Camshaft recommendation for PMO'd 2.4S ? Options

[amallagh]

I've recently converted my 914-6 to a 2.4S spec engine with PMO 40 carbs.

The engine/heads are standard S spec but with 2.2S pistons giving about 9.3:1 compression rather and the std 8.5:1.

Can any one recommend a specific cam profile/supplier that is optimised for use on carbs with an S spec engine ?
If the std S cams from an MFI engine are considered to the the best for this application then I would be happy to hear your experiences.
I'm looking for a fast road cam to give me similar performance to the std S cams on MFI. The car is used on the road as well as for competition, so I need something smoothy and tractible at low RPM.

Regards
Andrew

[brant]

Andrew,
have you driven an S cam MFI vehicle?

if so is that tractable enough and smooth enough for you at low rpm?
most of the competition (racing) cars will go with a cam even more aggressive than an 'S' for competition use

but they loose some of the low end tractability

so if you need something more tractable, then step down to a Solex
if the S cam tractability will work for you, it is a wonderful choice

(plus the 2.4 extra displacement tends to mellow out the 's' cam in comparison to a smaller 2.0 displacement)

if you want more horsepower and can give up some of the tractability then go for a GE60, or GE80, or GE100

the GE100 is probably way too much for anything streetable or below 7-8,000 rpm and not what your looking for.

brant

[J P Stein]

Solex or E cams are the cat's ass for a street/AX car.
S cams (& up) ain't worth spit with carbs unless it's a race car spending it's time above 5K rpms. Street cars spend 80% of their time on the idle jets.

Here's a chassis dyno graph of my 2.7L with Solex cams.

[0396]

Solex or E cams are the cat's ass for a street/AX car.
S cams (& up) ain't worth spit with carbs unless it's a race car spending it's time above 5K rpms. Street cars spend 80% of their time on the idle jets.

Here's a chassis dyno graph of my 2.7L with Solex cams.

'E' cams

[scotty b]

+1 for Solex (IMG:style_emoticons/default/driving.gif) S is great once you hit 6000. Under that and they're pretty blah

[Phoenix-MN]

(IMG:style_emoticons/default/agree.gif)

+1 for Solex (IMG:style_emoticons/default/driving.gif) S is great once you hit 6000. Under that and they're pretty blah

I have the "Solex" grind in my 2.5L, Lots and Lots of smiles. Pulls strong all the way to redline

Paul

[ArtechnikA]

...so if you need something more tractable, then step down to a Solex
if the S cam tractability will work for you, it is a wonderful choice

if you want more horsepower and can give up some of the tractability then go for a GE60, or GE80, or GE100

I agree the S cam is probably the hot setup once you learn to use some revs to get the car off the line. early MFI E's good too, and the Solex's are always nice.

The GE cams are 4-bearing camshafts, so I believe you're looking at some machine work on the cam boxes to make them work.

[amallagh]

Excuse my ignorance but what exactly in a Solex grind cam ?
T, E & S I understand, but Solex ? Was this a Porsche supplied cam ?

I am running an S cam but the car shows a a weird wavy form to the torque curve that can be dialed out by tuning, and the non original S cams are a suspect I would like to eliminate.
E cams were an obvious consideration but I was wondering if there was a cam profile that is better optimised for use with Carbs rather than MFI.
I don't mind S cams characteristics but wouldn't want anything more peaky. A little extra mid range torque would be OK as long as it did not compromise peak power too much. A sort of S/E hybrid optimised for carbs would be my ideal.

I am curious if someone can describe where the Solex cam sits on the T-E-S continuum in terms of power and performance characteristics.

Peak power for S and E cams in my 2.4 should be about 195 and 170bhp respectively as far as I am aware. (T's are too low to worry about !)

Another thing I was concerned about was running the milder S cams with a high compression engine with S spec ported heads. Would the bigger S heads compromise the breathing performance on E cams ?

[Phoenix-MN]

I think "Solex" was coined from the very ealy 911 engines cam spec that first used the Solex Carbs. The actual cam spec is somewhere between a "S" and a "E" grind. Someone can chime in if there is more to this.

(IMG:style_emoticons/default/idea.gif)

[ArtechnikA]

I think "Solex" was coined from the very ealy 911 engines cam spec that first used the Solex Carbs.

That is correct. The Solex (carbs) were spec'd for the original 2,0 911 (no-letters). The letters showed up around the time Porsche used the Webers to cure some driveability issues they eventually identified but the switch had been made. The Webers - originally made for an Alfa V6 (which is why the intake manifolds have slanty end-runners...) - gave improved tunability options and helped kneecap the T models so the top-of-the-line S was always faster (despite the T being lots lighter).

You really need all the numbers in any discussion of 911 camshafts, by which is meant Anderson's 911 Handboox and Frère's "911 Story" that have all the lift, duration, and lobe center data. Steve Weiner's Rennwerk site and WebCam's site also are great repositories of real data.

