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cam profiling?

ok.. I'm looking at changing my cam profile to change the characteristics of my stock engine. I've been told that a different cam profile can move the power curve up or down the RPM scale. I've read that a 911 S cam profile from the earlier years will give my 3.2 a smoother curve (the question is, why would a smoother power curve be beneficial?)

I'm struggling to understand what to choose... in that, I'm struggling to properly communicate what I want because I'm confused as to what exactly I want.

So help me understand how changes to a camshaft can do this.

I know a camshaft just opens and closes the valves... and I understand that the lift and duration are what you are changing, along with the overlap.

But what I don't understand is HOW the different changes actually effect the engine's performance.
The chamber can only hold so much air and fuel, and the piston can compress only so far... so how does the valve change the performance characteristic?

I've been to dozens of websites that show diagrams that look like I'm trying to understand the gravitational mass of a black hole while calculating its impact on tachyon waves through dark matter.
My point is... understanding the diagrams doesn't help me to choose the right profile (or keep from choosing the wrong one).

So when I call Webcams (or whoever), what am I needing to know I want, and how do I communicate that in a way that doesn't make the person on the other end of the conversation think I'm an idiot??
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Old 01-26-2018, 06:42 AM
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Trakrat, try this link for the best overview of changing cams has on the effect of the engine:

COMP Cams® - Sorry...

Cliff notes and highlights:

Longer duration moves the power curve up. Wider lobe separation makes for a broader torque curve with less peak HP. Narrower lobe separation makes for a peakier motor with highest peak HP. Narrow increases cylinder pressure, wider lowers cylinder pressures (best for NOS and boosted car)
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Old 01-26-2018, 08:23 AM
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If the rest of the motor is stock then you are limited with what you can do with the cams.
Starting with lobe center,
The injected motors with single plenum will require less overlap meaning a wider lobe center. 108 degrees up to stock 112 degrees work well. The early S cams have a lobe center of 98 degrees and typically creates upsetting pulses back into the plenum. Individual throttle bodies reduce the pulses keeping them contained to each cylinder so you can use a cam like the early S with ITB's though not ideal in my opinion.
Wider lobe center typically give you a broader torque curve.

Static compression ratio also plays a big part in cam choice.

The next spec to look at is duration.
Typically larger duration moves the rpm peak torque number higher and reduces low rpm torque.

The next spec is lift. When you go much higher lift then you may need stiffer springs.
A good resource is John Dougherty- DR Cams, (Cam Grinder on Pelican) he can recommend a cam for your needs.
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Last edited by RSstop; 01-27-2018 at 04:03 AM.. Reason: spelling
Old 01-26-2018, 08:27 AM
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Some of the more aggressive cam profiles will not make your Motronic injection happy (early S grind being one of them). Too much overlap (intake and exhaust valves being open at the same time), which creates the pulses that were mentioned above. I believe the grinds that work best in an engine with stock type EFI are lower on the duration, have wider lobe centers as mentioned above, and have more lift. John Dougherty (camgrinder here on the forum) is well regarded in the Porsche community. Maybe give him a call.

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Old 01-27-2018, 06:57 AM
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All of the above is true.

The thing about cams is you FIRST have to decide on what character you want in a motor. Torquey and responsive, high revving and high strung, or a compromise.

Then you must take into consideration all the other aspects of the motor. What compression and pistons, what induction, what exhaust will it have, will you need to meet smog requirements, are there race class rules to consider, etc.

Over the years Porsche has tailored their cams to their motors. So knowing how a Porsche cam is matched to their motors can be a good indication of how that cam behaves. You can look up those factory cam specs and see what motors they were used in.

Remember, it's all been done before. It is known that with the Carrera FI system, (and pistons), you are limited to a stock, 964, or a similar profile.

There are several recent threads on using 964 cams in Carrera motors.
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Old 01-27-2018, 09:49 AM
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Why re-invent the wheel?

