OK so I've read and know that the smaller the A/R housing the more quickly a turbo spools. (oversimplification, but it works for my question)

I have a SP74 (which is a PT74 modded by Sound Performance) with the .81 A/R housing and 47mm wastegate. It hits full boost around 5K.

My question is this:

If I switch to a .68 A/R housing, how much faster will it hit full boost? Is there an equation or rule of thumb to calculate this?

Thanks in advance,
Josh

 

I wouldn't say there is a rule of thumb, There are other ways to get better spool without shrinking down the turbine side. Dropping down to a .68 will be to restrictive for any good power out of that turbo. Your shooting yourself in the foot by running a large compressor only to put on the small exhaust housing.

 

got ya.

if you don't mind how else can i improve my spool?

 

spool is more of a relationship between the turbine housing and turbine wheel than just the housing size alone. The size of the turbine wheels needs to be as close to the size of the compressor wheel as possible. The smaller the turbine wheel the less leverage you have to turn the compressor wheel.

I have no idea what size turbine wheel or compressor wheel you have. I'm just feeding you some general information to absorb and decide if it can be benificial to you

 

engine porting has a ton to do with that but it sounds like its too late for that angle

this may sound like just plain common sense but the larger your intercooler is and intercooler pipes the longer it will take for air to fill those and get into the engine...some people run a smaller turbo to intercooler pipe to match the outlet of the turbo. Also having your boost controller and wastegate working perfectly together is HUGE and very tricky as well.

is your turbo a 74mm turbo? thats huge if so

 

got ya, so far.

here is another question:

if the turbo manafacturer offers the smaller turbo housing it should still work within the turbos efficiency range. i guess what I am saying is they wouldn't make a housing so small as to kill their turbo, or would they?

maybe I will just boost more to make up for the delay

and yes it is a 74mm turbo.

 

well the exhaust housings are made to match the engine that its going on. For instance a .70 exhaust housing is HUGE for a supra engine while that is very small for our engine and a comparable huge housing for us would be 1.15 or something. So the only real way to know if it is a restriction for you is to keep switching them and dynoing at various boost levels to see what happens hp wise.

it also depends on what you want from the turbo. I daily drive my car so I wanted fast spool and I hardly ever run racegas so I don't mind that my turbo dies out at around 22psi. Its one of the best for up to 18psi power wise even with the big turbos while having that quick spool that they dont. Sure I can't run 25psi but I don't want to put that much strain on the engine. See what I mean its all about what you're looking for it to do a turbo will act the same every time you floor it so you have to decide which one is best for you and changing all those little factors above will help but you cant change how the turbo is built and is going to respond for the most part.

 

I think I may run it like it is for a while.

Eventually, I will probably move to a smaller turbo setup.

I am more or less in the same boat as you. I am not really looking to run extreme boost or anything like that.

What kind of setup are you running?

 

Reducing backpressure after the turbine for one, manifold design is another as well as turbine wheel ratio. Which is kinda what SPO is getting at. I would say if your dead bent on keeping that size turbo rework the manifold and downpipe. This is of coarse not the cheapest solution though. Maybe selling the 74 and getting something else will work out to be better for you.

 

I'm thinking you are right. Thanks for the help guys.

 

No problem, 16psi at 5K seems a little on the slow side for response. For example I have guys seeing 1bar by 3800 on GT42Rs this is with a 1.15 A/r

 

I haven't had the car tuned yet. so I really didn't want to crank it too high. well really I can't since I havent done the 3 bar MAP conversion. I just had RX7store help me with a basemap for the 16psi. It is a brand new setup for me. Even after/when I get it tuned I don't think I will be going past the 20PSI mark.

I test drove the same setup in a friends car and remember it hitting around 4500-5000RPM. Of course his wasn't tuned at the time either.

I was just trying to determine if I wanted to change the A/R housing before I get it tuned this spring.

 

I'm running a 62-1 with 1.00 housing divided hks ss manifold and hks 50mm wastegate

its not a kit just parts thrown together : )

It really sounds like you need to switch out the turbo right now before tuning. Sell that beast to a supra guy for 1000 dollars and pick up something nice. Give Bryan at BNR a call and see what will fit on your manifold and talk with him about spool/power.

 

It looks like this mammoth turbo is going out the door. I was just on the phone with Sound Performance and he almost passed out when he found out the size of the turbo I was running. He said for my application and SP63 would be more than enough for what I am looking to do. He said that it could deliver up to about 600RWHP without a problem. Being that I am not looking to break the 500RWHP mark....EVER!! this should be more than enough and he said I could expect full spool by 3K. Which brings the question why would anyone want to keep stock twins?

Larry, at SP was astounded that I had such a large turbo on an 7 with such "small" aspirations.

Anyhow, I try to post back the results of the questions I ask, for those who like closure.

 

https://www.rx7club.com/showthread.p...960#post3972960

this would be a great option.... and is smaller and dual ball bearing. Has great manifold and dp design, was built by Sean. Just give it a thought. MMMMMMMM GT40R MMMMMM

 

Larry is a good friend, not to mention he is down the street but I do not see you making 600rwhp on a PT63.

 

well I am not heading that high anyway. If it will hit 450RWHP I am happy.

 

At what boost pressure.

 

umm, we didn't discuss boost so I am not sure what he had in mind.

What turbo would you recommend? He seemed to be confident that the SP63 could deliver the power. should I move up to the SP67? I would like to not have to break the 20PSI mark. Like I mentioned I am looking for probably 425-450RWHP.

Thanks for all your help so far.

 

The 63 should get you where you want to be at reasonable boost levels with good port work. 67 will get you there easily and with a well thought out setup shouldn't be too laggy either.

 

gt40r.... tell him Sean, cause its coming back to you anyway!!!

 

I made 410rwhp at 15 psi/475rwhp at 19psi with a 63. At 24psi it made 504rwhp at 6000 rpm but the boost dropped at higher rpms. At first I thougt I had ran out of compressor but it was later track down to a massive pre-turbine leak. The only good thing about the run though was that at 6000rpm the power increased by 80rwhp so should it have hold the boost to redline the power would have been a lot more. In time when I return to the dyno we'll be able too see what the turbo is capable of.
BTW. The actual configuration of the turbo is a 63mm compressor wheel in a 60-1(4inch inlet/ 2.5 inch outlet) compressor housing with a p-trim/.96a/r turbine housing.

 

Thanks for the info. If I can hit the 450 mark and stay below 20psi I will be a happy camper. Hell realistically if I break 400 I will be happy.

How is the spool on it(ie when are you hitting full boost)?

I have a line on a trade with a SP63 with a .68 (which I think may be a bit too small of an a/r housing, but that can be fixed for a couple hundred)

I really appreciate everyones help so far.