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Rear gear selection

7K views 41 replies 4 participants last post by  CrucialProspect 
#1 ·
While sitting here idle due to the virus watching the racing season go down the toilet, I’ve been looking a lot through dyno records, et slips, and 60’ times from the past couple seasons. I’m going to try to think through some issues on my e.t.

Currently run a full spool, bigger shafts, baseline suspension outlaw kit, etc, but still stock 3.27 gear. Also running a stock gear 4R trans with 28 10.5 m/t slick out back. My best 60’ to date (before aluminum block rebuild this year and some other weight reduction) has been 1.40, which I believe is pretty sad. I run mostly 1/8th mile which I shift into 3rd right at the traps at 6.668 at right around 100mph. I have the notion to swap in a 3.73 and see if it would be better. By raising the rear gear, I would not have the tendency to over power the tire at the shift point which, by looking at the datalog info, I see a spike before the shift which would indicate such. As I shift, it’s two-fold, rpm drops several hundred rpm and wheel speed decreases which would indicate it gaining traction again during the shift.

By looking at dyno curves, I’m hitting 615ftlbs at 4k and flatlines to redline, and 650hp at 5500rpm, also launching around 35-3800rpm, shifting around 6500, by the datalogging I can see the 1-2 shift drops quite a bit of rpm and I believe that’s where I’m losing the most power and time. I am able to build 10psi on the turbos off the transbrake at launch and it seems to hook, but as my suspension guru says, it doesn’t seem to launch very hard.

I have a lot of people telling me I should be going faster than I am and I’m trying to encompass the big picture. By raising the rear gear, I could possibly change my 60’ but would it change the overall picture? My thinking is the wheel speed would be slower throughout the 1/8th without the loss of traction, but because of such the rpm drop between shifts would not change.

I guess what I’m trying to see here is if a gear change is the right route, or if the rpm drop on shifts that is essentially dropping me out of the power and is the underlying issue. Obviously the only fix seems to me to be raise the shift point higher, therefore the 1-2 shift would still be in the peak power rpm band, but like I said, I’m suspecting loss of traction as a factor at the shift as a factor to the bogging of second gear.

Let me know all your opinions, I’m welcome to suggestions!
 
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#2 ·
Before track change (crappy prep) a tire change (Hoosier QTP to M/T slicks,) and baseline suspension, I was at a 1.98 60’ rolled through the traps at 98.7.

After all those changes early in the year, and some great tuning help from Kevin, In one weekend I went from a 1.98 to immediate 1.65 then finally whittled down to 1.40 A few weekends later at the end of the year. I believe the engine was at the end of its rope due to compression loss and ring gap erosion therefore warranting its rebuild this year. If it were making the power it did recorded during the time of the dyno sessions, it’s safe to say I’d break the 1.3 barrier.

I believe there is more in it I haven’t figured out yet and I should be deep in the 1.3’s maybe even 1.2 range, and should also have 110-115 traps. At least by some calculators, I’m WAY slower than I should be.

But who knows, it could be all pipe dreams and I need to turn up the boost. But I believe I’ve maxed out the suspension and it’s as good as it’s gonna get, so I’m looking elsewhere.
 
#3 ·
...I run mostly 1/8th mile which I shift into 3rd right at the traps at 6.668 at right around 100mph.
That is low rpm for the engine. You should target a 7500 rpm engine speed in the traps. Don't worry about MPH, worry about launch, 60 feet, and shift points / engine speed in the lights. You should be targeting a shift rpm of 7500 or higher. Don't worry about MPH it will take care of itself.

I have the notion to swap in a 3.73 and see if it would be better. By raising the rear gear, I would not have the tendency to over power the tire at the shift point which, by looking at the datalog info, I see a spike before the shift which would indicate such.
Increasing the engine's mechanical advantage with taller gears will aggravate wheel spin because you apply more torque to an already overpowered tire. Go up a tire size and gear the car, with the new tire, for 7500 rpm in the lights.

As I shift, it's two-fold, rpm drops several hundred rpm and wheel speed decreases which would indicate it gaining traction again during the shift.
Engine rpm drop after a gear change is solely associated with differences in transmission ratios. You can buffer it a bit with a high stall converter as long as your shift point in inside the converter stall window. If the converter is a 4000 rpm stall converter and your engine speed is 4500 after a shift the converter will provide no cushioning effect until it's stall speed is greater than the the engine speed after the shift.

