Discussion in 'Stage II Tech' started by Alky V6, Feb 18, 2012.
I threw more stroke at the sim. Boy, that was throwing a monkey wrench into things.
At the fear that this will start another disaster like the last theory thread, here's my opinion. It's been quiet for a few days anyways.
Like Dusty says adding stroke is going to shift the hp curve to the left on the rpm scale. If the hp is limited by the turbo, then in theory changing the stroke has no affect on hp it only changes the rpm it's made at. What does that do to torque?
Engine 1 makes peak power of 1650 hp with a 3" stroke at 9000rpm and has a 4" bore
Engine 2 makes peak power of 1650 hp with a 3.625" stroke at 8000rpm and has a 4" bore
Since HP1 of engine 1 equals HP2 of engine 2. Then TQ1*rpm1/5252 = TQ2*rpm2/5252
5252 is on both sides and crosses out.
Solving for the torque on engine 2
TQ2 = TQ1(rpm1/rpm2)
Since we said for the same hp the rpm on engine 2 is going to lower than rpm on engine 1. rpm1/rpm2 is going to be some number bigger than 1. Lets pick 9000/8000 = 1.125
So engine 2 is going to make 1.125 times more torque than engine 1. If the cam is right, this is mostly true for the whole torque curve, not just at peak hp.
Since torque is really what accelerates the car, my preference is always to have more torque.
Lets look at is from another angle. Rod load:
Obviously more rpm means more stress on a rod.
Imagine both motors need to make the same torque to accelerate the car to the same 60'.
TQ = Force * Stroke/2
Force = Cylinder pressure * Bore area
so: TQ = CP * B * S/2
Assuming both need to make the same torque for a given 60'. TQ1 = TQ2 so:
CP1 * B1 * S1/2 = CP2 * B2 * S2/2
If both motors are the same bore size then B1 = B2 and they cross out. The /2 is on both sides and also crosses out.
CP1 * S1 = CP2 * S2
Solve for cylinder pressure on engine 1.
CP1 = CP2 (S2/S1)
S2/S1 = 3.625/3 =1.21
CP1 = 1.21 CP2 That means the 3" stroke motor needs to make 1.21 times more cylinder pressure than the 3.625 stroke motor to make the same torque and thus the same 60'.
More cylinder pressure means more rod load. So for drag racing, where torque rules in the 60', you are stressing the rod more. And where hp rules on the back end, your stressing the rod more because of the rpm.
The other advantage to less rpm is the decrease in valve spring pressure and corresponding maintenance.
You tell me what RPM gains you? I've been told that you can do some intersting stuff with the torque converter as rpm increases, but I'm definitely no converter expert. In an NA motor sometimes you need that rpm to use all available cfm in the head. It's very debateable, but I don't think the same is quite as true for a turbo limited motor.
Interesting, Mike. Thanks for taking the time to post.
That all brings up another interesting question. How much torque do you need inorder to perform a respectable 60' with a 3300 lb car on a 10.5" tire with stock suspension? Will the 3.06 stroke leaving the line at 6200 rpm provide enough torque for the launch? With the long stroke, would you end up having to pull back a substantial amount of power to control the launch?
As things are with the 3.06 stroke, the drag sim is predicting a 1.16 60', with allowing the sim to pull back power to control traction at the beginning of the launch. I would imagine that adding more crank to the situation would be a waste.
I understand the durability issues. Those are important points to weigh.
Understand that I've established a max target cylinder pressure that I've used in the past, and I'm not allowing the sim, with these new calcs, to go over that limit.
The durability issues associated with high rpm are still a concern. I'm not worried so much about the rods (short stroke = slower piston speed) as I am about the valvetrain. That will take some careful planning on every aspect of the valvetrain.
There is also the option to temporarily install monster valve springs just to test max limits, then install lighter springs for more everyday action, limiting rpm, of course.
You are right, Mike. The longer stroke does move the torque curve left. I was a little surprised to see how drastically, though. It also tops out the compressor capacity and head flow capacity sooner.
The redline of the stroker motor would end up being about 1,000 rpm lower, as you eluded to in your post.
Maybe a 3.4" stroke wouldn't be a bad compromise.
Adding stroke does drastically increase the avg torque and hp throughout the rpm range. The big question is, for my chassis situation, is it needed?
If a person were entering an engine dyno contest where max output over the entire rpm band is the major concern, then I can see wanting to add crank. But, I'm trying to launch a 3300 lb car on 10.5" tires with a stock type suspension. How much torque do you really need?
The drag sim had to pull back throttle 69% to maintain traction at the hit of the launch using the most recent power curve I put together for the 3.06" stroke motor.
That depends on your track. But for the tracks here in the south east where a lot of 10.5 class racing happens, you can put down a lot more than you would think. Guys with 400"+ engines can leave with 6-8 lbs. of boost on a true 10.5. That's most likely near 1000 ft. lb. before the converter, trans ratio and rearend ratio does it's work.
If you can make the same power, but more torque with less stress and wear on the engine what is the disadvantage. It's always easier to back off the power than it is to try and squeeze out more.