Gordon,

If I were you, I would use an existing and provened engine configuration with minor modification unless you want to try new and novel cam design. Even if you are able to decide the desired cam profile specification, there is no guarantee you will be happy and satisfied with the result/s.

A Pelican Parts member asked me to rebuild his 3.0 SC motor from 180 Hp to 280 Hp (goal) sometime ago. This project was not cheap and was able to achieved the goal.







Tony
Old 01-28-2018, 06:08 AM
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What Tony says has merit. The fact is that the cam grinder companies are used to people who don't fully understand cams (I sure don't, not at that level anyway) calling them up and saying they want a cam which will double the HP and torque, do it all below 6,000 RPM, and give them 30 mpg as well, along with a smooth idle, and nice flat torque band. Sure, we can wave a magic wand and do all that, right? Don't politicians frequently make similar impossible promises? So they are used to all this - what are you going to use this engine for has to be the very first question. If you say you are street driving it, but want absolute max HP and are willing to shift at 8,500 rpm, you may be asked how frequently you plan on rebuilding the engine. Are you ready for 100 or 200 hour motors?

But the cam grinders, as well as a lot of engine builders who can refer to what their motors did on a dyno when done, are going to have useful suggestions based on what your use and preferences are - can you trade off a 1,200 rpm lumpy idle for some more top end? What kind of compression do you intend, and what fuel are you planning on? Are you OK with the MPG figures a power band between 5,000 and 6,000 rpm will give? Then you have to go with that, rather than trying to figure out what lobe center you want, what lift, how fast or gentle the rise off of and back onto the base circle you want, what overlap you want. In short, what guys have been saying - what's this motor for?

My 2.7 race motor (factory RS pistons, nominal 10.3 CR, Weber 46s, twin plug crank fire and pump premium, big Elgin 315 cam) chassis dynoed with decent headers and stingers at about 224 hp. I was hoping for a corrected flywheel HP of around 270 or a bit more, like Porsche getting 110hp/liter from their race motors, so this fell a bit short, but it performed well. And idled around the paddock just fine, so I think it would have worked fine, other than the fuel mileage I never paid attention to other than to make sure I had enough in the tank for a track session, on the street. And emissions not an issue, either.

I think most operate at this level of understanding. We don't have our own flow benches (though you can send the heads out to be flowed), or engine dynos so we can test before putting it into a car, and try out several cams to see which works best, or programs which would allow us to design a cam (though some do give you some ability to plug in a variety of cam specs) so we can tell a cam grinder what to do for us, and not just ask them what they have to meet our needs.
Old 01-28-2018, 12:28 PM
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Quote:
Originally Posted by Trakrat View Post
ok.. I'm looking at changing my cam profile to change the characteristics of my stock engine. I've been told that a different cam profile can move the power curve up or down the RPM scale. I've read that a 911 S cam profile from the earlier years will give my 3.2 a smoother curve (the question is, why would a smoother power curve be beneficial?)

I'm struggling to understand what to choose... in that, I'm struggling to properly communicate what I want because I'm confused as to what exactly I want.

So help me understand how changes to a camshaft can do this.

I know a camshaft just opens and closes the valves... and I understand that the lift and duration are what you are changing, along with the overlap.

But what I don't understand is HOW the different changes actually effect the engine's performance.
The chamber can only hold so much air and fuel, and the piston can compress only so far... so how does the valve change the performance characteristic?

I've been to dozens of websites that show diagrams that look like I'm trying to understand the gravitational mass of a black hole while calculating its impact on tachyon waves through dark matter.
My point is... understanding the diagrams doesn't help me to choose the right profile (or keep from choosing the wrong one).

So when I call Webcams (or whoever), what am I needing to know I want, and how do I communicate that in a way that doesn't make the person on the other end of the conversation think I'm an idiot??
All good points. But don’t forget the basics. Choose who you buy the cam from and what help they can give you.