If your wheel speed decreases after you shift, your car would be nosing over and slowing down, indicating a different problem.

By looking at dyno curves, I'm hitting 615ftlbs at 4k and flatlines to redline, and 650hp at 5500rpm, also launching around 35-3800rpm, shifting around 6500, by the datalogging I can see the 1-2 shift drops quite a bit of rpm and I believe that's where I'm losing the most power and time. I am able to build 10psi on the turbos off the transbrake at launch and it seems to hook, but as my suspension guru says, it doesn't seem to launch very hard.
If you want a smaller rpm drop on the 1-2 gear change then you need to either change the first and second gear ratios or go to a higher stall converter. The engine does not loose power on a gear change. It looses torque because of the reduced mechanical advantage in the higher gear. More boost at launch will give more torque at launch. Ignition is equally important. you are looking for MBT timing. You also need to visit your accelerator pump enrichment tables to experiment with how much additional fuel you are providing at launch.

.... By raising the rear gear, I could possibly change my 60' but would it change the overall picture?
Yes.

My thinking is the wheel speed would be slower throughout the 1/8th without the loss of traction, but because of such the rpm drop between shifts would not change.
Wheel speed and performance are linked at the hip. Anytime the car performs better the wheel speed is higher. If it is not then the car's performance is lower.

A simple example;

Assume it takes 100 revolutions of the tire to cover a finite distance, your race track. One revolution of the tire is 72 inches or six feet. That means your race track is 600 ft long, the tis 100 revolutions x 6 feet. Lets examine two different passes in the same car. Pass #1 is done in 6 seconds and pass #2 is done in 7 seconds. Both passes are 600 feet long. Pass #1 the wheel turns 100 times in.6 seconds (0,100 minutes) meaning it was spinning 1000 rpm, average. The wheel in pass # 2 covered the same distance but it took 7 seconds (0.1167 minutes) meaning it was spinning 857 rpm average.

Higher wheel speed => higher performance.

I guess what I'm trying to see here is if a gear change is the right route, or if the rpm drop on shifts that is essentially dropping me out of the power and is the underlying issue.
You should gear for a target rpm in the traps of 7500 rpm. You should shift at 7500 rpm or higher to keep the engine rpm closer to the engines peak power after a gear change,

Obviously the only fix seems to me to be raise the shift point higher, therefore the 1-2 shift would still be in the peak power rpm band, but like I said, I'm suspecting loss of traction as a factor at the shift as a factor to the bogging of second gear.
Raising the shift point is the correct answer. You need to shift above 7500 rpm.

I would give some thought to a 5000 rpm stall converter with a lockup capability for quarter mile tracks.

Ed
 
#4 ·
Thanks Ed, I’m running circle d’s multi disk 3800 stall. I don’t want the headache of tubbing for a taller tire, I've finally found one that seems to hook. That being said we were on the same page, raise redline/shift points above 6500.
Only reason I shift to third before the traps is because I’m on the rev limiter in 2nd right before I cross.

By raising shift rpm I’m likely to be in second through the traps at redline.

If I do get the opportunity for 1/4 mile, I still have another gear and lockup to.
 
#5 ·
Fact of the matter is, when shifting at 6500rpm, I drop to 3500rpm in second gear. Because I cannot change that EASILY, even by raising to 7k, would drop to 3800, 7500 would drop to a good goal at 4100rpm.

Because the car was set to redline at 6500rpm, what are any other limiting factors on raising shift rpm? Valve float, running out of fuel, timing are some things that come to mind initially.

Also, as I said, launching at 4k....should I start creeping up until I find the fine line between over powering the tire and the highest launch rpm possible?
 
#6 ·
Fact of the matter is, when shifting at 6500rpm, I drop to 3500rpm in second gear. Because I cannot change that EASILY, even by raising to 7k, would drop to 3800, 7500 would drop to a good goal at 4100rpm.
7500 is a walk in the park for these engines mechanically.

