LIFE WITH SIRS

[BMW RACER]

Looks like we're stuck with the 29mm SIR. I'm not happy about it, but I'm willing to give it a try. I'm on the list with Raetech to trade in my 27mm piece.

Any thoughts out there as far as mounting this thing? Obviously after the air mass sensor. How far away from the throttle should it be? Can we modify the wiring to move the AMS foreward? Can we run the air filter out through the hole below the left headlight? I'm thinking nope.

What affect will it have on the stock crankcase vent system?

Just thinking out loud.

[Z3_GoCar]

John,

I highly suspect that the reason the SIR in front of the MAF causes the problems that is does is that the there is a large velocity gradient in the middle. I'd say one would need to mount the SIR about 9-15 diameters ahead of the MAF, or you could use a flow straightener to make the intake tube shorter. Next factor to consider is any acoustic harmonics between the SIR and the valves. Actually the best way to deal with this factor is the same as what BMW has on the stock air box, a side branch oscillator. This is a chamber mounted perpendicular to the air flow that changes the natural resonant frequency of the intake tract. If you want to discuss this further and in more detail please feel free to pm me, I'd be glad to offer any help I can.

James

[dj10]

Looks like we're stuck with the 29mm SIR. I'm not happy about it, but I'm willing to give it a try. I'm on the list with Raetech to trade in my 27mm piece.

Any thoughts out there as far as mounting this thing? Obviously after the air mass sensor. How far away from the throttle should it be? Can we modify the wiring to move the AMS foreward? Can we run the air filter out through the hole below the left headlight? I'm thinking nope.

What affect will it have on the stock crankcase vent system?

Just thinking out loud. [/b]

John, this is how I'm going to mount the SIR:
Air Filter=AFM=SIR=======TB (AFM needs to read clean undistrubed air)
I think (this is wat I was told by someone who tested a SIR) there should be enough wire to move the AFM (AMS) forward of the SIR & towards the Air Filter. I'm wait for my car to get finished and the SIR to come in. This is as far as I can comment on. I was going to see if we can't keep the location of the air filter where it is now, just behind thne left headlight. I would think you like to keep the SIR as far away from the TB as possible, like just behind the AFM. I was also told the the car will be running rich and you won't be able to tune with a adjustable fuel reg., this means burning a new chip. I'll be able to tell you more when I get on the dyno with the SIR. If you guys want to go PM feel free.

[lateapex911]

I am not the expert you guys need, sorry, but my comments are free, so I thought I'd share them.

I think that extending the harness will prove problematic legally. But it is pretty long as is so it might be fine. I also think that gaining distance between the SIR and the TB is a good idea. I would look into it. From what I saw, the AFM needs to be upstream of the SIR, but there may be methods to make it work vice versa.

I might try to make the tract longer by bending the intake tube down, then back up between the SIR and the TB. Not sure if that will help, or is even possible, but I'd look at it if I had a BMW.

In tuning, I would consider steps to make sure I had a good broad tq curve. The car that we tested with showed good A/F ratios, but I imagine that is a case by case thing, as he had a custom chip.

After getting the thing to run the best it can on a dyno, I would think about gearing, if needed.

I know, most of this is obvious, but I'm just trying to help.

[BMW RACER]

Jake.

Good ideas, right now I'm waiting for the restrictor to get here.

My next race is May 5th. I'll keep you'all posted.

Thanks.
John

[David Finch]

Guys,

Put the Air Flow Meter first in line.

The 29 mm SIRs will be shipping by the end of this week.


Dave Finch

www.raetech.com

[BMW RACER]

Dave.

Since we can only move the AMS as far as the wiring will let us.
Is there a distance at which SIR stops having an adverse affect on the AMS if it's installed in front of the AMS?

Also I have a question for the "rules nerds"

Does the wiring loom have to remain in the original location? I'm thinking probably yes (IIDSYCDIYC) But flexability in that area yould help with moving the AMS.

Thanks

John

[Joe Harlan]

Dave.

Since we can only move the AMS as far as the wiring will let us.
Is there a distance at which SIR stops having an adverse affect on the AMS if it's installed in front of the AMS?

Also I have a question for the "rules nerds"

Does the wiring loom have to remain in the original location? I'm thinking probably yes (IIDSYCDIYC) But flexability in that area yould help with moving the AMS.

