Different inlet and exhaust cams in a 2V 308

Is this possible ?The exhaust port flows percentage wise more than the inlet port .Could you run the P6 inlet cam with more lift and duration ,and an early euro cam exhaust ? This would have less overlap and would it help the bottom end performance or am I just dreaming ?

 

Hi Dave,

I think Russ Turner, SNJ has been all over this one, as well as a couple of other guys, Basically the limitation to more power is not so much the cam as it is the intake tract, which just does not flow, (very well). More cam has been shown to really have a minimal improvement on hp.

The better approach seems to be larger intake valves, flow bench guided porting, then more cam.

Work with the search function a bit, it is amazing wwhat others have tried and what really works.

hth,
chris

 

While I dont have first hand experience with P6 cams in a "good" running engine, I do have limited experience with them in an engine with one completely dead piston and another piston about half gone. The engine had 9.7 LM Borgo pistons with stock unmolested heads and standard sized valves, complete with factory mismatched in/ex ports etc., and was so far to date the fastest and most powerful 308 ive been in including a QV. From everything I have gleaned the last 10 years or so from Doc Fung, James and Bill at Norwood, Butch Hooper and others, the consensus has been that P6 cams will bring power up to around 265-275 HP give or take, without really doing anything else to a stock carbed motor, to 300 HP and more with pistons, porting, etc... The 308GT/4-LM car for example, was making some 380HP with those same cams and 9.7 Borgos , although it may have used 208 heads. My car idled steady at 7-800 rpm with those cams, even with the dead cylinders. With all eight thumping evenly it will only get better. I will admit that low end power didnt have the response the stock early cam car has, and with two bad pistons its not a fair comparison in any way, but above 2500-3000 rpm it really got its fuse lit. I dont know how running a P6 intake cam with a standard exhaust cam would work, but it might prove interesting to try it. Maybe Mark would run a profile or three on his computer dyno simulator for us? Mark?

 

Give me the spec and I'll put it in.

To the original question, there is no reason in the world you couldn't use a stock exhasut cam with a P6 intake and expect it to be a good set-up. It sounds very reasonable to me and I will most likely be doing something very similar in my engine.

 

What do you need to know? So far as ive measured I would place the intake port flow at 82 CFM @ 10 inches W.C. as an average, and so far have zero knowledge of exhaust flow. Maybe you show different numbers or have exhaust flow knowledge??

 

I need the lift and .020 or .050 duration for the cam. Then what ever you have on the head for flows as various lifts. The more data I have the closer I can get, I know about what the exhaust is relative to the intake so I can guess at that and still come pretty close.

 

That's been the more modern thinking on hopping up similar DOHC cars for years. It preserves bottom end substantially while enhancing breathing.

 

Hi dave,

I could be a bit off base, (as above), but if i recall correctly, the us and euro cams were the same on the early, 75-77 308s, but the exhaust cam was timed 4 degrees differently.

If this is right, it could give you a larger group of car to source your exhaust cams from.

I have my cam covers off right now and will be doing the belts soon, so if someone has a spare set of P-6 intake cams laying around and would like to part with them, I am seriously interested

Frequently wrong, but never uncertain
chris

 

FWIW Even though it didn't last more than 175 hrs, my first cam setup was .406 lift intake and .350 lift exhaust with 242 degrees duration. Idled just fine also.

Next engine will have same exhaust cams, slight less intake lift with more duration.

These are weld/regrinds and I'm curious if they'll last . . . stock cams are back in the first engine that had the cam failure . .. about 100 more hrs so far with no issues.

 

For the P6 cams I show lift 9.5 mm (.374") intake, 9.35 mm (.368") exhaust. So far having checked 9 seperate intake ports with 9mm valve opening I have averaged about 83 CFM @ 10 inches W.C. P6 cam duration at .020" checking clearance are 290 degrees intake, 288 exhaust. Im sure with some mild massaging I could equalise all the ports to 90 CFM. IIRC, the stock early exhast cam has 8mm lift, and around 235 degrees duration.

