slate blue

Can somebody post a picture of these Level two headers for me, I need some ideas before making mine. Mine will be different but all info is handy. Thanks Greg.

Fastest928

Might go here for starters...

https://rennlist.com/forums/showthre...ght=devek+days

slate blue

Marc appreciate your help, I like the look of them, I also like the look of those centre mufflers that are for the S4. i.e the ones that go near the rear crossmember. How well do they fit? I was thinking about opening up S4 ones and re plumbing them for a larger pipe, 3 inches in my case. This is what I'm doing when it comes to my rear muffler on my S2. I'm putting new internals in, i.e new 3 inch perforated pipe.

This way I know the packaging will fit. My system will be stainless, 304 as I'm told 321 is a bitch and unnessecary. What are your mufflers made from and the fitment as far as the bracketing goes? The stock ones again will fit up nice but a lot of work too to replumb. Any thoughts?

Cheers Greg

BC

You know the Proffessor said the same damn thing. The stroke is what, 85mm? 89mm? I forgot.

Louie928

Brendan,
My thoughts on using the GTS crank rather than a 95mm "stroker" crank go like this. The 6.5L stroker motor makes lots of torque. If the engine is built toward performance with 968 valves, some porting, and a GT or B1 cam, the torque is easily high enough to be into the trans shredding territory. With the 95mm (3.74") stroke, the piston speed gets very high if you spin it to high rpms. 6500 rpm is 4052 ft/min piston speed. If you want to force feed this engine too, it will destroy the stock driveline if using sticky tires.

The GTS crank is 89mm (3.38") stroke. All else being equal as compared to the 6.5L motor, this motor will be 6.0L (with 104mm bore). It will make less peak torque, and the torque peak will be at a higher rpm. The lower torque value is less dangerous to the transmission. The peak hp will be very close to that of the 6.5L motor, but will occur at about 1000 rpm higher. To get to the same piston speed as the 95mm motor at 6500 rpm, you can turn the 89mm stroke motor to 7200 rpm. The 89mm crank vs. 95mm can use a longer rod which gives another slight advantage. Boosted, it will still make enough torque to hurt the driveline, if you get greedy, but still safer than the long 95mm stroke.

slate blue

By Louis Ott
Fisrt the GTS crank is 85.9 mm in stroke but what you are saying here is what I have been saying for some time, it was the reason I bought the Honda Rods (To offset grind either a GTS crank or weld and offset grind a standard crank, the welding would be minimal about a 2 mm build max and then hard chrome over the journals) I bought those 6.2" Honda Carrillos so as to make a 6.0 or 6.1 that would not have such a crappy rod to stroke ratio as a normal stroker. Its' ratio would have been around 1.75 to 1.

You will lose nothing in hp terms and infact with greater useable rpms you may well benefit by this because you can stay in lower gears and thus increase acceleration while decreasing drivetrain loads. You will probably also lower wear due to the lower skirt pressure and peak piston speeds that occur with short rod to stroke engines.

Cheers Greg

heinrich

Can you brainiacs explain the piston speed thing please? I think if the rear wheels are turning at a particular speed, then your crank is turning around its own axis at the same speed with stroker or GTS or S4 or S cranks. At that speed, the smaller throw cranks cause the pistons to move a shorter distance for that same axis rotation, thereby causing the smaller throw piston to have to move back and forth more often, vs large throw which pistons are still on their way somewhere upwards or downward?

Looking from the engine side, the combustion causes the large throw piston to move at the same rate as the small throw piston, let's say down the chamber. At the same rate of piston travel, the large throw crank should be pushing the crank at the same force, but for longer time, which means the .... ugh, I don't know

Vilhuer

It's simple math. For one crank rotation, pistons moves once up and down. Since both short and long stroke engines rev at same speed (giving that gearing is same) long stroke piston must cover longer distance for each revolution. This is done during same timeframe, meaning speed must be faster. Faster speed increased load piston is subjected to. It's accelerated to top speed and decelerated back to zero again twice during each crank rotation. As said, top speed for longer stroke must be higher for it to cover greater distance at same time.

heinrich

I think I get it. So it's like gearing the piston. Right? Combustion doesn't have to work as hard to push it along because it has a greater lever and isn't pushing as hard against the crank?

SharkSkin

H,

Maybe an example will help. Set a stock engine alongside a hypothetical stroker. Run both engines up to 6,000 RPM. Many of the internals on both engines will be under approximately the same stress, however the speed at which the piston on the stroker sweeps along the cylinder wall at it's fastest point might be comparable to what it woul be in the stock motor at 7,000 RPM. This may increase wear on the bores and pistons/rings, and will increase stress on rods, wrist pins, etc due to the greater accel/decel forces on each stroke. Piston speed is always 0 at TDC & BDC, but maxes out at ~90° when the crank is perpendicular to the bore..

heinrich

See Dave that's where you lose me. In order to turn that flywheel at 7,000 RPM, don't you have far greater stresses on the little short-throw rotating mass? To illustrate my thought, what if the crank throw were only 1mm. Wouldn't the piston have to work incredibly hard to push that crank? And conversely, if the throw is one foot, won't the piston have a relaxed time pushing and turning the crank? yes, I do see that there is greater distance covered per revolution, but is that distance not an easier one on the piston and rod and bearings, than the short throw scenario?

sublimate

Yes, that's essentially true. To get the same torque there has to be more force on the piston (due to combustion) of a short stroke motor than a longstroke motor (note that if the engines are the same displacement then the increase in force is spread out over a larger piston).

But the forces of combustion are much, much, much smaller than the huge inertial forces that occur when the piston has to change direction. That's why rods almost always fail in tension (they're stretched at the top of the stroke when they try to pull the piston back down) rather than in compression (which is the force that combustion puts on them as it pushes the piston down). In fact, larger combustion pressures tend to help an engine by reducing the tension on the rod on the first half of the power stroke (although it does put more stress on the head gasket).

atb

Heinrich,

If you use short rods/tall pistons on one crank and tall rods /short pistons on another with the same stroke, the tall rod motor will make more power even though the stroke is the same. The long rod motor's pistons will have a longer the hang time at TDC, and therefore can be tuned to accept more combustion forces before starting its down stroke. Also, once the long rod motor does start its down stroke, the crank will have advanced more than the short rod (edit: used to say stroke) motor. If you freeze both motors in time at this point in time (start of the downstroke), the longer rod will be applying the force of combustion at a greater angle on the crank (the crank throw will be closer to 90 degrees) than the shorter rod, which gives it a mechinal advantage over the shorter rod although they are both using the same combustive force.

Schools out. (Hope my analysis is correct)

heinrich

Thanks Victor. Now, if the problem is inertia at direction reversal, then wouldn't a piston that changes direction half as often, experience less strain over its life? Just asking, I don't know these things

heinrich

Adam thanks. That is very interesting. So, longer rods, shorter throw. Longer throw, shorter rods. I guess this stuff *is* rocket science.