Flowbench numbers for 3.0 heads?

[A Quiet Boom]

Being used to American V8's which always need more cylinder head flow than the stock non ported heads flow, I'm used to dealing with flow numbers both before and after porting. My question is this, approximately what are the flow numbers for a stock 3.0 head (1981)? Is their much room for improvement? (the ports look very smooth and straight for a stock head) And finally who (as in what shop) has a good rep for head porting? I'm piecing together ideas for the winter and might consider porting the heads along with other rpm/HP related mods. My car is very light ('66 912 body with lots of 'glass parts) and formerly had a 2.0E engine so I'm not concerned with losing a little lowend to gain some top end.

[CamB]

No numbers, but Steve Weiner has a discussion on the different heads and importance of porting (and not screwing it up).

http://www.rennsportsystems.com/~porsche/2f.html

http://www.rennsportsystems.com/~porsche/1-c.html

[jluetjen]

Quiet;
I've spent a lot of time researching this subject (abeit in a slightly different vein) and here's the thumbnail of the conclusions that I've come to.

When it comes to heads, the big deal is the intake ports since heads will exhale a lot better then the inhale. Unlike Detroit's heads, Porsche's heads are pretty good out of the box as long as your gas speeds (think: HP/engine size) stay within a fairly wide band. Specifically greater then 70m/s and less then 100m/s at the peak HP engine speed. As far as I can tell, as long as you meet this condition, porting is not going to do much for you.

One of my briefer summaries can be found on this thread. Assuming that you are using "early" SC heads with the bigger ports, a 3.4 engine putting out 240 HP at 6100 RPM would still be pulling only 97.9 m/s and would most likely not nead porting according to my analysis. On the other hand, a "late" SC head with the smaller intake ports on a 3.0 engine generating 210 HP at 6000 RPM would be pulling 109.9 m/s which sounds like it could gain something at the top end from some porting.

Those are my conclusions. I'm sure that lots of people have other opinions.

[A Quiet Boom]

What are you guys thoughts on using polished (not opened, just smoothed) 3.2 heads on a 3.0? Is the combustion chamber volume any different?

[jluetjen]

Well the ports are definitely bigger. I'd venture that they are most likely too big for a 3.0 unless you are going to be spinning it up over 7000 RPM with something like an S cam.

As far as polishing (the ports?)-- why? It looks good but I have yet to see anything that says it flows better. I have seen stuff that suggests if flows worse. Polishing the combustion chamber may help if you are building a 10/10th's motor, but once again I haven't seen any compelling arguments for spending the money except that it looks cool.

Someone else will know better then me if the 3.2 and 3.0 have the same combustion chamber volume.

- John

[A Quiet Boom]

My thought on this of course is that it take a certain amount of airflow to produce a given HP level respecting the fact that if engine size is not increased then the rpms at which said HP are gained will be higher. I in fact would like to increase the rpm and therefore peak HP potential of my motor and I fully understand the implications this can have on drivability and durability. That said however my car previously had a 2.0E with carburators and I find it hard to believe that increasing peak HP even 1000 rpms will yield less lowend than was exhibited by the 2.0 engine. The 3.0 has a very real and noticiable increase in both HP and lowend torque over the 2.0 that I had but niether engine has the brute lowend that I'm used to from performance V8s this is something I actually like about the 911 engine and only wish to enhance. Brute torque is fun but often difficult to control as well as somewhat less refined than smooth high rpm power that I so much enjoy from 911's. Another consideration is my chassis with a total weight in the 2100lb range is somewhat less susceptible to the drivability problems associated with the same modifications on a 2800-2900lb 911SC.

jluetjen made points alluding to the cost to value ratio of polishing the combustion chamber, here I need to make two points. First I absolutely will polish the combustion chambers not only to ensure they are cc'ed equally but also to ensure less chance of detonation (one of the best reasons for polishing BTW) Second I wasn't asking about the combustion chambers (I've made that decision and I'm quite capable of doing the work myself) but instead the ports themselves, I'm sorry I was not more clear on that issue. My question about polishing the ports was geared more at whether or not this would be necessary if I went to the better flowing 3.2 heads. My goal is to shift the motor somewhere in the 7000-7500rpm range for max power and to build the engine towards that goal. Does this make it a "10/10ths" motor? Probably but given a driver who enjoys the revs combined with a light chassis and I think it will yield a fun combination sort of in the spirit of the RS or RSR, high reving NA power in a light nimble chassis.

