Snowman -- You and Me -- Mano e' Mano!

[jluetjen]

I've been thinking really hard about your comment that the early T/E heads with 32 mm ports are good enough for tons of HP and the S heads being all marketing for anything less then a race engine. Here's my logic as to why I doubt that you'll be able to get more then about 175 HP from T/E heads, no matter how fast you spin a 911 motor. An also why you do need S heads to get the HP up to around 210.


Let's start with the T/E flow data that you have published previously:

Stock 911T w 46MM intakes 42MM exhaust
intake
lift flow (cfm at 28" )
0.1 " 60
0.2 " 121
0.3 " 174
0.35" 192
0.40" 200
0.45" 200

Note that BK911 flowed a stock T/E head and came in at about 150 CFM rather then your 200, but let's use 200 CFM at .4 inches. (Note that BK911 also flowed at least 8 S heads and they all came in at about 215+/- CFM) Let's convert 200 CFM to 5663 liters per minute.

How long is an E cam open for at it's peak HP engine speed? About 0.000105983 seconds. How much air can flow through a 32 mm port in that time? ~.4 liters. (Note that I've assumed that the flow profile is like a trapazoid with 1/3 of the time at full lift, 1/3 on the ramp up and 1/3 on the ramp down. So the flow over that time would be equal to 2/3 of the flow * time: (1/3T + .5(1/3)T + .5(1/3)T)

What is the capacity of a single 2.4 liter engine's cylinder? .4 liters.

Hmmm....

What happens if you put an S cam into that engine? Well the extra lift of the S cam won't provide any benefit over an E cam since an E cam's lift is .408 and an S's is .459 since a 32 mm port doesn't flow any more air at .459 inches of lift then it does at .408. But it does have more duration and overlap. At the same engine speed an S cam would have the valve open for 0.000114103 seconds. Going through the same math results in .43 liters of air being available for a .4 liter cylinder. So chances are the S cam would not quite be starting to run out of air at 6500 RPM like an E cam would. If you look at 6800 RPM where an S cam seems to peak, once again the ports would allow .41 liters of air into the cylinder which is within 3% of the cylinder capacity. Since the engine will be allowing full cylinder filling at a 5% higher engine speed, I'm guessing that maybe the engine will make 5% more HP or about 8.25 HP for a total of 173 HP. Keep in mind that you will lose some flexility at the bottom end since the torque peak will increase from an E's 6200 RPM to an S's 6500 RPM.

Now I've always maintained that the 2.7RS (which uses an S cam) was also port constrained even though it was using 36 mm ports. Repeating the analysis with a 2.7 engine results in the ports flowing .44 liters at 6800 RPM into a cylinder with a capacity of .45 liters. This might explain why 2.7 RS motors don't make proportionally more HP at then a 2.4. The answer is because at peak HP RPM the cylinders aren't getting a full charge of air, thus the BMEP (aka: torque/liter) at that engine speed is less then a 2.4S's. Incidentally at an RS's peak HP speed of 6300 RPM, the ports will flow .48 liters of air into the .45 liter cylinders.

[Jim Richards]

Interesting analysis, John. So, would a 2.7RS benefit from slghtly larger ports (e.g., 37 - 38 mm)? And what, if anything, would be sacrificed? Thanks.

[Wayne 962]

Just another thought here - you need to make sure that everyone is talking about the same thing. There many ways of measuring horsepower. Peak horsepower is typically at higher RPM, and really doesn't often mean much to the average street user. Low-end torque (available power) throughout the driving range is what often matters to most people.

For example, the 911 2.7S engine is rated at 175 HP, and the 3.0 is rated at 180 HP (peak). Yet these two engines feel very different when driving. Looking at the straight peak HP numbers, you would say there was only a 3% or so difference in them...

My point? Arguments about flow rates of heads are really only applicable to high-RPM ranges where you're talking about peak HP - not really applicable to everyday street driving.

-Wayne

[jluetjen]

Arguments about flow rates of heads are really only applicable to high-RPM ranges where you're talking about peak HP - not really applicable to everyday street driving.

I agree Wayne. Note my comment about putting an S cam into a 2.4E. I'm really just trying to understand the limit(s) of some specific cases. In this case the peak HP available through certain port sizes.

To make two extreme cases, first you could hypothetically attach heads with 32 mm ports up to a 3.6 liter engine and an E cam. This engine would generate it's full torque of 230 lb-ft. But it's peak HP of 176 HP would be at only about 4500 RPM. Above that speed and the cylinder wouldn't have enough time to fill. (Note that I came up with those numbers based on my previous observations that BMEP's at peak HP and peak Torque are very consistant for a given cam.) Anyhow, since most people drive below 4500 RPM 95%+ of the time, this would be an awsome motor with mountains of torque for a real kick-in the pants. But it would also be like driving a 911 with diesel truck motor since it will pull hard to 4500 RPM and then drop off fast.