Remember as well, like good places to take your car for service, there are good cam businesses and some that are not. Many do what is commonly called cut and paste designs, where they mix, and match known profiles onto the same shaft and give it their own name. Some cannot even copy good designs well.

Many of the well know Porsche “hot rod” shops including my company design our own cams, make our masters and call out tolerances that are checked when we receive the finished cams back in house. Today more and more cams are cut on a CNC grinder allowing the “master” step to be skipped and the actual finished part a lot more actuate. We are now having our cams and rockers DLC coated to help with friction and wear. As the designs are becoming more aggressive, so does the need to help lower the friction and wear rate.

Porsche 2V engines make the cam design somewhat harder as the opening and closing of each valve is controlled by the same cam. The lobe centers are fixed (LSA) and only the opening events can be adjusted by the installer. In a dual overhead set up, the LSA is fully controlled by the installer.

Many of the fancy nomenclature’s are nothing more than names given to the opening events. The commonly thought of “most important" event is the Intake closing event,(IVC). In these 2V Porsche engines this is controlled where you “time” the Intake centerline and the lobe configuration. If you time the intake centerline by opening lift, then the closing event will typically follow this by the lobes duration measured at the same lift off the seat. There are circumstances where this can change, “non symmetrical” cam lobes for an example.

This closing time controls the engine efficiency (VE) and the engines ability to produce peak torque. If you think about where the valve closing happens in relation to where the piston is, then it’s easy to understand how this event controls so much of the engines performance. It controls and can limit the amount of air/fuel mixture entering the cylinder, affects the dynamic compression of the engine along with the engines ability to produce torque at what RPM.

The opening of the Intake valve (IVO), is considered the 2nd most important event. This is like looking at the closing time but in the opposite reflection. It controls where the intake valve lifts off its seat in relationship to where the piston is in the cylinder. IVO helps low speed response, valve overlap, builds peak torque lower in the RPM range among other factors.

But the LSA also very important. In these Porsche 2V engines, there is often more consideration placed on the lobe details and not on the LSA. I have seen engines where the LSA killed the performance where the engine could not even attempt to make the power it could have. The exhaust valve in a lot of cases opens to early lowering the in-cylinder pressure or too late making the piston do more work on its rise to TDC, having to overcome higher back pressure.

There is a lot in this and far too much to include here. My advice is to buy the cams from those shops that have a reputation for doing high end work. They will have their own designs, understand how it all works and can tailor the cam you need based upon the engine you have. Most will be able to do simulation work to ensure the design is what will give you the best performance with the least compromises. You may pay more, but I can guarantee the results will be superior. You can be sure that each lobe is the same, the LSA on each pair is the same and they will probably ask you to send them your rockers for measuring.

Rocker pad radius and the pad length are extremely important to the engine performance. The correct rocker ratio and pad length controls the cam’s lift and duration. If the rockers are not to their designed specifications, then your cam selection is again based on flawed inputs. A lot of repaired rocker faces look great, but the ratios are all over the place. Small variations in lift, duration and valve timing make a huge difference in the engines performance. Many, place a huge importance on the lift number when setting the cams in position, forgetting that many of the other valves in the engine will open differently.

Check the basics, buy wisely and don’t get fooled by hype. You may lose a couple of foot pounds per cylinder but when multiplied by the number of cylinders and as the RPM rises and friction and other losses take over, your engines performance has dropped off by a considerable amount. It’s no secret why some of these high-end engine companies can produce engines with more performance. They simply understand the basics and their importance.

Last edited by Neil Harvey; 01-28-2018 at 02:16 PM..
Old 01-28-2018, 02:13 PM
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The chamber can only hold so much air and fuel, and the piston can compress only so far... so how does the valve change the performance characteristic?
Just to reply to this question: valves regulate when and how the cylinders are filled with air/gas and when and how the exhaust gases are removed.