Because the car was set to redline at 6500rpm, what are any other limiting factors on raising shift rpm? Valve float, running out of fuel, timing are some things that come to mind initially.
Valve float should not be a problem at 7500 rpm. More significant will be fuel supply which is a two dimensional problem. Dimension #1 is the fuel pumps. Do they have adequate volume at a base pressure of 39psi. Dimension #2 is fuel injectors. do they have adequate capacity to fuel the engine at an injector duty cycle of 75% or less. There has been a lot of discussion over the years about 85% and higher duty cycles. Buy big enough injectors you are not always hedged in by the injectors limiting your fuel supply to the engine.

To size your stuff go to the TToC and download a copy of the Fuel System Calculator and the instructions PDF document. The calculator will provide all the obscure, hard to get answers.

Also, as I said, launching at 4k....should I start creeping up until I find the fine line between over powering the tire and the highest launch rpm possible?
If you have a 4K still converter you should be launching at 4K. If you strike the tires you've got tire and chassis work to do to fix the problem. Put enough tire under the car and fix the chassis so it optimizes the launch with the new tires and the 4K stall speed.

Ed
 
#7 ·
Ed, I’ll check out the fuel system on the ttoc, I doubt I’ll have other issues.

Here’s another thought, by going with a 4.56 gear and launching in second, even with a 6500 redline I'm in the same ballpark as a 7500 redline.

I believe the only reasons the redline was set at 6500 was because it wasn’t making any more power, and it’s still stock computer, other than that I have no reason to believe it couldn’t turn 7500.
 
#9 · (Edited)
Ed, I'll check out the fuel system on the ttoc, I doubt I'll have other issues.
I suspect you are right but like an old carpenter somewhere once said, 'always measure twice and cut once'.

Here's another thought, by going with a 4.56 gear and launching in second, even with a 6500 redline I'm in the same ballpark as a 7500 redline.
Not a good idea at all! The overall ratio at launch will be decreased and your 60 ft times will increase. Additionally the ET and MPH will fall off. You are not making enough power in a light enough car to be able to do that.

I believe the only reasons the redline was set at 6500 was because it wasn't making any more power, and it's still stock computer, other than that I have no reason to believe it couldn't turn 7500.
You probably have an ignition reserve problem that is causing spark blowout around 7000 rpm. The fix is IGN-1 or IGN-1A coils. Your ECU will only support the IGN-1 coils. The -1A coils are not operable with the OEM ECU. Additionally the clock speed and internal architecture of the OEM ECU is 20 years old. Moores Law says your ECU compared to commercially available stuff today is right on par with a pre-historic stone axe - comparatively speaking. You ought to check out the current MS3Pro alternatives for the car.

Ed
 
#10 ·
I contacted the tuner to see what he could dig up, I doubted it was fuel, and I know the stock ecu “can handle” over 7200, with the right components. You are correct, it may have been snuffing the spark and that’s why it leveled off.

Regardless you are correct, about the only good way is to raise redline. Every other way is a bandaid.
 
#13 · (Edited)
What's the cam spec's? 7500 RPM may be stretching it for the cam in use.
My cam is spec'd to work with a 6500 max red line and by 6500 mine is comfortably done so I doubt it would be beneficial to go beyond 7000. Dyno pulls were stopped at 6200 or until I made the 900+ HP goal but a few lower HP pulls went to 6500 to check fueling.

Watching Mat's vids' the car doesn't explode off the line to get a hard launching 60'. It needs a lot more power in the form of a lot of boost or RPM or a combination of both. Now that he has good tires, a higher stall (4500 or more) would be fantastic because then some boost could be built up easily enough to try and over power the tires on the hit instead of 3-8ft out. Once that kinda hit is achieved then we can get the rest of the chassis dialed in to handle it. The first 6"-3ft is where it needs help.

ks
 
#14 · (Edited)
It is possible to install the cams to 'lay down' somewhere north of 7000 rpm, but you have to intentionally take that step, Kevin. My bet is it is a tuning and potentially an ECU issue. Most people would not and/or do not know how to install cams to do that. Usually when you do that, it produces a huge increase in low speed power because of how you have shifted the power curve downward in the engine's operating range. In Matt's case he is not dealing with huge low speed power issues, quite the contrary. The cams that JM uses are 12mm lift (0.470-iish) relatively smooth profiles installed to favor upper engine speed to allow his 10K+ rpm engine speeds. Low speed power is hard to evaluate on that car because it launches several thousand rpm higher than most modmotors shift at. I would be surprised to find that style of cam or cam phasing in Matt's engine.

