cant get more than 10psi boost why?
 

Ok as some of you know i have been working for some
time to get the boost up on my stock computer
run 20b.
I have a modified stock computer that will
boost to 16 psi in a cosmo but will not in my rx7.
the best boost i see is 9.5 psi max.
it hits 9.5 then drops to about 6 psi as the 2nd turbo
comes on then goes back up to about 8 to 9 psi
then as the revs pass 5500 to 6000 rpm the boost
drops off to 4 to 5 psi, i assume this is because the
stock turbo's cant keep up at the higher rpms.
anyone have any ideas as to why my 20b will not
get up to 16 psi and why a stock / modified 20b in
its cosmo body will?
Sounds to me like a 3rd gen like problem with a vac hose
off or a solenoid not working but i have replaced the vac
hoses with silicone and everything seemed to be working.
matt

mine was like that, and it turned out to be the 270degree bend ic pipe, that right there was worth 2psi. another thing to look at is the exhaust.

mike

the exhaust is 3" from front to back, no air box.
if it's the ic piping i should still see high boost pressure
at low rpm's . I am sure the ic piping has lots to do with
the low pressure at high rpm and that will get addressed
soon.

matt

you might also check the wastegate solenoid, its the one on the tb elbow, should be blue and red

mike

hey now, where did you get your modified ecu? im thinking of just running the stock ecu for a while. is it actually possible to squeeze 16psi out of the stock turbos? others were saying that some sort of turbo upgrade would be necissary (thats why im considering having BNR rebuild my turbos, but then i run into problems with the computer). a modified stock computer would be great! i would love to run the turbos in sequential mode if at all possible (is the modded stock computer set up for running sequential??) thanks!! -heath

yes this computer is a 20b computer modified with new
fuel and ignition curve as well as a boost controller function
that allows control of boost between 13 psi
and 16 psi. there are a couple other things that were
done to this computer but don't apply to a 20b in a
rx7:) and yes sequential turbo control like stock.
the stock computer with open exhaust and intake will
make almost 10 psi as it is,,, but i want 16 psi!!! :)

later
matt

now, may i ask where you got such a computer? this would seem to be the perfect solution for a 20b project (can anyone interject if they feel differently?). now, do you have modified turbos?? i have been told numerous times that the stock turbos (in there stock configuration) will not be able to support that psi as they run out of volume of air at a higher RPM.

also in response to your problem, i was thinking that since your computer is meant to go in the cosmo, maybe (and this is just a guess) people that were able to see 16psi were using the stock cosmo airbox and exhaust...therefore less flow throughout the system and thus they could build more pressure without having to worry about running out of volume. maybe look into BNR to do an upgrade of your turbos, or do what pettit does and swap the compressors (i think they use a ht18 for primary, and an ht 15 or 12 for secondary).

the computer mod comes from re-amemiya in japan.
I was thinking of a bnr upgrade he wants 1650 to do
them.
you talk about swapping compressors where would i get
these parts?
the car this modded computer is out of had 3" exhaust
and cone filter so we should be very close to the same.
I was thinking of putting a line on the turbo side of the
IC and see what the pressure is there maybe that is
where the problem is, maybe it is making lots of boost
and its not getting to the intake.

matt

warning: this might be bad. pull the wastegate line off, and run without it, that way you know if its the turbos/restriction or something else. be very careful it might overboost and kill it

mike

I was thinking of trying that but am unsure if the
modded computer is switching to the high map rom
and if i do get a burst of boost without the extra fuel
i was worried that it might blow up in a hurry...

when you say pull the waste gate line off where do
you suggest i pull it off of ? at the solenoid or the turbo.
guess it doesn't really matter, the one on the top would be easier i guess..
matt

now, are you sure this car that the ecu came out of was actually making 16psi? id be extreemly careful if you disable the wastegate...there has to be a safer way to test the turbos, but im convinced that they are running out of volume....well except for the fact that you cant make high boot at low RPM. hmmm, i wonder if its something with the computer thats preventing high boost?

as far as swapping compressors, i think thats what pettit does. i believe they use the hittachi ht-18 from the tII (this may not be accurate...i may be mixing things up) and the ht-12 or 15 (cant remember) from the FD (again i may be mixing things up, but i think they use the FD primary as a secondary???). please dont quote me on this cause i have a bad memory for these things. ill try to find the thread that talks about pettits solution. the BNR upgrade may be all you need but i havent talked to them yet and dont know how far you could push their upgraded turbos...or how much restriction there is due to stock exhaust manifold.

how much did you pay for the computer from RE amemia? did you send a core and they upgraded it? also, do you know if they are readily available? thanks again. -heath

