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Cool port verlosity

HI,

I am converting a 3.3 to 3.5 with 7.5 to 1. - I am also increasing the intake and exhaust ports to 38MM. They will be flowed etc.

Since this this a turbo I have been advised that If I Am concerned with performance before boost port size should remain small to maintain port velocity.

I need some help in understanding this concept of port velocity when used with a turbo engine or any engine. Can some one please explain the fundamentals to me.

I am just not familiar with the basics re port velocity?

Thanks,

Don
Old 10-29-2005, 06:46 PM
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In 30 words or less, having too big of an intake system for a small engine (which yours technically is in terms of effective displacement and fuel/air density, when there is no boost) will not be efficient at the lower RPMs.

-Wayne
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Old 10-29-2005, 07:34 PM
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Think about in terms of a straw. It's much easier to take a sip of a drink with a small diameter straw than with a big straw. Idle and low rpm's are sips.

When you want to take a big gulp you need a big straw but it's hard to take a sip from a wide diameter big straw.

The beauty of a turbo is you can take sips with small ports while your at idle or low rpm's but when you need a big gulp the turbos kick in and can give you more than the motor can even handle with those small ports.

Keep the ports small.
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Old 10-29-2005, 08:54 PM
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Don888

Technically speaking, it is called Bernoulli’s Equation, which says that an area (A) multiplied by the velocity (V) in one size pipe is equal to the A x V in another size pipe, if a constant pressure is applied to both. For a given engine displacement (3.5 L in your engine) each cylinder will intake a given amount of air (constant pressure). Looking at the intake ports, you are increasing the area from 32 mm diameter to 38 mm diameter. Let’s say that with the original 32 mm intake the air flows at a rate of 30 meters per second at 2000 RPM. So you can multiply the area (32/2 squared times PI = 0.08 square meters) times the velocity which we assumed to be 30 m/sec at 2000 RPM, we get 2.4 cubic meters per second of airflow. Now if you increase the size of the intake port to 38 mm, Bernoulli says that A32xV32 = A38xV38. In our example A38xV38 must equal the 2.4 cubic meters per second of airflow. The area of the 38mm port is 0.1134 sq m, so the airflow velocity in the bigger port must be 21 meters per second. So, at 2000 RPM, your intake airflow speed has dropped from 30 m/sec in the original 32mm port, to 21 m/sec in the 38 mm port, or your bigger ports flow speed is only 71 % of the smaller ports.

Not so technically speaking, if you turned on the garden hose, without a spray nozzle, the water would just flow out the end of the hose. Now if you put your thumb over the end, the water would spray out at a much greater speed, because the same amount of water is trying to get out of a much smaller opening, so it must go through the smaller opening faster.

This is probably way too complicated for a Sunday morning, but I hope it helps.

Rex
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Last edited by Rex Walter; 10-30-2005 at 07:30 AM..
Old 10-30-2005, 06:50 AM
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All good information.
I am not an engineer just a Porsche engine builder and I report what I have learned first hand.
On the dyno a stock 3.3 turbo makes 60 + hp before boost (3300 rpm) a stock 78-79 SC makes close to twice that. 32 mm port vs 39 mm port.
What I would add is that at 7.5 to 1 with 911SC cams a stock 930 turbo will function pretty much like a 78-79 SC. The only thing missing is larger ports. We have built many (not one or two) but many 930s as I describe and they work well with 38+ mm ports. We build them with 38+ mm intake ports and port the entire intake to 37 mm. The 1+ mm difference has an anti reversion effect that helps port velocity and they work.
Theory is cool but in practice the 38 mm port works and works well.

On a side note: We port the intake manifold to 37 mm because it works but it is important to note that 37 mm is also the maximum you can port a stock injector block without compromising it's structural integrity.

3.0 non intercooled, twin plug turbo.
Supertec twin plug distributor w/ BurnBros Marelli conversion
Taylor wires custom made
935 pistons
911 SC cams
SUPERTEC head studs
38 mm ports In and Ex
K27
B&B headers
1 MSD 6Al triggering two Blaster II coils.