The higher stall converter would be a significant benefit on two different fronts, one certainly is launch and the other would be cushioning the rpm drop between gears to provide a higher sustained power level to the tire. On any gear change that is below the converter stall threshold the engine speed would rise to the converter's stall speed. Higher stall means higher sustained engine operating speed (and power) throughout the rpm range. If the car is to be a dual purpose car high stall converters can make daily driving less enjoyable but race track operation will be quite nice. A lock up converter can go a long way to mitigating some of the bad manners a high stall converter has in daily driving.

The real need (like you suggest) is for more torque boost at launch and that is going to require mods to the tune and more sophistication in the ECU to allow access to and modification of the tune to produce and manage that. The stock ECU can get you well down the road but when the low speed power does start to rise the stock ECU lacks the sophistication to manage it, so you can actually get the car to launch hard w/o blowing the tires off - i.e. traction control among other things.

All that said Matt should still be able to see low 1.3 second, possibly better, 60 ft times. Like you suggested it is going to take more boost to get the steam in the boiler up for launch.

Ed
 
#15 ·
What a blessing having two of the most respected minds on my thread!

I guess Kevin can agree, the thread should be renamed to “why my 60’ sucks!”

Let me read everything y’all posted and gather my thoughts. Also, my tuner agreed to raise redline to 7500, he said it shouldn’t hurt a thing.
 
#16 ·
Attached are cam specs, and the dyno sheet.
Font Material property Number Symmetry Circle

Rectangle Slope Parallel Pattern Tints and shades

As I said, I've been steadily increasing launch rpm by the end of last year, it may very well have more in it that I didn't get a chance to see due to the end of the season.

I do know according to the go-pro when I'm at 4k about to launch, I've built 10psi on the transbrake.

One thing to keep in mind was that was I found out I had low compression in about 6 cylinders. So, was I down on power? Probably so. At least a bit.
 
#18 · (Edited)
You might want to check out where the cams were actually installed, Matt.

In one location, the cam card indicates the cams are installed straight up (0˚ Adv/Ret) which would imply a 2˚ overlap as illustrated by Mark Olson's CamChart S/W below;

Font Parallel Circle Symmetry Slope


More towards the top of the cam card, the card indicates the cams are ground on a 114˚ C/L for the intake and a 122˚ C/L for the exhaust which would be a 118˚ LSA advanced 4˚. That would look like the image below;

Font Parallel Circle Symmetry Slope


Of course neither of these two different phasing may be what the cams actually are or are installed at. It is possible they are different and were installed with a third phasing that was yet different than either of those above.

I am beginning to think it might be a good thing to check them out just to know what they really are and where they really are installed at. BTW they certainly are not shrinking violets in the valve lift department. :)

Ed
 
#19 ·
Ed, I installed them with the comp cams adjustable gears you recommended. And I installed them with dial indicator on valves, I believe you were holding my hand the whole time 😂

If you would like I can go back through that thread and my notes and pictures and get the numbers.
 
#21 ·
I went back and re-read your post (here is the link=> https://www.modularfords.com/threads/259311-Turbo-Engine-Re-Build/page5?p=2262567&viewfull=1#post2262567 I had forgotten that you did the phasing and you documented it. You are correct the cams are at 114.

When you describe cam phasing use the advance and retard representations as modifiers to the actual cam phasing. For example if the cam is cut on 114˚*centers as yours are, then installing them at 114 is putting them in straight up. If you install them at 112 and 116 then you have advanced the cam(s) 2˚. To get that 2˚ advance may require an offset at the cam of a different number like 4˚, 6˚, 8˚. That offset is typically compensating for one of two things.

  • The first. is a predisposition of some cam grinders to build a small advance into the cam for low speed performance. They know most of the cams will end up in street vehicles and poor bottom end is a kiss of death for the cam manufacturer follow on sales.
  • The second, is just a sloppy core supplier that did not get the cam keyway precisely at or near the zero point. Cam cores have additional material on the raw lobes which the cam grinder removes more or les of for a particular cam profile. If the core supplier is far enough off the mark then the finished goods cam can be a few dereees off of where you might expect it to be with the chains and dots "lined up".

