Max Compression for Cast Iron Cylinder 2.4??
 

17 years ago I prepped a set of cast iron cylinders, had a small overbore honed to fit 9.5:1 Wiseco pistons. The pistons are a different expansion ratio alloy for the cast iron cylinders. I local 911 builder told me recently 9.5 is too much. Should I trim the crown to lower CR? Motor will be used for exuberant street driving, PCA fun autocross but track duty is unlikely. It is a T specifications motor with E cams and 32 mm intake porting.

After 17 year child rearing hiatus I am back on the project. Seems nowadays over bores are pretty popular. Considering selling the cast iron P/C set and getting +Xmm over-bored Nikkis. Need to have the block align honed etc. anyway, so what is the max overbore if the block is machined? As opposed to if the block is not machined? The heads may also require machining for a larger bore or is that just not done!

TIA!
Bill

Another option would be to keep the pistons and just have your E cams reground to Mod-Solex. That compression ratio is about right for those cams.

Unless your E cams are already reground T cams. But you’ll love the Mod Solex grind. Just check piston to valve clearance. No idea what the valve pockets are like in your Wiseco pistons, but aftermarket are usually oversized, so I’m guessing it’ll be fine.

My E cams are reground T cams. From what I read the E cam timing makes for a lot of cylinder pressure and possible detonation with the 9.5 pistons I have (maybe?), hence my concern.

Alternatively or in addition to lowering the CR I could slightly retard the timing on the E cams? So many variables...

Depends on has you are going to run. Higher Octane fuel should be fine. Earlier 2.2 engines ran 9.8:1.

Early 2.2 S engine compression ratio is not 9.8 to 1 . I have measured numerous stock engines : 9.5 to 1 .

CR under 10 is ok with Super Unleaded only. Cams with more overlap will work great with that CR. MOD Solex (DC30) are a GREAT choice. i ran them and that car was FAST!

Also ran 9.5 CR on my otherwise stock 2.4 E engine and it eventually burned a JE piston. Need to keep temps down and not run lean.

73 911 E

The biggest challenge with cast iron cylinders on air-cooled 911 is head temps.
The fan is designed to blow air across the heads and cylinders to transfer heat to the air. The idea is to pull heat from the heads.
The laws of thermal dynamics tell us that heat travels from cast iron to aluminum.
That means the cylinders are not pulling heat from the heads as well as aluminum cylinders. As long as you don't make heat (horse power = heat) the issue will be minimal. As soon as you make horse power, the heads will heat and detonation will follow.

So I am considering two options and am back to my original question. Either way I will send the mag cases to ollies to be machined to application. Likely shuffle pinned.

Option: Oversized nickies (can I get to 2.5L?) or equivalent and a mod/solex cam and spend??

Option 2: use my already honed to size cast iron cylinders, the 9.5:1 wiseco pistons & E cams for the cost of cutting the crowns a bit.

I live in northern Ohio so prolonged high heat is not a problem. The car will see an occasional fun Autocross and vigorous backroads with the occasional highway burst. So what CR is best for commonly available pump premium?

I use iron cylinders on my modified 2.4S engine with 2.2S pistons, S cams, twin plugged, 40/38 mm ports, EFI+ITB. It makes easily over 220 hp, not a trace of detonation (I have knock sensors too). Of course that is not a prolonged track use car, just a spirited drive car.

IMHO, and assuming you are not twin plug, 9.5 is plenty with carbs and distributor, and pump 93 gas.

The dome starts to interfere with the flame propagation around 10. You need to go twin plug after that. If you want to make some power Also, you will have to back off on timing to be safe over 10.

I like to stay small on the magnesium cases bore-wise, and use the Porsche studs.

The iron cylinders do have air flowing over them. The air guides can always be trimmed a little to flow more around the top of the cylinder, but I think you will be fine as is. I like your idea of trimming the pistons down to 9, assuming there is enough material there to do it. It will be a real classic mag case 2.4.

The jury is in. I will stick with the cast iron cylinders. Likely drop the comp to 9.

The case has been helicoiled and they look good. So do I need to have the timeserts put in when I get the block align bored? Probably a stupid question.

My understanding of E cams is based on what I learned 15 years ago, that they are hard on valve springs. I had ollies redo the heads with valve springs I sourced 15 years ago from I don't know where. They were only $110 or so. Thinking of replacing them with "known quality" springs. Not looking for race springs, just correct for application quality springs. Any mfr brand recommendations?

I am also considering changing from the E-cams to a mod/solex. Can those be regroups from T cams or must they be bought new? I don't want an overly peaky motor. Upper-mid torque is best for backroad fun without attracting too much screaming-motor attention from the gendarmes.

Is anyone running retarded E-cams? I seem to recall piston valve concerns. The info in my memory is all 15+ years old...

I can't answer all of your questions but this one I can. I spoke with John Dougherty last week and he informed me that T cams can be reground to E cams but not to DC30 a/k/a mod-Solex. Mod-Solexes can be reground from E or Solex cams or from a new billet. I was intent on DC30s for my planned build but in the absence of finding E or Solex cores at reasonable prices, I have resigned myself to getting Ts reground to Es.

T TO E Cams with higher compression will be a nice upgrade and a crisp engine. E to DC 30 is the next step and a screaming engine.

Chris
73 E

"The case has been helicoiled and they look good. So do I need to have the timeserts put in when I get the block align bored? Probably a stupid question."

A Time-Sert gives you a larger thread area to distribute the head stud load. A common and very worthwhile change for the Mag case.

There is no downside to the timecerts. Not expensive and no chance of pulled studs ever.

I think most places use case savers vs timecerts today. Timecerts are finished by forcing the threads out into the material which can damage the threaded interface on old mag cases. Case savers have a course 14MM x 2 pitch outer thread and are thought to hold better on older soft mag.

john

Cast iron cylinder
 

Interesting thoughts on cylinder material.I agree with Henry on the heat transfer issue.I have done a bunch of 356 Race motors that made 165-174 hp.The cast iron cylinder was always the most stable and we would usually use the 356C non-counterweighted crank.In a 911 if you use the 911T non-counterweighted crank it just revs slow in comparison.Ciao

Sorry, I meat case savers. They are big and will not pull ever.