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Discussion Starter · #1 ·
What factors determine what boost an engine can handle? That is why can a wrx only boost about 15psi where the evo is doing 19?

Aside from the stock turbo not being able to produce higher than 15p.s.i. or so what can our enigine take? Evne if you forge parts like the Sti, Subaru still had to make it a 2.5 to get the power they wanted out of it without detonation. Right? So if that's the case then why can the evo at 2.0 tolerate a 19psi boost without pinging?

How does the relationship go. Why can the Supra boost 25psi on street gas and others boost only 5? How is compression related? What is the compression of the WRX and the Evo?
 

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Ill try to answer a few of your questions...

What factors determine what boost an engine can handle?
Alot. Everything from the turbo size, to octane of gas to everything else.
That is why can a wrx only boost about 15psi where the evo is doing 19? The wrx can boost higher, Its more to do with how much the turbo can efficiently push. Other factors are things like higher boost = higher temps= bad if you have a cat in the uppipe which can burn up and send pieces to the turbo. The evo has a stock front mount ic, which means better cooling which means it can handle more boost.

I think the reason the wrx was 2.0 and not 2.5 was to stay within rally specifications, and not to limit power. And when they made STis they didnt care, I dont know for sure, but its probably just marketing....People that wanted STIs had wrxes, and just wanted more power and didnt care about rally specs. Just a guess.
 

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They didn't HAVE to make it a 2.5L to get the power they wanted. Plenty of JDM STi's make ~300 hp. The 2.5L was, i think, an easy few ponies and helped them get that 300 within US emission regulations.
 

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Plus we have some pretty poor gas. Making 300hp safe on 91 octane is a lot harder than on the ambrosia they get in Europe and Japan...
 

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I'm just throwing it out there:

"The Evo is tuned within an inch of it's life"
(Car & Driver)

maybe not quoted exactly, but it's close...
Maybe DSM doesn't care as much about reliability and has less "safe guards"...I don;t really know...it's an observation.

from what I've read it seems that the main limit to boost (within reason) is the turbo.

and btw, the Sti does not have a cat in the uppipe...as god beared witness to on his.

From reading the C&D review, it looked like subaru really wanted to reduce the turbo lag ala VVT and the higher displacement...and "small turbo" (god), while DSM aimed for a rocket w/ mad handles...subaru just targeted a larger market, imo.

Hg.
 

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I think people pay too much attention to boost pressure. It doesn't matter. It's irrelevant. Sti makes 248 whp at 15 PSI, EVO 240 whp at 17.3 PSI. I can make 400 whp at 15 PSI with a big enough turbo. What's the point in looking at PSI?
 

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Discussion Starter · #7 ·
So then God it's really a matter of how much air your turbo can blow rather than the pressure it can hold.?. (cfm right?)

So we should really be looking at cfm and not pressure. And, a 'bigger' turbo gives us higher cfm's? Right.

So did DSM just slap on a bigger turbo on the evo?

God, when you tuned your wrx to 390whp was it on our crappy 91 street gas? How did (do) you overcome the detonation problems?

I think that Subaru did have to release the Sti with the 2.5 to get the power with emissions they wanted.

God, when you open up the exhaust via a cat free system or sports cats have you ever seen boost pressures go down?

Why do the MBC people always say it's best not to carank the boost up past 16 or 16.5 p.s.i.?

(thanks guys for your answers - I have a lot of questions about my car :) )
 

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meisnerboy said:
God, when you tuned your wrx to 390whp was it on our crappy 91 street gas? How did (do) you overcome the detonation problems?
I think you'll find that most every state in the US except for California can get 92+ octane "premium", so you're kinda on your own there.
 

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So then God it's really a matter of how much air your turbo can blow rather than the pressure it can hold.?. (cfm right?)

Kind of yes. THat is certainly more important than boost pressure. It also has to be matched to the displacement, compression ratio, rev limit, etc. . .

So we should really be looking at cfm and not pressure. And, a 'bigger' turbo gives us higher cfm's? Right.

