924Board.org

[larso]

Who has tried a cam in a 931? I know you'll get some lag but I am hoping someone has done a grind specifically for a turbo, high lift, not much duration? vice versa? what works good?

[924RACR]

Cams for turbos are not like cams for high-performance NA motors; a race-grind NA cam in a turbo motor will suck wind.

Crank the boost, and spend your money on plumbing an intercooler or water injection or engine management or whatever tickles your fancy.

The reason the cam grinds don't work the same way is because your head is pressurized; you're not dependant on the head flow characteristics to make power. The only limit is detonation! That's why turbos rule!

Better yet, spend your first $40 on getting the book "Maximum Boost" (see the Tech Section for the ISBN reference #).

_________________
Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype

[larso]

Vaughan, that's why I ask "why did the 931 have such a good flowing head".

[larso]

In other words, why did porsche put bigger valves in, and make the head fow so much better, doesn't that belong on the 924 NA?

[Rick MacLaren]

Vaughan, I'm not sure what you're saying. Are you saying a Stage 2 cam grind, for example, won't increase horsepower? Or are you saying the money is better spent elsewhere? Or both?

[924RACR]

Sure, the better head belongs on the NA, but never was applied to it. No reason not to except manufacturing and cost reasons.

As for the cam grind thing - mild grinds that boost low-end torque (before the boost comes in) can help, but the high-end gains in an NA engine are based on flow dynamics that do not occur at raised pressures like in a Turbo. This is the same reason why turbos have vastly different intake shapes than NA engines. A high-performance cam grind in a turbo won't necessarily help you, except perhaps before the boost comes in, and yes, the money would be better spent elsewhere. Also consider that, the more agressive the cam, the higher the power band - but then this conflicts with the turbo's boost.

Hang out with the Dodge turbo guys, they know their stuff pretty well... and they don't even have dodgy metric oil lines to deal with! LOL

_________________
Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype

[Rick MacLaren]

Yeah Vaughan, I can see how getting a proper grind for the 931 might be complex or difficult, but it doesn't make sense to say that you 'cannot' get a decent grind to increase performance. I mean, after all, controlling the timing of the intake and exhaust valves opening is quintessential engine tuning, isn't it? Thus, a decent performance cam "should", in theory, be possible.

Now, I've got this guy who's willing to grind a cam from a fresh billet to increase mid range performance (i.e., in the range after the turbo kicks in, 2500, to say 4500). Wouldn't such a cam, ground for midrange rather than top end, be ideal for the 924 Turbo?

I think that'd snap yer neck.
_________________
1980 Porsche 931 Bitched.
1979 Porsche 924 Tweaked.


[ This Message was edited by: Rick MacLaren on 2001-12-04 08:23 ]

[Rick MacLaren]

Vaughan...reply...reply! I need to get some closure on this cam thing - not meaning to quibble, but I need your thoughts on this one.

[924RACR]

Well, my silence was my reply!

I recommend you do some more reading/research on turbos before dumping more money in that direction; cams are designed and tuned for specific airflow characteristics. They're looking to optimize performance at a certain engine speed by acheiving certain fluid dynamics states in the intake as the valves open and close. Those fluid dynamics are quite dependent on MAP. Having a turbo totally changes your MAP, resulting in completely different fluid dynamics in the intake (and exhaust).

The same rules don't apply. The solution (for optimizing fluid dynamics at a given RPM) for an NA engine will not be the same as the solution for a turbo engine.

It's your money, spend it where you will, but I wouldn't spend my money there.

I'd spend it on an Electromotive HPV system, with programmable FI/ignition, knock sensor, MAP sensor, etc. I think that's where the gains on these cars are to be made, once you've added an intercooler and are controlling the boost.

NA motors make power by optimizing flow through the engine/airpump. The limitation is how much air you can get the motor to pump.

Turbos make power by turning the knob and raising the boost. They're force-fed, all you need to do to make more power is raise the boost. Very easy. The limitation is trying to make the engine stay together - preventing/controlling detonation through superior engine management. As you well know!

_________________
Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype

[Zuffen]

Larger valves, Port matched intake and exhaust. Trying to clean up the airflow and improve the dyanmic flow helps. You are forcing air, but if you can clean up the path, make the air flow smoother, less friction, less turbulence, greater volume at the same pressure.

