Catfood

Ok so after driving my 944S for awhile, and absolutely loving it! I need to better understand the car. I have rebuilt the head of the car and worked on the 4 valves per cylinder, the cams and the cam tensioner and I understand that difference. But what makes the car act like VTEC. As it revs up there is a power surge at 4K rpm and then even more at 5K and it continues to increase all the way to 6800. What is the cause of this, its not a variable cam, is it increased fuel mapping or how did porsche make this work?

thanks in advance!

Dan Shea

82-T/A

Correct me if I'm wrong, but I'm pretty sure that the 944S with the 2.7L does NOT have any kind of variable valve mechanism or controller. Although, you said "like" a VTEC, so maybe you just meant that it's similar to it.

When companies build motors, they typically have to build the motor in such a way that it produces the most power it can (while meeting certain restrictions) within an ideal power range for what the vehicle is going to be used for. An exmaple is... for a pick up truck, they ideally want loads of low-end torque, while in a light weight sports car, you typically want more top-end power. More specifically, an older Cadillac will want more low end and mid range power to get it moving (the basic setup for most all mid 70s to early 80s American cars). Smaller cars don't need as much low-end power, so the motors are built in such a way as to provide power in the mid and upper rpms where it can make better use of the power.

The problem is that... you CANNOT make your motor run at it's maximum efficiency throughout it's entire RPM range. You can have more low-end power at the sacrifice of upper end power, and more upper end power at the expense of low-end power (or somewhere in the middle). That's no longer an issue though... what Variable Valve Timing is, is basically "Cam Phasing". The cam has different "profiles". One profile is optimized for the lower rpms, and the other is optimized for the upper rpms. Now, when you say that the 2.7 "feels" like it's got VVT (or like Honda's VTEC), what you're referring to is the increase in power in the mid rpms. In a VVT motor, that's when the cam changes it's phasing to put it back in it's sweet spot. So it feels slimilar.

The difference is though, that in a VVT motor, you would have just as much engine efficiency in the middle of the lower rpms as you would in the middle of the upper rpms (if it was a decent VVT motor).

The 2.7L DOHC motor is really quite larger than most Honda motors... (I refer to that since you're talking specifically about VTEC). So, by it's sheer size, it will have more power in the lower rpms than a typical 1.6-2.2 liter Honda motor. DOHC motors are typically built to produce most of their power in the mid rpms all the way to redline. For example, the 2.4 TwinCam I had in my 4 cyl Grand Am back in 1997 used to run a flat 16 in the quarter. I could pull away from much bigger cars when pulling out of a turn because I'd just keep it at the top of the gear in it's sweet spot and I'd nail it and pull all the way from 3,500 rpms to redline. Those other cars were much quicker than mine, but I never had a problem pulling away from them out of a turn because they were at the bottom of 2nd gear, while I was in the middle of 1st.

However, the Pontiac Solstice that I have pulls hard from 1,500 rpms all the way to 3,000 rpms, and then from 3,500 rpms all the way to 7,200 rpms. The Solstice has VVT. I say it pulls hard, but I'm talking low 15s, not like 455 cubic inch Olds V8 pull... heheheh.

I don't own a DOHC Porsche, but I would speculate that they have built their motors like the rest, and that is to really take advantage of the engine from about 3,200 rpms to redline. It should really start to pull hard above 3,000 rpms all the way to the top. I've rambled a bit, but I hope it's helped explain how VVT and normal motors are built.


Todd,
2006 Pontiac Solstice
2004 VW Beetle Convertible (Wife's)
2002 Ford Crown Victoria LX-P74
1987 Pontiac Fiero SE / V6
1984 Porsche 944
1981 Pontiac TransAm WS6
1974 Volkswagen 412 Type-4 Estate Wagon
1973 Volkswagen Type-2 Transporter

redmund

Isn't the S 2.5L?

jgporsche

The problem is that you don't have a 944SMAX chip.... you would have LOTS more low rpm HP/TQ. What you are feeling is the nature of the beast.....that's how the 16V cars act... But you don't know what you're talking about until you drive a 968, then you won't think your S feels like a vtec car.