One thing to be aware of when comparing cam profiles - Porsche specifies timing at 0,1mm (near 'nuff 0.040"). The US 'standard' for cam timing is 0.050" so you need to read the small numbers and compare apples and oranges...

here's some cam data, copied by hand from WebCam, which I'm sure has already been posted to multiple threads by now...

CODE

Cam   intake        exhaust        lobe ctr
911T   216º .387" 207º .345    105º
911E   230º .408  222º .393      NA
911S   264º .450  236º .400    101º
Solex  244º .439  234º .406      97º
906    282º .465  252º .406      95º

the 906 kinda represents the "ultimate" small-bore 911 cam and didn't really start making power until 8000 rpm when used with 46mm Webers (and later MFI).

I can tell you from personal experience that E cams are very nice in a carbureted engine (BTDT).

[amallagh]

Great info, thankyou.

So is the Solex cam specifically optimised for use with Webers (PMOs) as well ?
I see the cam data posted above shows the progressively increased duration and lift from the T to the E to the S cam as you would expect.
The Solex grind however breaks this linear trend of these 3 cams with intake duration and lift sitting nicely between the E and S cam specs, but the exhaust duration is similar to the S and the lift is even greater. I see the lobe centres are also closer than on the S indicating more overlap. So the Solex is 'hotter' in some respects and 'milder' in others compared to the S cam.

Are these the characteristics that a Weber fuel system needs for peak performance ?
If the S cams make about 195bhp, and the E cams about 165bhp on a 2.4, then how much bhp would you expect with the Solex cams on Webers or PMOs ?

If the Solex cam is ideal for a PMO set up with S heads then the next question if where to I get a pair at a reasonable price ?

This has been really useful.

[J P Stein]

The big "problem" with S cams is duration and the consequent overlap. They work fine with MFI.....tho the bottom end is nothing to shout about, but carbs & MFI are horses of a different color.
MFI delivers fuel below the butterflies and has tall stacks. Carbs deliver fuel above the butterflies...close to the top of the carb, in fact, and have shorter stacks. What this all means is fuel reversion.....a nice spray of semi-atomized sitting in layers above the carb stacks. This gets sucked into the carbs when the engine starts to efficiently scavenge....around 5K rpms for an S cam. Below there, this reversion raises hell with the tune....particularly when enclosed in your....normal K & N filter with hat.....fuel bouncing all over hell & gone. One of the main reasons to avoid long duration cams in a street motor mit carbs.

The Solex profile is less prone to this as is the E. Tuning is still a stinker. Run the idles & mains rich enuff to avoid the dreaded Weber flat spot and you foul plugs regular like. The solution here is a hot CD ignition & coil (I use MSD) with a bit fat plug gap (.045-.050)....ya prolly ought to have some hi-po plug wires to handle the voltage....to burn thru the rich mixture....a bandaid that keeps your plugs clean.

The 2.4L motors had wimpy compression. Another point or so will reward you with more hp & torque and street gas will work fine....up to about 9.5:1.

Thas about all I think I know.

Note that the dyno chart above shows 225-230 hp at the crank (figuring a 15% drive line loss)& 210 ish ft/lbs of torque The famous 2.7L RS motor was rated at 210 & 188 at the crank(Probably conservative) with MFI. OK, so I run 10:1ish compression, Solex, carbs, & race gas. It's a great AX motor where low down (relatively) torque is king. The mileage sux.

[ArtechnikA]

So is the Solex cam specifically optimised for use with Webers (PMOs) as well ?

it's reasonable to assume that the only 'optimised' combinations are what came from the factory. You should expect to do some tuning with whatever cams you get, which can be great fun, especially if you have a chassis dyno... "Let's see - the engins is running a bit rich above 5500 rpm - do I fit a smaller main or a bigger air correction?" The only real answer is you try both and see which the engine likes better. Jets are available. Emulsion tubes, which (help) modulate the transition between the idle circult and the mains less so.

I see the cam data posted above shows the progressively increased duration and lift from the T to the E to the S cam as you would expect.
The Solex grind however breaks this linear trend of these 3 cams with intake duration and lift sitting nicely between the E and S cam specs, but the exhaust duration is similar to the S and the lift is even greater. I see the lobe centres are also closer than on the S indicating more overlap. So the Solex is 'hotter' in some respects and 'milder' in others compared to the S cam.

You also have to factor in stuff like what pistons were available at the times and what valves the respective engines used to determine how much lift could be used without 'contact.'

Are these the characteristics that a Weber fuel system needs for peak performance ?

