maximum lift

Given how bomb proof the PRV6 is, I reckon you could do something pretty spesh with a PRV8

Well they respond to increased lift, but you can't increase lift without increasing duration on a regrind, only with an original casting or adding material to the cam.

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Please explain - as long as you take material away evenly around the entire radius of the "heel" (ie the circular part) than you will not be increasing duration, but you increase lift.

Quite simple really. If a valve has opened further than standard in the same time (ie 360 degree revolution), then it has to have opened and closed 'faster', or started earlier and closed later, As the cam isnt physically rotating faster as thats a fixed variable.

The only other way you could get round this, would be to dish in the sides of the lobe, but this is just not done, and at one point, after the rocker has gone past the base circle of the cam, would make valve open and closing accelleration very high, as some part of the side of the lobe would be going almost vertical.

To make the valve open and close at the same time as it used to (ie keep the duration the same) after a regrind, surely you just have to make sure the heel is ground all the way around its circular diameter.... don't you?

I'm not sure I'm making sense here...!

But if you grind the 'heel' you, you also affect the width and sides of the cam lobe. Don't think of it as heel, you have to take material off the whole base circle. But by doing this, you are making the lobe narrower at the base, which means the sides of the lobe are now steeper, which then means it starts to open the valve earlier and faster, hence more duration but increased lift

The other issue on a regrind is, even you you have increased lift, the amount of time the valve is open at its peak is still very small, and gets smaller the more you increase the lift. You have gained duration, but only by the whole amount the valve is off its seat, not peak lift..

This is why the best cams (and the ones that have been used in proper race PRV's (ie Production Racer, Gp4 A310s, ie 240bhp +) have been proper machined blanks which will properly flow decent amounts, with good lift and proper duration, and hence why people struggle to get near the old race PRV power outputs because they are all regrinds today.

One of the main points of having a higher lift cam -is- the effect of opening the valve faster - the higher lift has less of an effect than opening the valves faster. This is the principle of the multi valve heads - you don't think in terms of x-sec area or lift, it's the circumference of the valve(s) x lift at time t plus-a-fraction, and the faster you can open the valve the quicker air can begin entering the chamber - but the valve is still only open for the same duration...

I'm not an expert here at all, but I did ask Kevin about this once and was told that you can regrind without affecting duration.... but I ain't gonna fight this one, don't know enough about it.

One of the main points of having a higher lift cam -is- the effect of opening the valve faster - the higher lift has less of an effect than opening the valves faster. This is the principle of the multi valve heads - you don't think in terms of x-sec area or lift, it's the circumference of the valve(s) x lift at time t plus-a-fraction, and the faster you can open the valve the quicker air can begin entering the chamber - but the valve is still only open for the same duration...

I'm not an expert here at all, but I did ask Kevin about this once and was told that you can regrind without affecting duration.... but I ain't gonna fight this one, don't know enough about it.

You don't want valves to open 'faster', but earlier at a smooth rate. You want the valve to be open 'longest' around peak lift, hence the max gains, and just after TDC. Fast valves (ie steep lobe ramps) opening just wears the valve train, causes valve bounce etc, then you need better spring rates, which then causes more wear..

I said in the post two up above, you can increase lift by duration, by dishing the edges of the lobe, but this increases the ramp accelleration.

Done a wee pic..



Pic 2 is a standard Lobe, and Ive marked the point where the valve will start to lift. Pic 3 is a reground lobe, as you can see no material is added to the tip, but the base circle is ground smaller, and the sides of the lobe matched to suit. As most people know, you then adjust the rocker arm to compensate for the smaller circle, but then that moves the arm and valve more when the cam lobe it as full lift. But if you look where I have marked, the point when the valve starts to lift is earlier than the larger lobe on the left, hence starts earlier, closes later, more duration.

Pic 1 is just a lobe with added material to the tip to increase lift, but as its more of a point the time its at peak lift is small.

The best cam lobe is a broad tip at peak lift, or you make a cam which lifts the valve more than would be ever required (ie past max valve flow), which sounds pointless, but it actally means the valve is open 'longer' at the max flow point.

area or lift, it's the circumference of the valve(s) x lift ...

The fact is it’s a combination of the two – a time-area integral. The same time area can be achieved by for example a slow ramp and long duration and lower lift, or fast ramp and shorter duration and necessarily higher lift. They both have different characteristics and a multitude of combinations thereof, which will be specific to an application and desired use. Its both art and science, and a whole area of study, I have been involved in time-area analysis as a hobby for many years, from 2 stroke kart racing up. I have built flow rigs, engine dynos etc in the pursuit of more. I started thinking I could beat everybody by the application of mathematical analysis – laugh, beautifully naïve then. Needless to say I wrecked countless engines in the pursuit of my magic calculated numbers. There are so many aspects I don’t think I will ever stop learning, and the field is ever expanding. You just have to go and have a look at the number of SAE papers that are published in and around this subject area. The reference list is just huge…. There are just so many factors, gas dynamics and gas laws, port velocities, pulse tuning, gas momentum, viscosities, hypersonic flow profiles, combustion, reversion, blowdown it goes on and on… At the end of the day, it all comes down to compromises, those that you will accept, those that you are prepared to pay (usually highly) to engineer out, and those you simply get stuffed by. Take ramp, it will be affected by stiffness of your support systems – resonances, valve float, masses, spring rates and damping etc. At the end of the day, especially with modern engines, the manufacturers are pretty much on the money, and have already exploited the best compromise in the package. Where that doesn’t stand will be where you change the operating envelope completely. I could talk about this all day, but unfortunately this stuff doesn’t pay the bills

All good fun though, and keeps us all speeding money! Laugh

Yep, like I said, a black art and even more so with the way the cams work on a SOHC V6 engine, which is actually very uncommon. Most of the V6/V8s in the past have been single cam, block based.

wow im glad i started this thread,some excellent advice from every body.one other question if i get acam made and want to keep the same base circle size and get the lift through more lift at top dead centre ,is there a point where the lobe wont fit through the cam bearing

It's easy to get fast ramps and high lift on a cam. Like Hengist Pod's wheels in Carry on Cleo you just square ones! Or do it like this guy



On a serious note solenoid actuation is meant to be on the way but most reckon they'll probably need 24v systems instead of 12v because it makes this so much easier in terms of smaller more powerful actuators.