H2

Since the 4-cylinder Porsche 968 can easily push out 250hp with simple mods...and ONLY FOUR CYLINDERS....why can't the mothership V-8 push out (easily) at least 400-450hp? What the disparity? Is it the variocam? 3.0 Liter jugs? While I think there may be an issue with putting the variocam on the 928....it shouldn't have been a biggie to produce a 6.0 liter 928. Other than perhaps the plug had been pulled on the 928 production pretty well by the time of the 968.

I question the variocam in the 928 because the 968 Turbo was built with essentially 944 heads, fewer valves, and I don't think it had variocam but I could be wrong on the latter. BTW, there were only a handful of factory 968 turbos made and I found it interesting that they used the 944 head rather than the 968 heads. There's probably an engineering reason for it.

But back to my original question, if they can get so many horses out of half the 928 engine for the reguglarly aspirated 968...why is it so hard to get reasonably equivalent horses from the 928 w/o going to forced induction?

Harvey

worf928

Sterling did it!

6+ liter with twin VarioCam.

The stock 968 was 240hp IIRC. So, an extra 10 hp with mods doesn't seem like a big deal.

In any case - IMO - the secret to the modern sauce of engine performance is modern engine management systems.

worf928

Sterling, is your motor still over-square? I don't remember where your bore and stroke numbers ended up.

The 968 motor was 88x104. I seem to remember that yours was ??x104?

It will be interesting to see where you end up. Let us know; we're all living vicariously through your project

I would think though - based upon what I've heard and read - that the base torque peak is directly a function of stroke versus bore with under-square motors having lower torque peaks than over-square motors all other things equal. Exhaust and other mods have some ability to move the peak around?

SwayBar

I think the 968 cam was about accomplishing both, having a 'big' cam with a great top-end, and 'detuning' the big cam for better bottom-end, idle, etc. The variocam gives you the best of both worlds, and I think it's awesome you got it to work in your car.

Do you know the duration and lift numbers of the 968 cam?

I think your projected torque-peak is a bit optimistic regarding the new exhaust; I'm thinking it won't change much at all.

I remember seeing the Devek car's dyno before the throttle-body installation, and with the Level 2 headers, etc, it's torque peak occurred at 3200-ish RPM, and rapidly fell off after that. It wasn't until after the installation of the throttle-bodies where the torque curve changed dramatically and did not look choked-off as it did with the stock intake manifold.

Louie's new engine output looks identical to the Devek car with an additional 25 hp.

The same occurred with Mark Anderson's racecar after he installed the carbon-fiber intake as it woke-up the engine and gave him 50 extra hp where he really needs it on the track.

Based upon those cars, it was the intake on the larger motors which significantly moved the power up into the higher RPM's, not the exhaust as they already had high-performance exhaust systems prior.

SwayBar

The 968 is an over-square motor at 104 x 88:

http://vista.pca.org/stl/968.htm

Remember, many stroker motors use the 104 mm pistons of the 968.

IcemanG17

Dont forget the increased frictional losses on the V8 vs the 4 cylinder.....same problem with a V12.....so an engine with the same displacement but more cylinders will have more frictional losses than a same displacement less cylinder engine.....

hacker-pschorr

You also need to factor in Porsche keeping the 928 within a certain performance window. Could the Cayman S be tuned to out perform the 911? Very easily - they simply do not want to do it. The 928 was plagued by being in the shadow of the 911 as soon as they realized it was not going to replace it.

There is no “performance mystery” to the 928 – it delivered the amount of power Porsche wanted in the car to keep it inline with the line up year for year. Considering they only gained 40ish hp after doubling the number of cams and increasing displacement by 300cc’s – this “theory” is pretty obvious (85 Euro S vs. GTS).

A better comparison is the 1987 944S vs. 1987 928S4. The 944S is a 2.5 liter, has larger valves, larger ports, higher compression, and I’m willing to bet a hotter cam = 188bhp. Porsche could have easily squeezed 376bhp out of the S4 motor, they simply didn’t want to.

worf928

egads. I had my over/under mixed up this morning. Time for another cup of coffee.

(Yes. I know that the 104mm is the bore.)

SwayBar

I knew something had to be up since usually your posts are spot-on.

928SS

iirc, the "std" 928 stroker crank is 95mm. 104/104.5 mm bores are common w/decent blocks, and most of us used 968 sized intake/exhaust valves

cams and porting help, but like you noted - to get the GT3's 136bhp/litre power, ITB's or other intake mods are required. iirc, devek and some others noted the OEM intake system is "only" good up to about 450rwhp or so...

iirc, mark A and some other's use the OEM management system at over 500rwhp, so it doesn't seem to be a big HP limiter compared to the intake.

heinrich

I doubt this Harv. 968 mods are almost a waste of time till you turbocharge them.

IcemanG17

With the shark tuner....the OEM management system is now able to handle higher HP much better than before...... Tim's beast runs stock management-airbox-intake (with spacers) & still makes 515whp! Of course it shakes the ground like an earthquake too!! :>)

heinrich

http://www.968s.com/engine.htm There is no way to improve 968 performance past a measly few horses, except turbocharging.