Large of small heads for 3.0?
 

So I'm contemplating my next improvement and not sure how far to go. The 3.0 in my 69 was rebuilt by PO to stock specs roughly 10k ago. '82 engine, stock SC cam and old style chain tensioners with new style ramps. On a cold start I sometimes get a puff of smoke. As well, I will get some smoke after running in high RPMs (tried staying ahead of a 930 on backroad twisties a few times) Other than that, my oil stick stays pretty constant. Anyway, during my last jaunt w/ the 930, my engine developed some pretty noisey chain slap that went away after i let the car sit overnight. My tech recommended we change to oil fed tensioners this winter. I'm thinking the timing chain may be a bit stretched too so when we open things up I might want to to further,

The improvemnt I'm considering is a new cam. Light car, 7.3:1 RP and tight gears get this car up and going quick. At max HP of 5800 RPM, I'm only going 118 MPH. I'm not complaing about max speed at all because I do my driving on the street and AutoX. My issue is the power just drops off a cliff at 6k. While I don't necessarily want to increase my max power peak, I'd like to extend the range of power. I have more options for cams than most because I have PMO 40s and am not limited to low overlap. I'm thinking either Camgrinders DC20 or DC21.

My next question is whether I use the heads on the car, or use early style heads. The later 3.0 had smaller heads & valves which makes better torque than the early style large heads which are known to breathe better in higher RPMs. I have read the "dyno run" thread and "3.2 SS rebuild" threads and see the difference in how these make power differently. A 3.2 set of heads are also an option and are available currently, but I'm not sure how they are different.

The car will be used primarily on the street. I've got 175 ft lbs at the rear wheels all the way from 3000 RPM to 5500 RPM now and like the streetability. The car sees occasional autoX use, but No track time... Yet.


Will the 3.2 head be similar to early 3.0 and make more power in high RPM vs. the small heads which scrifice high power to keep low torque strong? Any reliability issues to consider with 3.2s?

signing-up

Hi Joe:

Like most things in life, one makes tradeoffs and these situations are no exceptions,....:)

The early SC heads had larger ports (all used the same valves) than yours and the small-port ones were optimized for more bottom-end and mid-range HP. One can use the early heads for better pulling power above 5500 RPM and in fact, those large-port heads (as well as Carrera ones) will support up to about 330 HP.

You can use a 964 or 20/21 cams for improved mid-range and upper-end power, too.

Since you have carbs, I would install the larger port heads and make sure your intake manifolds match the ports,........:) For what you are doing, I don't think you'll notice any lack of low-RPM torque and it will pull hard to 6500 with the right camshaft selection (and carb configuration).

Hope this helps,

Steve, Thank you for the reply. It helps. I was thinking about using some DC-40's. Right now I have a p&c issue to solve. Was hoping to be able to have my cis pistons pocketed for the task, but like all things "Porsche", it turns out that I need new p&c's. The question now is which ones. Do I stay with 95, or go to 98's. As far as the heads go, it seems that the opportunity costs aren't there. By the time I pay for a set of early heads, and ship them over, it exceeds the cost of having mine opened up.

A friend of mine opened up his own ports with an air powered grinding wheel. It looked crude but was very effective. He now has as much power as any of our 3.0 spec motors.

-Andy

Jon:

The P/C question is one that only you can answer. Just remember that they are NOT all the same and you get what you pay for. :)

I will tell you that simply enlarging the ports doesn't always result in a linear improvement. Size isn't the whole picture; airflow is, and I've seen plenty of beautifully done heads that didn't flow very well on our bench.

Its FAR easier to ruin a set of heads with some well-meaning efforts using a die grinder than make real improvements and thats why I usually recommend buying & using factory SC (early) or Carrera ones.

Unless one has the budget to have them ported AND flowed, its not worth the efforts, IMHO of course.

Getting back to MY original post here, I checked some specs and discovered the Carrera valves are still bigger yet than the early SC heads. What affect does this have on a stock 9.3:1 engine, if anything at all? I'm trying to keep this to a heads only improvement and not get into P&Cs.

I originally thought I'd do a nice 3 angle valve job w/ a little polishing to my later 3.0 heads since hogging them out might not give an improvemnt in flow. But since I've discovered this car has tighter gears along with 50 series tires, I'm not really concerned with much below 3k.

I've also read that the Carrera heads had issues w/ valve guides. What improvements must be considered to decrease wear?

I'm sorry Joe! Can't believe I just jumped in on your post. Very sorry.

i am not sure where you read that, but any spec. book will correctly state both SC and Carrera heads use the 49/41.5mm valves. look up any parts catalog, they will state valves are for '78-89, no change in '84 w/ the 3.2.

There is nothing special about 3.2 heads vs. SC regarding valve guides. They are the same guides, same valves, same springs. If you want to maximize your valve guide wear life, use Manganese Bronze guides. They are harder than the std. bronze, last longer but do not cool quite as well..

Hey Jon, Not to worry. Just didn't want to head off in a P&C conversation.

Scott,

I guess I misread the stats I had. As I reread, 49mm in & 41.5mm ex is correct for SC heads AND Carrera heads. It's the port sizes that differ. My stats show early SCs had 39 mm In & 35mm ex ports. Carreras had 40mm in/38mm ports.

Not sure where I read about the valve guides. Maybe it was the buyers guide. I seem to think it had to do with extra heat and cooling ability of the valves. I'm no engine guru, just an early car go kart junky!:D

Joe:

The final choice of heads will lie with your cams of choice and expected RPM range. :)

I would simply do the regular factory 3-angle valve job, install Phosphorus-Bronze guides and hand-hone them to fit. We've made our own guides from this material since 1978 and its been singularly successful in a wide variety of street and racing engines. The factory guides wear very quickly.