993RS

P.S. in terms of the "nuts" factor...

Jim Morton

Hey guys !

I need to post a correction (Thanks Steve W.).

I mentioned how the 993 cams have narrow lobe centers. I was thinking overlap while I was typing and picturing the lift curve plot. Imistakenly stated lobe center in place of overlap. I was backwards....damn technical dislexia!

The cams have a very narrow overlap as needed for emission reasons, hence wide lobe centers (NOT NARROW). Sorry for my mistake. The lobe centers are about 114 degs. In comparison, some of thet "HOT" cams run down at 103 - 108 degs. With the wider lobe center (narrow overlap), the 993 engine does not take much advantage of cylinder scavenge due to the exhaust gas / intake charge momentum.

Sorry for any confusion.

Regards

914und993

Another reason for low overlap is for more reliable electronic fuel injection performance. With more overlap, higher amplitude pressure waves propagate back up the intake manifold to the mass air flow sensor, and this can degrade accuracy of this sensor.

I know high tech race vehicles use electronic fuel injection and "hot" (high overlap) cams, but I'm not sure how they deal with this problem. Maybe they don't try to sense mass air flow, but use a fixed map of some sort - I don't know - perhaps someone can educate us a bit.

Chip

Bill Verburg

It's very easy an inexpensive to backdate the cam drive gears to the 964 ones. That is is long as you buy new cams. That's what we did on my engine. And yes, the temp. use of mech. lifters makes it fairly straight forward to set the cam timing.

Bill Verburg

It's very easy an inexpensive to backdate the cam drive gears to the 964 ones. That is is long as you buy new cams. That's what we did on my engine. And yes, the temp. use of mech. lifters makes it fairly straight forward to set the cam timing.

fixnprsh

hmmmm, Ive built several 993 motors and backdating is THE ONLY WAY to adjust cam timing, a 993 uses a holder piece that bolts where the pulley goes with a PIN in it with a bar connected that fits into the intermediate shaft opening that will only lock the crank in ONE position. then you install the holders into the back of the cam housings that have an offcenter SLOT with a special bolt so it can also only be set in ONE certain position. Then you install tensioner plungers to a preset depth in the holes where the chain tensioner piston would go, then lock down the bolts. Its so simple a burgerking employee (like myself) could do it in his sleep as there is no way of changing the settings or screwing it up, so unless your backdating your cams, you're getting bent over.

TomF

I love the sparkplug cam-sprocket holder. Is that a Porsche special tools part! LOL
Bill, your engine project sure looks like a lot of fun.

JasonAndreas

I don't know what the newer race cars are using (maybe Alpha-N? + MAP?) but the newer Bosch hot-film air-mass meters (HFM5, etc.) supposedly measure direction of air-flow as well as quantity.

Kevin

Folks;

There is another way to check the hydraulic profiled camshafts.. Yes, you need to use the old traditional methods like Bill has shown. However, if you check this link out, you will see a hydraulic 993 rocker arm that I converted so that you can check cam timing.. It is very critical to do this if you plan on squeezing the last few HP out of your engine.. It will also smooth out your idle..

https://rennlist.com/forums/showthre...7&page=4&pp=15

I also posted the actual factory tools used to do the quick and easy way.. The shop should only be charging a 2-4 hours to CHECK the cam timing with a tool like I made.. If it was my motor I would install the early cam sprockets and dowel pins, like from a 964.. Some of the early 993 and 993TT came with them.. The later lock and load style can be easily changed out..

993RS

The valve timing on my car was done this weeik. As mentioned above, a problem was detected. Timing differences between the left and the right cylinder bank.

The correct amount of lift apparently should be 1mm. The left bank measured 0,82mm and the right bank measured 0,94mm. Not so hot. They will now set the timing right, but it entails one day's work apparently. Should make a big improvement in the engine.

flatair

Aww c'mon 993RS, you've slipped most way down the slippery slope, why not RS cams?

993RS

Well, never really considered them, because I assumed, perhaps erroneously, that the torque curve would become more peaky, whereas I prefer a near-to-flat torque curve...

flatair

Yup you're right, the RS cams will make the bottom end a little softer. I don't have access to a dyno curve for a modified VRAM engine similar to yours, but here's a comparo of stock vs RSRS's VRAM with intake, headers, mufflers, RS cams and custom chip:

RPM............RSRS.........stock VRAM
2000.............62...................68.....+6
2500.............79...................87.....+8
3000.............108.................106....-2
3500.............149.................143....-6
4000.............177.................162....-15
4500.............207.................190....-17
5000.............232.................210....-22
5500.............253.................234....-19
6000.............267.................241....-26
6500.............275.................237....-38
7000.............261.................233 *extrapolated from redline number

Of course this isn't a perfect comparison, as I'm sure the mods you've made to your engine have made some differences across the rev band, but looks like below 3000 it's a bit softer than normal, but abour 4000 there are some good gains to be made.

Then again, it's definitely not a flat torque curve (is that possible in a 993?? )

When do you get your car back?

flat

993RS

I hope my car will be ready by Friday. Re-gearing: done, LWF: done, currently setting valve timing. Repainted my big reds (they were a bit chipped), red seat belts and turbo S ducts still to mount...
Can hardly wait... Been playing "Need for Speed" for an outlet...

FlyYellow

so is there any way to get a RS cam to pass smog?