Cam Lift?

[Anonymous]

I have a question about cam lift.

Let's say you have a 260° (210°@.05") with a lobe lift of .450" w/ ford 1.6:1 rockers. If you use a higher ratio rocker (1.75:1) how does that affect the engine? I know it lifts more, but what about VE, torque, HP, ecetera?

Will the higher lift promote higer VE due to the valve be open more?

 

[StrangeRanger]

For torque to go up, VE has to go up. Torque id proportional to BMEP and the only way to significantly increase BMEP once you've established the engine configuration is th find a way to stuff more fuel/air mix into it.

As to whether VE and therefore BMEP and torque will go up, the answer is it depends. :roll:

If the intake valve lift was the bottleneck in the intake system, then yes you will see an increase. Usually, this is not the case if you've done moderate headwork and put in an aftermarket cam. If you've gotten carried away with the porting job and increased the flow area of the ports so that it is more than the flow area of the valve at full lift, than the higher lift will help. You are likely to also need more duration in that case though, so a bigger cam overall is probably called for.

At the risk of repeating myself yet again, if you select the cam that has the characteristics you need and want in the first place then you won't need to deal with the issues imposed by higher ratio rockers.

 

[Anonymous]

Thank you SR. I know that torque is dependent upon VE & BMEP.

I figure a stock 300 has a BMEP around 115 - 125?

I guess my question is how will high lift affect these characteristics?
What affect does short duration / high lift on an engine? Most cams that I see with valve lift approaching .500" are in the neighboer hood of 280°

If the lift is higher, duration longer, and the head / runners have been optimized, then BMEP should go up nicely, correct? And since BMEP, VE, and Torque are fairly linear, just how do we optimize those for a 2V / pushrod/ poor flowing head with only ~15in^2 of inlet valve area?

A BMEP of 200 is hard to acheive on our 300's I would think. That would yeild ~400 ft/lbs. Over 200 on a 2V/cyl pushrod engine is even harder and would require some pretty trick peices.

I have read several times where you wrote (sorry to make you repeat yourself) to chose a cam that has the lift you need, instead of using rockers as a crutch.

I'm just trying to understand more about a cams lift vs. it's duration (@.050") and what difference in will make to the powerband, cylinder filling, ect.

 

[StrangeRanger]

It all comes down to flow area and flow velocity at various RPM.

Ricardo showed long ago that VE is maximized when the intake velocity is approx 140 FPS and is in the acceptable range when it's between 40 and 240. For the 300 with heads that have been reworked to open up the flow bowls to the ID of the valve seats and the ports blended to match, these velocities occur at about 830 (40 FPS), 2900 (140 FPS) and 4985 (240 FPS) when you look at port/bowl size alone.

To get the same flow area through the valve it has to be opened far enough to give the same flow area as the bowls/ports. For a 300 this works out to .392". The stock cam produces .400" lift which means it will support the same flow as the ports only when it is at max lift. The higher lift cams do two things: They usually have a longer duration at or above .392" meaning that the cylinders will fill better over a larger part of the intake stroke. They also, by virtue of their higher lift, reduce the pressure drop through the valve and seat; any reduction in pressure drop tends to improve cylinder filling.

The problem with long duration cams is that they tend to raise the RPM range in which the engine operates. They have greater overlap which necessarily reduces torque at lower RPMs.

So it becomes a balancing act: how to get increased lift without increasing duration to the point where you lose low RPM characteristics. One way is to increase the lobe center separation and thereby reduce overlap. That costs high RPM performance though.

My favorite example of this is two cams that Crane makes for the 302 (I know it's heresy, but allow it just this once.)

The 2040, also sold as the FRPP "E" cam is 282° advertised duration, 220° @ .050" and .498 lift with 110° lobe centers

The 2030 is 270/278° advertised duration, 216/220° @.050", .533/.544" lift with 112° lobe centers.

Both are installed with a 4° retard; both are smog-legal.

On otherwise equivalent engines, the 2040 makes way more top end power and will win in the 1/4 every time but the 2030 produces massively more and broader midrange and quicker throttle response. It is much more streetable and is an almost perfect autocross cam. You pays your money and you takes your pick.