why such low lift?

[stuffy1256]

Ive tried searching, but to no avail.

Ive been out of the DSM loop for a while, but recently have come back. I was looking at the available cams, and was wondering why all the lifts were so low? The highest ive seen has been around .400. Reading Polk's 1g head flow write up it seems like, even in stock form, that the head flow continues rapidly past .450 where he stopped testing. When he tests his race head and keeps going, he gets incredible air flow by going .500+. Why is it that we have such low lift compared to, dare i say, the domestic market. More flow the better right? so why are our cams so puny. There seems to be a lot had, in my mind at least, by going to higher lift.

Is the lower lift due to revving high and the valve springs cant hold up? that would seem like the only logical thing to me. However, what about the 2.4 builds that dont need to rev as high, where 7000 is the where most people will be shifting and it wont be as bad on the valve spring. Or could it be that we just have weak springs? I think I remember reading that the dual springs are only 90# springs? 105# and 130# springs are not uncommon in the domestic world, why only 90#? Seems a little weak to me.

Just been bugging me for a while, like I said, I tried searching under "cam lift" and "higher lift" in this forum to no avail.

 

[Big Woo]

A poppet style valve is no longer a restristion to air flow once it has been lifted 25% of its diameter. Of course this is just a rule of thumb, and there are many other variables that determine when the valve no longer becomes a restriction to air flow through a given port.

Domestic cars (IE small block chevy, and other two valve engines) use much larger diameter valves. Therefore they must be lifted higher so that thay become less of a restriction.

four valve engines have smaller diameter valves, therefore less gross lift is necessary.

Valve spring pressure is a function of RPM and the mass of the system.

 

[stuffy1256]

but even if a valve is no longer a restriction, wouldnt you still gain some hp because of the increased air flow of the lift? more air in means more fuel which means more HP. Maybe there is just something im not getting here.

 

[Defiant]

When you add in the diameter of the stem, the lift doesn't even have to be 25% of the seat diameter. Also, having a pressurized intake feeding two valves does a lot to make up for any lack of lift; lower lift makes for easier-turning camshafts, and less parasitic loss in driving them.

 

[92awddsm]

With 4 canted valves, you can only go with so much lift before the ntake valves start hitting exhaust valves during overlap. Im not sure what the limit is on a 4g63 so I cant tell you that part.

 

[nanokpsi]

There are also physical limitations in the dsm head, such as guide length, that limit the amount of lift your cam can have before you have interference issues. On stock style guides it tends to be a .440 lift limit.

Smaller motors also pull less vac that large motors at a given rpm.

 

[stuffy1256]

There are also physical limitations in the dsm head, such as guide length, that limit the amount of lift your cam can have before you have interference issues. On stock style guides it tends to be a .450 lift limit.

Smaller motors also pull less vac that large motors at a given rpm.

well then what about aftermarket guides? Do they exist? For the limited vac, you would just need a vacuum canister like some of the wilder v8s use when they have radical cams.

 

[nanokpsi]

There are aftermarket guides but they might have to be trimmed as well. The Fox Lake/Shep head is good to .500 lift. The other limitations still exist. I was just adding more information.
You'll also run into other issues. Driveabilty will start to suffer, valve float, etc. The cam manufactures didn't get together and say "lets make non aggressive cams for this car".

That being said, Crane or Comp will make you any cam you want.

 

[stuffy1256]

i knew there was a reason why they didnt make a high lift cam, I just didnt know why.