Can lightweight/aluminum flywheels cause jerky cruising?
03-19-2016, 04:00 AM
#1
Can lightweight/aluminum flywheels cause jerky cruising? I’ve been searching for the solution to this problem for years. Ever since I installed this engine, it’s been jerky/bucky while cruising. Now I’m at the point where I have literally ruled out all other causes and this is all that’s left. I’ve been avoiding it because of the cost involved and it never seemed as likely as other possible causes.
The car lightly jerks forward and backward, not side to side, while cruising. Usually goes away around 55-60mph. Decel is fine, acceleration is outstanding, idle is fine. Even though it smooths out at freeway speed, under light load at those speeds, it will buck. It will also smooth out at lower speeds under high load. The bucking is not consistent, sometimes it’s harder, sometimes it’s softer, and sometimes it’s extremely frequent, sometimes there are short pauses between jolts. Sometimes (usually during the first 1-2 minutes after a cold start), it will be completely Cadillac smooth. These changes can happen at any time. The inconsistency is the main reason I thought it was something else. When I first put the engine in, I had a Spec stage 2 clutch and aluminum flywheel. Now I have a Mcleod clutch and Fidanza aluminum flywheel.
So can a lightweight/aluminum flywheel cause this? I really like the way the car drives and behaves in all other conditions besides cruising (which I’ll admit this is my DD and cruising is 90% of what the car does). I’m just worried I’ll put a 25-30lb flywheel on and it won’t fix my problem and make the car less fun to drive, then I’ll be out a ton of money for nothing.
The car lightly jerks forward and backward, not side to side, while cruising. Usually goes away around 55-60mph. Decel is fine, acceleration is outstanding, idle is fine. Even though it smooths out at freeway speed, under light load at those speeds, it will buck. It will also smooth out at lower speeds under high load. The bucking is not consistent, sometimes it’s harder, sometimes it’s softer, and sometimes it’s extremely frequent, sometimes there are short pauses between jolts. Sometimes (usually during the first 1-2 minutes after a cold start), it will be completely Cadillac smooth. These changes can happen at any time. The inconsistency is the main reason I thought it was something else. When I first put the engine in, I had a Spec stage 2 clutch and aluminum flywheel. Now I have a Mcleod clutch and Fidanza aluminum flywheel.
So can a lightweight/aluminum flywheel cause this? I really like the way the car drives and behaves in all other conditions besides cruising (which I’ll admit this is my DD and cruising is 90% of what the car does). I’m just worried I’ll put a 25-30lb flywheel on and it won’t fix my problem and make the car less fun to drive, then I’ll be out a ton of money for nothing.
03-19-2016, 04:23 AM
#2
9 Second Club
You say you've been searching for a solution....so what have you tried so far ?
I'd say it's more than likely tuning related, but really without being in the car etc it's almost impossible to say from afar.
I'd say it's more than likely tuning related, but really without being in the car etc it's almost impossible to say from afar.
03-19-2016, 06:14 AM
#3
I use HP Tuners and the things I have tried are running spark everywhere from 19*-50*, running everything from 0.9 to 1.06 lambda, running SD, MAF only, open loop, running IAC more closed and more open, adjusted cracker/follower/rolling idle, closed loop PID's... you name it, I adjusted it in the name of smoothness. Now I have the tune set to MAF only open loop with afr at 14.42. The tune is dead nuts. In closed loop the fuel trims are within 3% of each other, in open loop afr is right at 14.42.
As far as the physical engine, I've tried different injectors, different fuel pressure regulator, upgraded from stock LT1 engine management to 24x, checked for vacuum leaks, checked cylinder compression, tried different spark plugs, checked for exhaust leaks, you name the sensor, it's been ruled out.
The only things I can think of are the light flywheel or something in the long block. Everything I've found about lightweight flywheels only talks about the differences in starting from a stop compared to a heavier flywheel. I haven't found anything online where someone complains about jerky cruising but a few people have mentioned in my other threads over the years that a lightweight flywheel COULD cause my bucking. Now I want to ask here in the manual transmission section if anyone has heard of this or has experienced it themselves.
