Riddle me this...guess the cams!

[kenikh]

I recently bought a group of parts that came from one engine; the heads have pretty unusual port specs which got me wondering. I want to ask you what cams you think were most likely attached to them. Here are the details I have:

• Ports: 38I/35E (this is the odd part; I have never seen ports more than 1mm apart)
• Twin Plug
• Cut for 89mm P/Cs
• Hi rev springs (haven't tested them see what max lift is)
• Titanium retainers

Some other details:

• The mechanic who sold me the parts said the motor came in running fne, but was running a cam way too big for regular street use
• The motor was running a twin plug Electromotive HPV-1 set to cut off at ~7700RPM
• Pistons used in this motor were JE 89mm at ~10.5:1

So that is what I have to go on. Your thoughts?

[304065]

Short-stroke (66mm) 2,5 with GE100 cams. What do I win?

What is the casting number on the heads, i.e. what did they start with? What valves? As you well know a 2,0 S is 36/35, that would be a good starting point for somebody who wanted a 38mm intake port like a 901/2X. I agree that it's odd, however: air that comes in has to leave somehow, which is why I guessed a camshaft with a very (comparatively) long exhaust duration.

[kenikh]

You are thinking what I was thinking, except that I am certain that it was built on a 70.4mm crank (I have the rods, which I forgot to mention explicitly). My guess was a GE/DC80, but on a 2.5, a 100 makes perfect sense. The exhaust ports just seemed a tad too small, but with long exhaust duration on the cam, that would solve the last piece of that puzzle.

As far as what you win? The sense of accomplishment of being the first to answer?

As far as the casting # goes, I will have to double check. They are 1974 heads.

[jluetjen]

Exhaust ports tend to be pretty forgiving for being undersized. While it takes a lot of effort to get air to flow into an engine, exhaust gasses have been known to push their way past cylinder head gaskets to get out of a combustion chamber. So a couple of mm of exhaust port diameter really isn't that consiquential. Slightly "undersized" exhaust ports can also be helpful for keeping the exhaust gas velocity up, along with the associated inertia -- which will then improve the exhaust scavenging of the combustion chamber, and increase the sensitivity of the engine to tuning the exhaust and intake paths.

So I wouldn't be so quick to condemn those 35 mm exhaust ports!

[kenikh]

Originally Posted by jluetjen

So I wouldn't be so quick to condemn those 35 mm exhaust ports!

Definitely not condemning! As we know, gases under pressure tend to find their own way. As we also know, the intake side doesn't have that luxury unless turbo/supercharged. I figured there was a rhyme to the reason on these heads, I was just looking for a clearer response than my anecdotal reference library (my head).

[davidppp]

Hello all.

The other good reason to keep the zorst ports small is to give a good antireversion step to the headers...1/4 inch may not be too much if you want to optimise this..and this can help avoid bad holes in throttle response when revs fall off the cam with a very wide overlap..

It all points to a BIG cam..

Any clever stuff in the portwork?

swirl, offset ports etc?

Kind regards
David

Kind regards
David

[kenikh]

Originally Posted by davidppp

Any clever stuff in the portwork?

swirl, offset ports etc?

Kind regards
David

Kind regards
David

Please define how to recognize swirl and offset ports. Like I said, my understanding of porting is pretty simplistic.

[davidppp]

Hi Kenikh.

Basically, anything done to induce a twisting motion of the air..can be an oval port which twists as it arrives at the valve seat, or can have been ground away one side of the valve guide..and in the bowl... no doubt other clever things too..

Just intrigued to know what degree of ingenuity might have been in the porter's mind!

Kind regards
David

[kenikh]

Once I get them cleaned, I will make sure to post some photos.