Another potential whoops you want to be attentive to, as the engine builder (particularly on the 4V engines), is the fit of the exhaust cams in the heads. Some core suppliers will provide the exhaust cams with five ten thousandths (one half thousandth) additional on the cam journals. I have never gotten a good explanation of why. When you install the cams in the heads the intakes spin effortlessly. The exhausts have a very noticeable drag associated with rotating them. The drag is the additional five, ten thousandths of an inch diameter on the journals. Most of the time not a problem but then there is always the one time it is a problem. In my experience it has only been a problem when I build the engine. Everyone else seems to get a pass on the problem.

Matt's engine is already built so the next comments are for everyone else who later reads this thread. The last thing to get oiled in an OHC engine is the cams. That means during the build cycle the last thing you do as you assemble the cams to the heads is to add a single drop of ProLong (<= clickable) to each cam saddle and cam cap to provide lubrication at cold start before the oil system can deliver lube to the journals. Then always run ProLong in the oil to protect the top of the engine when you garage the car for a week or more because the lube will run out of the space between the cm journals and the cam saddles asking the cm to scuff the saddles.

Parting thoughts on the Prolong. A 12oz bottle costs somewhere between $15 and $20. The gallon bottle on the clickable link above costs $64 and has 128 oz of ProLOng in the bottle. Do the math - its way less expensive to by the gallon bottle.
 
#22 ·
I also like to pre-lube using the Peterson pump a few minutes prior to starting.

As Kevin stated it’s simply not leaving in a hurry. That may be because it was getting low on power (low compression) or there simply wasn’t enough launch rpm.

Because the 60’ is done solely in first gear I want to focus on it more than the 1-2 shift.
 
#24 ·
Ed,
Regarding the Pro Long lubricant. Aside from the single drop on the cam saddles, do you add it anywhere else during a new build? Also, is it safe to say you do not want to add any to the oil on a newly built engine until it has had a full break in and the rings have seated? Conservative assembly locations aside that is.

Thanks
Ken
 
#25 ·
Ed,
Regarding the Pro Long lubricant. Aside from the single drop on the cam saddles, do you add it anywhere else during a new build?
No.

Also, is it safe to say you do not want to add any to the oil on a newly built engine until it has had a full break in and the rings have seated? Conservative assembly locations aside that is.

Thanks
Ken
Absolutely none to the oil until the rings are seated. At assembly use TotalSeal's QuickSeat ring seating secret sauce.

Ed
 
#26 ·
Ed, I have changed the shift point to 7500rpm, will see how it does but it should keep the engine and boost in the powerband and take care of the 1-2 shift.

With that being said, the 60’ is ALL 1st gear, so I’m still on the hunt.

With 10psi boost (all I can build on transbrake) and launching at 43-4500rpm I’m going to go out on a limb and say that watching the past videos, it has a bog. I say that because it physically does not have the power at the launch to break the tires loose and traction is too good.

Since I starting going to a better track and changed to slicks, I’ve never had the issue with traction, and that may be the reason I’m seeing this “bog.” The bog is defined as “high rpm, then release the t-brake, the suspension does its thing, transfers weight, and tires crinkle....then rpm drops and about 20’ out starts picking up again.“

I also have the option of launching at a higher rpm instead of changing gearing, but I admit, I’m an amateur at all these minute detail that go into the launch. What I’ve read, studied, and seen is that I need to be on the very fine line of spinning off the line and maximum traction, and as of right now with no immediate changes to power or boost, I’m just getting maximum traction by weight transfer and wheel speed is too low to actively move the car forward.
 
#28 ·
Ed, I have changed the shift point to 7500rpm, will see how it does but it should keep the engine and boost in the powerband and take care of the 1-2 shift.

With that being said, the 60' is ALL 1st gear, so I'm still on the hunt.

With 10psi boost (all I can build on transbrake) and launching at 43-4500rpm I'm going to go out on a limb and say that watching the past videos, it has a bog. I say that because it physically does not have the power at the launch to break the tires loose and traction is too good.

Since I starting going to a better track and changed to slicks, I've never had the issue with traction, and that may be the reason I'm seeing this "bog." The bog is defined as "high rpm, then release the t-brake, the suspension does its thing, transfers weight, and tires crinkle....then rpm drops and about 20' out starts picking up again."