Gases entering and exiting the cylinder follow complicated paths and by controlling well the "whens and hows" it is possible to improve the way cylinders are filled and make sure that the burning gases make as much force on the piston as possible at the right point in the stroke.

Old 01-29-2018, 01:16 AM
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Thanks all for the replies.
I'm contemplating going to 3.4 pistons with a 9.8 compression. But not sure if it's worth the investment.
The engine will see a few track days, but nothing competitive. The only competition I hope to do is some auto-x. Other than that, the engine will see most of its time on spirited drives on the street.

Obviously, my goal is to get as much hp and tq that I can get... but the question I'm having to figure out is do I design the cam around the engine??? Or do I design the engine around the cams??

Can I ask that my cams reach peak power at 5000rpm? Is that possible?

I guess the problem is, when someone asks me "What do you want your engine to do?"... It's like a financial planner asking "What do you want your investments to do?"... the answer is ALWAYS going to be for it to reach its maximum potential as quickly as possible.
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Old 01-29-2018, 05:52 AM
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Cam that reaches peak power at 5000 RPM, to me, would be garbage. I have to ask, why?

The stock cams peak at around 6100, and are not that good IMO.

These short stroke motors need RPM to make power. If we were talking an old Big Block Chevy, yes, you can make decent power at 5K peak. But not a 3.2.

To achieve big power at 5K, means a very short duration cam that will require monumental lift.

The kicker though, is lift is severely limited due to small combustion chambers, large valves, and the geometry of the valves.

2nd, if you had a high lift cam with short duration, your cam lobe ramps would be very sharp putting a lot of strain on the rockers and would require very stiff valvesprings exasperating the total stress on the valvetrain.

Another words, it's possible, but far from practical.
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Old 01-29-2018, 06:23 AM
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Originally Posted by Trakrat View Post
I guess the problem is, when someone asks me "What do you want your engine to do?"... It's like a financial planner asking "What do you want your investments to do?"... the answer is ALWAYS going to be for it to reach its maximum potential as quickly as possible.
If you can define precisely what you mean by "maximum potential" when talking about an engine, that's half the question answered.

There is a trade-off that you need to navigate between max power, torque at low-mid rpm, engine cost and reliability.

Some engines spend all their lives between 6-8k rpm on the track with a close-ratio gearbox and are rebuilt every few dozen hours. Others are used for sporty Sunday drives on open mountain roads and are expected to last for many years with simple maintenance.
Old 01-29-2018, 06:53 AM
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It would be awesome if you were to define exactly your intended use and desired maintenance/reliability requirements and let aircooled engine experts chip in with what engine mods they would advise for a budget of $2k, $5k, $10k, etc.

Maybe changing cams is not an optimal mod for your budget?
Old 01-29-2018, 06:58 AM
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Originally Posted by Tippy View Post
Cam that reaches peak power at 5000 RPM, to me, would be garbage. I have to ask, why?
Thanks.. that's exactly what I'm trying to learn. I don't want to ask something that doesn't make sense.
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Old 01-29-2018, 07:39 AM
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Thanks.. that's exactly what I'm trying to learn. I don't want to ask something that doesn't make sense.
I think what no one has identified is that you want the engine to feel the strongest at 5000 RPM and not have its highest HP at 5000RPM.

I hope my learned friend Steve Weiner will jump in here also and add some weight to my input.

An engine does not make horsepower. An engine produces “work” which is a result of two simple measurable inputs. Force and distance. Work is actual force multiplied by distance. Force is the pressure placed upon the piston by combustion and distance is the crank length or stroke. This “work” is known as torque. An engine makes torque only. It increases due to the force acting upon the piston until friction and other parasitic loses increase and overcome the work done.

However, as these parasitic forces increase so does the engine speed. This new factor, RPM is a function of the mathematical equation that calculates horsepower. Horse power increases only because the RPM does. In fact, the work done by the engine will either stay consistent, (our hope in every engine) or drop of unfortunately, typically.