The other extreme case would be the 2.0 rally motor which used an S cam and 32 mm ports. At the S's peak HP speed of 6800 RPM, this engine's cylinder(s) need .33 liters of air while the 32 mm ports can deliver .41 liters of air. So even with 32 mm ports, the cylinders are getting enough air. The engine can't take advantage of the extra lift of the S cams, but the duration and overlap will still be of benefit. The result is an engine that generates almost as much HP as a 2.0 S's 170. In fact, with carbs these engines made 150 and with the extra 10 HP (rule of thumb) for MFI, you could expect close to 160. It might even pull a little better then an S at lower rev's given the extra intake velocity, but I haven't modeled that yet. Even with a 906 cam, the 2.0 engine with 32 mm ports should pull to almost 8000 RPM. You won't be making 906/911R HP though since you won't be able to take advantage of the extra lift. Where's that leave you? Maybe at around 190+/- HP. The conclusion is that smaller engines can take better advantage of a head's flow capacity since they are taking smaller bites of air. But even then, it's a case of deminishing returns since the engines can't make use of the additional lift available from the more radical cams.

[Scott Clarke]

John-
That's great information. What is the math like if you assume that bk911's numbers are right?

[jluetjen]

In the case of the 2.4E, the ports will be flowing the cylinder capacity at 5500 RPM.

[Wayne 962]

I don't consider myself an expert on 911 head flow rates. However, I will make the following observations. All of the Porsche factory racing engines had large ports on them (906, 911R, etc.). I would think that the factory knew what it was doing when it designed those into the heads. Afterall, they won quite often back then.

-Wayne

[neilca]

John,

That is a very interesting analysis. Let me include one more variable to your work. That is, the boundry layer of the flow increases as the speed increases. So as the speed of the air increases the port actually starts to choke itself off. Thus, the need for the larger port if you plan on high rpm operation. On the Sport 2000's I used to run we were restricted on the port size by the rules. The Weber 32/36 carb had a fuel tube from the float bowl to the throat of the carb. At high speeds fuel would be siphoned from the bowl into the throat to increase the intake mass, this I was told would decrease the boundry layer and allow more air to flow. At first I didn't believe this ,but then I thought "what good is more fuel without more air?" I disconnected this tube once and could really feel the difference. By the way this mod is illegal for IT classes, we checked!I have seen this mod done to IDF carburators as well.

Just some food for thought,
neilca

[snowman]

Hmmmmn,
You say "If you look at 6800 RPM where an S cam seems to peak, once again the ports would allow .41 liters of air into the cylinder which is within 3% of the cylinder capacity. Since the engine will be allowing full cylinder filling at a 5% higher engine speed, I'm guessing that maybe the engine will make 5% more HP or about 8.25 HP for a total of 173 HP. "

I will agree with the crude analysis you presented, but then ask the question ONLY 8 more HP AND you have to spin it higher than 6500 RPM to do it!!! ALL that Porting, All that money for WHAT?!!! 8 HP and a loss of torque at more driveable RPMs. Tell me again thats not marketing for a street engine.

Using a more accurate method, eg engine dyno 2000, the actual cam profiles, the actual measured air flows, the difference becomes nothing.

You did state an interesting bit of info. That someone measured the max airflow of the T heads at 150 cfm instead of 200. If this is true then there would indeed be a very big difference in power. From what I have seen of Porsche heads, the ones that measured 150 CFM must have come from a Chevy. Ie a lot of flashing left in the ports. It could be true, but I doubt it is true in general for a Porsche.

[jluetjen]

You say "If you look at 6800 RPM where an S cam seems to peak, once again the ports would allow .41 liters of air into the cylinder which is within 3% of the cylinder capacity. Since the engine will be allowing full cylinder filling at a 5% higher engine speed, I'm guessing that maybe the engine will make 5% more HP or about 8.25 HP for a total of 173 HP. "

but then ask the question ONLY 8 more HP AND you have to spin it higher than 6500 RPM to do it!!! ALL that Porting, All that money for WHAT?!!! 8 HP and a loss of torque at more driveable RPMs. Tell me again thats not marketing for a street engine.

Snowman, you mis-read me. The case you quoted is the example of an early 2.0 Rally motor that used 32 mm ports. There was no porting done. This was an example of the diminishing returns (even on a smaller engine) of using a longer duration camshaft with 32 mm ports. Yes you will get more HP, but it's not like there are tons of HP unused in the heads. On a good day, you'll be stretching pretty hard to get 180 HP using 32 mm ports.