Thanks

John
[/b]
John, I am not gonna quote any rules here but I think anywhere the stock portion of the loom can plug in would be OK. I had thought of the possibility of running a hose across the front tof the engine and back to the stock location and putting the MAF on the end. I think the tubing is free these days and the additional length may alllow the MAF to work correctly with the SIR. This is just a thought but the proven (dynoed)deal is Airfilter====MAF====SIR===TB...

[dj10]

John, I am not gonna quote any rules here but I think anywhere the stock portion of the loom can plug in would be OK. I had thought of the possibility of running a hose across the front tof the engine and back to the stock location and putting the MAF on the end. I think the tubing is free these days and the additional length may alllow the MAF to work correctly with the SIR. This is just a thought but the proven (dynoed)deal is Airfilter====MAF====SIR===TB... [/b]

Joe, in your opinion, doesn't it seem logicial that with the SIR the engine should run rich since the AFM is reading good volume and clean air but the SIR is shutting down the air, thus the computer is throwing more fuel into what is actually less air? Am I looking at this correctly?
Thanks
dj

[mlytle]

Joe, in your opinion, doesn't it seem logicial that with the SIR the engine should run rich since the AFM is reading good volume and clean air but the SIR is shutting down the air, thus the computer is throwing more fuel into what is actually less air? Am I looking at this correctly?
Thanks
dj
[/b]
if the sir is shutting down the air, then the air will stop going through the maf and it will read the correct amount of air. air has to go somewhere...if it can't get through the sir, then is won't get past the maf either, even if the maf is in front of the sir. it is a "sealed" system.

in theory....

[dj10]

if the sir is shutting down the air, then the air will stop going through the maf and it will read the correct amount of air. air has to go somewhere...if it can't get through the sir, then is won't get past the maf either, even if the maf is in front of the sir. it is a "sealed" system.
in theory.... [/b]

So if I read you correctly, if the maf is reading less air then it is telling the computer to put less fuel to the engine thus creating a lean condition? Does this sound right? If this is correct then engine should start going lean from about 3500- 4000 rpm up.

[DoubleXL240Z]

I may be incorrect, but, I don't believe that the MAF will read correctly because it was designed to work in the 4 inch(or so) air intake system, calculate velocity, and determine volume with respect to that area. With the velocity being considerably greater due to the restriction readings will have to be adjusted accordingly.
That is why the MAF doesn't want the SIR in front of it, as it will direct the airstream only on the middle of the wire. Causing way out readings.
This is just a SWAG(Scientific Wild Ass Guess). I pick up my E36 this coming weekend and am looking to learn all I can, and help where ever possible.
Thanks

[mlytle]

So if I read you correctly, if the maf is reading less air then it is telling the computer to put less fuel to the engine thus creating a lean condition? Does this sound right? If this is correct then engine should start going lean from about 3500- 4000 rpm up.
[/b]

not really. maf is reading less air because less air is going into the engine through the sir. this leads to computer putting less fuel in to try to keep the mixture at the correct ratio based on the maps in the computer. this is where the custom chip is critcal and just playing with an adjustable fpr isn't very effective.

of course, i am not a professional engine tuner, but i hung out with one during several tuning sessions on my car last year.... :D

[dj10]

not really. maf is reading less air because less air is going into the engine through the sir. this leads to computer putting less fuel in to try to keep the mixture at the correct ratio based on the maps in the computer. this is where the custom chip is critcal and just playing with an adjustable fpr isn't very effective.

of course, i am not a professional engine tuner, but i hung out with one during several tuning sessions on my car last year.... :D
[/b]
I wonder who that might be? ;) I think I'm understanding this a little better. From what I've heard your enginer tuner/ builder should get his 29 mm SIR this week. I had him burn me 2 chips last year and only 1 will be any good (the one without the FPR) lol. Damn is that a long day for me if I have to go to his dyno again to get another chip burnt (620 miles round trip). He's such a unique and likable guy would be worth it if I had too.

I may be incorrect, but, I don't believe that the MAF will read correctly because it was designed to work in the 4 inch(or so) air intake system, calculate velocity, and determine volume with respect to that area. With the velocity being considerably greater due to the restriction readings will have to be adjusted accordingly.
That is why the MAF doesn't want the SIR in front of it, as it will direct the airstream only on the middle of the wire. Causing way out readings.
This is just a SWAG(Scientific Wild Ass Guess). I pick up my E36 this coming weekend and am looking to learn all I can, and help where ever possible.
Thanks--------------------
Chris Leone
E36 ITS underconstruction
Remember half of the people you call friends are considered below average.

