Holiday ride of shame

[Dan Epperly]

You didn’t mention it but he will cc the chambers and measure the cylinder seating surfaces in the head to make sure they are in the same plane?

I will ask him to CC my heads after he cleans them up.
All he said was "the valves were hitting"
I'm not sure how I can figure out the volume of those Mahle Pistons since they have been altered, which I would need to do to figure compression.

[Martin Benade]

At least chamber volumes will tell if they are particularly small compared to what Porsche stamped at the factory.

[Vic Skirmants]

You didn’t mention it but he will cc the chambers and measure the cylinder seating surfaces in the head to make sure they are in the same plane?

I will ask him to CC my heads after he cleans them up.
All he said was "the valves were hitting"
I'm not sure how I can figure out the volume of those Mahle Pistons since they have been altered, which I would need to do to figure compression.

Dan; forget about theoretical calculations based on phase of the moon, etc.
You will have to bolt a head onto the engine and fill the combustion chamber with fluid; Ron Ladow can tell you all about it and even supply some trick parts to make it easy. ONLY THEN will you know FOR SURE what your compression ratio is.

[Dan Epperly]

Cam has R200.5 stamped on it.
Vic I read copious amounts about how to calculate CR and the merits of this or that method, I'll try filling the chambers when I get everything back from Dennis.

[Vic Skirmants]

Cam has R200.5 stamped on it.
Vic I read copious amounts about how to calculate CR and the merits of this or that method, I'll try filling the chambers when I get everything back from Dennis.

NOW we're making progress. Thanks for listening.

[C J Murray]

Cam has R200.5 stamped on it.

Well, well, well! That is the dopey cam that was popular in the 1980s I think called Tricom or some such name. The lobe separation is 104* vs 108* for the stock cams and 11* more duration. Why this matters is because the cam takes up more room in the chamber, the valves are closer to the piston by virtue of both of those changes. Guess what cam put those marks in the pistons in my picture?

Dan, I commend you for posting all this! You are doing a great service to those who would go down a similar path of thinking that everything will work out fine if I just bolt it together. Just bolt it together ONLY works by luck or if you are dealing with 100% stock virgin parts. This is why many knowledgable people advise to keep your engine stock. The point is not that all rebuilds should be done to stock specs. The point is that if you don't check everything very carefully your machined parts and aftermarket crappy parts are going to bite you in the ass!

If I can help in any way please email or call. There are others that can help here. You have already helped many people here and I hope the do-over yields a fine result. Throw that cam in the trash.

[Dan Epperly]

Hi Cliff,
I don't mind being the negative example. It's all a learning experience for me and I've learned much reading about other people's disasters over the years.
I basically got this engine in a box with the car. I wonder if I had assembled it as it was designed if it would have worked? Meaning the heads, odd cut industrial Pistons, the cam...maybe I threw a wrench in the works using the AA big bores?
So now I need to decide how to proceed. If I junk the cam and use the Mahle modified industrials with a stock cam am I going to have a different set of problems, such as too low compression?

[C J Murray]

I did a short tech session at the Ocean City NJ ECH where I covered the potential problems associated with building an engine. There are 2 basic issues, an engine where lack of attention to clearances causes self destruction or an engine with mismatched component choices that may run forever but always run poorly.

Clearances are pretty clear cut. When the factory built the engines they could rely on the dimensions being very close to correct and they could mostly just assemble the parts. I am sure they learned to double check certain clearances but they could build an engine very quickly, much more quickly than checking all the things we have to check today with our abused parts. Now everything must be checked carefully. As a side note, Porsche tested the completed engines and if they were sub-standard they were carefully rebuilt by experts in finding the problems, so they slapped them together quickly with virgin parts and in the rare case that the result is not good they go back and approach the engine carefully. We have to start with the latter approach.

Mismatched component choices are very common mistakes made by today's builders. There is a planning process. First be honest about how you drive and where you drive. Don't build a 7500rpm engine for 4500rpm driving. Even F1 and MotoGP engine builders detune their engines because they need broad power delivery to achieve the greatest average power within the rpm range used while racing.

Porsche thought that the S/S90/SC/912 cam was as radical as was sensible for street use. They had Mezger do a lot of work on the 616 engine which resulted in some beneficial changes along with the C cam which yielded a big improvement over the Super results. If he felt he could improve the S90 cam he would have done so.

Your cam has 11* more duration than the hot stock cam. That pushes your usable rpm range up at least 500rpm and it reduces power in the lower rpm range. The reason your cam was changed to 104* LC is to compensate for the excessive duration. Changing/tightening the LC closes the intake valve closer to the same point as the stock cam to try to retain some low end torque and the overlap promotes acceleration. That doesn't really work and it puts the exhaust valve very close to the piston.

