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Recon1342
Recon1342 SuperDork
5/24/22 11:20 a.m.

Daily is a '97 F-150, 4.6 V8, five speed manual. Small motor, so it doesn't like being lugged AT ALL, and is happiest shifting around 2500-2750 rpm. The downside is that I can count about one and a half Mississippi's before the revs come back down and I can shift. Being a 4.6 mod motor, there are tons of flywheel options out there. Is it worth considering a lightweight flywheel? 

therieldeal
therieldeal Reader
5/24/22 11:37 a.m.

In my opinion a truck is the wrong application for a lightweight flywheel. 

You want that flywheel weight to help get you moving smoothly from a stop, especially when loaded or towing.

pointofdeparture
pointofdeparture UltimaDork
5/24/22 12:01 p.m.

Yep, I would not put a lightweight flywheel in any vehicle that has to haul or tow.

buzzboy
buzzboy SuperDork
5/24/22 12:58 p.m.

Factory is 32lbs from what it looks. Heck, I'd go heavier if it was an option. But that's just me. Smooth torque is what I want in a truck(or really anything I drive).

Curtis73 (Forum Supporter)
Curtis73 (Forum Supporter) MegaDork
5/24/22 1:01 p.m.

The weight of the flywheels stores inertia.  Lighter flywheels often means longer clutch engagements using the friction to apply torque to get moving.  Like most things it's best used when other things are light.... like the gross combined weight of the rig.

The opposite is true any time you drive a Taco with the 22re.  They compensated for the heavier truck and low-torque engine by using a really heavy flywheel.  It's a whole new learning curve driving one.  You clutch to upshift and the RPMs hang way up high making re-engaging the clutch smoothly a chore... but the flipside is you can pretty much pop the clutch from a stand-still and it doesn't stall, it just lurches forward.

ProDarwin
ProDarwin MegaDork
5/24/22 1:03 p.m.
Recon1342 said:

I can count about one and a half Mississippi's before the revs come back down

Are you sure that is flywheel?  Every car I have experienced that in was due to programming.  IAC or throttle plate control for emissions reasons.

79rex
79rex HalfDork
5/24/22 1:07 p.m.

Im just here to say we have a very different thoughts on what a small motor is 

GameboyRMH
GameboyRMH MegaDork
5/24/22 1:25 p.m.
ProDarwin said:
Recon1342 said:

I can count about one and a half Mississippi's before the revs come back down

Are you sure that is flywheel?  Every car I have experienced that in was due to programming.  IAC or throttle plate control for emissions reasons.

This. You can partially work around it by lifting off the gas slightly before disengaging the clutch.

Curtis73 (Forum Supporter)
Curtis73 (Forum Supporter) MegaDork
5/24/22 1:31 p.m.
79rex said:

Im just here to say we have a very different thoughts on what a small motor is 

In truck speak, 4.6L is pretty small.  Life begins at 5.7L.  Fun begins at 8.2L

Curtis73 (Forum Supporter)
Curtis73 (Forum Supporter) MegaDork
5/24/22 1:39 p.m.
GameboyRMH said:
ProDarwin said:
Recon1342 said:

I can count about one and a half Mississippi's before the revs come back down

Are you sure that is flywheel?  Every car I have experienced that in was due to programming.  IAC or throttle plate control for emissions reasons.

This. You can partially work around it by lifting off the gas slightly before disengaging the clutch.

I've never seen anything in the programming that suggests IAC stays open. The IAC is pretty "dumb."  It stays closed and only signals to open when the TPS reports zero and RPMs drop to idle.  At least on stuff up to 2003.  Not sure I've played with newer stuff.

This was also a problem with carbureted 300s, 302s, and 351s where there is nothing allowing extra air in.  When the throttle is closed, it's closed.  A flywheel that is heavier than necessary can cause the same problem.  The added inertia sucks harder against the closed throttle plates.

ProDarwin
ProDarwin MegaDork
5/24/22 1:45 p.m.
Curtis73 (Forum Supporter) said:
GameboyRMH said:
ProDarwin said:
Recon1342 said:

I can count about one and a half Mississippi's before the revs come back down

Are you sure that is flywheel?  Every car I have experienced that in was due to programming.  IAC or throttle plate control for emissions reasons.

This. You can partially work around it by lifting off the gas slightly before disengaging the clutch.

I've never seen anything in the programming that suggests IAC stays open. The IAC is pretty "dumb."  It stays closed and only signals to open when the TPS reports zero and RPMs drop to idle.  At least on stuff up to 2003.  Not sure I've played with newer stuff.

This was also a problem with carbureted 300s, 302s, and 351s where there is nothing allowing extra air in.  When the throttle is closed, it's closed.  A flywheel that is heavier than necessary can cause the same problem.  The added inertia sucks harder against the closed throttle plates.