As far as the physical engine, I've tried different injectors, different fuel pressure regulator, upgraded from stock LT1 engine management to 24x, checked for vacuum leaks, checked cylinder compression, tried different spark plugs, checked for exhaust leaks, you name the sensor, it's been ruled out.
The only things I can think of are the light flywheel or something in the long block. Everything I've found about lightweight flywheels only talks about the differences in starting from a stop compared to a heavier flywheel. I haven't found anything online where someone complains about jerky cruising but a few people have mentioned in my other threads over the years that a lightweight flywheel COULD cause my bucking. Now I want to ask here in the manual transmission section if anyone has heard of this or has experienced it themselves.
03-19-2016, 07:30 AM
#4
9 Second Club
So what is the total weight of your clutch/flywheel combo ?
Of course it is possible it could contribute to it...but it's more then likely tuning.
Easiest thing to do, is drive it and datalog to see if there are any anomalies when it is occurring where something control related might be the cause.
Of course it is possible it could contribute to it...but it's more then likely tuning.
Easiest thing to do, is drive it and datalog to see if there are any anomalies when it is occurring where something control related might be the cause.
03-19-2016, 08:19 AM
#5
I would think that if it were flywheel related, it wouldn't "jerk" but it would be a continual shaking directly related to engine RPMs. Does the engine do odd things when idling, or you rev it slightly? How about in neutral with clutch or no clutch?
Are you sure it's not driveline or axle? Like a bad pinion angle on cruising/no squat, or maybe the diff has a lot of play? Just my thoughts.
Are you sure it's not driveline or axle? Like a bad pinion angle on cruising/no squat, or maybe the diff has a lot of play? Just my thoughts.
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03-19-2016, 04:11 PM
#9
So what is the total weight of your clutch/flywheel combo ?
Of course it is possible it could contribute to it...but it's more then likely tuning.
Easiest thing to do, is drive it and datalog to see if there are any anomalies when it is occurring where something control related might be the cause.
Of course it is possible it could contribute to it...but it's more then likely tuning.
Easiest thing to do, is drive it and datalog to see if there are any anomalies when it is occurring where something control related might be the cause.
I would think that if it were flywheel related, it wouldn't "jerk" but it would be a continual shaking directly related to engine RPMs. Does the engine do odd things when idling, or you rev it slightly? How about in neutral with clutch or no clutch?
Are you sure it's not driveline or axle? Like a bad pinion angle on cruising/no squat, or maybe the diff has a lot of play? Just my thoughts.
Are you sure it's not driveline or axle? Like a bad pinion angle on cruising/no squat, or maybe the diff has a lot of play? Just my thoughts.
I thought so too but I tried to mimic those cold start conditions when the engine is warm by advancing fuel injector timing and setting afr to 13.2. When the engine is warm it didn't matter, still jerky. I even have a professional from over seas who's been helping me through email for the last year and he couldn't tune it out. Now he's thinking I need a heavier flywheel with more inertia to get smooth cruise conditions. I can see here though that jerky cruising isn't a common light flywheel problem.
Last edited by AdsoYo; 03-30-2016 at 12:46 AM.
03-19-2016, 07:32 PM
#11
9 Second Club
A tuner thousands of miles away isnt going to fix it. You need an experienced and competent person in the car tuning it, experiencing exactly what you're referring to as the problem when it occurs.
Trying to fix something like that remotely really is pissing into the wind. It isnt to say you may not get lucky, but it's just a small chance.
Trying to fix something like that remotely really is pissing into the wind. It isnt to say you may not get lucky, but it's just a small chance.
03-20-2016, 07:11 AM
#12
Sorry forgot to answer. The flywheel is 12.5lbs and the clutch and pressure plate are the Mcleod Street Pro clutch kit, don't know the weight on that.
Thanks for your vote.
He offered to help for free and has been doing a great job. We were able to rule out a lot and he really dialed in the tune.
