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I have had my 1985 928S 5L 32V 4Cam 5-speed for about 14 months, and coming from high revving flat sixes with 80 to way over 100 horsepower per liter displacement, I find the 928 V8 kind of dull... (from the factory it has 57Hp/Liter displacement)
Plenty of power and torque, but it doesn't have the rev happy character of the flat sixes. And the typical American sounding burble exhaust note, due to the odd firing nature and "impossible" exhaust scavenging. Could that be changed?
Would it be possible to have a flat plane crank made, and modify the rest to fit? In my simple mind it shouldn't be rocket science... The flat plane crank is basically an inline four crank with two rods on each journal, and two four-cylinder banks in a 90-degree angle. The 944 S2 and 968 are basically 1/2 928 V8. With to my knowledge the same cylinder heads.
With a flat plane crank, each of the two cylinder banks should behave exactly like a four cylinder, with the same firing order. Which again means that 944/968 cam shafts could be used. Or maybe something with more aggressive duration and lift? Would it be that simple? Or would you need balancing shafts like the 944/968? Can some of the engine gurus chime in on this?
Of course the ignition firing order must be changed, but that is the easiest thing. And the even pulsating intake plenum must be changed to something that looks more like the 78-84 CIS intake. Or ITBs...
Has anyone tried this? Wouldn't it be awesome with a matching numbers V8 engine, with the flat plane crank sound? Revving to 7,500 or 8,000rpm... Is it doable? A crankshaft is not that expensive, and a set of Carillo rods are around $2,500, and lightweight pistons about the same. Four camshafts should be around $2,000. With a little ingenuity and clever engineering, it should even work with the existing LH and EZF ECUs. Or a MoTec...
Nobody has done it because the engine would rattle itself to pieces. The rotating assembly has way too much mass (hence the slow revving) to safely be converted.
If you want it to respond quicker you can do everything you described sans flat plane crank or possibly install a supercharger.
To clarify about vibration. Any 90° V engine has PERFECT primary balance. Each pair of cylinders that share a crank journal has this perfect primary balance, for any even number of cylinders from V2 upwards. How many actual cylinders and how each pair are phased with respect to the others is another matter that will determine the higher order balances (secondary, tertiary) and of course the sound. A 'flat plane' V8 crank is like an in-line 4, but with a pair of cylinders on each journal and although it will have perfect primary balance, the secondary etc. imbalances inherent in the in-line 4 engine will still be there. In fact to the best of my knowledge, higher order imbalances can only be reduced by the use of balancers.
In essence, it is a mistake to think of the primary balance of a 90° V engine as anything to do with the balance of each bank on its own. The inherent balance comes from each pair of cylinders at 90° on the same journal. Any angle other than 90° is a whole lot more complicated.
A flat or 'boxer' engine has the same perfect primary balance, but can do so with less counterweight on the crank.
I'm not sure what advantage a 'cross plane' V8 would offer since each bank has less inherent balance than a 180° in-line 4. For several years Yamaha (who also 'invented' the 5 valve layout and licensed it to Ferrari who used it for some years) have used a 'cross plane' crank in their R1 (1000cc in-line 4). This is to enable better grip characteristics due to the uneven firing order that results. However the additional vibration caused by the 'cross plane' crank requires balancer shafts that a regular 180° 'flat plane' in-line 4 in otherwise equivalent bikes does not require. 'Cross plane' however can be done in different ways and maybe in a V8 it can be configured to reduce secondary vibes compared to a 180° 'flat plane' crank design. I've not thought about that in depth so I couldn't be sure either way. However, whether 'flat plane' or 'cross plane', a 90° V8 will always have perfect primary balance.
As a side note, Yamaha chose 5 valves to avoid copying Honda who used 8 valves in their racing NR500 motorcycle (and subsequently on the very exotic NR road going 750) and I believe had patents on other numbers - except 5. All somewhat irrelevant now as all those users of more than 4 valves per cylinder have dropped the idea and reverted to using just 4 valves. Anything more simply wasn't worth the additional complexity (especially the oval cylinders Honda used in order to fit in the 8 valves).