I also have the option of launching at a higher rpm instead of changing gearing, but I admit, I'm an amateur at all these minute detail that go into the launch. What I've read, studied, and seen is that I need to be on the very fine line of spinning off the line and maximum traction, and as of right now with no immediate changes to power or boost, I'm just getting maximum traction by weight transfer and wheel speed is too low to actively move the car forward.
A good portion of your problem is likely ECU related. The OEM ECU was not designed to properly launch the car with turbos at the track. It was designed to meet EPA mandated emissions. When you finally switch to an aftermarket ECU you will find it easier to build boost at launch, control the engine power at launch, after launch, mid track and so on.

That said you should be able to build more than 10 psi at launch with the OEM ECU. What happens just after launch will not be as easy to control as it will be with an aftermarket ECU but you will have some limited control. Additionally I would wait until you can build launch boost and then check your converter stall speed performance. The additional boost will give you more low speed torque and more stall as a result. I think you will find the stall creeping up to the low 5000 rpm range which is getting pretty close to a sweet spot.

With respect to building boost, a number of the turbo guys at different times have commented that retarding the timing at staging allows the turbos to build higher boost for the launch. At the hit the timing is restored sufficiently to just creep up on the tire's traction threshold and then as rapidly as possible move up to MBT as you go down track. Thes kinds of tuning practices for race track operation will almost always require and aftermarket ECU with the features built into its software. Absent that you will only be able to get so far with the OEM ECU.

You might want to take a hard look at the MS3Pro or the MS3Pro PnP versions that DIYAutoTune offers. The PnP version will run the instrument cluster and use the OEM wiring harness, The non-PnP version will not run the instrument cluster and requires you to build the wiring harness from their kit pieces.

Ed
 
#27 ·
Don’t get me wrong, it’s not a bog similar to a clutch drop at 6k rpm in a Toyota Corolla on slicks, It just seems to struggle between the power of the engine and the traction at the ground.

Ideally I believe (and I could be wrong) that when you launch at “X” rpm you should not lose or stay at that rpm for very long. I feel like I drag out that launch rpm a good 20’ until it starts building more steam, and is able to move the car forwards faster.

Again, I could be completely going about this wrong, but I feel if I can instantaneously raise rpm off launch and wheel speed off the line it would yield a faster 60.’

It may be worth looking back through the datalog info and seeing if there is an rpm drop on launch, and if so how long the dip stays before increasing. By comparing rpm to wheel speed for that first 1.4 seconds I should be able to deduce whether or not I’m applying enough power to make a clean acceleration or if I need to take in factors such as launch rpm or gearing. Even though it may only be milliseconds, in my understanding you should never lull or dip in rpm off launch.
 
#29 ·
Don't get me wrong, it's not a bog similar to a clutch drop at 6k rpm in a Toyota Corolla on slicks, It just seems to struggle between the power of the engine and the traction at the ground.

Ideally I believe (and I could be wrong) that when you launch at "X" rpm you should not lose or stay at that rpm for very long. I feel like I drag out that launch rpm a good 20' until it starts building more steam, and is able to move the car forwards faster.

Again, I could be completely going about this wrong, but I feel if I can instantaneously raise rpm off launch and wheel speed off the line it would yield a faster 60.'

It may be worth looking back through the datalog info and seeing if there is an rpm drop on launch, and if so how long the dip stays before increasing. By comparing rpm to wheel speed for that first 1.4 seconds I should be able to deduce whether or not I'm applying enough power to make a clean acceleration or if I need to take in factors such as launch rpm or gearing. Even though it may only be milliseconds, in my understanding you should never lull or dip in rpm off launch.
I suspect the data log will not show the engine loosing rpm. More likely it will show the engine laboring around launch rpm for a second or so and then ramping up the rpm scale as the car moves down track. The phenomena is caused by too tall of a first gear / ring gear / tire size combination for the torque the engine is capable of producing at launch.

If the engine rpm and vehicle speed is about right at quarter mile, then your tire / ring and pinion selection is pretty close to the mark. The problem lies with the available engine power (torque) and first gear mechanical advantage. Because you can not change first gear, you only have engine torque to play with and that. requires more boost. This brings us back to the tune and the ECU issues in my previous post.

Ed
 
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