That is correct all other things being equal.

So did DSM just slap on a bigger turbo on the evo?

Yep, the EVO is a pretty nice size turbo. THe STi turbo isn't bad size either.

God, when you tuned your wrx to 390whp was it on our crappy 91 street gas? How did (do) you overcome the detonation problems?

It was on 93 octane and just tuned the timing and fuel properly. I can do it easily with your car too. Turbo XS tuned my WRX with nothing more than a Unichip.

I think that Subaru did have to release the Sti with the 2.5 to get the power with emissions they wanted.

God, when you open up the exhaust via a cat free system or sports cats have you ever seen boost pressures go down?

Usually they go up a tad.

Why do the MBC people always say it's best not to carank the boost up past 16 or 16.5 p.s.i.?

Because the turbo is part it's efficiency range. IT's just heating air at that point and too hot and it will ust detonate.
 

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Friend of mine pulled his boost connector hose while running tolyene(sp?) probably about 117 octane at the autox. Peaked around 18psi on a clipped stock turbo. G
 

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0260B4U said:
Friend of mine pulled his boost connector hose while running tolyene(sp?) probably about 117 octane at the autox. Peaked around 18psi on a clipped stock turbo. G
wouldn't that be blowing a lot of hot air?
 

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Discussion Starter · #12 ·
So we should really be looking at cfm and not pressure. And, a 'bigger' turbo gives us higher cfm's? Right.
That is correct all other things being equal.
Then it would be best (InMyNaiveOpinion) that we have a cfm gauge rather than a boost. Maybe that's why people think boost is the be all end all God. Cause that's the kind of gauge we have

God, when you open up the exhaust via a cat free system or sports cats have you ever seen boost pressures go down?
Usually they go up a tad.
Wow that totally goes against my intuition (but thanks for the answer). My lousy intuition would think that if you opened up the pipes then they would flow easier and pressures would go down.

God, when you tuned your wrx to 390whp was it on our crappy 91 street gas? How did (do) you overcome the detonation problems?
It was on 93 octane and just tuned the timing and fuel properly. I can do it easily with your car too. Turbo XS tuned my WRX with nothing more than a Unichip.
So what do you think the limits are on a dialy driver with 91 octane and stock ic? With say an VF30 or 34 what whp would you be running without knock? To do this is it as simple as retarding the timing so that pinging stops? Then the difference being the higher octane gas you have the more you can advance timing? What are the limits of this approach?

Thanks
 

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I'm a novice so I just started on this. A book I have says view the engine as an air pump. The CFM is important as long as you have the supports- intake, exhaust, fuel.

The IHI VF series- 24/30/34 flow sorta similar air volumes. The VF24 around 415 CFM and the 30/34 I think is around 430. The PE turbos flow around 625 CFM. Running a PE with a stock downpipe is an extreme example of "constipation". It disrupts the efficiency of the air pump. An inefficent set-up will inhibit the turbo from boosting in its efficiency range. And more air= more O2 meaning more fuel so you run lean without sufficient fuel delivery.

I believe a VF30/34 can run 280whp with a good set-up. Check out the turbo XS site. When they were initially doing their stage 4 they found that the injector duty cycles were pushing max. So they put a larger fuel pump and larger injectors- tuned it and got more power. I think they do this at 16 to 17lb boost- which is the upper safe limit of the stock internals.

I run a VF24 with a MRT TMIC. Turbo XS also has a graph showing how upgrading to their TMIC improves performance. as far as octane goes, the factory ECU parameters are gauged for 91 octane, I believe. Running 93 pouts you in a bonus situation, not a penalty.
 

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0260B4U said:
Friend of mine pulled his boost connector hose while running tolyene(sp?) probably about 117 octane at the autox. Peaked around 18psi on a clipped stock turbo. G
it's Toluene and Toluene itself is 114 Octane, but you can't run completely on it...... more like 30% of your Tank...... so he was actually probably running 98.4 octane.

wouldn't that be blowing a lot of hot air?
since the turbo was clipped it can flow more air...... but yes, it was probably flowing a little too high of temps....... but that's why the guy was using Toluene, so he could run at higher boost and temps without detonation.

