I will be dropping a new street ported motor with 3mm apex seals into my FD this month. Mods I already have include:

PFS Powertrain Management System (purple computer)
PFS intake
PFS intercooler
Efini Y-pipe
Majestic rebuilt turbos with 10% clip
PFS downpipe
PFS midpipe with Borla XR-1 resonator
PFS catback exhaust
Competition Mazda motor mounts
Unorthodox Racing pulleys
PFS silicone vacuum line replacements
Cosmo fuel pump
K&N washable billet aluminum fuel filter relocated to under hood for easy cleaning
Taylor 8mm spark plug wires
J&S knock sensor w/ monitor
Crane HI 6D ignition amp
Competition Mazda radiator
Pettit aluminum AST

In addition, I plan to fit 1300cc secondaries and replace the PFS PMS with the Haltech E6K. Apart from careful tuning, what else do I need to run 15psi safely for street and occasional track day use? My nightmare would be blowing the new motor the first time I take it to the track (road course).

 

The only fuel mod I can see that you are missing is an adjustable fuel pressure regulator. I wouldn't go as far to say that you actually need it, since you are going with 1300 cc secondary injectors, but it can assist you in tuning. If you are going to run 15 psi with your stock injectors for a while before you get the bigger injectors then I would recommend a FPR just to be sure you have enough fuel.

 

With the correct fuel system and tuning 15 PSI on the motor will not be a problem. BUT, the problem lies with the turbos themself. They will probably do 15 PSI but they wont last too long doing it, and I'm sure they will be out of their efficiency range. So if you really want to run higher boost you would be better off going with a single that is designed to opperate efficiently at higer boost.

 

How much boost can the stock twins take? I will probably upgrade to single in the future, so if the twins can last a year at 15psi that will be sufficient.

 

I have run anywhere from 13-15 psi on my stock turbos for 3 years and haven't had any problems with them. I wouldn't worry about them not lasting a year. I have 50K miles on mine. The biggest thing is the extra heat you will be feeding your engine at 15 psi. They just won't be efficient. In other words you won't be making as much HP as you would with a turbo designed to operate at 15 psi. If you plan on going single in the next year or two then I wouldn't worry about running the turbos at 15 psi.

 

Me too, I have over 30,000 miles on my stock turbo's and I'm boosting 15lbs w/ 16lb spikes. Just don't go around town everyday under wot.

 

I run 14 to 15 on mine in this cool weather and have been doing so for a few months

I always hear that the turbos are "outside their effeciency range" at this boost, which may be true, but here is something to think about....when you run a straight through 3 inch exhaust like I am doing, and alot of guys do, there is MUCH less back pressure on the turbos....so even if your boost gauge says 14-15 psi, the turbos arent really working any harder than if you has the stock pre-cat, main cat, and restrictive cat-back on there at 10 psi, at least that's my take on it

when I put my down pipe and cat-back on I got a 2-3 psi jump from that alone, up from 10 psi to 12-13 psi, just becasue of less backpressure....I don't think the the turbos are working harder....they are likely flowing more air, but I think it's the backpressure that puts the stress on them and causes the seals to blow, right?

I guess this is a little counter-intuitive....you would think that uncocrking the exhaust would result in LESS boost pressure....is it becasue the flow goes way up?

 

mine lasted 4 months at high boost....they are cooked.

Id bet that you fry the wheels from high heat and ....well after that comes plenty of leaking...

the only way you will get them to last is to use high boost sparingly.

its m2 for me....fark the stock brittle junk....three sets of turbos for me in the past two years.

jason

 

I don't agree with this. When you decrease back pressure, the volume and velocity of the exhaust flow is increased significantly. This causes the turbos to spool up quickly and is the reason why you get 2-3 more psi at the same setting. With a freer flowing exhaust the turbos are actually working harder since they have to spin faster to maintain a set boost pressure. Think about it this way. Try blowing through one of those small coffee straws. It is relatively easy to maintain a given pressure. Now blow thorough a larger soda straw and try to mantain the same pressure. I think you'll agree that it takes more breath (air) to hold a given pressure through the larger straw, therefore you are doing more work.

 

My answer to your question would be... You need luck, I didn't see that anywhere in you list of parts, so you still need to purchase some from some where, get your car blessed, maybe throw some holy water on it etc..

