sublimate

So it occurs to me that with a pair of 944 (or 968) heads and a custom crank that one could easily turn a 928 motor into a flat-plane V8.

Sure, without balance shafts it won't be too smooth, but it'd be worth it too hear that howling rev-happy goodness. And I think some of the Ferrari's didn't have balance shafts so it might not be too bad, particularly for the track.

The crank shouldn't cost too much since there's no twist and no counterweights (less windage-induced oil problems, yah). And hey, might as well add some stroke to it while I'm at it.

And the heads seem pretty straight-forward. Weld up a water hole and maybe a few other little mods.

What am I missing? It seems too easy.

perrys4

$$$$$$$$$$$.$$ ?

PorKen

$³ - but a cool idea!

slate blue

It certainly wont be too easy, I think you would need to be a very brave man. No sorry a brave and weathly man. The 968 heads are quite good with excellent flow chracteristics, remember these were designed after the 928 heads were. The ports have a more D shape to them like the GT3.

Basically when you look at the 86.5 heads then the s4 heads and then the 968 heads you can really see an evolution, I think you could use the 968 cams but you would need atleast to do the cam mod that is required when converting the 85 - 86.5 cams to suit the S4 and later heads. The spacing on the cam gears (belt drive may not be correct also)

To be quite honest I don't think I would do this project, not enough upside with plenty of catastrophies waiting in the wings.

Good Luck.

Greg

Vilhuer

Beem thinking about it little. Certainly doable as there is one 7+ liter Corvette in here with such setup. Before hearing about it I thought so large engine would shake itself to death. Obviously not and Ferrari gets 4L to rev past 7k rpm also.

slate blue

All I would say about the vibration problems is that when F1 went to the V8 they were all worried and had vibration concerns. 4 cylinders have vibration problems and that is what you have well two of them. Now I know Sublimate is a cluey guy so I'm not teaching you to suck eggs, it is just that you might find yourself in the R&D lab, and that is expensive.

Smokey Yunick didn't think a flat plane crank was worth it for a Nascar engine, now I know things have moved on, if you read Race Engine Technology you will realise this, (you may also have other knowledge too) just a tid bit about the Nascar engines is that they have different camshaft profiles for each cylinder.

Cheers Greg

sublimate

I think I'd try it with a 4.5L block and 944 2v heads first as a proof of concept.

Anyone know if the intake will bolt up to the early 944 heads? I did a search and saw that the 968heads don't match up to the S4 manifold, but nothing about the 2 valvers.

If the intake bolts up then it seems the only big cost is the custom crank and a flat-plane crank costs only about half that of a cross-plane one. It'd basically just be a 944 crank with throws wide enough for 2 connecting rods (and maybe an extra bearing journal??? - how many bearings does a 944 have?).

Since the injectors are batch-fired there wouldn't need to be any changes to the computer. And the order of plugs on the distributor could just be switched around to get the firing right. What else is there?

So if all is perfect then for a couple of thousand $ you'd have a totally unique fast-reving motor. I agree there wouldn't be much increase in power (a little from the decreased rotating mass and windage, and perhaps another little bit from the better exhaust pulse timing). But if it worked out well then it would open up a way to much bigger HP w/ 968 heads and a stroker crank.

As for vibration, I've read that the vibrations of the 2 banks will tend to cancel each other out some, although I haven't done the math yet to be sure that is correct. Do any 944 racers remove the balance shafts to gain horsepower?

sublimate

How's the mini-stroker project going?

slate blue

By Sublimate
Funny, well I hope to order a crank very soon 3.55" stroke honda journals, the aim is revs and better breathing due to superior rod to stroke, who knows it may eleviate some of the manifolding problems the 928 has? Restrictive issues that is.

Cheers Greg

P.S I don't think you will get a flat plane crank any cheaper than $2500 which is what I will be paying for my crank.

sublimate

Who's doing your crank?
Is it going to be a twisted or non-twisted forging?

mark kibort

you cant use 4 cylinder cams on a V8, can you? i know the heads can be modded to be used on some conversions.

tell me about a flat plane 928 engine. i dont understand. are we talking about a boxer conversion or something.

what are the gains trying to be achieved by some kind of alternative design?

how about doing the 928 top end on a chevy bottom?

mk

Vilhuer

Cross-plane V8 (928, US V8's etc)


Flat-plane V8 (Ferrari)


More complete explanation of different crank configurations.

With flat plane it's possible to use 944 or 968 cams as engine is in effect two four cylinder engines which share common crankshaft.

mark kibort

I think i understand now. so, the balance is much different, but how is it better or worse again? if ferarri can do it, and you can use 968 parts, thats kind of cool. bigger valves, higher lift cams (i think?), and all you have to do is twist up one of our cranks?

then, what about spark timing ? just use the sensors and computers from two 968s in parallel?

mk

sublimate

The advantage of cross-plane is:
1. No counter-weights on crank = low rotational inertia (rev happy), low windage losses, lower over-all engine weight, smaller crank so engine can sit lower.

2. Even firing between banks = more efficient exhaust scavenging
A cross-plane V8 like our fires (L=left bank, r=right bank) LRLLRLRR. So looking at the exhaust from one bank it looks like:
PULSE-nothing-PULSE-PULSE-nothing-PULSE-nothing-nothing-PULSE-nothing....
Not even at all, so there no way to really design an efficient exhaust without a very complicated header which combines both banks. It's amusing that people spend $$$s to design equal length headers for cross-plane V8s when the pulses are so unequally timed.
A flat plane fires LRLRLRLR so each bank sees even exhaust pulses.

Shouldn't need to do anything other than change the order the the plug wires are connected to the distributor. You're still firing a cylinder every 1/4 revolution, just a different one sometimes.

mark kibort

so my equal length headers really didnt buy me much??

anyway, sounds interesting. I have a block,a crank, some rods and pistons from the euro 4.7.
lets build one!

how do we flat plane that crank. i got some big hammers in the shade tree garage!

MK