When I was running the stock PCM I had it dyno'd by one of the best known tuners in my state and he wasn't able to smooth it out. I feel like I'm pissing into the wind no matter what.
If anyone else with aluminum flywheel experience runs across this thread, please chime in.
Thanks for your vote.
A tuner thousands of miles away isnt going to fix it. You need an experienced and competent person in the car tuning it, experiencing exactly what you're referring to as the problem when it occurs.
Trying to fix something like that remotely really is pissing into the wind. It isnt to say you may not get lucky, but it's just a small chance.
Trying to fix something like that remotely really is pissing into the wind. It isnt to say you may not get lucky, but it's just a small chance.
When I was running the stock PCM I had it dyno'd by one of the best known tuners in my state and he wasn't able to smooth it out. I feel like I'm pissing into the wind no matter what.
If anyone else with aluminum flywheel experience runs across this thread, please chime in.
03-20-2016, 07:33 AM
#13
I ran the 12lb fidanza flywheel and a mcleod rst twin which was 33lbs together so all in a ll a pretty light setup and never had jerking. Once you got going it was just like stock. My bet is its in the tune.
Another thing to consider is that if your cam is wild and prone to surge you may be feeling that. A heavier clutch will dampen some surging from the engine due to its inertia. The lighter setup doesnt have as much mass to do so....Even in that case I would never go back to a heavy clutch though
Another thing to consider is that if your cam is wild and prone to surge you may be feeling that. A heavier clutch will dampen some surging from the engine due to its inertia. The lighter setup doesnt have as much mass to do so....Even in that case I would never go back to a heavy clutch though
03-20-2016, 12:17 PM
#14
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I ran the 12lb fidanza flywheel and a mcleod rst twin which was 33lbs together so all in a ll a pretty light setup and never had jerking. Once you got going it was just like stock. My bet is its in the tune.
Another thing to consider is that if your cam is wild and prone to surge you may be feeling that. A heavier clutch will dampen some surging from the engine due to its inertia. The lighter setup doesnt have as much mass to do so....Even in that case I would never go back to a heavy clutch though
Another thing to consider is that if your cam is wild and prone to surge you may be feeling that. A heavier clutch will dampen some surging from the engine due to its inertia. The lighter setup doesnt have as much mass to do so....Even in that case I would never go back to a heavy clutch though
03-20-2016, 02:32 PM
#15
an EFI reprogram is always suspect but...
i would offer yes it is the lightweight flywheel, and if you have an aftermarket drive shaft without a torsional damper then that would further add to the problem.
simple fact a flywheel is there for a reason, it is a "massive object" that contains momentum to help smooth out the natural pulsation of the engine. you start reducing that mass and there should be no surprise pulsation and NVH gets worse.
4 cylinders are the worst and would have the most pulsation, not up until you get to the v-12 where that is said to be naturally balanced but it still has a flywheel. how much mass you need in the flywheel for a given engine to reduce the pulsation enough that you as a driver do not notice... i don't know.
I do know with new s60 rear and aftermarket aluminum drive shaft on my 2002 with manual trans that i have more nvh and i attribute it most to not having a torsional damper on the trans slip yoke.
with reduced flywheel mass and increased pulsation, i also suspect the PCM now has to deal with that, and it would notice that pulsation from the crank sensor signal which indicates engine rpm. as that pulses and changes, the computer will try and correct that, and would not surprise me if it gets into a hysteresis loop where you then have the engine fuel/ignition management not helping the problem go away and might make it worse. What makes me think this is you say it's not happening after cold start, which is when things run in open loop and there would be no feedback... would be like on an old sbc having a carb where you lock the distributor in at 10-12° timing at idle and it idles super smooth, because there is no feedback causing more problems than helping.
if the application is drag strip where it's full throttle 100% of the time then ok not a big deal with a reduced mass flywheel. but for a street car that you want to drive nice on the highway, where the engine is running only at 10%-30% load then flywheel mass is definitely a positive, along with torsional dampers to absorb vibes and pulsation.
