How wild are the stock 928 16v cams? Even the S2 don't seem that wild comapred to some of the chevy after market cams I've looked at in magazines. However, the chevy cams where measures with .050" lift, while the Porsceh where at 1mm. Not exactly apples to apples. My metric conversion says .050" = 1.27mm. Course, I could be wrong.

Why didn't Porsche put the varo-cam on the GTS in 92, when they put it on the 968. My understanding is it the heads are nearly identical, whats the problem? Lets see, a 5.4l engine tuned to the same level as the 968's 80 hp/l = 432 hp. That's a viper killer, and the Corvette doesn't even come into the picture.

Anyone ever heard of Rhoads Lifters. According to the write up in the magazine, your typicaly sunny, BS blurb, they are a varible duration lifter. They through around claims of giveing you a better top end, and low end. The website talks about 100,000 units sold. Sounds too good to be true.

Anyways. It's all intersting.

 

I haven't seen the specs on the 16v cams, but one thing to keep in mind is that the 928 was designed to be a refined machine. I'm not sure that the rough idle and a monster mid range hit from a hotter cam would have enticed too many doctors and dentists in the day. (Although it would be cool to see now)

Porsche probably didn't variocam the 928 for the same reason they didn't include a tip-tronic option. My guess would be that the '28 was already on the outs by then and already too expensive for the market by that time. Of course, there are some conspiracy theorists that would opine that these were further restrictions put on the 928 by the turbo crowd, which together with going to milder cams than the GT, would keep any HP gains brought on by the extra .4L of displacement on the GTS to a minimum.

In the first small block chevy I ever built I installed rhoads lifters. The lifters plungers bleed down at lower rpms essentially taking up some of the cam lobe before moving the pushrod, allowing hotter cams to idle. At higher rpms, and under higher oilpressure, the lifters pump up transmitting the full lift of the cam to the pushrod. The rhoads were louder than he** when run, and to me, didn't have the same cool sound of a mechanical lifter. Sounded almost akin to a deisel. I switch to the Crane version which didn't have as much bleed down, but were dead silent.

Memories....

 

the 928 cams are very mild ....even Euro S 16 valve...

 

Any guess what a wilder cam, and a set of crane lifters would do to a 928?

Are such lifters on OEM application? Other than noise, was is the downside?

Sounds like an intersting idea anyways.

Thanks.

 

I'm a big fan of Rhoads lifters. I have used them in three applications of my other passion--Pontiacs--without the noise and other downsides some have experienced with Chevy applications.

They make a race cam "streetable" by bleeding down and reducing lift and duration at lower RPM's, and pumping up as described to act near solids at higher engine speeds. In Pontiacs, they work great and are quiet. I could never run the type of cams I have without them.

I don't think there is any way to apply such a concept in OHC motors like Porsche water-cooled applications however. Lifters are used in OHV engines to manage the geometry and valve lash (if hydraulic) between the cam and the pushrod. The OHC Porsches don't need such an animal, as pushrods and rockers are eliminated. Someone would have to configure a variable hydraulic tappet between cam and valve that also bleeds down, and I don't know that anything resembling this technology exists (I could be wrong).

They (and the Crane's mentioned previously) are a type of rudimentary "variocam" technology however, and work as advertised. Cool cheap technology.

You can also run some serious compression. I am running 12+:1 in Pontiac, and am only able to do so with huge cam bleeding off cylinder pressure--and can only run huge cam due to Rhoads.

Jeff
'87 5-spd.
RMB, chips, Ott, 993 wheels
'92 968 Cab. 6-spd.

Porsche slowly replacing Pontiac as primary passion.

ps. Someone has got to want a 914 project free. Anyone know a Boy Scout troop? Shop class?

 

the normal 968 3 liter makes 240 hp or 80 hp per liter ; with the same cam profiles theoretically it would have been relatively easy for the GTS 5.4 liter to make about 432 HP with an improved intake manifold . Note the 968 passed emissions , idles well is not very radical at all (does have vari-cam) . Same "logic" the 6.5 liter monster motors if built to Porsche design standards using Porsche camshaft profiles street technology really should be making about 520 Horsepower .

 

the normal 968 3 liter makes 240 hp or 80 hp per liter ; with the same cam profiles theoretically it would have been relatively easy for the GTS 5.4 liter to make about 432 HP with an improved intake manifold . Note the 968 passed emissions , idles well is not very radical at all (does have vari-cam) . Same "logic" the 6.5 liter monster motors if built to Porsche design standards using Porsche camshaft profiles street technology really should be making about 520 Horsepower .

 

Sterling I do recall someone having done just that, or at least ALL the GT cams into a GTS. I do not recall any "numbers"

4 Billet cams + The EMU to run it = 7k or thereabouts (DTA, Autronic, etc, excluding motec, as thats 10k and above)
=380rwhp, since the cam can be pretty wild with the computer change.

One Supercharger, withs stock computer= 8k or thereabouts.
=525rwhp

One Supercharger + the EMU to make sure you get the most out of the boost
=600rwhp.

I honestly think that a stroker really needs cams a hell of a lot more than a supercharged situation. Sure, more duration (not overlap) would be needed for some more filling of the cylinders, but people have proven here that a stock 928 engine can be a GREAT foundation from which MANY different ideas, i.e., cams, stroking, exhaust, intake, supercharging, and turbocharging can be added.

There is a guy on the 996TT board that is doing about 750 on a 996TT. I don't believe the motor has been opened. Supercooling of the intake charge, big turbos, big intercoolers, and some different gears in the trans.

I don't like 1/4 mile drags as a data point, but I think he is under 10.
Yes, he has all wheel drive.

But we have a V-8.
I think 900hp is anice goal for a worked bottom end and some good boost.

 

A local PacNW'er (Garth Hadley) has GT cams in his '93 GTS.
Here is the dynograph showing his stock cam run against a GT cam run:

 

Oops, let's try this:

<img src="http://boards.rennlist.com/upload/GarthDyno.jpg" alt=" - " />

 

If they're so great, why hasn't GM put one in a Corvette, or one of their trucks? It would seem to me that there would be money to be made...

I'm just curious, as something just seems too good to be true.

Thanks.

 

I share Sterling´s thinking and it arises a lot of doubts in my head.

In a few days, I´ll have a set of GT cams I plan to install on my GTS.

I wonder If I will extract more and better power leaving the GTS´s exhaust cams in place, installing ONLY the GT intake cams instead of the full set.

Would this combination REALLY make more power?
Would it be too agressive, ruining the drivability of the car?
What issues could arise?

Any help or comment would be greatly appreciated.

 

I believe it's the other way around guys, you use the gts intakes with the gt exhausts, well that is what I have been proposing anyway.

Why would you use the other combo? I think you would lose power.

On another topic, I have been thinking, now that's dangerous I know, but I think I know the easiest way to convert a 928 engine to Variocam.

The way I would do it is to use a custom 180 degree crank with an 88 mm stroke so that you can use the 968 rods and pistons.(A flat crank, deck heights would also need to be checked to make sure they are the same) and then just use the standard 968 cams. This way you can keep as many factory parts as possible so that reliability and avaibility should not be of concern. A 88mm stroke is probably optimium for about 7000 rpm. I say that also considering the 968 valve timing. One problem the car will sound like a Ferrari!

As I mentioned before the GTS cams should be good upto about 8000 rpm. As long as the intake tract can be kept as short as possible. The shorter stroke of the GTS allows that type of RPM limit.

 

Yep!
Rigth!
Sorry for the mistake, Greg. <img border="0" title="" alt="[Wink]" src="wink.gif" />