Why DOHC and not SOHC ??

[roadsailing]

it appears to me that 3D cams would wear out faster, as it is more like a point load than a line load.

still, ferraris aren't typically high mileage cars i guess.

[Nim]

it appears to me that 3D cams would wear out faster, as it is more like a point load than a line load.

still, ferraris aren't typically high mileage cars i guess.

I guess there's that. I mean, there must be reasons it isn't being used, as it's an awesome idea.

[myne]

There'd have to be a way to put some form of angle tolerating balljointed rocker or even roller rocker.
The Ferrari design need not wear as badly as you may expect since it only has to apply the AVERAGE profile of the contact patch at any one time.

[fuzz!]

it appears to me that 3D cams would wear out faster, as it is more like a point load than a line load.

The Ferrari design need not wear as badly as you may expect since it only has to apply the AVERAGE profile of the contact patch at any one time.

yeah i don't see how they'd wear out any quicker than a normal cam. you reckon because the contact patch is always at a slight angle? same pressure on smaller surface area (i guess that's what you might mean with point load/line load. still the valve side contact could be angled for a greater contact area.

[ed]

y'all should see the base circle profile on the 1uz vvti : whereas the normal cam lobe has a load transfer surface width of say 12mm, the base circle lobe width reduces to say 4mm to help reduce friction during non valve lift rotation...

[wraith]

Hmm, there was something in Peformance Buildups mag about a year ago about a helical cam? Have to go dig for it. Sounds similar to this 3D camshaft business. At the end of it though, DOHC is better than SOHC because it gives greater options to the cylinder head guys to make the engine run "better" according to their design brief. Personally, I feel that the less crap their is between the camshaft lobe and the valve, the better off one is in a high RPM enviroment. My limited research appears to correspond with this Even if Toyotas love affair with direct acutation is coming to a close, they have left us with a legacy of excellent engines. Want torque? Power? Stupid redlines? Boat anchors?

cheers

Ash

[fuzz!]

perhaps someone here can educate me (many have tried and failed) a little on the "scissor gear means non-independant cam timing" thing because as far as i can tell it suffers the same problems as adjusting regular dual-independant cams.

discussed earlier was the possibility of moving the scissor gear over a tooth. now supposedly the change in angle was far too great.. fair enough. but how do you do fine adjustments on independant cams. they still have a finite number of teeth on the cam gear (whether they be for a chain or a belt) which would result in equally large angle changes. i have never pulled a cam gear off the shaft but i imagine there is some sort of notched 'seat' for the gear to sit in so it won't slip. again rotation here would be finite (i imagine even moreso than on the gear teeth).

perhaps a combination of the two would lead to finer adjustment possibilities, which couldn't be achieved with the scissor gear setup.

if anyone knows what i'm rambling about and has an answer for me i would love to learn more.

[roadsailing]

there is no problem adjusting scissor gear driven cams, just takes a bit more work to make them adjustable, and a bit of brain power to work out which way to rotate them to adjust.

normal cams only go on one or two ways, cams are never adjusted by advancing or retarding by a whole tooth (unless you are doing major backyard stuff)

[fuzz!]

there is no problem adjusting scissor gear driven cams, just takes a bit more work to make them adjustable, and a bit of brain power to work out which way to rotate them to adjust.

normal cams only go on one or two ways, cams are never adjusted by advancing or retarding by a whole tooth (unless you are doing major backyard stuff)

so all cam timing adjustments are done by arc-spraying and regrinding the lobes? if that's the case it applies to both scissor gear/independant cams. i'm just trying to get closure on the earlier conversation piece.

[roadsailing]

no no no, adjusting cam timing is done by moving the drive sprocket/wheel relative to the position of the cam shaft itself.

in this way you can adjust the timing in any way you want, scissor gear driven cams are really no different, but i imagine they are more difficult to modify to make them adjustable.

do a search, there are lots of pictures of adjustable pulleys on these forums.

[river]

Hi,

How about a disc? I did a search and couldn't find anything about it, but it wasn't the most extensive search i did.

The top of the piston is a rotating disc with a hole in it, that is over the inlet for induction and over the exhaust when expelling the used charge. This would only be a rotating disc and thereby eliminates the up/down and inertia problems of valve and push rod assemblies.

A twin-disc system would have two discs on top of the engine, one for inlet and one for outlet, where you can adjust each to provide the necessary tuning required. Furthermore, you can make these contra-rotating discs to cancel out the spinning inertia.

The holes don't have to be circular, they can be oval shaped, to provide more time to induct or expel. The discs would be chain driven by a common chain rotating at 1/4 engine speed.

The discs don't have to be flat, they can be hemi-shaped to match the top of the cylinder head. Also, the cylinder head would have two holes on it that line up with the discs, so the discs don't have to take the entire pressure of the cylinder.

Crikey, I better check that USA patent database to see if some dog has beaten me to it!!

seeyuzz
river

[roadsailing]

thats been done, can't remember how exactly.

once again, friction is a problem.

i think i said it before, but the problem with such valves is:

my hazy memory tells me there was once a drag car that only had exhaust valves, air and fuel were injected in liquid form on the "induction stroke" or something similar.

[fuzz!]

no no no, adjusting cam timing is done by moving the drive sprocket/wheel relative to the position of the cam shaft itself.

in this way you can adjust the timing in any way you want, scissor gear driven cams are really no different, but i imagine they are more difficult to modify to make them adjustable.

do a search, there are lots of pictures of adjustable pulleys on these forums.

righto, this came to mind first but i thought there would be a risk of the cam gear slipping on the shaft if it wasn't somehow locked into place, throwing off timing and possibly ending in disaster. i will have a search on the issue.

[Norbie]

Crikey, I better check that USA patent database to see if some dog has beaten me to it!!

Too late, someone has.

If you can figure out a way to make such a system seal properly you'll be a millionaire!

[o_man_ra23]

just because you don't like 2 strokes doesn't mean they are wrong they have very high power to weight ratio, but often have small power bands.. think of motorbike or model plane 2 strokes.. shit for lower half of rpm, then they come on song in a big way (something to do with the way the lubricating oil combusts?)

WOW!! something i know more than Stu about!! This is a miracle. The power band on 2 stroke engines has to do with the port size and placement relative to each other and to the stroke of the engine. It also has to do with the expansion pipe (big ass exhaust with small outlet). Basically, the exhaust gasses come out hot, and in pressure waves. The frequency of these pressure waves are dependant on the RPM. As the hot and expanded gasses come out, they fill the expansion chamber. From there they will cool, and contract. This process will seal the combustion chamber when the gas is hot and expanding, and then draw the gasses out of the combustion chamber when it contracts. During the power band of a 2 stroke, the expanding and contracting cycle of the exhaust gasses works to increase combustion chamber pressures during combustion, and draws out the gasses, sucking through fresh gas from the carbie during the exhaust cycle. So the exhaust is pressurised during the up stroke, and vacuum during down stroke. It acts a little like a supercharger, hence the high power nature of the 2 stroke motor. Unfortunately, this procedure can actually reverse itself and create a bad dead spot, hence the super lack of power outside of the power band. The power band can be increased by means of a variable port timing (shift up/down one of the ports) similar to the variable valve timing. Yamaha did this in thier race prepped bikes in the mid 80s with a thing called YVPS. This system worked on changing the position of the intake port.

2 strokes arent wrong, they just work with pressure waves, and have principles based on frequency matching, something most people dont understand (you cant be an electrical engineer without understanding frequency matching).

Cheers, Owen

PS sorry for the OT