SWAG--------Good one Chris! Man did you pick a good time to do a E36 ;) .
This is a reason maybe not to mount the SIR to close to the MAF but not to close the TB either. I know there is only a limited amout of space to mount everything needed.

[Joe Harlan]

If you could move the SIR upstream far enough to allow the velocity to return to normal (fill the tube) you could run the SIR out front and likely not see the thing until it goes sonic. This is why the location between the MAF and the TB seems best at this point the airspeed will remain constant through the tube and the MAF until the air stalls at the SIR. I believe at this point the It will act like an over rich revlimiter since the MAF will have seen then air before the stall. I think the problem we are having getting our arms around this is that it is not like a FPR that screws up the airflow. The Idea is this type of restrictor is allowing all the air the engine can actually use until it goes sonic. Even at this point the leanest we should get through ECU control would be stoich on a stock program.

[DoubleXL240Z]

I believe thats why we see formula/ sports racers using any restrictor in front of everything and use an airbox of sorts, for attenuation or cleaning/straightening the flow of air, as well as acting as a "holding tank" for a decent supply of air. Leaving sub sonic supplies for the engine. It would be nice to have that kind of room available!!

[Joe Harlan]

I am not sure you don't have room if your creative. I don't think it will net much of anything but I think you could run enough tube to run the SIR out front.

[lateapex911]

Thinking in terms of Cubic Feet per minute, we know that your engine uses hundreds.....(think about a 650 holley on a Chevy, that makes twice the HP) so, if you are flat down a straight for 20 seconds, you'll use a lot of air, LOL. A lot more than any box can house.

If you are doing experiments in this area, I imagine that the concept will be to get the speed of the air stabilized and smooth across the hot wire resistor.

My understanding of the genious of the hot wire design is that it automaticaly adjusts for all changes in the conditions. Basically a current is sent thru it, and the resistance is monitored. The computer seeks to maintain the temp of the wire. More air flowing over it, whether due to speed, incresed density due to temp, or incresed density due to barometric pressure will couse the computer to adjust. The adjustment is merely then translated into a fuel need.

In testing, I think the SIR, when placed in front of the MAF, presented a stream of very high velocity air/turbulence, that had not slowed and smoothed, to perhaps one section of the wire, and the wire was over cooled, so to speak, causing the computer to assume that it needed to add more fuel. Thats why we saw issues with black smoke, and CEL triggers relatively low in the rev range.

The safest thing to do is place the MAF upstream in clean flow, but I think that a closer study could yeild a different arrangement with the SIR upstream.

Again, I'm not an engineer, just reporting what I saw, and just trying to help.

[dj10]

Thinking in terms of Cubic Feet per minute, we know that your engine uses hundreds.....(think about a 650 holley on a Chevy, that makes twice the HP) so, if you are flat down a straight for 20 seconds, you'll use a lot of air, LOL. A lot more than any box can house.

If you are doing experiments in this area, I imagine that the concept will be to get the speed of the air stabilized and smooth across the hot wire resistor.

My understanding of the genious of the hot wire design is that it automaticaly adjusts for all changes in the conditions. Basically a current is sent thru it, and the resistance is monitored. The computer seeks to maintain the temp of the wire. More air flowing over it, whether due to speed, incresed density due to temp, or incresed density due to barometric pressure will couse the computer to adjust. The adjustment is merely then translated into a fuel need.

In testing, I think the SIR, when placed in front of the MAF, presented a stream of very high velocity air/turbulence, that had not slowed and smoothed, to perhaps one section of the wire, and the wire was over cooled, so to speak, causing the computer to assume that it needed to add more fuel. Thats why we saw issues with black smoke, and CEL triggers relatively low in the rev range.

The safest thing to do is place the MAF upstream in clean flow, but I think that a closer study could yeild a different arrangement with the SIR upstream.

Again, I'm not an engineer, just reporting what I saw, and just trying to help. [/b]

Damn I think the CRB should tell us how thay want us to mount for it to be legal. With all this expermentation don't we deserve a little more time to make this damn thing work? Hell it's been 6 years, what another month or 2?

