[Adam65]

Straight up timing is defined as when the lobe separation angle and the intake centerline angle are the same. Most racing cams are ground with no advance in them, so if they are installed "dot to dot", that is straight up: a 110 lobe separation cam will be installed at 110 intake centerline. Most aftermarket street cams are ground with 4 degrees advance. If you have a aftermarket street cam with 110 degrees lobe separation angle and put it in dot to dot, your intake centerline will be 106 degrees. You would have to put in a 4 degree bushing and retard it back to 110 degrees intake centerline to be straight up, even though dot to dot is what everyone thinks is automatically straight up timing. It is dependant on how the cam is ground.

[randolphi]

I like the reed valve idea, but feel that locating them between manifold&head port would be more effectual, however, I do not know of any " reeds" that could control up to a 4in pistons airflow or pressure "needs"

[Dirty Biker]

Well we were talking about putting the reeds in the head next to each intake port on the head I think, thats the only way it would work I reckon. As far as them being an obstruction in the air flow, (compared to no reeds at all) that may be true, but at higher rpms when they are no longer needed, they would simply be open all the time so it would be a matter of having a big enough reed cage to flow the cfms needed for the hp desired. My 38 mm carb on my husky for example could theoretically flow a whole bunch, not sure exactly sure how much and at what vacuum but if each cylinder had a reed cage like a rm250 suzuki that could make 57 horsepower, that would be about 456 horses right? To flow more you could have a reed cage with 6 reeds in it per cylinder (my husky reed cage only has four reeds) and get more flow that way. There may be bigger reeds that would be better I dunno. Its all just kinda a new idea I guess! Anyways for sure at least it would let you run that long duration cam without the reversion at low rpms and still have the high rpm magic once the intake runner inertia kicked in at higher rpm. Kinda the best of both worlds I guess.

As far as handling a four inch piston, I am pretty sure the positive(reverse) intake pressure on a bigger piston is not that much more than the two stroke motor with no camshaft at all. If it is any credit to this, my husky motor has nearly a 3.5 inch piston. The reed valves they make for these motors are made out of some crazy amazing stuff, they can go for lots and lots of miles "flapping in the breeze" or whatever you wanna call it. The ones I took out of it when I upgraded to boysen dual stage reeds looked like the original ones from 1981 according to Andy at the husky shop I go to.

Between this reed stuff and the spanny chambers we are talking one crazy motor here... Crazy!!!!!

[benwantland]

All the theoretical discussion going on here is fun, but I think this gets to the meat of the original question:

Originally Posted by Adam65

Straight up timing is defined as when the lobe separation angle and the intake centerline angle are the same. Most racing cams are ground with no advance in them, so if they are installed "dot to dot", that is straight up: a 110 lobe separation cam will be installed at 110 intake centerline. Most aftermarket street cams are ground with 4 degrees advance. If you have a aftermarket street cam with 110 degrees lobe separation angle and put it in dot to dot, your intake centerline will be 106 degrees. You would have to put in a 4 degree bushing and retard it back to 110 degrees intake centerline to be straight up, even though dot to dot is what everyone thinks is automatically straight up timing. It is dependant on how the cam is ground.

They're just words, and a lot of this phraseology isn't exactly textbook, but the widely-accepted meaning of installing a cam "straight up" is that the Lobe seperation and Intake centerline are the same, or, in other words, at TDC at the end of the exhaust stroke, the cam is perfectly centered between its intake and exhaust events.

The general consensus among folks who seem to know is that installing the cam 4 degrees advanced from straight up is usually optimal for street applications, and - as mentioned - most street performance cams are ground this way. Most. And most timing sets are made so that installing dot-to-dot doesn't give any additional advance/retard. Most.

But then, the cam grind could be off a bit, or the timing set could have an advance or retard built in that you're unaware of. Or you could have an adjustable timing set and not be sure how to set it. This is why we "degree" cams when installing them. To make sure they're installed in the intended phase relative to the crankshaft.

FWIW - I've always heard that some of the blame for low-compression emissions engines being dogs in the 70's and 80's was because of retarded (at least relative to what we consider normal, i.e. 4 degrees advanced) cam timing.

In general - if all other aspects of the engine are supportive of the change - retarding the cam pushes the power curve farther up the rpm range (moving valuable power out of the lower rpms), while advancing the cam slides the power closer to the bottom end (due to closing the intake valve sooner, letting the engine build more cylinder pressure, i.e. torque).

[topwrench]

Originally Posted by benwantland

All the theoretical discussion going on here is fun, but I think this gets to the meat of the original question:



They're just words, and a lot of this phraseology isn't exactly textbook, but the widely-accepted meaning of installing a cam "straight up" is that the Lobe seperation and Intake centerline are the same, or, in other words, at TDC at the end of the exhaust stroke, the cam is perfectly centered between its intake and exhaust events.

The general consensus among folks who seem to know is that installing the cam 4 degrees advanced from straight up is usually optimal for street applications, and - as mentioned - most street performance cams are ground this way. Most. And most timing sets are made so that installing dot-to-dot doesn't give any additional advance/retard. Most.

But then, the cam grind could be off a bit, or the timing set could have an advance or retard built in that you're unaware of. Or you could have an adjustable timing set and not be sure how to set it. This is why we "degree" cams when installing them. To make sure they're installed in the intended phase relative to the crankshaft.

FWIW - I've always heard that some of the blame for low-compression emissions engines being dogs in the 70's and 80's was because of retarded (at least relative to what we consider normal, i.e. 4 degrees advanced) cam timing.

In general - if all other aspects of the engine are supportive of the change - retarding the cam pushes the power curve farther up the rpm range (moving valuable power out of the lower rpms), while advancing the cam slides the power closer to the bottom end (due to closing the intake valve sooner, letting the engine build more cylinder pressure, i.e. torque).

BOTH ARE CORRECT!!!

[Duntov]

Originally Posted by Dirty Biker

Between this reed stuff and the spanny chambers we are talking one crazy motor here... Crazy!!!!!

You're right Biker, it sounds a bit crazy, but with both you and Tech in on it I'm going to follow to see what it comes to. Makes a pretty good thread.