Keep in mind that polishing ports has pros and cons. Laminar flow is important in effeciency. You can polish too much or change a flow pattern which reduces fuel atomization and burn.

Tha cam can't change things a whole lot on forced air induction. You have to look at the displacement and the required cubic air volume required at a range of fuel delivery to set your air fuel ratio. A race engine on full boost dosn't require near as clean a burn, so enviormetnal factors are built into the cam design as well as the fuel system. Tuning an engine and then adding a turbo are pretty intensive on the math. If you can improve the normally aspirated engine to peak effeciency then a turbo will just expontentially increase the performance. Raising effeciency is not always related to HP gains and vice versa.

It is complicated with all the factors involved

_________________
Bob Dodd - 924turbo@cox.net
931 1982, 944 1982 euro, 924S 1988SE, 93 968 tip 06 Silver Cayenne S, 06 Black Cayenne S

I have Way too many cars, parts for the 931,944 and 951

[Rick MacLaren]

Damn. I was trying to avoid getting that engineering degree!

[Peter_in_AU]

Rick,

forget the degree just get some good engine simulation software. I think Dyno2000 is the best value for money tho it's oriented more towards normally aspired engines.

A while back I got my copy to calculate the "best" cam for a supercharged 924 I have built a simulation of and after 4 hours and trying literally hundreds of millions of combinations the power gains it came up with were quite small. Doing the same thing for a N/A engine gave a much greater percentage increase in power.

[Zuffen]

Peter if you took that program and set some known values like cam lift and duration, does the software help plot out where the best gains would be.

CR increase?

Larger valves?



_________________
Bob Dodd - 924turbo@cox.net
931 1982, 944 1982 euro, 924S 1988SE, 93 968 tip 06 Silver Cayenne S, 06 Black Cayenne S

I have Way too many cars, parts for the 931,944 and 951

[Peter_in_AU]

head over to http://www.motionsoftware.com/ and download the Dyno2000 full manual to find out what it can do.

Basically what you do is set up your base-line engine by entering all the details - number of cylinders, fuel system type, intake details, exhaust details, valve sizes, cam lift, timing and ramp, compression ratio, piston shape, head port type, port flow (just key in the flowbench figures directly from Jon's site) etc etc. At that point it gives you dyno-type graphs of the hp and torque. Although a lot of the selections are oriented towards big-iron the manual equates most of the terms to things that non-hot-rod guys could understand (hands up who knows what a tunnel-ram manifold is).

It is very accurate in the engine outputs it calculates. With the data I gave it, it picked my Euro NA as giving 127hp @ 5500rpm and 141ft/lbs @ 4000rpm. Haynes quotes the official Porsche figures as 125hp @ 58000 and 141ft/lbs @ 3500 (and we all know how Porsche prefer to quote slightly lower output and speed figures).

OK, so once you have your base-line defined and saved you can have some fun.

Change the valve size, instantly see the result. Change the exhaust type, the compression ratio, the port flows, the valve lift and instantly see the effect.

The real fun part is getting it to calculate a cam for you. You tell it the minimum and maximum lift you'll accept, the earliest valve opening and the latest valve closing. You then tell it the step size to use e.g. 1mm for valve lift and 1 degree for valve opening and closing. You tell it where you want the power and whether you're after hp or torque. Then you hit the "do it" button and go and relax. Depending on the "step" sizes you chose and the power of your PC the calculation can take from seconds to hours. Realistically, you start with big steps to identify the area that works and then fine-tune the numbers with smaller steps.

You can also do the same sort of thing by varying the bore, stroke and compression (less useful to the non-V8 community).

It really is a ton of fun and for around US$50 really good value for money

[larso]

OK, so what is up with this interference 931 engine? the pistons and the head in the 931 are purely for flow...so why don't we just take 924 pistons, and a 924 head, which is 8.5 comp, or 8.0 isn't it (in the 70's) and stick it on the 931? No more interference...

Did porsche design the 931 head and pistons to help the heat flow out better so that there would be less head problems? Maybe the heat needs to flow out for those reasons more than the HP reasons???

[ This Message was edited by: larso on 2001-12-08 13:35 ]