Jake, I'm not sure if there was enought room to get the sir forward enough to not cause turbulance to the AFM. Bending tubing like a header system might work but the bends would have to be perfectly smooth with absolutely no kinks.

[lateapex911]

Well, if it were me, I'd run to the Home Despot, or the bLowes, (kidding) and pick up some PvC plumbing parts to "prove/disprove" the theory parts of it. I think you can fab up a pretty fast set of parts, with some rubber hoses and some U and 90 degree bends to try a bunch of different configurations out before dyno time.

Then on the dyno, you can just swap the parts out to get some quick direction.

I'd love to take credit for such a concept but I have seen some very bright engineers do it to make sure that their theory matched reality before commencing with more time consuming methods. (One guys dyno results were strong enough that he actually raced the "mock up" for a while before finalizing it)
I have developed a relationship with my local NAPA store, and they allow me to "putter" around the back area looking for the weird bends of pipes and hoses I might need for whatever project I am working on. Look at large truck radiator style hoses as well.

Once you have what you think is the final direction, you can get fancier with the right silicone hoses and such.

[Knestis]

The "attenuation volume" issue is precisely why the SIR sits way out on the front of the airbox, where we have seen it in F3, FIA sportscars, etc. - or so I've read. Remember that air is a fluid - it flows and eddies and stuff. I can see how it might confuse things, putting the SIR close enough in front of the MAF that it in essence creates a blast of increasing-velocity air from that little hole.

Particularly if I didn't have a bottomless engineering budget, I'd be tempted to build the biggest airbox possible, and use the SIR as the inlet. Put the filter between the SIR and airflow sensor, to leave things as much like the OE MAF is used to seeing them, absent the restriction, and count on the flexibility built into the system to cope with some of the variables that remain, short of sonic velocities being reached at the SIR.

K

[dj10]

The "attenuation volume" issue is precisely why the SIR sits way out on the front of the airbox, where we have seen it in F3, FIA sportscars, etc. - or so I've read. Remember that air is a fluid - it flows and eddies and stuff. I can see how it might confuse things, putting the SIR close enough in front of the MAF that it in essence creates a blast of increasing-velocity air from that little hole.

Particularly if I didn't have a bottomless engineering budget, I'd be tempted to build the biggest airbox possible, and use the SIR as the inlet. Put the filter between the SIR and airflow sensor, to leave things as much like the OE MAF is used to seeing them, absent the restriction, and count on the flexibility built into the system to cope with some of the variables that remain, short of sonic velocities being reached at the SIR.
K [/b]

You can't put the air filter between the sir and maf, the rule states all air must enter through the sir. If you put the air filter between the sir and maf you would be drawing in air that would not be going through the sir, making it illegal.
[/QUOTIt's on page 40 of the GT section of the GCR, called the "GTCS"
it says:

QUOTE
GTCS 17.1.2.F.4.i.10. Single Inlet Restrictors
a. The intent of this rule is to have a sealed system from
the Restrictor to the Intake Ports of the Cylinder Head.
All of the air entering the Intake Ports shall pass through
the specified Restrictor. Modification or addition to any
part of the Intake System that allows the introduction of
air into the Intake Ports that has not passed through the
specified Restrictor is prohibited.
b. The Engine Air Intake System must be fitted with an
aluminum air restrictor. The Intake System is defined
as an assembly of parts, including but not restricted
to: the Restrictor, Restrictor Housing, Ducting, Filters,
Air Box, Velocity Stacks, Throttle Body, Carburetors,
Manifold and Manifold Gasket up to the Intake Ports on
the Cylinder Head.
c. The Restrictor must be round in shape. The maximum
ID of the Restrictor is listed on the vehicle’s spec line.
The Restrictor’s maximum ID must be maintained for a
minimum length of 3mm. Restrictor mounting/placement
within the intake system is free, but must allow
accessibility for measurement. It is acceptable to have
some minor disassembly of the intake system to provide
access to the Restrictor for measurement. Measurement
device and restrictor shall be similar temperatures when
used.
d. Sealing the Restrictor from its supply of air must cause
the engine to stop within 4 seconds. This check is to be
made at an engine speed of approximately 2500 rpm.
The sealed airbox must withstand this test. Pressure
sensors present inside the intake system must be
disconnected during this check.
e. All GTL cars that have either an IR or SIR size (restricted)
listed on their spec line shall utilize an SIR for National
competitions.