The 944S has a 2.5L engine.

Catfood

You kind of missed what I was talking about, even though I appreciate your guys' response. I understand that my car does not have VTEC, and I also understand the principles of VTEC, however what Im wondering is how does the car make more power at high rpm. What causes it to suddenly pick up the pace

jgporsche

------^

V2Rocket

You SO did not just compare a P-car to a Honda.

82-T/A

Well, I kind of did. Your motor is designed specifically to produce power in the mid to upper rpms. The reason it's as quick as it is in the lower rpms is because of it's displacement.

The fact that it has 16 valves allows the engine to breath a lot better in the upper rpms than it normally would if it was just an 8V NA.



Todd,
2006 Pontiac Solstice
2004 VW Beetle Convertible (Wife's)
2002 Ford Crown Victoria LX-P74
1987 Pontiac Fiero SE / V6
1984 Porsche 944
1981 Pontiac TransAm WS6
1974 Volkswagen 412 Type-4 Estate Wagon
1973 Volkswagen Type-2 Transporter

V2Rocket

BLASPHEMY.

No engine is superior to the immense amounts of bottomless power and torque produced by the supreme 8v NA motor. If internal combustion were a religion, this finest piece of machinery would be its temple.

82-T/A

Hahah... well, I have an 8V too, but truth be told, it is possible to make just as much horsepower with an 8V as a 16 valve. The ONLY thing a 16 valve has over 8 valve is more cyl head flow, and that's because it has more open valve area. The only reason why they go to 16 valves is simply because it's easier (more room) to stick two equally sized valves right next to eachother (on each side). It's not the number of valves that makes the engine breath better, but the open area of the valve. So, you could in effect install larger valves in an 8V that would allow it to breath JUST as easily as a 16 valve, and it would probably have more power too since there would be less rotating mass and friction with one less cam.

The ONLY problem is that I see though, is a limitation on size. It's a LOT harder to stick two huge aposing valves apart from eachother because you run out of surface area. This is why probably in this particular case, they needed two intake and two exhaust, to better make use of the available space. In theory it would work though.

Like for example, on a hemi-spherical cyl head... you could put much larger valves.


I have a 1969 455 cubic inch Oldsmobile Rocket-V8 engine that I pulled several years ago. It's the same block and heads as a 68 Hurst / Olds, but the cyl head received smaller valves because it was used in a full size vehicle. That said, the port sizes and everything are the same. I had the machine shop put larger valves in it than what even the Hurst Olds put out. It's going to be sick... (to the tune of 700 foot pounds of torque).


Realistically though... if the 2.5 Porsche 944 motor was AS popular as say, the small block chevy... you'd be able to buy replacement high-flow cyl heads with larger ports and better camshafts. The 2.5 has the displacement to at least get the car rolling, but the motor is built on the bottom end that it COULD (and should) support lots of top-end power. It's the cyl head that chokes it.



Todd,
2006 Pontiac Solstice
2004 VW Beetle Convertible (Wife's)
2002 Ford Crown Victoria LX-P74
1987 Pontiac Fiero SE / V6
1984 Porsche 944
1981 Pontiac TransAm WS6
1974 Volkswagen 412 Type-4 Estate Wagon
1973 Volkswagen Type-2 Transporter

AznDrgn

Combination of fuel mapping and air flow characteristics in the intake and head that make it surge in the higher RPM's. I highly doubt it was completely intentional but it is what it is.

Jfrahm

Tuned intake/airbox, header, etc. Porsche put a lot of effort into intake tuning.

AznDrgn

Well in that case it sucks that they didn't bother doing more tuning in the lower end.

yellowline

You're not racing if you're running at 2500 RPM.