I am quite sure the computer was making 16 psi boost in
the car it came out of, it came from lance warren of
3rotor.com i seen a couple videos of it working in his car.
I would love to test it in a stock cosmo but there are no
cosmos around my area:( the cost of the mod from re-amemiya is very high, i was going to send
my stock computer over and have it done but i thought
getting the computer already moded used was a better
deal,,, i might have been wrong about that, will have to
see if i get it figured out.
I would love to know more about the parts needed to m
mod the turbo myself i would love to try and see what
happens. so any more details you can find the better:)


matt

Just a quick question,,, is there a way to control the boost
on the sequentials? is there a boost controller that
could be used on the 20b? i assume there should be
if the computer is chipped for 16 psi i would assume that
controlling the boost with some type of controller on the
waste gate should be possible?

matt

well a boost controller for an fd would work wouldnt it since they are sequential? do you have any sort of ballpark range for the RE Amemia upgrade? ok check out this thread -> https://www.rx7club.com/showthread.p...highlight=ht18 there seems to be some disagreement on what the actual turbos are on the 20b. can someone who is knowledgable interject and let all of us know what turbos are used for the stock 20b primary, and secondary? thanks guys! heath

ht15 and ht10, they are "upgraded" with the fc's ht18 and one fd ht12. i dont think it would be too hard to do, but i dunno never tried

mike

i wonder why they dont use the ht-18 and the ht-15, or 2 ht-15's? more boost no? do you know (for the 18 & 12 that pettit uses) which one is the primary and which is secondary ? wouldnt you get faster spool up with a small primary and larger secondary cause you could prespool the larger one? im gonna talk to BNR and see what they say. it would be great to have them swap the compressors and THEN do their upgrade for better flow and balance the whole assembaly.

the bigger one is primary. on the 20b all 3 rotors go to the front turbo and then upon transition rotors 1 and 2 keep going to the front turbo and #3 goes to the rear turbo

mike

thanks for the info Mike!! wow, thats quite a complicated design.... it doesnt sound like the most efficiant. also, wouldnt it make sense to use a smaller turbo for your primary, and since its sequential, pre-spool up the bigger one as you approach transition? i see why they used the smaller one since its only run off of one rotor but the design just seems really bad. i would dump everything into one pipe, then open/close vents to use each turbo. mazda always takes a strange approach to things but they must have chosen it for a reason.

by the way mike, what is your turbo setup? if your using the stockers, what has your expierience been with them, and how many PSI can you run? thanks again, heath

well my opinion is that 3/2 is inherently flawed, theres no easy way to do it. mazda as always was looking for a flat torque curve, so they picked a primary that spools fast by itself.

i'm running the stockers too, until they die. right now i dont have any boost control so i'm only running about .5bar (7ish psi), my goal is to run .7(10psi), maybe .8-9 (12-13) in the midrange and let it fall toward redline. the previous setup would run .8 in the midrange but due to a bunch of little things (weak spark, restrictive ic pipes) would fall to .4 at redline no matter what i did, its better now. i never tried to go higher; i want to go easy on the motor

mike

mike where would be the bast place to get
fc's ht18 and fd ht12 turbo parts... i assume that to mod the 20b stock turbos all that is being done it to use the
compressor wheel from the above. or is there more?
will these fc and fd compressor wheels fit without
machining the turbo housing?

matt

to get the parts prolly get used set(s) of turbos. i think the fd turbo would go in pretty easily. the inlets are the same diameter, so it might just be minor machining of the housing? or use the fd housing? i'm not sure about the fc turbo, i dont know if they are just putting the bigger wheels in the stock housings or using the fc housing. if you're lucky then the ht15 and ht18 have the same cartridge, and shaft size

mike

something to look into,,, the fc turbo is that a 88 or the 89?
i cant remember now but i think they were different.

matt

they have 2, 86-88 and 89-91. they both use the same compressor, the 89+ has a slightly different exhaust wheel in a redesigned housing

mike

so would a guy just change the compressor wheel or
both the compressor and exhaust sides?

matt

im guessing the whole thing? isnt it one balanced unit. wouldnt changing just one side upset the balance? and how would you even do that? hopefully all the turbos are interchangeable...which would make for a much simpler swap. this is all pure speculation on my part as i havent even pulled the motor from my front clip yet:biggrin:. im assuming we would want the newer ht-18? how about keeping the 15 and moving it to secondary, and adding an ht-18 to the primary side?

the only thng that worries me about messing with the turbos is from what mike said a few posts up

"the bigger one is primary. on the 20b all 3 rotors go to the front turbo and then upon transition rotors 1 and 2 keep going to the front turbo and #3 goes to the rear turbo"

if this is so and we change compressors, wouldnt we mess up the back-pressure ratios between the rotors. i mean with only the primary running its not a problem, but since the secondary is only run off of one rotor, wont we end up with rotor 3 seeing more/less back pressure than the others? how would this affect anything if at all? if rotor 3 was seeing more back pressure than the others wouldnt put it under more stress and make rotor 3 much more likely to fail? anyway, if back pressure ratios are not an issue then might as well keep the 15, move it to secondary, and add an 18 to primary. also if bnr was to rebuild/upgrade them we'd get even more boost :wink: -heath ps i cant imagine a used ht18 would be all that much... im gonna check out prices just for fun in the fc section...ill report back.