All those cams were good at what they were good at and less good at others. E was a good compromise. S has good peak power, crappy fuel economy, iffy driveability, and will foul plugs in light driving if jetted for peak power. it's all tradeoffs. You can also tweak things A LITTLE by advancing and retarding the cams a bit, a game even the factory played from time to time, but these engines do not respond to that trick like some do. Still, if you make a guess and decide you only missed what you wanted by a little, it's a cheap and easy thing to try.

If the S cams make about 195bhp, and the E cams about 165bhp on a 2.4, then how much bhp would you expect with the Solex cams on Webers or PMOs ?

at what rpm? with what venturiis? and what exhaust? You can speculate endlessly, and the fact is that all this stuff is more than a little nonlinear and the only real answer is to build up the engine and ask it, on a dyno. This more than anything is where the 'real' engine builders earn their big bucks.

If the Solex cam is ideal for a PMO set up with S heads then the next question if where to I get a pair at a reasonable price ?

you can't and you wouldn't want them if you could find them. Real Solex cams date from the days when Porsche was oiling cams by sending pressurised oil through a drilling in the center of the cam and letting it out through port in the base circle of the lobes as well as the bearing journals.

What you do is get a cam grinder (like WebCam) to grind that profile on either new billet or rewelded core cams. WebCam has been doing that game since the 50's and it's a trusted, reliable method. There are other grinders too, I've just researched WebCam a bit more than I have the others.

This has been really useful.

It's all a part of doing the homework.

[ArtechnikA]

MFI delivers fuel below the butterflies and has tall stacks. Carbs deliver fuel above the butterflies...close to the top of the carb, in fact, and have shorter stacks.

As I know you know, the big difference here is that MFI delivers the fuel once always, in a shot calibrated to rpm and throttle position, and has no airflow sensor at all.

Carbs work by pressure differential of the passing airstream sucking in the gas.

The airstream passes the fuel delivery stuff once when 'on the cam.' When you've got reversion, as you do with big overlaps, the airstream passes the fuel delivery stuff once on intake, then again during the reversion, and the fuel metering stuff just knows pressure differential - it does not know the direction of the airstream. Finally it's sucked back in yet again, passing the fuel delivery bits a third time, picking up even more gas.

No wonder the suckers run rich...

[J P Stein]

Tho I can't prove it, I suspect the MFI, S combo has a certain amount of reversion.
The tall stacks & the extra bit of length the delivery point provides would help keep this contained where it does no (or little ) harm.

The S cams came out for one year only (IIRC) with IDS carbs. From what I'm told, it was not a happy marriage.

[brant]

You've opened a big mystery box now
there are lots of tricks, and lots of ways to get the most out of it.
and it really all boils down to intended use:

what RPM will you really be at
street or race, because you can't maximize both....

by the way, you can get GE grinds for 3 bearring set ups
most of the guys I know run GE60's on the street
and most of the guys I race with run GE100's (on 1967 2.0/6 motors)

you can really really help jetting by using IDS style auxillary enrichment devices, and also by running tall 2ndary venturi's

you can work around flat spots by changing primary venturi's or changing gearing.

not to mention the use of a wideband type system to actually get it perfect.

I do all of the above
I love the auxillary enrichment tubes, and I run an F/J/O/S/V gearbox that leaves me in the power band at all times...

[scotty b]

Tho I can't prove it, I suspect the MFI, S combo has a certain amount of reversion.
The tall stacks & the extra bit of length the delivery point provides would help keep this contained where it does no (or little ) harm.

The S cams came out for one year only (IIRC) with IDS carbs. From what I'm told, it was not a happy marriage.

I think your thinking of the 67-S which was a one year only car. Short wheelbase S spec engine. No issues there other than yes the carbs can be a bit difficult to tune because the cams suck ass at idle

[ArtechnikA]

Tho I can't prove it, I suspect the MFI, S combo has a certain amount of reversion.

I have no doubt the airflow experienced reversion.

The BIG difference is that carbs add more gas to the air each time it passes through the venturiis (total of 3). MFI sends a single high-pressure shot to the port on each intake stroke. Some of that mixture might bounce out and get sucked back in, but you get the right amount of fuel - not 3X.

[ArtechnikA]

by the way, you can get GE grinds for 3 bearing set ups

cool deal, good to know!

most of the guys I know run GE60's on the street

the GE80 is a little milder than a RSR Sprint cam and one of the contending profiles for my 2,8.

...by running tall 2ndary venturi's

did we ever establish if those are available for 46's? 'Cause I have a pair of those in case I get impatient while saving my nickels & dimes for an RSR MFI setup...

[brant]

[quote]
...by running tall 2ndary venturi's[/quote]
did we ever establish if those are available for 46's? 'Cause I have a pair of those in case I get impatient while saving my nickels & dimes for an RSR MFI setup...
[/quote]


Rich,
I know they exist
I wish I knew you were looking
my buddy just sold a set about 3 weeks back.. fairly reasonably too...
(his were running on a 906 cam'd 2.0/6 GT 914 racer)