Cams, Compression and Such. A Discussion.

[koko64]

Recently needed to adjust the ignition again on a 944 hi comp 1992 SL/SS.

 Some years go I had set an ignition map on the Ignitech for this bike after a full rebuild by a mate and mentor of mine. The bike has FCR39s on long manifolds, it also had stock cams advanced/dialled in by Brad Black and someone in the past skimmed the heads giving a very high comp with the JE 94mm pistons. The map worked well and the bike ran very strong pulling 3rd gear wheelies. It was a hard starter with very high cranking cylinder pressure and we rebuilt the starter and replaced the sprag bearing. With advanced stock cams, skimmed heads and 94mm JEs it was a beast to turn over. It ran premium fuel of course.
We decided it had such high comp that it could take more cam and I got some 900ie cams off speeddog and a local bloke fitted them to stock specs. I added an enlarged airbox. We hoped the hotter cams would bleed off some cranking compression and give the side effect of starting easier and give some more top end too. It worked well and the bike turned over much easier, because the cranking compression was reduced. There was also a stronger midrange surge which felt good when overtaking.
Some upper mid range pinging appeared so I knocked off a degree or two from 5000 rpm. This raises the issue of cranking compression Vs potential compression at higher rpm. Someone suggested that the enlarged airbox also contributed to this as it increased the sub filter airbox volume and working with the hotter cams added to the potential compression at higher revs.
I have only dealt with half a dozen or so hi comp desmodue motors and would invite comment and discussion from those who know these motors well.

[koko64]

Was hoping for an around the campfire with a few drinks type discussion. Its winter here and shed time for projects. Theres not many people available to discuss this stuff.

[Speeddog]

Well, it'll be up to 100 degF here soon, so I'll pass on the campfire. 

I think the main change that helped the cranking would be later intake closing.

More later......

[koko64]

100
I knew the timing numbers would help starting. The thing had so much comp we wouldn't lose anything down low with hotter cams either, that and it is a very light bike. The owner was questioning how a "hotter" cam would lower cranking compression and got a second opinion which of course backed me up on the cam's numbers with later intake closing.

But the comp went up elsewhere in the rev range due to the increase in effective compression.

[Howie]

Another reason for variable cam timing.  My bike with 10.1:1 compression ratio and stock cams is about all the battery can handle. 

[Dirty Duc]

I'm of the impression the starter/starter circuit is barely up to the task of a "normal" compression. The "blower" bike single was a hard starter with a lightened flywheel, but when I switched to a "stock" weight flywheel it started much easier (it ate the sprag, so needed apart).

[Speeddog]

Another reason for variable cam timing.  My bike with 10.1:1 compression ratio and stock cams is about all the battery can handle. 

Not a big surprise there, as the starting equipment is not far beyond that found on a Pantah.
It has been upgraded a few times, there are different gearsets.
Those cams are very small overlap and the intake closes early, so they're going to make high cranking pressure.
Brad played with cam timing on a 1000SS and found that the OEM spec was good.

Quite a few of his cam fiddling sessions seem to find that the intake closing at 65 deg ABDC is a pretty good setting.

I fitted some adjustable pullies to my 750 a while back, and advanced my cams from ~118 intake centerline to 114 (which puts closing at 65 deg ABDC).
It felt like it gained a little power, and also ran a little smoother at low rpm donking around town.

[koko64]

I think it builds low rpm cylinder pressure giving a bit more pep through to the mid range at least. On a very hi comp motor its too much of a good thing. Must help a stock motor substantially.

Does the DVT retard the cams at start up, advance them through the low and midrange rpms and retard them again for top end rpm?

[Charlie98]

100

Hopefully, that's not Celsius.... although here in Texas it's feeling like 100C.   

[Speeddog]

I think it builds low rpm cylinder pressure giving a bit more pep through to the mid range at least. On a very hi comp motor its too much of a good thing. Must help a stock motor substantially.

Does the DVT retard the cams at start up, advance them through the low and midrange rpms and retard them again for top end rpm?

As a lower cost alternative to the longer cams, retarding the ones it had would help the starting, as the later inlet closing is the big factor.
May have been about a wash as far as power.

AFAIK, the Ducati DVT is controlled hydraulically by a servo-valve, powered by oil pressure.
I don't recall seeing any detailed explanation of the strategy.

[NAKID]

Another reason for variable cam timing.  My bike with 10.1:1 compression ratio and stock cams is about all the battery can handle. 

Kickstart mod!

[ducatiz]

Another reason for variable cam timing.  My bike with 10.1:1 compression ratio and stock cams is about all the battery can handle. 

wasn't there a bike that had a valve on the cylinders that stayed open until the starter revs got up to a certain speed?

[Dirty Duc]

wasn't there a bike that had a valve on the cylinders that stayed open until the starter revs got up to a certain speed?

http://www.dansmc.com/comp_release2.htm

also
http://shop.pitsterpro.com/frdez-d-compressionsystem.aspx

[DarkMonster620]

Was hoping for an around the campfire with a few drinks type discussion. Its winter here and shed time for projects. Theres not many people available to discuss this stuff.

For some reason, this just showed up in my unread posts notice . . .

Now, if you take care of food and drinks, I can join you sitting by the campfire to discuss these issues . . .

[Speeddog]

wasn't there a bike that had a valve on the cylinders that stayed open until the starter revs got up to a certain speed?

I think nearly all of the new Harleys have 'em.

XT660 Grizzly ATV has one.

My XT600 has one that works off of a cam system on the kickstart.
Had no idea how effective it was until the cable broke.