if you can easily reprogram your PCM, i would try locking everything out regarding closed loop and any feed back from O2 and any other sensor. Take that out of the equation while running on the highway, if you still have the same jerk and pulsation then that would further prove flywheel could be the problem. If you ever were to do a new flywheel, give a look at corvette dual-mass flywheels.
i would offer yes it is the lightweight flywheel, and if you have an aftermarket drive shaft without a torsional damper then that would further add to the problem.
simple fact a flywheel is there for a reason, it is a "massive object" that contains momentum to help smooth out the natural pulsation of the engine. you start reducing that mass and there should be no surprise pulsation and NVH gets worse.
4 cylinders are the worst and would have the most pulsation, not up until you get to the v-12 where that is said to be naturally balanced but it still has a flywheel. how much mass you need in the flywheel for a given engine to reduce the pulsation enough that you as a driver do not notice... i don't know.
I do know with new s60 rear and aftermarket aluminum drive shaft on my 2002 with manual trans that i have more nvh and i attribute it most to not having a torsional damper on the trans slip yoke.
Sometimes (usually during the first 1-2 minutes after a cold start), it will be completely Cadillac smooth
if the application is drag strip where it's full throttle 100% of the time then ok not a big deal with a reduced mass flywheel. but for a street car that you want to drive nice on the highway, where the engine is running only at 10%-30% load then flywheel mass is definitely a positive, along with torsional dampers to absorb vibes and pulsation.
if you can easily reprogram your PCM, i would try locking everything out regarding closed loop and any feed back from O2 and any other sensor. Take that out of the equation while running on the highway, if you still have the same jerk and pulsation then that would further prove flywheel could be the problem. If you ever were to do a new flywheel, give a look at corvette dual-mass flywheels.
03-21-2016, 06:46 AM
#16
I ran the 12lb fidanza flywheel and a mcleod rst twin which was 33lbs together so all in a ll a pretty light setup and never had jerking. Once you got going it was just like stock. My bet is its in the tune.
Another thing to consider is that if your cam is wild and prone to surge you may be feeling that. A heavier clutch will dampen some surging from the engine due to its inertia. The lighter setup doesnt have as much mass to do so....Even in that case I would never go back to a heavy clutch though
Another thing to consider is that if your cam is wild and prone to surge you may be feeling that. A heavier clutch will dampen some surging from the engine due to its inertia. The lighter setup doesnt have as much mass to do so....Even in that case I would never go back to a heavy clutch though
This was my thought as well. I went through something similar. I have a pretty choppy can and installing light clutch exposed some surging that wasn't there before. The lighter stuff improves throttle response, so your IAC motor moves a little bit and the motor surges faster than the IAC can react.
an EFI reprogram is always suspect but...
i would offer yes it is the lightweight flywheel, and if you have an aftermarket drive shaft without a torsional damper then that would further add to the problem.
simple fact a flywheel is there for a reason, it is a "massive object" that contains momentum to help smooth out the natural pulsation of the engine. you start reducing that mass and there should be no surprise pulsation and NVH gets worse.
4 cylinders are the worst and would have the most pulsation, not up until you get to the v-12 where that is said to be naturally balanced but it still has a flywheel. how much mass you need in the flywheel for a given engine to reduce the pulsation enough that you as a driver do not notice... i don't know.
I do know with new s60 rear and aftermarket aluminum drive shaft on my 2002 with manual trans that i have more nvh and i attribute it most to not having a torsional damper on the trans slip yoke.
with reduced flywheel mass and increased pulsation, i also suspect the PCM now has to deal with that, and it would notice that pulsation from the crank sensor signal which indicates engine rpm. as that pulses and changes, the computer will try and correct that, and would not surprise me if it gets into a hysteresis loop where you then have the engine fuel/ignition management not helping the problem go away and might make it worse. What makes me think this is you say it's not happening after cold start, which is when things run in open loop and there would be no feedback... would be like on an old sbc having a carb where you lock the distributor in at 10-12° timing at idle and it idles super smooth, because there is no feedback causing more problems than helping.