What cars?

I used to mess with Saturns, and all of the 1.9L cars do that (starting in 1991).  I (and many others) made a block off plate to remove the IAC for this specific reason.  The difference is night and day.  I know many other cars suffer from this as well.

Pete. (l33t FS)
Pete. (l33t FS) MegaDork
5/24/22 5:47 p.m.

In reply to ProDarwin :

Every computer I have been into would hold the IAC at a certain percentage, makes the return to idle less likely to undershoot and overcompensate.

A lot of the OE tunes have BAD throttle hang.

APEowner
APEowner SuperDork
5/24/22 7:01 p.m.

Rapidly closing the throttle creates a lean condition which causes high levels of NOx emissions. The throttle hang is done in the engine calibration to prevent that.

mdshaw
mdshaw HalfDork
5/24/22 7:35 p.m.

In reply to Curtis73 (Forum Supporter) :

Wrong Curtis, in truck speak fun begins with diesel-5.9, 6.7, 7.3.

Pete. (l33t FS)
Pete. (l33t FS) MegaDork
5/24/22 10:41 p.m.
APEowner said:

Rapidly closing the throttle creates a lean condition which causes high levels of NOx emissions. The throttle hang is done in the engine calibration to prevent that.

Rich condition, no?

You need an accelerator pump (or acceleration enrichment) because when you open the throttle, the pressure in the manifold goes up, causing fuel to condense out of suspension and cling to the inside of the manifold.  This stays in equilibrium until you close the throttle again, at which point it re-evaporates under the higher vacuum and gets pulled into the engine.

Decent EFI hardware models how much fuel is wetting the manifold, which has got to be fun to set up smiley

Given that every EFI scheme I am aware of does deceleration fuel cutoff, killing the injectors entirely under closed throttle, I do not think NOx is that much of an issue especially at such low loads.

Recon1342
Recon1342 SuperDork
5/25/22 7:38 a.m.

In reply to Everybody :

So, it's probably not the flywheel then?

wink

Curtis73 (Forum Supporter)
Curtis73 (Forum Supporter) MegaDork
5/25/22 10:16 a.m.
ProDarwin said:
Curtis73 (Forum Supporter) said:
GameboyRMH said:
ProDarwin said:
Recon1342 said:

I can count about one and a half Mississippi's before the revs come back down

Are you sure that is flywheel?  Every car I have experienced that in was due to programming.  IAC or throttle plate control for emissions reasons.

This. You can partially work around it by lifting off the gas slightly before disengaging the clutch.

I've never seen anything in the programming that suggests IAC stays open. The IAC is pretty "dumb."  It stays closed and only signals to open when the TPS reports zero and RPMs drop to idle.  At least on stuff up to 2003.  Not sure I've played with newer stuff.

This was also a problem with carbureted 300s, 302s, and 351s where there is nothing allowing extra air in.  When the throttle is closed, it's closed.  A flywheel that is heavier than necessary can cause the same problem.  The added inertia sucks harder against the closed throttle plates.

What cars?

I used to mess with Saturns, and all of the 1.9L cars do that (starting in 1991).  I (and many others) made a block off plate to remove the IAC for this specific reason.  The difference is night and day.  I know many other cars suffer from this as well.

2003 F150 with a 4.6L

IAC on the scanner was 100% closed at all times except when the ECM showed zero on TPS and approaching idle.  Like it might start fluttering at 1000 rpms on decel to start transitioning to idle which was around 800.  This was not a full diagnostic test of the programming parameters, I was just diagnosing a high idle problem.  Conditions were pretty much the same both hot and cold (open and closed loop)

my 96 LT1 would hold the IAC at about 3% for one second on decel, then full shut until the same parameters as above.  Every once in a while it would cycle the IAC to a couple percent and fire the injectors on decel which I assumed was for maintaining catalyst heat.

Curtis73 (Forum Supporter)
Curtis73 (Forum Supporter) MegaDork
5/25/22 10:20 a.m.
mdshaw said:

In reply to Curtis73 (Forum Supporter) :

Wrong Curtis, in truck speak fun begins with diesel-5.9, 6.7, 7.3.

that's right you know - Morgan Freeman "he's Right u know" | Meme Generator

APEowner
APEowner SuperDork
5/25/22 3:44 p.m.
Pete. (l33t FS) said:
APEowner said:

Rapidly closing the throttle creates a lean condition which causes high levels of NOx emissions. The throttle hang is done in the engine calibration to prevent that.

Rich condition, no?

You need an accelerator pump (or acceleration enrichment) because when you open the throttle, the pressure in the manifold goes up, causing fuel to condense out of suspension and cling to the inside of the manifold.  This stays in equilibrium until you close the throttle again, at which point it re-evaporates under the higher vacuum and gets pulled into the engine.