But the physical valve timing with P6 cams has been quoted all over the map. I have the LM car listed at 51-58-64-44, and another chart for someone else's setup showing 48-62-64-44, and I think there are a few other ones people have tried. Probably setting to lobe centers as you have suggested elsewhere would be the best approach??

 

Sean,
There are cams with durataion and more lift sold and in use for the QV heads that work and live out on the track so I'm pretty sure your problem was a hardness issue of some kind.

 

A tidbit I just learned is that P6 cams will/do flip the shims out of the stock buckets. I would consider them a shim under only grind at this point.

 

Dont know if you can input ignition into your simulator, but the LM car specs show 34 degrees engine advance @5000, with 13 degrees full distributor advance @3600 distributor rpm. But it doesnt show any other figures to know what actual full engine advance would be, so maybe we could guess at 40-42 degrees @7200? Also, the duration and valve timing figures I posted above are at a checking clearance of .50 mm.

 

I dont know, the LM car ran standard valve shims and tappets at Le Mans, and mine had all stock valve train. All the valve train stuff in mine looked in pretty good shape, and though I have no clue how long it had been together that way, it had obviously been like that for quite a while. I think the problem could be to much lift and not enough duration?? There have been a lot of threads and discussions of different people or shops trying to modify the P6 grind, but most of those were problematic as far as I can tell. With that kind of lift but with too short of duration it kicks the shims out, or in Lucky's case, wipes the surface too hard?

 

Ignition is assumed optimal, so I can't do anything with that. Wifey says we're going "out" now, but I'll plug everything in twhen we get back.

 

You don't know what vavles or springs they ran though to make it work. I have confirmation that a daytona running P6 factory cams on the street has flipped shims out at least 3 times in over the past 20 years. Heavy springs or lighter vavles would probably solve it but the cams are borderline.

Sean's issue is a hardness mis-match I'm prtty sure, his grind shold work and other are running more radical grinds without issue.

 

Without jumping through a lot of threads, some things that have come up about cams here in the last five years:

Kermit showed that on a 2 valve, flow does continue to rise with lift IN A 2 VALVE engine past lifts of 0.400 lift. I showed that IN A 4 VALVE engine that there is no sig. increase in flow over .348 lift or so due to flow limitations of the stock head.

The safest lift for LONG TERM USE in a Ferrari head is about .360 as shown by Laurie at WebCAM; Yes, you can run higher lifts but many people have shown reduced relative operating lives due to wear with these cams and increased chances of shim-flipping; photos of the dangerous camlobe/tappet contact patches at increasing lifts have also been posted. Bottom line: you can run lifts well above 360, but you are also accepting the increasing risk of increased wear and shim spitting. Not a problem so much in a race engine, but could be an issue for long term owners.

It's been common and successful practice in these engines that they will well respond to increasing just the intake cam and keeping stock exhaust cams due to the stock set-ups higher than optimal exhaust:intake flow ratio.

Philip Airey ran P-6s in his 308 race car with tolerable loss of low end grunt, even keeping the stock 8.8:1 CR and more open carburation. Ideally, P-6s on the street do need a higher CR.

Although not directly comparable, Paul's 82CFM measurement ON A 2 VALVE head does jive with many measurements of flows in the low 90s CFM ON A 4 VALVE HEAD. Porting on TWO VALVE heads does bring the flows up to the QV flow levels, which explains why guys that even alone port/polish their 2 valve heads see QV levels of power even with the K-jet, more with carbs and less restrictive intakes. In my ported and polished FOUR VALVE head, the flows are a bit above 100CFM, or about 10% on the bench (IMPORTANT:with carb manifolds); as soon as the motor is built, we'll put her on the dyno and see what she does.

The most hp that K-jet is typically good for in an UNBLOWN engine is about 270 - 280 hp.

One less obvious thing I have learned in all of these engine mods everyone has done is that there is always a "cost" that the modification takes. The original engines are a compromise to meet a lot of parameters, progressively with emissions as one. As examples - More cam duration may have higher top end power, but low end suffers; there is an increased chance of pre-ignition with large amounts of boost or increased advance; increased camwear may occur over .360 lift; carbs will give increased power and better sound/response, but at a cost of possible poorer fuel economy; blown engines may wear faster and blow up if fuel is not right. And of course, while EFI is provides best power and flexibility, it's expensive, finicky, looks like crap with all those wires/computers and is in general just less manly than Webers.