The planning process is pretty simple, consider use requirements, choose cam, choose CR that matches cam, size carbs correctly to match the package. Because Porsche carburetors are so expensive and their appearance is important when originality is a factor then the plan for the engine could have the carb choice make the cam choice for you.

What carbs do you have? What CR will you have with the pistons and heads that you have? No guessing! Answer those questions and I will tell you what I think you should do.

[Wes Bender]

If an award was given for "Thread of the Year", this would probably be it.

[Dan Epperly]

Here's what I have and what I know.
--Zenith carbs S specifications rebuilt by CFI
--Stock original muffler and tailpipes
--B heads. Chamber volume unknown at this point
--85.5 Mahle industrial Pistons and cylinders. Cylinders cut down. Will need to measure and report back. Pistons machines with slight dome and deeper exhaust pocket.
--stock B crank uncut.
--stock rebuilt BR18 distributor
--stock gear box

What I want is a reliable touring engine. I don't need heaps of horsepower, but want a little more than what a normal produced. Want something that runs on regular gas.

[C J Murray]

Let's see what your CR is, accurately, but a stock C(not SC) cam may be just the ticket for what I think your CR will measure. Wait until you know the CR!.

Make certain that your head angle matches the angle your pistons are, important!

Measure your CR with a head to piston minimum(tightest point) clearance between .035" and .050", important!

[Martin Benade]

From other discussions I am still not clear, are N and C cams identical, only differing very slightly because of rocker arm ratio?

[C J Murray]

From other discussions I am still not clear, are N and C cams identical, only differing very slightly because of rocker arm ratio?

Not exactly. I have my own Cam Doctor data at home, not here, but I think Willhoit has it right. The N and C are close but different. We know that the Super and the C make the same peak power but look how much milder the C cam is. TORQUE! This is a big reason why people like the way a C drives. If you have Zeniths the C cam is a great choice. Many aftermarket cams are snake oil voodoo. Some are good but you had better have a plan and understand what the result will be. Also, extremely important, the numbers of lift and duration and LC do not allow a true evaluation of how good a cam is or how well designed it is. Porsche factory cams have sensible numbers and that is why my father drove 4 of these engines from new to 100k miles each with zero engine work.

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[Mike Horton]

Cam has R200.5 stamped on it.

Well, well, well! That is the dopey cam that was popular in the 1980s I think called Tricom or some such name. The lobe separation is 104* vs 108* for the stock cams and 11* more duration. Why this matters is because the cam takes up more room in the chamber, the valves are closer to the piston by virtue of both of those changes. Guess what cam put those marks in the pistons in my picture?

Dan, I commend you for posting all this! You are doing a great service to those who would go down a similar path of thinking that everything will work out fine if I just bolt it together. Just bolt it together ONLY works by luck or if you are dealing with 100% stock virgin parts. This is why many knowledgable people advise to keep your engine stock. The point is not that all rebuilds should be done to stock specs. The point is that if you don't check everything very carefully your machined parts and aftermarket crappy parts are going to bite you in the ass!

If I can help in any way please email or call. There are others that can help here. You have already helped many people here and I hope the do-over yields a fine result. Throw that cam in the trash.

+1

I had one of those RO200.5 cams, in a set of parts I'd bought, and sent it as a core, to be reground. I concur with C.J., on the extra duration, and lobe centers, hence, Ray's likely reason, to deepen the EX. valve relief.

At least, we've ruled out the AA or any other big bore sets, as culprits. Even those Industrials, are original German big bores, just initially set up as low C/R.

Now the question, can the heads be saved?

I helped a friend with a "crankshaftectomy, in a Ray Litz '58 N, which he converted to a S, in 1975, per the invoice, using factory stock 616-12 cast iron cylinders, and the 8.5:1 cast pistons, Mahle brand, that RO200.5 cam, but... the heads, looked nothing like these pictured. The compensating cut, to the inner fin surface, was conventional, like in Ron's photo...

I do have a set of core 912 heads, from a vintage racer, which have that same circular cut, as in Dan's pic...,

[Dan Epperly]

Mike,
I asked Dennis about the merits of just machining the relevant area vs the whole face and he said leaving the material provides a heat sink. As long as there is clearance for the fins and the top of the cylinder is seating properly in the head there is no real need to remove so much material.
Maybe I'm missing something, but why would it be better to remove more material? What problem do you see with the heads other than the circlular machining and the step inside the chamber?