For those patient souls (968 heads)
03-27-2008, 10:56 PM
#16
Drifting
Flat plane cranks have bad harmonics. Audi tried bending 90 degree forged V8 cranks into flat plane cranks in the DTM series in the early 90s and ended up with lots of engine failures. The only production cars that use them are Ferraris and the Esprit V8. Both are relatively small displacement. Anything larger than about 4 litres and it shakes itself to death.
03-28-2008, 12:18 PM
#18
Drifting
Flat plane cranks have bad harmonics. Audi tried bending 90 degree forged V8 cranks into flat plane cranks in the DTM series in the early 90s and ended up with lots of engine failures. The only production cars that use them are Ferraris and the Esprit V8. Both are relatively small displacement. Anything larger than about 4 litres and it shakes itself to death.
03-28-2008, 09:13 PM
#19
There is next to no advantage with a flat plane crank, as mentioned before the harmonics are bad news, you wouldn't want to try and solve these by yourself, you would need an R&D department. Use the Tri-Y headers to get around the exhaust pulsing issue. This is what I am going to do on my 2 valve stroker engine.
As to using 968 heads I only think this would be an advantage if you are going to rev the engine higher than what is the current norm. That itself requires a lot of work, for me I will run dry sump and a moldex crank with the honda journals so less bearing speed but with out getting off topic you may be able to fit larger than 39 mm valves to the heads of the 968, I would aim for 41 mm and leave the exhaust how it is or maybe 35 m.
The GT3 uses 41 mm and 34 mm but it has a better exhaust port. Also you may save some money on the porting front with the 968 head as it probably only needs the ports cleaned up and valve area blended to the new larger valves. If I was to build a 4 valver, (one day I hope) I will also change the lifter setup and use the GT3 lifters, this is not that hard as you will need new custom cams you could get a GT3 grind as a master.
The GT3 lifter is radiused and as such allows a very fast ramp rate similar to a roller cam. This is the ultimate in bucket cam operation, (the bucket weighs 31 grams full of oil) the absolute ultimate being the finger follower design. So then if you use the GT3 profile and have heads that flow in proportion to the GT3 you will get by my calculation an engine of about 6.1 to 6.2 liters using a 90 mm stroke crank. This stroke with a high piston pin position will allow high revs in the order of 8000+ like the GT3.
This should deliver about 115 to 120 hp per liter and as such result in about 700 flywheel hp with nice drivability and very little strain as the mass forces have been reduced, also the higher revs should allow for longer between gears and less strain on the drivetrain which is where I have always been worried in regards to durability and the associated costs of rebuilding and or replacing gearboxes especially as these parts will dry up eventually.
I don't know how I got on to this train of thought, (I blame Brendan) as I need to get back to my old ancient 2 valve heads. Which are flowing very nicely BTW.
Greg
As to using 968 heads I only think this would be an advantage if you are going to rev the engine higher than what is the current norm. That itself requires a lot of work, for me I will run dry sump and a moldex crank with the honda journals so less bearing speed but with out getting off topic you may be able to fit larger than 39 mm valves to the heads of the 968, I would aim for 41 mm and leave the exhaust how it is or maybe 35 m.
The GT3 uses 41 mm and 34 mm but it has a better exhaust port. Also you may save some money on the porting front with the 968 head as it probably only needs the ports cleaned up and valve area blended to the new larger valves. If I was to build a 4 valver, (one day I hope) I will also change the lifter setup and use the GT3 lifters, this is not that hard as you will need new custom cams you could get a GT3 grind as a master.
The GT3 lifter is radiused and as such allows a very fast ramp rate similar to a roller cam. This is the ultimate in bucket cam operation, (the bucket weighs 31 grams full of oil) the absolute ultimate being the finger follower design. So then if you use the GT3 profile and have heads that flow in proportion to the GT3 you will get by my calculation an engine of about 6.1 to 6.2 liters using a 90 mm stroke crank. This stroke with a high piston pin position will allow high revs in the order of 8000+ like the GT3.
This should deliver about 115 to 120 hp per liter and as such result in about 700 flywheel hp with nice drivability and very little strain as the mass forces have been reduced, also the higher revs should allow for longer between gears and less strain on the drivetrain which is where I have always been worried in regards to durability and the associated costs of rebuilding and or replacing gearboxes especially as these parts will dry up eventually.
I don't know how I got on to this train of thought, (I blame Brendan) as I need to get back to my old ancient 2 valve heads. Which are flowing very nicely BTW.
Greg
03-28-2008, 09:42 PM
#20
Rennlist Member
Thread Starter