I appreciate that you think the method is pretty rough cut, but how does the software that you mentioned describe that you can get there? What sort of engine configuration will allow "Big HP" (200 HP+) in a 6 cylinder, 2 valve per cylinder engine using 32 mm ports flowing 200 CFM?

[BK911]

Just as a side note:

When I flow heads I don't use a bellmouth on the intake. Mainly for consistancy. I have used one to see the difference and the results averaged about 5% higher. That still doesn't get me near the 200 cfm range on a T head.

Anybody else have flow data on any stock 911 heads? I'd like to compare and see how close my numbers are. The flowbench I use does get calibrated annually so I am fairly confident it is at least close. Maybe operator error? Wouldn't be the first time!

BK

Edit:
I checked my notes. On an S head I was getting almost 10% more airflow with a bellmouth.

[350HP930]

I have picked the SC330 cams for my turbo project.

I will let you know how it turns out in a few months once its all together.

[snowman]

Originally posted by BK911
Just as a side note:

When I flow heads I don't use a bellmouth on the intake. Mainly for consistancy. I have used one to see the difference and the results averaged about 5% higher. That still doesn't get me near the 200 cfm range on a T head.

Anybody else have flow data on any stock 911 heads? I'd like to compare and see how close my numbers are. The flowbench I use does get calibrated annually so I am fairly confident it is at least close. Maybe operator error? Wouldn't be the first time!

BK

Edit:
I checked my notes. On an S head I was getting almost 10% more airflow with a bellmouth.

The ports I flowed were 46 mm T ports see first post in this thread.

The flow bench, a Superflow 600, also calibrated yearly. Also other heads have been flowed, ones with well known flow rates, such as chevy, ford, with results consistant with commonly accepted numbers, eg typical 350 head about 160 cfm unported, 200 to 225 cfm ported (depending on head and valves) AFR heads compare within a couple of percent of advertized figures. And yes the spark plug was in the hole and the leakage measured almost zero with valves closed. Clay was molded to a radius of 3/4 inch and fitted to the ports to smooth airflow. One last final note. I flowed my 912 Race car heads and the actual power to the rear wheels, measured with a dyno at sea level, temp 70 degrees F, Relative humidity 50%, 140HP at 7800 RPM, for 1725cc, 12:1 compression, cam 285 degrees at 0.050" lift, is right on to what Dyno 2000 using the measured air flows of the heads and the measured cam profile predicts.

[jluetjen]

Snowman, While working out this morning I think I came up with a "real world" confirmation of your maximum of "lots more then 170 HP" claim for 32 mm ports.

The example would be the FIA restricter sizes. In 2003, NAV8 engines in the FIA series were limited to 2x33.1 mm intake restricters and made "around 600" HP. This is "around" 300 HP per restricter. So yes, it is possible to get 300 HP from a 33.1mm intake. The difference is that between the FIA restricters and the cylinders is a plenum which smooths out the pulses from the individual cylinders. Also when the engine is creating "around 600 HP", the air through the restricter is constantly at the sonic limit.

In the case of a 911, there is one cylinder per 32 mm port with no plenum between the two. As a result the 32 intake port is seeing very strong pulses from the cylinder and it is the air flow during the pulse which is limiting HP rather then the average flow. An interesting idea that I've seen investigated on motorcyle engines is to create separate plenums of a specific volume for each cylinder. This would reduce the affect of the pulse through the restricted port and allow some additional HP.

[snowman]

It sort of boils down to the following:

1. If you keep the displacement small, the RPM goes out of sight to get HP. You just can't maintain any driveability with large ports on a small displacement engine.

2. You increase displacement, so you can have much larger ports and a lot of HP at low RPM. Then you have a Chevy, not a Porsche. There is a company that sells Chevy conversions for Porsches. One more reason CA is on the decline.

[A Quiet Boom]

Just a little info for you guys to think about. This relates to Ford V8's specifically but can be of some use for the 911. Also I forget all the math involved in determining neccessary airflow for a given HP so I won't get into that.

Typically street/strip and pure strip heads are compared at .600" lift even though most street engines don't open the valves that far. 250-300cfm at 600 is considered good for street/strip and over 300 is a must for a strip only motor (small blocks 300-400 cubic inches). My drag motor produced in excess of 600HP with 315cfm at .600. Cam lift was .690 intake/.710 exhaust with duration of approx.301/309 at .050" lift. Engine size was 410CI with 4.040" bore and 4.00" stroke. That's close to 1.5HP per CI.

