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GM LE2 engine conversion

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wsimpso1

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Yes Bill, I will be spinning the water pump impeller at a higher equivalent speed than the engine but will not be anywhere near redline RPM on it. The tip speed will still be well below cavitation speed. I am running smaller than stock diameter coolant lines so will see more restriction. This was also because the pulley I used is an aluminum one I can source easily and shed some weight but also fit the alternator where I want. It's all a compromise isn't it?
Critical cases on cooling modern cars are usually protracted idle and Davis Dam/Baker Hill, all corrected to 100F with AC on Max. Davis Dam and Baker Hill are miles at 7% grade and 35 mph on a 100 F day with the AC in max. This is typically around 2000 rpm and medium throttle, the transmissioncooler and AC condensor are dumping heat on the radiator and even the radiator is usually sized based on this test condition. The pump size and spin speeds are selected to be a little bigger than needed for that. Once you have enough flow for Davis Dam and Baker Hill, the other conditions are usually well covered.

Ross over at SDS (rv6ejguy on here) has a bunch of experience with high performance ground vehicles and auto conversions in airplanes, with thousands of his systems running. Good reading... I do not remember him ever talking about raising accessory speeds on any conversions for airplanes, and he uses small coolant tubes too.

Airplanes cannot even run at the critical point on car engines (30-35% rpm/75% torque/low air speed) that the car guys live in - a good flight propellor can not make much torque/power at 30% rpm - they need to run in the 75-90% rpm range for flight, where we have a lot of airspeed and a lot of pump rpm.

Unless I had some good data or a downtown level analysis indicating I needed the coolant pump to be spun faster than stock, I would be really reluctant to do it. Costs you horsepower better spent turning the prop and has to shorten bearing and seal life in the coolant pump too.

Billski
 
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Chris Matheny

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Bill, My main reason for the pulley diameter change was weight, packaging, (with the alternator right below it) and cost. I have emailed a vendor about another pulley sized the same as stock but in aluminum and when i make the alternator bracket i may be able to safely increase the diameter to stock. I do have more cooling restriction than stock but since this water pump is the same as the 1.5L engine also i'm guessing it to be very ample for this application. I am familiar with the Davis Dam testing, especially with the towing tests they do there, its amazing. With the pump being inexpensive I just looked at it like a wear item to be replaced every year or two when a coolant drain was done. I am probably a little over zealous on maintenance though, always have been that way. Thanks for the advice guys.
 

Chris Matheny

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I got a good bit done tonight on the torsional vibration damper and should have it machined by the end of the weekend with pics to come. I'm starting off with the simplest of my 3 ideas, I think it will absorb the high spikes that a 4 cylinder has and tame them more than enough for my gearbox to handle them. Pics to come, just updating this so people know its still moving forward.
 

Chris Matheny

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As promised, I got the crankshaft side of the damper done this weekend. Here's a couple of pics. It will be using a rubber isolation bushing from Moog. Snapchat-27459981.jpgSnapchat-1321425852.jpg
 

pictsidhe

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I'd also not be speeding up the water pump. Slowing it down is a better idea. If it really needs to spin fast due to packaging, could you find a smaller one?
 

Chris Matheny

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I have sourced another pulley (original diameter but aluminum) but will not know if clearance will be an issue with the alternator until a little later in the project. I am ultimately trying to make all these parts easy to source if anyone decides to try this same conversion in the future. Thanks guys
 

plncraze

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Thank you for sharing your experience. Doing things online can get interesting so anytime someone shares a real project that is going through development and construction it is appreciated.
 

Chris Matheny

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What was you selection criteria for the rubber bushing? Have you calculated the required compliance?
They are 2.700" from center, with 130hp at 4200RPM 163#TQ with the impulse torque of a 4 cylinder they will be seeing a peak shock of 890# per bushing and a mean of 240#.

Bushing deflection.
100# 0.037"
200# 0.051"
300# 0.064"
400# 0.077"
500# 0.088"
600# 0.098"
700# 0.109"
800# 0.120"

800# is as high as I was able to test with my setup but looking at the pattern from .400 on it seems fairly linear at .010-.011 per 100# so at the 890# it will see in this application I would speculate it would be in the .128-.130 range. I will say that determining where its natural frequency will occur is beyond my math ability so if anyone can determine that and for see any issues I am all ears. Thanks
 
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wsimpso1

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They are 2.700" from center, with 130hp at 4200RPM 163#TQ with the impulse torque of a 4 cylinder they will be seeing a peak shock of 890# per bushing and a mean of 240#.