But seriously the stock turbo system is very restricted on the exaust side, this cause very high exaust pressure (bad), lot's of Heat (bad), and lots of heat soak (really bad)... any kind of continus spirited driving and it will knock and you'll have a cool rotory clock... If your smart you'll just keep the boost down until you get your single...

 

2 things silver.

1-When you increase the size of the pipes exhaust velocity does not go up. it goes down.

This doesnt matter though because the exhaust has already passed through the narrow turbo manifold and turbine housing at high speed.


2- The turbo does not pump exhaust. The exhaust drives the turbo. A smaller "straw" has much more back pressure, which means more effort flows the same amount of air, adding stress to the turbo compressor. Zero back pressure would be ideal

 

I said when back pressure is decreased, not when size is increased.

This does matter. The exhaust pipes after the turbo have a lot to do with how the turbo operates.



The turbo does not pump exhaust, but it pumps intake air which is turned into exhaust. It works as one system. If a turbo sees back pressure then the engine will be pressurized sooner. It may be flowing less air but it will be able to hold a set pressure easier because of the exhaust bottleneck. When the exhaust is freed up, the turbo must spin much faster to keep up with a given pressure. The faster the turbo has to spin to keep up, the harder it is working.

 

For those of you guys running high boost, don't you have problems w/ fuel cut?

 

car you run 18lbs on race gas on stock twins.

 

not for very long you cant....youll cook those little suckers. trust me. Im now on the fourth set...no more rebuilds EVER. Rebuilds are a waste of money and that is what youll do running 18lbs of boost on stockers.


j

 

Pettit's unlimited chip completely eliminates fuel cut. All of the stand alone units do as well.

 

The speed of the turbo spool isnt what kills them, its the heat and resistance associated with back pressure. Less back pressure allows for better thermal efficiency, therefore less stress on the turbo, and the capability for higher boost psi.

Less backpressure is Always better w/ a turbo.

I quote Corky Bell of Bell Turbochargers, pg. 131 of Maximum Boost : "Back pressure in an exhaust system is evil"

 

Why run so much on the stock twins? You wont be gaining much horsepower with 15psi than if you were to run 13psi. Like artguy said, you're just gonna be cooking your turbos. Stock turbos arent efficient enough to run more than 14 psi unless you ceramic coat the turbos and/or clip them.

I suggest buying a new set of twins or go single if you wanna run that much boost.

 

I have been running 13.5-14 lbs.for quite a while now usually just at the track for the past
3 summers without any problems.
It's generally agreed that beyond 14lbs.with stock units and injectors you don't go faster.
I have 31k on '93 and have run a best of 12.3 @116

 

who makes stock replacement turbos that will allow 18-20lbs of boost on twin set up.

 

silver7,

I have been doing some more thinking about this....based on my admittedly limited knowledge of how turbochargers work, I think what actually happens is that, as soon as you uncork the exhaust system, of course the engine flows much more air (drastically reduced backpressure/friction loss)

a portion of that increased flow is diverted into the turbo housing to spin the turbos...the increased flow spins the turbos faster, so they produce more boost at the same rpm as before you "uncorked"....that is why you get progrssively more boost as you add a cat-back, downpipe, and midpipe....the flow of air through the engine jumps each time you pull of a restrictive element of the exhaust (or add a low restriction intake, for that matter)

question is, are the turbos "working harder"?

well, it stands to reason they are certainly spinning faster...however, they are also under less backpressure (spinning more freely)...I would think backpressure is what puts more "stress" on the turbos, as indicated by the quote from Corky Bell's book (I need to get that book)

of course, if you run really high boost, even with a wide open exhaust, the diminutive stock turbo rpm starts skyrocketing, to the point that the little suckers probably just melt down...a good synthetic oil like Mobil 1, that can withstand high temps, would likely help somewhat to prevent meltdown

but I think the high percentage of these cars with premature turbo failure as a result of keeping the very restrictive stock downpipe/cat-back on tells you how damning backpressure is to turbos

 

Here's the deal as I've understood it from reading various posts, turbo books, and having to tangle with every god damned bit of my twin turbos. The stock 10 psi boost level is limited to that by several factors.