What makes all this relevant to 911 engines is that most of them produce roughly 1HP/CI which, by most standards, makes them a high performance engine. Just popping a 3.0SC up to 225HP put's it in race engine territory meaning that street manners diminish. As Wayne stated earlier torque is usually better for street driving with one exception. If you have enough torque already then losing a little won't be as noticeable, this can be accomplished with a longer stroke (stroker motor) or forced induction. A good street head will have ample flow or area under the curve in the range BELOW peak cam lift because the engine only sees peak lift briefly and most of the cylinder filling occurs at lower lifts. A good head porter can improve low end airflow without going overboard add to that port matching the intake and exhaust and significant, streetable gains can be made. I personally have seen noticable (improvements in ET) with mild port work and matching even on aftermarket aluminum V8 heads with engines turing less than 6000rpm.

Snowman's last reply is the real truth (kinda sad for us 911 guys) high HP in a 911 means high rpms, remember these are already performance engines and already peaky compared to a V8, the only serious streetable gains come from a swap to a larger motor.

While I know that everyone's definition of streetable may be different take this into consideration. My drag car was a total blast at the strip yet a total pain on the street, anything below 3000rpm and the motor would buck and shake the whole car, above 3000 and with a little throttle and I couldn't keep the tires planted. Believe me driving around town in second gear above 3000 in no fun with a V8 and not a lot of fun in a 911 either (a big reason why I upgraded from a 2.0E with carbs to a 3.0SC engine) so unless it's a weekend warrior save the race heads for racecars and spend your money making the car lighter or the engine bigger.

[snowman]

A Quiet Boom has made a serious understatement when he said any street engine can flow as much as 300 cfm per cylinder. Stock V8 heads can flow as little as 160 cfm per cylinder, and as much as 225 cfm with light porting. Heavy duty porting and or race heads start in the 260cfm up catagory. and 300CFm is a race engine.

Of course real drag race engines flow as much as 600 CFM per cylinder, naturally aspirtated!!! Of course drag racers can't be satisfied with that so they supercharge it as well!!! At over 10,000 HP they stop counting.

Funny thing is that a stock chevy with mild porting, the same duration cams as a Porsche, headers and good carb (and thats a 307 chevy, not a big block) makes over 400HP. But of course they break the first time one steps on it.

[A Quiet Boom]

My point about 300cfm on a race engine is geared towards V8's not 911 engines. Please note I put 300-400CI in parenthesis. A small block V8 has ample torque and can be driven with a mild cam and 300cfm ports. I seriously doubt the same of a 911 engine because torque is already at a premium. I'd bet 225cfm would be about max for a drivable 911 but given that 200 or less will get the job done there is little point. I do recommend port matching and a mild cleanup to get the airflow below peak lift a little higher, this allows less cam with the same cylinder filling or more cylinder filling without too much loss to the bottom end.

[snowman]

Nice thing about 911 ports, they need almost no port matching, almost no cleanup, in fact a stock 911 port would be considered "ported" if it were on a chevy or ford. The only significant thing I have found is a slight, and by chevy standards, I mean very slight, bump on the short side radius, the same short coming a chevy has but on one tenth the scale. Additinal porting on a Porsche is really port reshaping, a risky business. Most Porsches only benefit if ported to run at 7500 plus RPM, which basically means larger ports, poor low end performance, and peaky high end performance.

Don't get me wrong, even that "little" bump on the short end radius can yield another 5 to 10 percent performance on a Porsche (On a chevy or ford it could yield as much as a 40 or 50 percent increase, no sweat). But don't expect that chevy bang for the buck on a Porsche.

[jluetjen]

Snowman, you haven't responded to my original observation. This discussion of SBC's and V8's isn't really germain (as opposed to German ) to the discussion since the drag motors that you are talking about are completely different animals compared to a 911 motor.

Drag Motor............911 Motor
.82 liters/cylinder.... .45 liters/cylinder
Rectangular ports.... circular port cross section
"Bathtub" chambers... Hemi head (OK, Hemi's have Hemi heads, but SBC's and Fords don't)
Blown ..................... I'm discussing NA 911's, not Turbos

Aside from both examples being internal combustion engines, they have very little in common and I don't see what it's adding to the discussion.

Keeping to the subject at hand (NA 911 and 912 motors), do you see errors in my logic or the way that I'm looking at the problem? I'm trying to understand how to get over 200 HP from a 911 motor using 32 mm intake ports since I'm sure that there are a lot of people on this board who would like to know. Conversely, "if you cahn't get theah from heah" (as we say up here in New England), that's a good thing to know also.

You have steadfastly maintained that you can get there (over 200 HP in a NA 6 cylinder engine with 32 mm ports) and I don't see a path to do so. Show me the way! What cam would you recommend? What cylinder size? Which head configuration (2.0 or 2.2/2.4?)? What CR?