So, I think you want your engine to make its peak torque at 5000 RPM and stay somewhat consistent over the next 2000 RPM. That should net you the engine I think you want.

To achieve peak torque at 5000 RPM many factors come into play. In general, any 2v Porsche air cooled engine will produce its peak torque around 5000 RPM if the cam you choose has enough LSA built into it, its initial setting (IVC) event are in a typical range. But some basic parameters are required.

So, what should you aim for. The maximum lift is limited by two main factors. Rocker arm pad length and piston to valve clearance at a given compression ratio. The rocker pad length is critical, so the nose of the cam does not run off the pad. The valve to piston clearance is determined by the piston dome volume with pocket depths that still give under crown thickness.

Today, many race engines run lifts of over 15mm with seat to seat duration over 280°. These cams produce power above 500 HP at engine speeds reaching 9000 RPM. This sort of design has its drawbacks too. Very high cam velocities are built into these designs putting the valve train under tremendous force. Something like this will not work in your engine.

Do we want lift or duration? Shorter duration cams will make a better driving engine, more responsive throttle response and make more torque. What is a short duration camshaft? And how is it measured. Most cams in the US are measured at zero lash or what is known as seat to seat, and at 0.050” so the opening ramps do not affect the measurements.

If we lower the lift and increase the duration, we want the opening ramp speed to increase so that the cam lifts the valve off the seat quicker allowing the area under the lift curve to be as big as possible. Remember this is directly proportional to the maximum airflow through the port past the valve.

It’s impossible to best match the cam to your engine without knowing your actual driving needs, the spec on the engine and the air flow through your heads. But my guesstimate without knowing these parameters would be to suggest a cam with approx. 0.470” intake lift, 0.430” exhaust lift, and 0.050” durations of approx. Intake duration of 236° and maybe about 227° Exhaust. The LSA is more difficult to guess, but a number around 113° could work, but to really know you need to know head flow numbers. This is going to have a big effect on the peak numbers. Where you set the cam at is down to testing but a little retarded could help here to move the peak torque up to 5000 RPM and above 5000 RPM have it not drop off too much.

You can buy something off the shelf and know no better could be had, or have it made custom to suit your engine and have it max out the engine potential. Ask a proven engine builder what you need. My bet is, the number of his questions will out number yours.
Old 01-29-2018, 06:15 PM
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Was briefly mentioned above, and perhaps *not such* an issue with 911's, as less people play with gearsets and final drive ratios than they do in my neck of the woods with small, light sportscars....my old Westfield and other 7-esque type of cars provide you with the opportunity to really play with all sorts of gear combinations, from mundane production 'boxes through to dog boxes and "Rocket" ratios, through to a fully sequential box like I ended up using.

You start messing around with cam design and moving the power bands up and down the rev range, and all of a sudden you find yourself with a car that can be horrid to drive, purely because of the gearing you're using; a peaky engine with "stock" ratios (ie, long) isn't going to be much fun...so then that takes you down the path of different gearsets and final drive ratios....depending on what is then determined, that can either fall into a category that's well troden and relatively available, or you can be sourcing a final drive CW&P that becomes very time consuming (batches of 10 made every year or so...) and expensive.

So....per the other guys input, you really need to decide what your realistic wishes are...."max hp and max torque" isn't it....personally I'd rather sacrifice a good amount of hp for low down torque and a wide power (torque) band, good road manners, and something that doesn't stress the engine to the point of rebuilding every few hundred hours. You mention that it's primarily road use; not sure what it's like in your area, but here in the UK it's becoming increasingly difficult to find places where you can really stretch the legs of a revvy, top-end motor....too much traffic, speed cameras, or just not the safe conditions to do it when you do find that place where there isn't traffic and cameras! I became increasingly frustrated with my old 996 GT3; it was all top end hp, and not *that* many places to use it, without driving like a complete bell-end....the M5 on the other hand is all about face-melting torque (well, and a loony top end as well), but I can use that FAR more of the time, and in a safer fashion. Needless to say, I love the M5 way more than I ever did the GT3. Horses/courses.
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Old 01-30-2018, 01:18 AM
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Originally Posted by Neil Harvey View Post
I think what no one has identified is that you want the engine to feel the strongest at 5000 RPM and not have its highest HP at 5000RPM.
That would be my guess as well. I would advise the OP to talk at length with someone having lots of experience in engine modification who can elicit his requirements.