Just had to sneek that last bit in to poke at Mark.

Anyway - it's all about balance. There was one FChatter some years ago that did a full up race engine for the street - wild cams, HUGE carbs, everything. While a stunningly spectacular final engine, it was quite unwieldy and difficult to get just right for everyday use. I have chosen a much more modest route focusing on reliability and simplicity in everyday street use with matched components for a balaned total package.

jmho

 

Here's what I get. The top graph is P6 intake and exhasut and the bottom is P6 intake with early stock exhaust, both with webers. I see no difference at all.
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One thing to add is that Laurie was looking at a QV head with 33mm buckets. The 2V buckets are 37mm so they will handle .080" more lift making the 2v number .420.

 

Thank you very much Mark, that is really cool to see. Thats a weird looking power curve, sure stays flat across the top, and interesting to see power coming on at as low of rpm as it shows. What actual parameters did you use, such as CR, port flow CFM, etc? I'm curious how increasing CR to 10:1 or more, and/or increasing port flow would effect that graph. I think it would also be of interest to see what dropping lift down to 8mm and maintaining duration would do. Where did you get this dyno computer thing anyway, it looks like fun. If I could beg, could you run one test at 100 CFM intake flow@10 inches of water with 10:1 pistons?

 

Thanks Mark
Very interesting , as Paul is going to run higher comp (10:1) I was considering going a bit higher than that (10.5:1 possibly 11:1 can't decide how brave I am ) and if Paul can show us how to get a 100CFM out of the heads . Looks like we could have a reasonably streetable motor .
Once again thankyou

 

I was in a hurry last night and guessed at the vavle sizes.....and I guess too big. Here is the P6 intake with the stock exhasut with the correct valves. The top graph is with EFI and the bottom much lower hp one is with the more manly (I guess Russ meant girly man ) webers.
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4 graphs
1. P6 intake, stock exhasut, webers, 10:1cr
2.P6 intake, stock exhasut, webers, 10:1cr, 100 cfm head
3.P6 intake (cut to 8mm lift), stock exhasut (cut to 8mm lift), webers, 10:1cr, 100 cfm head
4.P6 intake, stock exhasut, EFI, 10:1cr, 100 cfm head

A couple things to note. The 40mm weber runs out of flow and becomes a choke point at around 90 cfm. You can get 95ish cfm through it as long as the head and intake are flowing over 100-105, but it's still the choke point. That limits the hp to about 280 if you run those carbs.

On the simulations, anything you see on the graph that is above your redline is garbage and I strongly suspect anything below about 3000 rpm is very questionable. The software is a $150 dynosim sportcompact and it works to get an idea of the hp and torque peaks if you know what to plug in everywhere, but it doesn't work well across the entire rpm band.
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Still very interesting, and another big thank you, Mark. Now I question if the 380 HP Micholetto reportedly got from the 308GT/4 LM car is an accurate number. The specs I have show it with 9.7 pistons, 40mm carbs with 36mm venturis, but no clue how it was ported. But your findings do help confirm the P6 to give over 275HP to a basically stock carbed engine. Do you have any idea how they could have achieved over 350 HP with this setup?? And I assume these graphs are indicating flywheel HP, right??

I had another thought too, though slightly off topic. The noise the carbs make when you open them. Ive had one throat of the carb opened fully on the flowbench at nearly 20 inches and it dont make a peep. I wonder how much airflow it needs before it will honk, or if the sound comes from the process of the valve fluttering? Thoughts??

And I wonder how much more airflow Russ big 44's would offer. Another 40% flow??

 

IMO anyone claiming to get 380 hp with 40mm webers out of a 3.0 liter engine is either lying or has completely gutted and ported the carbs, cut the throttle shafts down and a few other little things that would make them useless below 4000 rpm.

The graphs are flywheel hp.

The sound you hear from a running engine is gas bubbling and resonance from the valves so you won't hear any of that on the flow bench.

A 44mm carb has 21% more area so it should flow about 21% more air.