As it is with the Stage I motor, I cannot use all that is available from that motor at the launch. I do have to control it back. I'm guessing I'll have to pull back with the Stage II heads, too. I'm guessing stroking it would be a waste, if I'm already pulling back.
One consideration that I feel is very important when going with a stroker crank, is the split wrist pin arrangement of the even fire crank. I believe that the split pin arrangement becomes weaker as the stroke is increased. Consequently, the split wrist pin arrangement is stronger as the stroke is decreased. To me, that is a much more important consideration than what the rods will have to go through at high rpm.
You could build an Odd fire motor. Several others have gone this way. Then you wouldn't have to worry about the structural integrity of the crank as the stroke length increases.
At this point, I'm not interested in building an odd fire motor. I like the sound of the even fire with tuned exhaust too much. If I were building a max effort low seven second attacker, then I would seriously think about going that route. I also have to add, that I think in that sort of situation, odd fire would be the only way to go.
Using that justification wouldn't you just stick with the M&A heads? Are you saying you don't want to make more power?
This ignores the cheek of the crank and any fillets, but worst case.
With a 2.25" rod journal split 30 degrees. The 3.06" stroke has a cross section between journals of 2.232 in^2. The 3.625 is 1.928 in^2. The 3.400 is 2.047 in^2. The 3.625 crank has 86% of the area a 3.06 does. The 3.625 is 94% of a 3.400 crank.
Cross section area and strength are directly related. So 86% area means 86% of the strength.
I did some hand calculations for shear using 4340, but I question the answer. The strength looks way too high for even the 3.625. I'm getting over 20,000 psi of cylinder pressure for the 3.635 on a 4" bore to break a heat treated 4340 crank. Normal turbo cylinder pressure is I think around ~3000psi. Or something like 30,000 ft.lb. of torque. I must be missing something.
In reality you would break the crank due to cyclic loading. But the math for that is more involved than I want to look at right now. But the two factors that determine life cycle are cycles per second and max and min load. When you turn the rpm up, that cycles per second number goes up and life goes down. It's a trade off. Short stroke crank with more area, but high cycles per second. Or long stroke with less area but lower cycles per second. For two motors that make the same power, they likely have the same life.
Very good information, Mike. And, very good points. As I was reading, the fatigue factor came to mind, but you brought that up later in your post.
There's room for improvement after the launch and on the top end. Particularly on a good track. Besides, I just have to do this Stage II deal.
The plan is not to take the motor up to 9,200-9500 rpm on every pass. I'm really thinking that what I'll end up doing is have a special 'killer' spring set on the shelf for special events, or whenever James calls me out , or when I know I'll be visiting a track that can handle it. Otherwise, I'll have a tamer spring set in the car and limit rpm to 7500-8000 rpm. Even then, I think I'll still have about 100-150 more hp at that rpm level than what I have with the Stage I heads now. That will be more than enough hp to play around with at my local track.
Thanks for the graphics, Mike. The illustration helps to visualize the difference rather dramatically.
Some interesting facts about piston speeds and G force differences between 3.06" and 3.625" strokes.
RPM = Piston speed (ft/min);
7000 = 3570
7250 = 3698
7500 = 3825
7750 = 3953
8000 = 4080
8250 = 4208
8500 = 4335
8750 = 4463
9000 = 4590
9250 = 4718
9500 = 4845
RPM = Piston speed (ft/min);
6000 = 3625
6250 = 3776
6500 = 3927
6750 = 4078
7000 = 4229
7250 = 4380
7500 = 4531
7750 = 4682
8000 = 4833
8250 = 4984
8500 = 5135
8750 = 5286
9000 = 5438
9250 = 5589
9500 = 5740
Piston Gs at 9000 rpm with 3.06" stroke = 4350
Piston Gs at 8000 rpm with 3.625" stroke = 4210
Piston Gs at 8250 rpm with 3.625" stroke = 4480
This is interesting. The shift point I commonly used with the Stage I engine was 7200-7400 rpm. The piston speed at 7250 rpm with the 3.06 stroke is the same as the piston speed would be with the 3.625" stroke at 6125 rpm.
And I was worried that the rods might be stressed by the rpm. If they check out, they're going back in.
Donnie, you need to design some combo that is going to be reliable, higher rpm is going to be less reliable. You may want to reduce the rpm and just increase boost, and air flow though turbine and header size etc,
Norbs, you obviously have a different idea of what makes a racing engine. To each their own.
You build your engine the way you want to, and I'll build mine the way I want to.
The light blue and red trace lines are using a 3.625 stroke with a 218/218 dur, 115 l/s camshaft. It was installed somewhere between 3-5 degrees advanced.
The green and dark blue trace lines are using a 3.06 stroke and my crazy cam specs.
Both are using my manifolding specs, although the manifolding specs are having far less effect with the smaller camshaft.
Both are using the same turbo. FI91X.
Life of a racing engine doesn't start 'till after 5,000 rpm.
Even if I use lighter springs and limit my rpm to 7800 rpm again, check the graph.
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