If you want a less dull, sportier, more agile engine, in short do the following.
1. Install a set of 48mm individual throttle bodies.
2. Port the heads with 39mm intake valves.
3. Install hotter camshafts and springs.
4. Install a set of well-tuned headers.
5. Build and install a dual 2.5 inch exhaust system.
6. Install a 10mm spacer at the oil pan.
The oscillating parts of the engine are heavy. The wrist pins and the small end of the connecting rods can be lightened.
Crankshaft need to be rebalanced.
The crankcase ventilation need modifications.
The engine lubrication system need modifications.
Åke
If you want a less dull, sportier, more agile engine, in short do the following.
1. Install a set of 48mm individual throttle bodies.
2. Port the heads with 39mm intake valves.
3. Install hotter camshafts and springs.
4. Install a set of well-tuned headers.
5. Build and install a dual 2.5 inch exhaust system.
6. Install a 10mm spacer at the oil pan.
The oscillating parts of the engine are heavy. The wrist pins and the small end of the connecting rods can be lightened.
Crankshaft need to be rebalanced.
The crankcase ventilation need modifications.
The engine lubrication system need modifications.
Åke
Here is a nice picture of John Gill´s amazing 5,4 liter race engine. 374 KW or 509 HP at the wheels. Close to 600 HP at the crank.
That is not a dull engine.
Åke
Plenty of power and torque, but it doesn't have the rev happy character of the flat sixes. And the typical American sounding burble exhaust note, due to the odd firing nature and "impossible" exhaust scavenging. Could that be changed?
Any one...???
This is all Kind'a by design, the 928 was a German GT car designed to have the a sprinkle of Ferrari GT Style crossed with Mercedes brutalism, hence the relatively large and quiet V8, lazy steering, cushy ride.
Yes, it all can be changed but the question is how much time and money are you willing to commit and do you have the know how or access to people that have the knowledge to successfully re-engineer a high revving flat plane V8 from the 928's block
I would argue much else needs to and can be done before getting to the engine itself, flat plane or otherwise.
1. Engine:, S300 Chips and Fuel system Upgrades.
2. Exhaust: Complete exhaust system redesign... X pipe and convert to 2.25" Borla Straight through mufflers, Ditch all other silencers. Really provides an exotic sound. I added a vacuum valve which closes 1/2 the system at cruise so the drive is dead silent at highway cruise and at WOT, it's thrilling.
3. Steering: Lots of the dullness in the 928 revolves around the heavy, imprecise and slow steering feel. Solutions include solid bushing and steering rack re-design. (controversial position)
4. Suspension: LCA bushings are a travesty and make the suspension slow to react. LCA and UCA's be re'bushed with aftermarket Powerflex pieces (controversial position)
5. Driveline: I hope you have a 5 speed. There's no un-dulling an automatic IMHO. Consider diff gears, the later 928's were geared too tall for thrills.
6. Brakes: GTS brakes ad good pads at the minimum.
7. Wheels / Tires: Good UHP tires go a long way in the un-dulling process, not a lot of great tires in the OE 16" size so you'll need 17' or 18' wheels which brings you to the Front offset / wheel problem.
You'd be starting from a decent position though, the 85's (assuming American) have decent cams that can provide a good sound and the body isnt burdened with as many creature comforts as the later cars. The 85's also have a front suspension that is slightly more tolerant of the incorrect front offset wheels.
I've already gone down this path for a few years and I'm up to the engine now but I'm going with a traditional cross plane 6000 RPM stroker. I dont have the wherewithal to pull off an exotic rev'y V8 and n'or would I want to. I'm going with the Torque and brutality of a 6.0L using mostly American hot rodding principals. Part of the reason I dont want a high strung V8 is that I already have one in a off topic 6 speed car. I've also come to know that I prefer torque for the street and for the type of HPDE driver I am. The other reason is I'm smitten... I built a V12 / 6 speed manual, straight piped and off topic car for a friend and there is nothing else like it. It's has a rhythmic sound, power delivery and smoothness no V8 can match, so building one for me is in the works next.