Blue
 

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hey

there are more things to think about. You can bump your boost up in a wrx to anything you want, but on a stock wrx there is no point to bump the level ligher than 15.5psi because at that point when the engine is running hard and the boost is at it's max(15.5) the stock injectors on a wrx are maxed out on their duty cycle. If you bump your boost higher withought gettin a figher flowing fuel pump and injectors you risk injector failure and engine damage.

Once someone changes their fuel system(to one like on the sti) and get a nicer intercooler(like the one on the sti) you can safely bump your psi upto 18. If you get a front mount intercooler you can bump your boost upto 20 with a vf34 because it cools down the air better, and there is less chance of yoiur cylinders melting from the heat. If you get a cooler more efficient turbo, you can bump it up higher with the same settings.

a note on the sti/evo deal. If you really think about it, the stock sti's in america are tuned very conservatively. They can easily make 17.5 psi(which would make it much quicker than the evo) of boost with no mods other than a manual boost controller, unlike the evo which is maxed out. Also people say that the max speed on the evo is higher than the sti, but the sti is governed and the evo is not, so that is debateable too.

I hear the evo handles nice though:)

also, I don't think the pe turbos flow that much air at all. I think they flow 5 hundred and something. The sr40 on the other hand flows 6 something, and does it more efficiently than any other turbo I know of.
 

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Clilpping a turbo refers to clipping the turbine fan blades at an angle to reduce the restriction of exhaust gas flow through the assembly.

It's like taking some scissors to a pinwheel that's being sprayed on by a garden hose. More of the water will get through (less pinwheel material to block it) but the pinwheel will spin slower because there is less surface area for the water to push on...

It increases flow at the expense of spool/lag.


also, I don't think the pe turbos flow that much air at all. I think they flow 5 hundred and something. The sr40 on the other hand flows 6 something, and does it more efficiently than any other turbo I know of.
Hippy, how do you know how efficiently the sr40 flows air? Have you found a real compressor map for it?
 

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hey

I haven't, but my friend has an sr40 and I have a vf34, and at the same boost my utec says the air going into the mas is a lot hotter. I attribute that to its efficiency. Also everything I have read about the sr40 seems to be true when I look at my friends car. I'm not saying it's the god of turbos, but I can say that if you wanna run 20psi of boost instead of 18 you should get the sr40:). And if you want to run more get a front mount and an sr40(and maybe a nice stroker kit and some gears:)).....I dunno I just like it. If I was going really big I'd get that or the sr50...there are lotsa turbos out there they are just nice:), and some good power makes everyone happy.
 

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So what do you think the limits are on a dialy driver with 91 octane and stock ic? With say an VF30 or 34 what whp would you be running without knock? To do this is it as simple as retarding the timing so that pinging stops? Then the difference being the higher octane gas you have the more you can advance timing? What are the limits of this approach?

Thanks
[/QUOTE]

Well the timing is part of the equation. It has to be advanced in certain areas and retarded in others. The VF30/34 is around 270 whp properly tuned with stock upper mount and fuel system upgrade. With the bigger top mount I'm getting a tad more. With front mount 285-290 whp is possible on 93 octane. Not bad for such a modestly sized turbo. With the PE turbos we are getting around 280-285 with stock IC and 300-310 with front mount.
 

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hey

hey god,
I have a vf34, and for some reason I think that your estimates for horespower are very modest If you have the right other mods, ie-front mount, some headers, some light pulleys, exhast, intake, fuel, light wheels, and some spark plugs I think you can easily get more than 300hp.

At least I think it could push 300+hp, not that you ever use that hp. I mean most of the time, the higher the rpm the higher the horsepower. I don't want to tell anyone how to drive, cause everyone seems to think that to get the best times you need to rev your engine to 8000:), but using the peak horespower of your engine isn't always making your car accelerate at a peak rate. Torque is good too:).
 
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