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Teach me about camshafts


Phill

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21 hours ago, Phill said:

Are SSIs classed as large headers?

No.  SSIs are a repro of the HEs from the early 70s which were a perfect fit on the little engines up to 2.7 litres.  They still work fine on 3.0s and 3.2s but they are not large headers.  The "right size for modified IBs is 1 5/8 inch primaries, but its more a nice to have than a must have if you have SSIs already.

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Thanks. I have SSIs (dansk).

The way it looks at the moment it seems sensible to wait until the engine is opened up before deciding which cam to go for. If new pistons are recommended then go for pocketed ones and a higher lift cam. If pistons are fine then just go for best cam  available to suit existing pistons. Which brings me to the question, which cam  is best for standard pistons where CIS is not a factor?

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Ask John Dougherty. He is very knowledgable and happy to answer enquiries. IF it's the engine you want then the cost of replacement pistons to allow bigger cams shouldn't restrict you IMHO. Compared to a rebuild they are not THAT expensive..... my 2p as always.....

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16 minutes ago, Leicestershire said:

Are you doing the work yourself?

I would love to but I'm not sure my skills extend that far. The other reason is that I don't have the space to drop the engine and then work on it. This could change by the time I come to do it but unlikely. I could rent a space for 6 months when the time comes of course.

My idea was to fact find and start buying parts to soften the blow but there seem to be too many variables to do that safely. Fall back is to fact find, plan and save money but at the rate parts are going up in price this is not ideal.

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If you are paying labour costs it makes sense to get as much done as you can for your £.

Pistons will set you back around £1k but you won’t pay any extra for fitting.

If going down the DIY road then pistons will be a large % of the total cost.

Basic rules help manage a project.

Plan your build.

Decicide what you want.

Measure what you have. 

Keep a track on your build with regular updates from the guy doing the work. If DIY log all costs.

When complete measure the end results.

 

Looking forward to following this build and seeing what gains you can find.

 

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Be careful about deciding on a direction only once the engine is opened Phill. 

My P&C's turned out to both be hosed when they opened my motor up (although it still made 216 bhp - gotta love an SC engine) 

Would have liked to go 3.2 ss at that point but the lead time on the Max Moritz pistons was indeterminate and my engine builder wasn't able to have my car sitting around in his workshop for an extended period waiting for parts so I had to go with parts that were readily available. 

A missed opportunity for me :(

Edited by GaryH
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My advice would be to shun the crazy pricing of many southern builders and look to someone like Unit Eleven in Cheshire. The pricing will be a pleasant surprise, and with ITB's needing to be set up, Wayne is right on the doorstep so a win win.

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22 minutes ago, ras62 said:

My advice would be to shun the crazy pricing of many southern builders and look to someone like Unit Eleven in Cheshire.

Jonny and Danny at Unit 11 rebuilt mine. 

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That’s a really interesting thread with some information that’s hard to come across!

I have often wondered how does one know which pistons will have enough clearance for a given high lift cam. Is there a table somewhere?

On 8/18/2019 at 9:14 PM, Leicestershire said:

Stock takes 7500 rpm. How fast do you want to go? The majority of cam profiles matched to use in a road going vehicle are all done by 6800 rpm.

That’s quite impressive! Does this mean that a stock SC engine with ITB/EFI and custom electronics could be made to rev up to 7000 rpm without risking immediate damage?

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18 minutes ago, WP0ZZZ said:

That’s a really interesting thread with some information that’s hard to come across!

I have often wondered how does one know which pistons will have enough clearance for a given high lift cam. Is there a table somewhere?

That’s quite impressive! Does this mean that a stock SC engine with ITB/EFI and custom electronics could be made to rev up to 7000 rpm without risking immediate damage?

Stock SC engine already revs to nearly 7k rpm, so yes, a healthy, fresh engine can run a 7k rpm redline with just a little additional risk.  Even the 3.2 CS, which is same internals as 3.2, with smaller rod bolts and heavier pistons compared to an SC was redlined at 6840rpm IIRC.  The intake system and engine management have nothing to do with what is a safe redline.  In a rebuild, its how you spec your rod bolts, valve springs and retainers that give you extra headroom for revs and then it's about lightening components.

Re, the how do you know what cam with what piston Q, I haven't seen a table, which is why you ask a cam supplier what they know works from experience and then you ask the same Q to your engine builder.  Then they measure it during the build to be sure it works.  Very little in the world of air cooled engine recipes hasn't been done before - you just have to search a bit to find the recipe.

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11 minutes ago, Richard Bernau said:

In a rebuild, its how you spec your rod bolts, valve springs and retainers that give you extra headroom for revs and then it's about lightening components.

This ^^^^^

Luckily your SC engine already has beefy rod bolts :)

Edited by GaryH
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Very good advice and info coming out here - thank you.

I have just read MaxDiesel's engine rebuild thread which is great. It seems that a careful and methodical approach could mean a DIY job is on the cards. I would like to get a leakdown test done and my valves definitely need checking as it's not been done in my ownership.

I will order Wayne's book in the next few days too.

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Thanks for your comments, chaps!

I just learned that ignition cut-off for SCs is at 6800 ± 200 rpm except for MY80 where it is 6500 ± 200 rpm. And my car is MY80 😕 I'll need to check whether there is something special in the MY80s that justifies such a difference...

Once upgrade time arrives I'll need to do some serious homework on camshafts and pistons. Ideally I would like something with a strong mid range but which is not out of breath at 7000 rpm. Not very worried about a large headline power figure but I would like some healthy area under the curve between 3500-7000 rpm. Usage would be a combination of hard mountain and track driving on a car weighting 1000-1050 kg. Any cam suggestions?

 

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Cutoff is in the rotor on an SC at 6600 doesn't matter what year!

One from a 3.2 fits & wont have a cutout!

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Just now, Leicestershire said:

Rotor arm from a Vauxhall Royale will add RPM to your SC!

1980 SC’s are the 188 hp model. Bit of an odd ball motor with a unique distributor and bigger oil cooler. Most of the 188 cars I have seen have 34 or 36mm ports so worth checking what you have before planning any engine development.

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5 minutes ago, Type911 said:

Only 2 dizzys, one for the 180bhp & 188bhp and then one for the 204bhp version

 

Could it be that Porsche have rationalised parts stock?

Happy to be corrected on this but from memory, the original 1980 distributors looked very narrow, had a single clip retaining the cap and a single bob-weight on the main shaft.

Visually, the pre 1980 SC’s look very similar to the 1981, similar diameter with 2 clips retaining the cap.

 

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My ’79 has no vacuum advance.


Sent from my iPhone using Tapatalk

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7 hours ago, Type911 said:

Cutoff is in the rotor on an SC at 6600 doesn't matter what year!

One from a 3.2 fits & wont have a cutout!

You can just take the rev limiter slider part out of the standard rotor.  This retains the correct rotor resistance of 5k Ohms.

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8 hours ago, Leicestershire said:

1980 SC’s are the 188 hp model. Bit of an odd ball motor with a unique distributor and bigger oil cooler. Most of the 188 cars I have seen have 34 or 36mm ports so worth checking what you have before planning any engine development.

Is there a mix of port sizes within the same model year?

I seem to remember that small port sizes are preferred for performance builds (which sounds kind of counterintuitive.) Is that right?

Edited by WP0ZZZ
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