Cam profiles & "Lofting" the valve

I guess I've slipped out of the loop a bit these days because I'm thinking that sounds like a really bad idea that would hammer the snot out of the cams and buckets. Just playing out the concept in my head, it would seem like a 1 specific rpm you could design the cam to throw and gently catch the valve....but at any point above critical (the valve is thrown), but below tuned, the valve would crash back onto the cam. Then above tuned, the valve would end up in the air quite a long time (think ski jump) until the flare-out on the bottom of the lobe catches it, and it would be going REALLY fast by then.

With I have run 270 duration at .500 lift on a street 2 valve and a friend had 290 at .050 and both engines had plenty of low end (lots of static compression, 12.5 in mine, 15 in my friends, both run fine on pump gas),. 270 duration (at .050) is a 8500-9000 rpm cam in a 2 valve 308 engine (heads flowed to match of course), I can't imagine even wanting more duration than that.

 

I have a set of these 2V cams which came in my first car that also had 9.7:1 Borgo pistons. The car idled below 1000 rpm quite smooth, and actually drove quite responsively. I think if I would have had a better engine and been able to tune it better it would have ran fantastic. But I must add a caveat.

Number 8 cylinder had ZERO compression, and ZERO holding with leakdown. I even hooked the airhose directly to the spark plug adapter, and with 100 psi pressure it blew right through. Number 7 was holding about 60%. So at best I had a 6 1/2 cylinder engine. The primary header tube on number 8 would be ice cold even have after a hard run, and number 7 would only get luke warm. Yet I bet it would have still walked a QV. Myself I knew better than to drive it, I was sure it needed to come apart, but I was advised by a lot of top Ferrari people to try driving it in the hope it was only frozen rings and they would break loose.

Back to these cams, this is probably as radical as anyone would want to really go with a street car. Any more lift or duration and any hope of idle quality is going to rapidly disappear.
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What Mark said. While "controlled" lofting provides greater area under the lift curve it only works in a narrow RPM range. Have you got a spintron, dyno and software like http://www.profblairandassociates.com/pdfs/4sthead-Insight.pdf to engineer this?

 

DLC?

 

Like these guys? All the euro companies opened US offices in the Carolina's because of NASCAR.
http://www.bekaert.com/bac/Products/Diamond-like%20coatings/Racing%20Engine%20Parts.htm

 

Sean,
Are your heads and manifolds ported yet? If not, you are leaving a LOT of hp on the table while beating the snot out of your valve train with big cams. 230 at .05" duration should be about right for around a 7000-7500 rph hp peak.

I still don't like the lofting idea. It seems a bit brutal on the parts for the street and unnecessary for the track.

 

Do a search on P6...there have been numerous threads, but most of the info is conflicting.

There seem to be several schools of thought
1.it is a ferrari race grind, it must be the best you can do.
2. it's a race cam and only an idiot would use it on the street
3. it's the best street cam ever.


I personally think they are all mostly wrong. I think any good performing engine will have parts that are well matched.

Simply dropping a P6 race cam in a street engine does seem to make a bit more peak hp, but also pretty much guarantees mismatched parts which results in significant losses at lower rpm and serious tuning issues with carb set-ups. Also don’t forget a P6 is a 30+ year old cam grind, there are better modern grinds available.

Here’s a recipe for success.
Start by putting the heads on a flow bench whether you are going to port them or not. The head flow is almost always the limiting factor and it certainly is on both 2 and 4 valve 308 engines.

Once you know the flow, you know what rpm the peak hp wants to be at and a cam can be chosen. When the cam matches the head flow properly, you will have about 2500 rpm between the torque peak and the hp peak. As you try and increase the cam duration to get more hp, the peak torque rpm will go up proportionally with the duration increase. The hp peak on the other hand is flow limited and will go up in a diminishing relationship peaking at about 10% more hp and 500ish rpm over the optimal point when the cam is about 25 degrees at .05” more than optimal. By this point the torque peak is going to be about 1000 rpm below the hp peak, making a pretty peaky engine. This is a stock 2V engine with P6 cams.

For a 308 with stock redline and properly flowing heads the optimal cam timing is about 210 degrees at .05” lift. Ferrari already over cammed early carb 308s and the QVs to make up for poor flowing heads, more lift, less to equal duration will point you toward optimal.

Once the cam is chosen you can set the compression ratio. More duration means you need more static compression…so clearly stock pistons would be less than optimal with a P6 or similar cams. Compression is really a minimum value, running more is pretty much always better is you can get gas with a high enough octane. If you have built an optimal engine and want to run on pump gas, about 9.5 is about all the compression you will be able to stand without having to pull the timing back to stop it from knocking. Add about 1 point to the compression for every 10 degrees you add to the cam duration and you’ll be pretty close.

 

As most know, the 308 engine is basically 2/3 of a 365 series V-12, as used in the Daytona for example, with a cylinder capacity of 365cc or so and a bit wider V (90 degrees vs 60 degrees). The origin of the P-6 cam as I have read was that it was basically derived from the 365 GTB/C Daytona competition cam, which IIRC was also called P-6.

The P-6 cam in 308 racing was used in the aborted Chinetti NART LeMans entry, and the set-up of that engine are well documented here on FChat, and amazingly, are similar to a Comp. Daytona.

There are several who have gone to P-6 cams in 308s for track use, notably Phillip Airey, who have had some good results. Again, without doing a search, I seem to remember that Philip's race set up was not street friendly as he had stock compression, but was not unmanagable in any sense. There have been other FChatters drop MUCH wilder cams in, with varying results. If you have the cajones (and compression) for it, Frank Capo in Australia runs by report 260+ duration (in a qv!!!), .410 lift in a qv with 360 hp all day long -- on carbs, OBTW.

As Mark says, you have to match the components, most especially as you begin reach the edges of each components flow envelope. My biggest mistake in the past was to put in hotter cams when the head was already max flowed - thus, the hotter cams gave the exact same hp numbers as stock because the head was the limiting factor. Now that I have ported the head, I am hoping for a better match with increased compression as well to keep a strong lower end.

 

Maybe.

You need to know the flow numbers before an intelligent cam choice can be be. With heads that don't flow very well, particularly at low lifts, more duration will work. The ported QV heads picked up 100% more flow at .050 lift which effectively makes the cam act as is you added about 10-15 more duration. The difference is the engine is much more efficient at low rpm with good heads and a short duration cam than it is with poor heads and a long duration cam.

 

If the valve is going to loft near redline, how much of an over rev is it going to take to get piston to valve contact??

Makes me nervous.

 

Your heads s*ck.





Actually, they are better than the stock 4v heads in some ways. a stock 4v head is all done at .300 lift and 89 cfm. The ported 4V heads are 112 at .350 lift, which is still a bit low for a 7500 rpm engine. A 4v heads needs so much less lift to be work well.

My XR harley (2 valve) is nearly the same bore and stoke as a 308...and flows 148 at.450 lift. The cam in that is 270 duration at .05 lift and .580 lift and it makes about 95 rwhp at 8600 rpm....that would be 380 rwhp (450 crank) for an 8 cylinder.

Back tp you. You need all the lift you can get to get access to the higher flow numbers. The flow is reaaly leveling out over .400, so I see way that was selected and I agree it's a good choice. If you could add 20-30 cfm of head flow to go with that cam, you would have a 340+ hp engine at around 7500-8000 rpm.

 

Not with the stock intake, I think it's about already max flowing. But yes, after seeing the 2v intake ports, you should be able to get the flow up another 20 cfm, maybe more.