Last edited by icsamerica; 03-29-2022 at 12:51 PM.
If you really want a flat plane, high revving engine in a 928, it would be easier to put a flat plane Coyote or Chevy engine it with a corvette transaxle. And probably cheaper than converting the Porche engine.
[QUOTE=Plenty of power and torque, but it doesn't have the rev happy character of the flat sixes.[/QUOTE]
Actually, it can be rocket science, especially if you enjoy spending lots of money on things that can blow up. You've bought and sold a lot of Porsches. Why don't you buy a GT3 or GT4 varient of the 911 or Cayman? I think you'll get "rev happy" in spades and you will probably be able to sell it for more than you paid for it (less the high cost of maintenance). The 928 is what it was intended to be. Why the need to reinvent history?
I mean no disrespect by this, but I think you are chasing the wrong class of car here. You have a very nice spec, and besides turning it into a weapon through weight reduction, bigger tyres, a custom exhaust and a supercharger..nothing else is really worth the time, effort and of course money.
You would extract more enjoyment from the 911(s) you already presumably own, or perhaps look at scratching your itch with a classic V8 Ferrari model.
I will say though, and I think most will agree with me on this, that these cars do not suffer from dull steering. At least the early cars don't. The steering in my 81 is heavy and direct, with great feedback and just enough powered assistance. My factory shocks on original 16" wheels also provide a tremendous amount of road feel, actually a bit too much for a grand tourer on some surfaces. These cars also rev quite freely as well.
Perhaps look at checking and adjusting the above first before looking into more complex modifications. It may be that something or several things are a bit off. If all is good and it's still not the drive you enjoy, then the 928 may just simply not be for you in stock form.
I think the abject craziness of trying to re-engineer a crankshaft (which will require new pistons, maybe rods, cams, a different firing order, a different computer control, etc etc etc) for these cars is awesome.
A waste of money? Perhpas. Insane? Undoubtedly. But why NOT tackle the engineering challenges?
It'll never be a 308, but maybe you can make the engine into what you want. Is that wasteful insane than supercharging, or twin turbo charging, or building a 6.5 litre stroker? I think it seems so only because no one has done what you suggest, and people have already figured out everything about the other modifications. But why not take such a crazy plunge? You might end up with exactly what you want.
I think the abject craziness of trying to re-engineer a crankshaft (which will require new pistons, maybe rods, cams, a different firing order, a different computer control, etc etc etc) for these cars is awesome.
A waste of money? Perhpas. Insane? Undoubtedly. But why NOT tackle the engineering challenges?
It'll never be a 308, but maybe you can make the engine into what you want. Is that wasteful insane than supercharging, or twin turbo charging, or building a 6.5 litre stroker? I think it seems so only because no one has done what you suggest, and people have already figured out everything about the other modifications. But why not take such a crazy plunge? You might end up with exactly what you want.
I agree completely,
having built many things that made no financial sense/will never pay me back for the time invested.
Semi-retired, as of Feb 1, 2023.
The days of free technical advice are over.
Free consultations will no longer be available.
Will still be in the shop, isolated and exclusively working on project cars, developmental work and products, engines and transmissions.
Have fun with your 928's people!
I have probably posted this before, but...
The DTM (German Touring car racing) stipulates an engine derived from a production car. Somebody remarked once that an Audi V8 sounded like a flat plane crank, when the production engine was a 2 plane. When they asked Audi they were told, Yes, we take a prodution crank,press it flat and heat treat and stress relieve it.
i believe i have read that GM usedto make flat plane cranks to order, the same way.
Almost all Ferrari V8 cranks are flat plane, an I dont think they are famous for being rough running engines?
jp 83 Euro S AT 57k
03-29-2022, 12:56 AM
Flat Plane Crank in a 928 V8...???
