First I'd like to say hello to evolutionm. First Post!

I've done a lot of research on this site to aid my EVO purchase, I thank you all for the information. I'm super excited to be the proud new owner of a WW SSS GSR! I'd post a pic but Im sure you all know what a stock evo looks like lol.

I come with a long history in the Honda world. I drive (and built) a sleeved and fully built Integra putting down 490whp on E85 (28psi, GT3076R). I'M READY FOR AWD!

My first question is about boost... In the Honda world it's very common to have creep issues due to manifold design. And by creep I mean more boost than desired as revs rise. Why does boost taper off on an Evo? Can anyone tell me why there's such a difference? I've seen so many posts with people saying "I run 26psi tapering off to 19psi..." ¿por qué?

Thanks!

Here's my Integra if anyone cares:

 

Basically because we're pushing relatively small turbos so hard with good head/cam designs. Remember that boost is back-pressure, not airflow.

 

Yea, a stock turbo cant keep up those boost pressures at higher rpms. You are definitely getting the most you are going to get out of it then.

 

I see. Maybe I don't fully understand backpressure.. I thought turbo cars want less, i.e. using a large 3-4" exhaust where an NA car wants more backpressure so they use a smaller exhaust? Is this not true?

 

I'm talking about intake back pressure, not exhaust back pressure.

 

Ahh... I get it. So as revs climb the engine uses more air meaning the turbo has to work harder to maintain the same boost levels.?. I guess I never had this problem because I never used a 'small' turbo on my Honda.

Even still, a stock evo turbo can flow 30 something psi. For example if you tune for say 23 psi, shouldn't the turbo be able to spin faster/more than that as revs rise to counteract the decrease in backpressure? I see why psi would drop as revs rise if the wheel spins at the same speed but why doesnt the wheel spin faster as revs go up to maintain the desired 23 psi? Hopefully you see what Im saying...

 

Turbo Side:

The stock turbo may be able to reach 30psi but only for the briefest of moment falling like a rock afterward, I recall most being around the 4500rpm range.

You got a couple of issues really holding back the stock turbos in evo's. First the exhaust port is pretty small so not a lot of flow can get past it. Secondly the compressor on the turbo is small this means it would going well past mach 1 speeds to achieve 30psi at 8000rpm.

A compressor wheel hits the wall so to speak when it achieves this mach value and the energy to spin the turbine at this point becomes counter productive. This is why in addition to the size of the things we get big boost drop off.

You need to start thinking of turbos in terms of CFM to really get the whole picture. The stocker for an 8 is only really capable of around 440CFM or so which is way below what you need for 30psi at redline.

Backpressure:

I've said this so many times before but to reiterate back pressure is BAD ALWAYS. The term everyone is searching for is called "Scavenging" which as it turns out is pretty ineffective in a turbo environment. Why you may ask?

Well there is really large piece of metal in the way of exhaust gases getting out into the world called a turbine . The turbine in a turbo effectively removes most of the energy from a pulse of exhaust fumes and creates a pretty big restriction. As a result you'll see most turbo cars geared toward minimizing any additional back pressure by having super flowing exhausts.

NA cars don't work like this without a turbo in the way the speed of the pulse can drag fresh in from the intake into the exhaust further cleaning the chamber for more fuel making more power.

 

Thanks for the detailed response. You said an 8 is capable of about 440CFM, how bout a 10? I'll probably run E85, what's the most boost I can run 'safely' on this turbo? What boost do you think I'll see at redline?

 

Not sure to be honest never saw a compressor map for that turbo but I'm sure one could be found along with its CFM rate.

There was an excel spreadsheet floating around here with a pretty kick *** map of showing you what CFM is required to maintain boost at some rpm. Just have to look for it .

When you get a comrpessor map what you want to do is not go off the right hand side of the chart which will putting the turbo into the mach zone.

 

Did a little bit of research

Seems the stock turbo for an Evo X is a td04h 15g with a fancy compressor cover on it.

Specs:
Advertised flow (lbs/min): 42lbs/min (thats about 460 or so CFM)

This is really almost identical to the evo 8-9 turbo flow rate but in all likely hood mitsubishi made their engine flow better than the 8 and 9 with that fancy dual mivec head of theirs. I expect fully this turbo will have trouble going beyond 21psi of boost at redline. Beyond this and the air is giong to get really hot as its outside of the effecient ranges.

Seems like if you want to really put some power down you're going to need to up the anty with the turbo and get something more like a FP green/red/black to get some more boost. Figure out your power goals and choose a turbo from there

Stealing this from a honda board:

 

Thanks for the research. The lines on the map help make it easier to read. Do you happen to have a link that teaches more on how to read compressor maps? I keep asking about the stock turbo because I don't think I want to upgrade it, just push it. I have my integra to blow up lol. I want this to be a reliable daily driver. Just out of curiousity what's the flow of the fp green vs my 3076r?

 

The green in flow is nearly identical in flow capacity only about a couple pounds less.

If you want to max the flow you need some higher boost pressures though, e85 does a good job making power on stock motors as you can up the boost much more safely.