I questioned flowbench numbers to help me to pinpoint areas that may or may not need improvement to acheive my goal, flowbench numbers are frequently used when buliding race motors an I essentially plan to build just that without getting to crazy with cam timing and compression, perhaps a 9/10ths motor LOL

[jluetjen]

Quiet;
I believe that both the 3.0 SC and the 3.2 heads both have 90cc combustion chamber. But of course this should be checked when you're building your motor since there is always some natural variation in the process. As you mentioned, the 3.2's ports are 40i/38e versus an early 3.0 SC's 39i/35e and a late SC's 34i/35e.

You are correct that HP is a function of air-flow (assuming that the rest of the motor is designed to use the airflow potential of the heads!). As to whether it is worth the trouble of polishing the ports, I'll leave that decision up to you. Here are a couple of examples that you might find interesting.

1) Assume a 3.0 engine with 3.2 heads, an S cam which generates peak HP at 6500 RPM and rev's over 7000 RPM. This engine will be pulling air at about 86m/s when generating an estimated peak HP of 250 HP at 6500 RPM. The maximum torque will be about 200 lb-ft at 5100 RPM. For comparison, a 2.2T pulled 87.9 m/s and a '72 RSR pulled 85.9 m/s.

2) The same engine with a full race cam like a GE80 will generate it's peak HP at almost 7800 RPM and would need to rev to about 8200 RPM on overrun. It would also put out about 227 lb-ft of torque -- but at 6200 RPM. This engine would be pulling 103.2 m/s of air at 7800 RPM which is pretty fast for a 911. But at 7500 RPM it would be pulling about 100 m/s. In order to make it's maximum HP of 305 HP at 7800 RPM it would most likely need the ports enlarged*. If this was not done, the motor would most likely put out a maximum HP of 295 HP at 7500 RPM and it would still be doing something close to that at 7800 RPM. For comparison, a '72E pulled 100.3 m/s and a normal 74 2.7 w/CIS was pulling 105.8 m/s.

3) If you just put a stock 3.6 liter 911 motor in your car, you would have 250 HP at 6100 RPM and 228 lb-ft of torque at 4800 RPM. BTW - This engine pulls about 90 m/s of air at its peak HP RPM of 6100 RPM.

So. You can get 250 HP by spinning a 3.0 to 6800 RPM+ or a 3.6 to 6100 RPM. If you go the high strung 3.0 route, you will need to spend a bunch of money on the crankshaft, rods and rod bolts to keep them together at the higher forces resulting from the higher rev's. In general it will have a shorter life since there is a pretty direct inverse correlation between rev's and engine life expectancy.

If you go the 3.6 route, you will get more torque, and at a more useful engine speed and you will most likely not even need to open the motor.

* Note: When I say the enlarge the ports, I mean not just the area near the manifold face, but also the area near the valve guide. In the early 911 ports that I've looked at, the port diameter increases by about 1mm for every 1/2 inch in from the manifold face. The widest diameter is near the valve guide boss where the air needs to go past the valve and around a corner.

BTW - My comment about a 10/10th's motor refer's to a motor built for maximum HP where the capacity, rev's or intake air are limited -- usually because of technical regulations. Some example are SCCA or IMSA GT cars. In order to be successful, these motors are usually built for maximum rev's (7500+) and maximum CR (12:1 or more). Generally they are built within a mm of distruction to eek out the last 1/2 a HP.

Most engines on this BBS are not built in the high-strung fashion for one of two reasons. 1) Generally the different Porsche clubs' rules are written in such a way that the money spent on this doesn't net much gain. Or 2) The owners are not competing with their cars, or drive in venues where the rules are not limited such as DE. In both cases, the result is engines tend to be built bigger and turn fewer rev's since that is the easier recipe for making HP.

[A Quiet Boom]

jluetjen, I agree whole-heartedly with your points on the cost vs value of a high strung 3.0 to a 3.6 and in fact would recommend that route to most in search of more HP. Along those same lines my mustang drag car has a bored and stroked 351w with a total displacement of 410" being a full race motor I shift it at a rather high 7000rpms for a stroker engine, it's a lot of fun but very hard on the rest of the drivetrain given this engines significant grunt (600+ lbs. ft torque) conversely my friend runs nearly identical times as I with a slightly lighter chassis and a high strung 302 shifted at 9000 Both these engines of course are built for track duty only and I'd prefer to stay in the big inch/lower rpm range for drag racing but on my P-car I have different goals. I don't intend to go drag racing with it and eventually will DE it but my goals are not toward winning races but instead I'm seeking the overall driving experience and feel of a high reving six. Because I'm after this "experience" and not a particular HP level I could spend more than a 3.6 conversion and still be very happy with the result, my previous 2.0E had that "racecar" feel of higher rpm power that I seek to enhance with a high strung 3.0. These mods are aimed squarely at enjoyment and personal preference not cost/benefit or winning races, I fully understand the consequences of a high strung motor and am willing to endure them if I can attain the "feel" I'm after. Obviously my late model 3.0 heads are going to run out of breathing capacity as I inch toward this final goal, the question is by how much and does it make more sense to go with 3.2 heads and possibly port and polish them again putting costs on the back burner and the goal of high rpm power at the front. Thank you for your help so far.