First the stock exhaust being as restrictive as it is is very limiting to the efficiency range of the turbos. Hence if the turbo is spinning out 12 psi with stock exhaust it has to work that much harder to produce it. Think of it this way. If you're blowing into a hole with a plug at the end of it, the air you blow in will come back at you. If the hole goes straight through, the air you blow into it doesn't come back. Again try breathing through a small straw, you can do it for a very short time and then it becomes laborsome to your lungs, because of all the effort you're putting into breathing. Now get a 3/4 in PVC pipe and breath through that. You most likely won't get tired breathing through it, unless maybe you're running a marathon. Exhaust is energy and if energy has to be put to pushing gases through a straw it's not going to the flywheel.

Two, the stock wastegate port is TINY. This is why when you put a good flowing exhaust on it, that you get a good 2-3 psi increase. The wastegate simply can't get enough exhaust around the turbo so it can't maintain stock levels of boost. That should illustrate exactly how restrictive(especially the precat) the exhaust is.

That being said, what you run into with the stockers running high pressures is that because they are so tiny, they are being subjected to very high speeds. Stock boost runs the turbos around 140,000 rpm and of course the more pressure the faster they have to spin, which increases the wear on the turbos. So for any given higher boost level they will wear out that much faster.

As far as what boost pressure to run on stock turbos, remember that pressure and air flow are not the same thing. I've heard between 14-17 psi as far as running out of efficiency. I've heard of people hitting 20 psi spikes etc., though that doesn't mean they were flowing much more air than at 17psi. Remember these are two TINY turbos. Personally I don't think running much more than 14-15 psi is going to get you anywhere as you're probably in the end of efficiency and any more pressure isn't going to get you anymore HP's without compromising the turbos anyway. I would limit to 15 psi and work around the turbos until you decide you want or can afford a single.

Hope I've cleared things up a little

 

Was this quote referring to stress on a turbo or making power with a turbo. I have not read this book, but I am having a hard time understanding how back pressure hurts a turbo. I see how it can rob power, but all back pressure does is create less exhaust flow causing the turbo to spin more slowly, and this is just not taking full advantage of the turbo system when trying to make power.

 

I think the best analogy for this comes from walking with a small child....

You are 6' tall and your son is 3 years old with a height of about maybe 2.5'. Go walk at a brisk pace. Study the speed of your legs compared to your son's legs. Who's legs are moving faster just to maintain the same speed? Your son.

Same thing with a turbo. Our stock turbos need to spin much faster to supply those 15psi than does a larger turbo. Faster spinning turbines = more friction. More friction = more heat. Heat = bad. With no back pressure, those turbos are also spinning a LOT faster than what originally designed for. Rub your finger across your desk 20 times at a pace of about 4 inches per second. Now rub your finger across your desk 20 times at a pace of about 36 inches per second. You rubbed your finger across the desk the same distance, but which felt as if heat was building up in your finger? I think you get the idea.

Now, HOW many times do you go around driving at WOT? For the most part, a majority of us have well modded FDs, and I think you can agree, that it doesn't take us not even 1/2 throttle or anywhere near 10psi to be able to easily over take someone on the hwy and on the streets.

So the question remains, HOW many times are you going to actually floor it causing your turbos with no back pressure to over work themselves?

 

Ill say it again.....high boost on the stock turbos will cook them like my girlfrend cooks pizza. Black. If you dont believe me Ill post a pic of my last set of turbo wheels. The set had under 8k on them and they are entirely warped on the edges of the fins and are very very brittle from the heat. They look like something went through the motor even though nothing did. (I had thought I went thru my apex seals but it was a side seal that contributed to my low compression). The heat from such high boost also contributed to that side seal damage. Dont waste your time with high boost on the stock set. (I went thru three sets in 90k...two in 15k once I adjusted the boost above factory). You will find yourself with leaks and oil smoke out your exhaust.

it is my simple opinion that you will need larger injectors and a better pump. Im sure the larger fuel lines and such will help as well...however I do not agree that you should depend on your stock injectors at high boost even if you get the pump.. Spikes hit up to 18lbs on my set during my death run at the track. The detonation cooked my motor...pure and simple. I learned the HARD WAY. Two motors and all the parts within the past two years. You dont have to do it the way I did...or the other boys who have learned the same lesson.

jason
94 fD

PS. M2 puts together a larger ball bearing twin setup. I will have mine on my car around the first of the month (they are being built this week). Ill let you know how they work out. www.m2performance.com is the website.

j