Decent EFI hardware models how much fuel is wetting the manifold, which has got to be fun to set up smiley

Given that every EFI scheme I am aware of does deceleration fuel cutoff, killing the injectors entirely under closed throttle, I do not think NOx is that much of an issue especially at such low loads.

I should have typed idle hang not throttle.  The hang isn't necessarily a result of the IAC being open but rather from fuel being added to what would otherwise be a lean condition.  As you pull your foot off the throttle a mass of air enters the chamber without any fuel. When you do reapply throttle, it creates a lean condition which generates NOx. Delaying the fuel cut-off adds fuel to keep the mixture correct and avoids the lean condition to prevent the NOx from occurring.

 

Fupdiggity (Forum Supporter)
Fupdiggity (Forum Supporter) Reader
5/25/22 9:58 p.m.

Another vote for "not the flywheel"

My first lesson on rev hang vs rotational mass came when I had a 96 powerstroke & 90 volvo 240, both manual. The powerstroke would drop revs quickly between gears, the volvo would hang annoyingly. One time I disconnected the MAF (or afm, can't remember) on the volvo and it stopped hanging between shifts. The powerstroke allowed quicker shifts than the volvo, mostly because the volvo wasn't running 100%, but that told me that rotational mass of the big 'ol diesel wasn't enough to noticiably slow down shifts. 

Now that I think about it, the powerstroke had much closer gearing due to the small power band, not sure how that factors in to percieved shift speed.

Curtis73 (Forum Supporter)
Curtis73 (Forum Supporter) MegaDork
5/26/22 9:20 a.m.

It's a little hard though to compare apples to apples.  A diesel with 18:1 compression, not throttle, and full defeuling at zero throttle is hard to compare to a modern EFI gas with all kinds of possibilities.

I agree.  My 7.3 strokes all snapped to idle pretty quickly despite having a pretty hefty flywheel but I think that was primarily due to the nature of the diesel's internal mechanicals.

I think what we've determined about this is "it depends."  It's obvious that some ECMs tailor the "rpm hang" with IAC or TBW and some don't.  

I can say this much without question.... if you have a carbureted engine and do nothing other than change for a lighter flywheel, the RPM hang is much less pronounced.  The engine wants to return to idle against the vacuum of the closed throttle and if you have less inertia strapped to the crankshaft, it will return faster.  That's just basic physics.

I think what we need to know here is what the scanner says the IAC is doing on this Ford before we can definitively say how a flywheel will affect that particular combo.  I foresee two possibilities, assuming the ECM is using the IAC to slow the RPM loss:

1- the ECM has a pre-determined set of parameters assuming a certain rotating mass.  It keeps the IAC open on X curve for Y seconds.  In this case, the RPM drop should be hastened when using a lighter flywheel
2- the ECM has an algorithm for keeping RPM on a certain curve and will compensate for the lighter flywheel by keeping the IAC open longer.  In this case the lighter flywheel won't likely change the hang.

Either way, I don't think that lightening the flywheel is the smartest move for a tow vehicle.  You might find that it's excessively heavy and maybe dropping a few pounds would be a better match for RPM drop and ratio spread, but I think we would need to know what the ECM is doing (if anything) to keep RPM hanging.

Recon1342
Recon1342 SuperDork
5/26/22 9:47 a.m.

I love this forum so much. You guys are awesome!

wspohn
wspohn SuperDork
5/26/22 12:51 p.m.

Flywheel weight is relative.  Traditional British cars tended to use quite heavy flywheels - the Austin truck engine used in the Austin Healey 100 had one that weighed somewhere around 30 pounds!

I use a 10 lb wheel on an MG race engine and while very responsive on the track I think it would be tedious on the street.  It depend on the engine.

BTW, although I have used alloy wheels for racing, you can also have a steel wheel lightened by 'windowing' as well as thinning.   This is a Spitfire wheel reduced to around 9 lbs.

 

 

GameboyRMH
GameboyRMH MegaDork
5/26/22 12:55 p.m.

Relevant issue, I just put some lightweight pulleys on my 86, which has a relatively small but still noticeable amount of rev hang even with the aftermarket tune - maybe something that's hard to tune out or maybe the e-throttle isn't capable of closing quite as fast as a cable throttle? The pulleys have done, at best, nothing to improve this - it had been a few weeks since I drove the car so I can't do anything like a back-to-back comparison, but I wouldn't be surprised if they actually made it slightly worse. Maybe it's because I'd been driving an E30 that revved like a sportbike in a simulator recently, but I'm really noticing the need to lift off the throttle early when shifting now.

Recon1342
Recon1342 SuperDork
5/26/22 9:57 p.m.

In reply to GameboyRMH :

I don't think the F1Shifty is DBW. It just doesn't return to idle as quickly as I think it should. The good folks here at GRM have me wondering if it can be tuned out...

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