Quote:
Originally Posted by Neil Harvey View Post
An engine does not make horsepower. An engine produces “work” which is a result of two simple measurable inputs. Force and distance. Work is actual force multiplied by distance. Force is the pressure placed upon the piston by combustion and distance is the crank length or stroke. This “work” is known as torque. An engine makes torque only. It increases due to the force acting upon the piston until friction and other parasitic loses increase and overcome the work done.
At the risk of going off-topic, I would like to qualify some of the things in the paragraph above.

First: work and torque are two different things (although one can use the same units for both).

Second: an engine makes both torque and power. Both are equally real or artificial. Forces and torques can be understood intuitively but the are not more "true" than work and power. The two approaches are two sides of the same coin. When studying advanced physics or mechanical engineering, the immense value of the work/power approach becomes apparent. (See Wikipedia article on Analytical Mechanics for more on this...)
Old 01-30-2018, 09:09 AM
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Quote:
Originally Posted by Neil Harvey View Post

It’s impossible to best match the cam to your engine without knowing your actual driving needs, the spec on the engine and the air flow through your heads. But my guesstimate without knowing these parameters would be to suggest a cam with approx. 0.470” intake lift, 0.430” exhaust lift, and 0.050” durations of approx. Intake duration of 236° and maybe about 227° Exhaust. The LSA is more difficult to guess, but a number around 113° could work, but to really know you need to know head flow numbers.
In case anybody's wondering what those numbers equate to, the specs he mention are basically the typical 964 cam numbers
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Old 01-30-2018, 09:15 AM
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I think what no one has identified is that you want the engine to feel the strongest at 5000 RPM and not have its highest HP at 5000RPM.
I just picked 5k RPMs as a starting point. But more specifically, I don't want to have to wait all the way until the engine reaches 6k+ RPMs just to have the power come on, otherwise I'll have to make some changes to make the engine rev faster (lightweight flywheel, lighter engine components, etc..)

Quote:
Originally Posted by Neil Harvey View Post
It’s impossible to best match the cam to your engine without knowing your actual driving needs, the spec on the engine and the air flow through your heads. But my guesstimate without knowing these parameters would be to suggest a cam with approx. 0.470” intake lift, 0.430” exhaust lift, and 0.050” durations of approx. Intake duration of 236° and maybe about 227° Exhaust. The LSA is more difficult to guess, but a number around 113° could work, but to really know you need to know head flow numbers. This is going to have a big effect on the peak numbers. Where you set the cam at is down to testing but a little retarded could help here to move the peak torque up to 5000 RPM and above 5000 RPM have it not drop off too much.

The numbers I am looking at is 0.485" intake life, 0.452" exhaust lift, intake duration 238, exhaust duration 226.

But I'm not sure what stock is...maybe 0.450", 0.400" and 232, 222?
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Old 01-30-2018, 12:01 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #19 (permalink)
KTL KTL is offline
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drcamshafts.com or webcamshafts.com are good references just to see what some typical specifications are for various cam profiles.

Dougherty Racing Cams Porsche 911, 930 and 964 camshaft profiles

www.webcamshafts.com

Sort like putting a name to the face so to speak.
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Kevin L
Present: '86 Carrera, '79 911SC widebody conversion rolling racecar shell
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Old 01-30-2018, 12:09 PM
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