Bushing deflection.
100# 0.037"
200# 0.051"
300# 0.064"
400# 0.077"
500# 0.088"
600# 0.098"
700# 0.109"
800# 0.120"

800# is as high as I was able to test with my setup but looking at the pattern from .400 on it seems fairly linear at .010-.011 per 100# so at the 890# it will see in this application I would speculate it would be in the .128-.130 range. I will say that determining where its natural frequency will occur is beyond my math ability so if anyone can determine that and for see any issues I am all ears. Thanks
I would be happy to do the calcs. We need the mass moment of inertia for the engine side (crankshaft and flywheel minimum) and then gear ratio, and tHe rest of the parts. Input shaft parts and gear, output shaft and gear, and propellor. If you model the crank, flywheel, and shafts in SolidWorks, get the right density into the program, it has a tool to give you MMOI about the axes used in modeling the parts. The prop, well, let's hope the prop maker has that number for you. The other way is to research a torsional pendulum for estimating MMOI of the parts. This is all sqrt(k/m), and we have k, need the m's.

Billski
 

Chris Matheny

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Billski, I will take you up on that as soon as all the parts are done. It's not the fastest build as you can see so it may be a bit before I have numbers. I do not have any modeling software as of now but I believe i will be investing in some soon. You're not too far from me so maybe ill just drive the prototype up there and have you look it all over when its done.
 

pictsidhe

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A torsional pendulum is pretty easy to set up. Hang your thing from 2 wires, st moving and time the oscillations. I have a spreadsheet to crunch the numbers. The hardest bit is getting it to rotate cleanly without swinging. I'm tempted to contrive something better than my fingers next time I try that...
 

Chris Matheny

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Working on the output flange for my turbocharger. Wrote the program and machined an aluminum prototype tonight. I'll probably cut the titanium one tomorrow if I get time. Still trying to progress a little at a time.
 

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wsimpso1

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I finally set up calculations for your proposed damper. Before I get into numbers, let's get into specifics and then what I think about this stuff. First, I want to check the numbers and my assumptions:
  • The forces listed are for one bushing loaded radially;
  • There are three bushings in the assembly, with them assumed to be equally sharing load;
  • The working radii is 2.7";
  • The aluminum piece is bolted to the crank flange;
  • There is a flywheel bolted between the aluminum piece and the crank flange;
  • The gearbox side will have a similar piece with forks that straddle each bushing;
  • And the engine is a four cylinder making mean torque of 163 ft-lb;
I can estimate mean firing accels of the engine from this data and from your peak firing torque of 840 ft-lb.

Now for some discussion. This system has "soft" elements deliberately included and so we shall assume a "soft" system where the 1st natural order in torsion must be at least one-half octave below idle firing frequency. This puts the system in the isolating side of the classically taught frequency response curve. If it really there, the actual torques made through the isolating system will be mean input torque plus the firing "swing" of the crankshaft times the the spring rate in use. Swing is the second integral of the firing accel - usually we assume the firing accel curve is close enough to a sine wave, that we assume sinusoidal and deflection becomes alpha/omega^2 (in radians). Alpha for normally aspirated gas engines usually make peak torque around 1 ft-l/in^3 and accels peak at about 2500 rad/s/s. For turbocharged engines, the accels scale approximately with peak torque. Now if we have a map of firing accels or P-t curves for an engine, we can be more precise, but these rules of thumb work pretty well if we do not have engine based specifics.

Torques delivered to the gearbox should be a modest amount above mean torque. While we can usually figure out what max torque is from the P-t curve, and this torque will be seen by the bolted joint between crank flange and flywheel, it is not seen through the rest of the isolated system.

Chris' job from here is to get the inertia of the parts upstream of the isolator (crankshaft and flywheel) and the parts downstream (gears, shafts, prop flange, and prop). Somewhere in the thread, I am sure I saw the gear ratio, but I do not recall where and am reluctant to read some 96 posts again. My job is to use Chris' data to estimate the natural frequency of the system based upon the bushings for spring rate. If the first natural frequency is more than a half octave below idle firing frequency, we can then go about checking peak torque etc. If natural frequency is close to desired ground idle, we might want to get the spring rates of the other pieces too. If the natural frequency is higher than firing frequency at the idle, the system must have either substantially lower rate springs as the soft elements or substantially more inertia on the engine side (usually the flywheel) or both.

Since the flywheel and spring elements are part and parcel of everything between the crank flange and the gearbox, I suggest that we would want to do this analysis before making finalizing design of the cases that bolt to rear face of block.

Billski
 
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