting is likely the easiest and fastest route to get results. A variable length can also be achieved by a circular portion that can be rotated.

Last night, I wrote a pithy, insightful, beautifully-worded post (and if I recall correctly, it made me laugh and cry, and opened my eyes to what a beautiful place the world can be). But then I hit some pair of keys that made the whole thing disappear. What a loss to human history.

But in any case, what I said was something like this:

The intake tract is likely far too short to worry about acoustic tuning effects -- unless the engine looks really funny, with the carbs back behind the rear wheel. However, the exhaust system, especially with a straight can, could be returning a positive pulse (reflected from a negative pulse exiting the far end of the exhaust system) which happens to work to stuff the exhaust port right at the closing point at the roughly 3000 rpm. I'd suggest running the same test at the same motoring rpm, with and without the back half of the exhaust (or even without the entire system -- which might make me a little nervous) to see if there is a noticeable difference in compression pressures. It would also be interesting to see the compression figures at a low motoring speed (1000 rpm).

The difference in pressures seem much greater than I would expect at such a low RPM, and in fact seem to be about the reverse of what I would expect -- but then I have never done compression tests at such a "high" (for cranking) rpm.

Also worth considering is the differences in the timing of intake valve closing, which can affect effective compression dramatically. (The Prius engine uses this technique to achieve Atkinson effect, in which the effective compression ratio and expansion ratio are different rather than symmetric.) (How many times can I use affect or effect in a sentence?)

Most high performance engines suffer from some blowback through the carbs, and that blowback ends up being reinducted and a bit rich, I've theorized, having been carbureted twice. (The Prius would suffer from the same problem were it not injected.)

I'm puzzled. It's as if bike is "on the cam" at 3000 rpm, and dramatically so, stuffing in about 50% more mixture than one would expect. The amount of overlap, however, would suggest the the bike would only be on the cam at very high rpm. So maybe the answer is in the intake valve closing timing or exhaust sytem acoustics. I'd love to know the results of changing parts of the exhaust.

Also would be good to know if the valve timing points in each case are at the different cam grinder's suggested check points (e.g. 1mm off the seat) or at a certain point (again e.g. 1mm) for each cam setup, independent of the grinders recommendation... if that makes any sense at all. (Some cam grinders ease the valves down, where others let them slam into the seats with more vigor.)

As an aside, I'd like to see more threads like this and fewer "This morning, I turned the key and nothing happened." Great to have all the detail in the orginal post and the feedback as the thread develops. I like threads, like this one, in which my answer boils down to "I don't have a clue... seems like FM to me."

This thread also makes me long to be road racing again... but I'm chicken. Even the bike I loved to race most would now be considered "vintage" (cough cough), but turned 14,500 rpm hour after hour, clutch used only on downshifts, without missing a beat.

Hi Keith, Floram and Switchman

Thanks for your input its given me some good ideas to go have a look at for sure.

Yes it is so close to being spot on except for the flat spot which as usual requires 90% of the total effort to get the last 10% gains!!

Its an exciting bike to ride, the Bakker chrome moly frame is nice and tight and fairly short wheelbase so it handles real positive and turns fast.

I'll report our results soon.

Cheers