On the test drive we made .1 bar of boost @ 1800 RPM climbing my hill.
Full boost of 1.0 bar @ 3400.
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Last edited by Henry Schmidt; 10-30-2005 at 07:58 AM..
Old 10-30-2005, 07:40 AM
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Ask this question and get a thousand different answers. There are several different areas of a cylinder head to look at other than velocity. I would recommend Bill at Xtreme cylinder heads in Florida. He does the heads for the fastest Porsches in the country. From 200hp Porsches all the way to the latest being in the 1000rwhp range from a 3.6. My advice is to send them to a shop that knows what they are doing bottom line. They will discuss with you what you are trying to accomplish. Your cam specs, turbo sizing, intake design etc will play major roles in the port design.

Eric Hood
Old 10-30-2005, 04:43 PM
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I always enjoy the technical side to these discussions and have a car that confirmed every bit of the previous posts. My 930 intakes are out to 3.2L specs and with super C2 cams, a stock Honda will beat my car in the first 3500 rpm. Since I don't drag race the car, this never happens. Even on the track, I desperately try to keep the the tach above 3500 rpm.

However, anything above 3500 rpm, not much can keep up and those big ports and that big turbo stretch their legs - right to red line.

My point is, if you want a drag car or something with good torque, the 930 is not it.

When I instructed at Barber (Birmingham AL), I was continually spanked by 2200lb 911's - great technical course. I only used 2nd and 3rd gear (ah, wish I had a G50/50). But at Road Atlanta it was a very different story.
Old 10-30-2005, 05:01 PM
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Don put a 2 step on that car and you will be an animal down low and even meaner from a roll

Eric Hood
Old 10-30-2005, 05:04 PM
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Yep...
I wonder how much of a bigger pig this car would be down low if I didn't bump the compression to 8, stroke it with a 964 crank and bore it out a bit....

It's only money....
Old 10-30-2005, 05:36 PM
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I would seriously wire in a 2 step to your car. It is a cheap deal to run. Put a switch under your clutch pedal. That way you can build boost from a dead stop or even better from a roll to make sure you leave on boost...this is the secret to eating the bikes I am not sure if the tec3r has a built in 2 step or anti lag feature? I know my gen 7 system does. Compression and cubes really do help in spool up time

Eric
Old 10-30-2005, 05:40 PM
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Guys,
1) This is the information that I exactly needed. Henry, I got the idea of going with 38MM rather than 40MM when I discussed it with you earlier this year. I am glad to see that you set up works so well. I am running a really high gear in first (930 4 sp) It winds up too fast! I suck up rpms.

Eric Hood,
2)I am not familiar wit the 2 step. Can you expand on how this works and where I can get the set up or what type of parts I need. Are you recommending I put in an electronically activated second low end profile turbo on my engine remote controled.

3)Does anyone make a Magneto setup for a Porsche. Would there be a larger benefit in using one or two magnetos rather then using duel plugs of some of the other ignition alternatives for a CIS System.

Don
Old 10-30-2005, 08:36 PM
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Rex,
Great techincal reply. In regard to Bernoulli’s Equation what if you factor in the the higer lift C2 cams and the increase compression from 7:1 to 7.6:1 into the velocity equation at low RPM plus the larger displacement which you already factored in.

Don
Old 10-30-2005, 08:53 PM
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No there will be no extra turbos 2 step rev limiter work like this.... you set it lower then the normal rev limiter so you can hold it to the floor and motor will only rev to a set point. Say 3500 rpm then the 2nd rev limiter is the redline revlimiter(5500). when you take off the first rev limiter shuts off. leaving just the redline rev limiter. On a turbo car it will help build boost on the line so you can come off the line with boost. You can vary the amount of boost you launch on by the selected rpm you choose. What it is doing is retarding the timing or killing spark depending on the system you choose. The unsed fuel explodes in the downpipe causing the turbo to spool up. If you have ever seen a pro style turbo car launch you will notice how they pop and make all kinds of racket on the line...this is why. In the early days of turbos the secret to beating a turbo car with a stick was to stage before them so they could not get up on the 2 step in time. This would cause them to launch without boost among other things

Eric Hood
Old 10-30-2005, 08:55 PM
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This is why you NEVER want to race a turbo car with a 2 step from a roll! Advances like these can give you the best of both worlds depending on the type of racing you choose. I street race and drag race so it would be more of an advantage in this arena than the typical Porsche autocrosser would find.
Old 10-30-2005, 08:59 PM
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Eric,
Thans for the information. Does your shop set up a 2 step for a Turbo like this. I realize that you are in Oklahoma. But can you provice all of the parts ands and the instructions or do you need the car?