So, all air goes through it.

[lateapex911]

With all due respect, I disagree.

Here's the pertinent quote:


QUOTE
GTCS 17.1.2.F.4.i.10. Single Inlet Restrictors
a. The intent of this rule is to have a sealed system from
the Restrictor to the Intake Ports of the Cylinder Head.
All of the air entering the Intake Ports shall pass through
the specified Restrictor. Modification or addition to any
part of the Intake System that allows the introduction of
air into the Intake Ports that has not passed through the
specified Restrictor is prohibited.[/b]

So the key is that all air that gets to the intake ports has to pass thru the SIR.

So having the SIR in front of the air filter is fine, as long as the box is sealed that houses the air filter and the SIR. And it has to pass the 4 second rule.

Fab away!

[M3 Pete]

I've seen SIRs mounted through the middle of the front of the car, right out where the main radiator intake opening is located. But they also had a custom low mounted radiator to accomodate the plumbing.

It seems kind of frustrating to be required to use this "trumpet" somewhere in the intake tract and not have any flexibility with respect to the location of the MAF with respect to the wiring loom.

Maybe the solution is to mount a "unicorn" horn way out in front with the SIR at the end. Don't follow too close though, you'll cut down on the airflow to the engine. :D

[lateapex911]

Only if the stock air inlet was outside the engine compartment...

[Knestis]

You can't put the air filter between the sir and maf, the rule states all air must enter through the sir. If you put the air filter between the sir and maf you would be drawing in air that would not be going through the sir, making it illegal. ...[/b]

Um ... huh?

That doesn't have anything to do with anything. All of the air goes through the SIR, then all of the air goes through the filter, then all of the air goes into the engine. You street car filters all of the air that the engine breathes, right? I'll bet that the air goes through some kind of snorkle or intake hole before it gets to the filter, and that the intake tract is essentially sealed between that hole and, well, the end of the exhaust pipe for all practical purposes. Think SIR instead of snorkle hole and you're there.

That, and a couple decades of SIR-controlled Formula 3 cars - all of which do exactly what you claim isn't possible - MIGHT be some evidence that it is.

I don't have the faintest idea what you are talking about.

K

[dj10]

Um ... huh?

That doesn't have anything to do with anything. All of the air goes through the SIR, then all of the air goes through the filter, then all of the air goes into the engine. You street car filters all of the air that the engine breathes, right? I'll bet that the air goes through some kind of snorkle or intake hole before it gets to the filter, and that the intake tract is essentially sealed between that hole and, well, the end of the exhaust pipe for all practical purposes. Think SIR instead of snorkle hole and you're there.

That, and a couple decades of SIR-controlled Formula 3 cars - all of which do exactly what you claim isn't possible - MIGHT be some evidence that it is.

I don't have the faintest idea what you are talking about.

K
[/b]

K,
I was thinking of a in line air filter and not using something like stock air filter box which would seal it from taking air from anywhere but the SIR. Building something like this to meet the 4 sec rule and keeping inside the engine compartment & legal could be an answer, It also will be a lot of time for experimentation. Maybe I can drop my car off with you and let you piss around with it. :D I should get it done by May 1st, this year, right? ;)

My next question is what would the air filter box do other than cut down on turbulence that would be created by the SIR? It seem to me that we spend money getting rid of the air filter boxes to use K&N cone tyoe filters for better flow. Anyone have a tuned header system for this? :D Hell a 100 hours of testing (including Dyno time) and you might be able to sell 2 or 3 of these. If you would want to keep your K&N cone type air filter would the SIR also work as well 1/2 between the AFM and TB. If you have to thein of air like a fluid isn't it best in a straight line?

[BMW RACER]

Good morning campers!
Just a quick note to any E36 racers who are going to try the SIR deal. We have to be ready to race in one month! Not sure about you, but I haven't even got my 29mm SIR from Raetech yet.

I've written to SCCA asking for more time to install, dyno, burn chips, re-dyno and then track test the car with the SIR. I'm not talking about optimising the whole deal. I'm just talking about making the car raceworthy without risk of damaging the motor. If we get enough people writing maybe we can get a little more time.

Please write to: [email protected]

Thanks.