[jluetjen]

Quiet;

I guess you could go for a 3.0 with 3.2 heads and an S cam and not worry about porting them (per my case 1). I'd argue that you are not into porting territory unless you are going to be running race cams per my case 2.

It sounds like it should be a fun engine!

[A Quiet Boom]

Originally posted by jluetjen


It sounds like it should be a fun engine!

I sure hope so Although I'd like to look into a custom cam profile once I have flow numbers in hand as well as intake and exhaust sorted out. My experience with Fords has been that what is a huge cam for a 302 is a mild cam in a 400+ stroker but that doenst take into account that 911 heads have gotten progressively better over the years which probably offsets my experiences in the past. Once I've sorted everything I'm gonna take a long hard look at cam profiles.

[jluetjen]

... I'm gonna take a long hard look at cam profiles.

How are you planning on comparing them? I have yet to see anyone make a good comparison between duration/lift/overlap and the affect it will have on an engine. I'd be curious to see how you plan to do it and what you come up with.

[A Quiet Boom]

Hence my need for flow numbers which in this case will be after I've finished work on the heads. Basically two things need to be determined when selecting a cam, first what is the desired HP level within the flow capabilities of the heads and second what is a maximum rpm range in which you intend to use the engine. After that the cam is tailored to that end. Lift is very much determined by the flow at various lifts the heads produce, it's important to have the valve open as long as possible in the sweet spot of the heads flow range so often times cam lift will exceed the lift at which maximum flow is attained such that you pass maximum flow twice on each valve event. Duration comes into play here as well but is also closely tied to the rpm range. Overlap can be changed via lobe center angle which is the angle between peak lobe lift on intake vs. exhaust. Wider lobe center angles (less overlap) tend to produce broader torque and hp curves with smaller peaks while the inverse is true of tighter LCA. All of this assumes a symetrical cam grind in which the lift and duration numbers are the same for intake and exhaust. In many cases either the heads flow poorly on intake or exhaust or there is some reason for needing more flow from one side. An example of this would be as supercharged engine, the intake charge is assisted by the supercharger but the exhaust is "on it's own" so quite often lift and duration will favor the exhaust side, on engines which have some sort of restriction to the intake the cam will favor the intake side. Ideally the ratio between exhaust flow and intake flow is optimized via porting or better cylinder head design and the cam doesn't need to play the role of a "crutch". I could go on and on for pages describing all this but suffice it to say that my application in which the desired goal is peaky high rpm HP (again just a preferance thing) the cam will feature a tight LCA, longer duration and lift deep into the flow range of the heads. What do I consider to be a "big" cam? well my racecar with it's 410" motor and fully ported heads has a custom Crane grind, without looking up the specifics it's approximately .690" Intake/.705" Exhaust Lift with 300 Intake/305 Exhaust degrees duration at .050" lift and a 108 degree lobe center. The intake ports flow 315cfm and the industry standard .600" lift, it's also ground 5 degrees retarded. This effectively pushes the powerband upwards by about 800 rpm over my previous cam which produced too much low-end torque for the traction ability of my chassis. For the P-car I'm thinking of a duration similar to the S cam but assymetrical favoring the weaker port per final flowbench results, lift will also be dependent on the flow of the ports but limited for valvetrain durability, LCA is up in the air until I decide on a few other issues such as gearing etc.

[A Quiet Boom]

It's important to note here that the engine I intend to build makes absolutely no sense for most people in a street car. The better choice would most likely be a warmed over 3.6 but that's just not my desire here. I actually love a rough idling monster that screams like a banshee from 4000-7000rpms and is generally "bad mannered" LOL. My goals are more about the "racecar experience" than streetable torque and I'm prepared to sacrifice drivability to do it. Generally it's much better to build torque and "talk" HP with regards to street and most racing vehicles but I've been there/done that, now I want to build fun and just enjoy it.

[CamB]

Yeah, I hear that.

I have now have an engine (3.2 with S cams) that is pretty torquey (as in more than a 2.7 CIS in my experience) from 3000- 5000rpm, then comes on cam. And all hell breaks loose.

It kinda seems silly, but is fun... It is a little annoying since it is somewhat fussy below 2000rpm, but I guess that is the price I have to pay.