don
Old 10-31-2005, 11:24 PM
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Quote:
Originally posted by Rex Walter
Don888

Technically speaking, ...........................So, at 2000 RPM, your intake airflow speed has dropped from 30 m/sec in the original 32mm port, to 21 m/sec in the 38 mm port, or your bigger ports flow speed is only 71 % of the smaller ports.
.....................Not so technically speaking, if you turned on the garden hose, without a spray nozzle, the water would just flow out the end of the hose. .........................
Rex
I was just thinking, four cycle engine pulse!
All of this ignores the fact that intake ports do not flow in one direction. Intake ports have pulses and these pulses and the control of these pulses have as much to do with cylinder filling ( the goal ) as port size. At low rpm, cam overlap has more to do with intake pulses than port size. In short control the pulses and you create cylinder filling. Port size and head flow are only a small part of this important equation.

Garden hose, I never thought of an intake port as a garden hose.

One last thought: If you want a dragster buy a Hemi !!
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Last edited by Henry Schmidt; 11-01-2005 at 04:58 PM..
Old 11-01-2005, 04:54 PM
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I should have added something about volumetric efficiency to my previous post. My example assumes the cylinder is 100% effective at filling completely when the intake valve opens. This is obviously not the case in the real world, because if it were, a turbo would not increase HP. When Henry says that 32 mm ports work as well as 38 mm ports simply implies that the engine is pretty efficient to begin with, and the cylinders are filling just as well with the slower intake airflow speed as the faster one – other factors (as Henry points out) also come into play like reversion, intake valve speed vs piston speed, overlap, etc…now you’re getting out of my comfort zone. Theory only goes so far, there’s no substitute for experience.

That said, part of the fun of this forum is bench racing, and putting together combinations that may or may not have been tried before – so don’t get me wrong, I enjoy reading all of it. But I remember in High School when we would just slap the latest and best advertised parts on our Mustangs and Camaros. Unfortunately, they never seemed to make the cars go any faster. I have learned that the mismatching of major components can prevent an engine from making the most power. Find someone you trust, and stick with their recommendations.

Rex
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Old 11-02-2005, 05:21 AM
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LOL, I've been reading Henry's posts for a while and I am totally convinced that he really knows his stuff. But..... I just looked at those pictures he posted and realized I've been to his place, twice! DOH.

Talk about a place you could spend countless hours, he has Porsche stuff everywhere! Not a bad view of the coast either.
Old 11-02-2005, 08:05 AM
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Turbo charging is about putting ten pounds of st*t in a five pound bag.
100% volumetric generally refers to complete cylinder filling at 1 atmosphere.
With a turbo 100% volumetric efficiency is not the goal. 130 % and much more can be achieved.

This is how you explain 500 Cubic inch engines making 6000 hp.

BTW: we are no longer in Orange County. The shop is now located on a hill top in Fallbrook. It's not an ocean veiw but it's still nice. Theo is not a pelican but he want's to be.

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Last edited by Henry Schmidt; 11-02-2005 at 09:29 AM..
Old 11-02-2005, 09:17 AM
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Henry nailed it. Another point is that VE is directly related to the torque curve. With most turbocharged engines the torque doesn't fall off as soon or as dramatic as it does on a N/A engine.
With a N/A engine the only thing pushing the air into the cylinders is atmospheric pressure (14.7 psig) and to a much lesser extent momentum. When the engine runs out of breath the torque is falling off because the VE is falling off. Basically the engine can't pull the air in fast enough to fill the cylinder at high rpm.

With a turbo you are increasing the pressure that is pushing the air in, and the air is also more dense. that means that restrictions and design deficiencies in the intake system have a less dramatic effect. Since you are pushing the air in harder you can fill the cylinders faster and at higher rpm. That is useable horsepower in a nutshell.
Not only do you acheive a higher VE with a turbo, you can make it last much longer.

Peak HP and torque are basically meaningless, what matters is getting the torque to come in early and stay flat all the way up. Make the torque start early and last to redline and you will have a monster.
Old 11-02-2005, 09:47 AM
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