[buldogge]

I emailed off a letter this morning John...NO confidence it will do any good...but...it's done.



Good morning campers!
Just a quick note to any E36 racers who are going to try the SIR deal. We have to be ready to race in one month! Not sure about you, but I haven't even got my 29mm SIR from Raetech yet.

I've written to SCCA asking for more time to install, dyno, burn chips, re-dyno and then track test the car with the SIR. I'm not talking about optimising the whole deal. I'm just talking about making the car raceworthy without risk of damaging the motor. If we get enough people writing maybe we can get a little more time.

Please write to: [email protected]

Thanks.
[/b]

[JeffYoung]

I did as well. You guys deserve the time.

[David Finch]

The 29 mm SIRs are available from RAETECH. John's shipped yesterday after he gave me a call.


www.raetech.com

734 665 2215



Dave Finch

[BMW RACER]

Dave.

Just got it, looks nice, too bad I have use it !!

Well it's off to the dyno (when I find one)

[steve s]

should have ours on friday, then to dyno. if everything works as advertised we'll do some track testing at roebling's test day.

[lateapex911]

Just to let you guys know, I have posted on our own forum (ITAC) about this. I think that there is support for an extention from 4 members, but as we don't have a con call with the CRB for weeks, I am not sure what we can do. If I can get a clear majority to support, perhaps we can send the CRB an ITAC request.

But, I'm new on the ITAC, so I'm not sure if thats an approved procedure, and even if it were to be, whether the CRB will agree.


Would a month be a helpful period?

Just thought you should know that your thoughts, letters and the situation isn't being ignored.

(And that I appreciate the letters that are reasonable and well thought out, as opposed to those that call us morons, LOL. )

(As a side note, I don't know who reads the letters along the way, but the CRB does occasionally stop in on the ITAC board and very occcasionaly posts a comment.)

[dj10]

Just to let you guys know, I have posted on our own forum (ITAC) about this. I think that there is support for an extention from 4 members, but as we don't have a con call with the CRB for weeks, I am not sure what we can do. If I can get a clear majority to support, perhaps we can send the CRB an ITAC request.

But, I'm new on the ITAC, so I'm not sure if thats an approved procedure, and even if it were to be, whether the CRB will agree.

Would a month be a helpful period?

Just thought you should know that your thoughts, letters and the situation isn't being ignored.

(And that I appreciate the letters that are reasonable and well thought out, as opposed to those that call us morons, LOL. )


(As a side note, I don't know who reads the letters along the way, but the CRB does occasionally stop in on the ITAC board and very occcasionaly posts a comment.)
[/b]

Jake, I've had the pleasure of emailing thoughts on these matters to, let me say, who I think, someone of importance up in the SCCA. It is refreshing that someone like him takes the time to actually listen. Even though we don't 100% agree on everything, I think he is open minded and sincere, I'm sure he hopes things work out for what only could and should be better racing. I don't know for sure if my engine is built well enough to be competive. With my SIR otw I guess it's time to find out after testing, etc. etc.. I think 1 more month would be enough if the SCCA would grant us that. But if they don't, all we can do is get ready, test with what we have and know, then go for it. Either way, I've come this far, I'm stubborn and I want to see if this thing will work.

BTW JAKE, WHO YOU CALLING MORON!!!! ;)

[ShelbyRacer]

Thinking in terms of Cubic Feet per minute, we know that your engine uses hundreds.....(think about a 650 holley on a Chevy, that makes twice the HP) so, if you are flat down a straight for 20 seconds, you'll use a lot of air, LOL. A lot more than any box can house.
[/b]

Yes, *however*...

One of the big issues is how the restrictor affects throttle transients and response. If you have a large enough volume intake tract, you can provide a "reservior" of air that can be tapped prior to pulling large flow through the restrictor. Think of it as an accumulator in hydraulic terms or a capacitor in electrical...

BTW- this is also why you want the Air Flow sensor (be it MAF, Vane Air, or other) to be in close to the stock location if using the stock computer. If going custom (which I pray you guys are), you can play a little. For carb guys, it's like changing intakes to one with a large volume under the carb- you need more pump shot because it takes the pressure "signal" longer to get to the carb jets... This is a MAJOR advantage of custom programming in ECU's- you can easily manipulate not only the amount of accel enrichment, but also the duration and other factors...