if the application is drag strip where it's full throttle 100% of the time then ok not a big deal with a reduced mass flywheel. but for a street car that you want to drive nice on the highway, where the engine is running only at 10%-30% load then flywheel mass is definitely a positive, along with torsional dampers to absorb vibes and pulsation.
if you can easily reprogram your PCM, i would try locking everything out regarding closed loop and any feed back from O2 and any other sensor. Take that out of the equation while running on the highway, if you still have the same jerk and pulsation then that would further prove flywheel could be the problem. If you ever were to do a new flywheel, give a look at corvette dual-mass flywheels.
i would offer yes it is the lightweight flywheel, and if you have an aftermarket drive shaft without a torsional damper then that would further add to the problem.
simple fact a flywheel is there for a reason, it is a "massive object" that contains momentum to help smooth out the natural pulsation of the engine. you start reducing that mass and there should be no surprise pulsation and NVH gets worse.
4 cylinders are the worst and would have the most pulsation, not up until you get to the v-12 where that is said to be naturally balanced but it still has a flywheel. how much mass you need in the flywheel for a given engine to reduce the pulsation enough that you as a driver do not notice... i don't know.
I do know with new s60 rear and aftermarket aluminum drive shaft on my 2002 with manual trans that i have more nvh and i attribute it most to not having a torsional damper on the trans slip yoke.
with reduced flywheel mass and increased pulsation, i also suspect the PCM now has to deal with that, and it would notice that pulsation from the crank sensor signal which indicates engine rpm. as that pulses and changes, the computer will try and correct that, and would not surprise me if it gets into a hysteresis loop where you then have the engine fuel/ignition management not helping the problem go away and might make it worse. What makes me think this is you say it's not happening after cold start, which is when things run in open loop and there would be no feedback... would be like on an old sbc having a carb where you lock the distributor in at 10-12° timing at idle and it idles super smooth, because there is no feedback causing more problems than helping.
if the application is drag strip where it's full throttle 100% of the time then ok not a big deal with a reduced mass flywheel. but for a street car that you want to drive nice on the highway, where the engine is running only at 10%-30% load then flywheel mass is definitely a positive, along with torsional dampers to absorb vibes and pulsation.
if you can easily reprogram your PCM, i would try locking everything out regarding closed loop and any feed back from O2 and any other sensor. Take that out of the equation while running on the highway, if you still have the same jerk and pulsation then that would further prove flywheel could be the problem. If you ever were to do a new flywheel, give a look at corvette dual-mass flywheels.
Why go dual-mass vs steel? Looking around online my only options seem to be stock iron from Luk, a bunch of steel choices ranging from about 23lbs to 33lbs, and a bunch of aluminum choices from about 12.5lbs to 18lbs. Couldn't find any dual-mass. I should mention that I had this lightweight drive train with my stock engine and the car still felt like stock with no jerkiness or bucking whatsoever.
I like your idea about the crank sensor picking up pulses, never thought of that. My firsthand experience with other performance cars is pretty limited but the most hardcore aspects of this engine are the 11:1 compression, cam, and displacement and those really are not impressive. I feel like cars with way crazier set-ups than this have stock street manners.
03-24-2016, 08:34 PM
#20
I suspect it's cam and gearing related and a touch of tuning and I'll explain my reasoning. When you're at 50-60mph your probably loading and unloading the driveline due to engine rpm and light load needed to keep the car at a constant speed. When you are in a different gear or at a higher speed in the same gear you have constant load on the engine and driveline. It wouldn't surprise me if you said you were turning less than 1850rpms when it happens. Cams play a huge part in it because many grinds don't smooth out until above your cruising rpm.
One other thing not mentioned that could be a factor is ring gear play, If you have an excessive amount of pinion/ring gear play it will make the problem worse.
One other thing not mentioned that could be a factor is ring gear play, If you have an excessive amount of pinion/ring gear play it will make the problem worse.