I am building a 400CI big block and was looking at running twins but decided I might go with a single instead but cant figure out what to run I was thinking maybe a GT42? I plan on running a top rpm of around 6500 max.How fast would I be able to spool a GT 42 or would it be the wrong turbo for my application? I am looking for around 800-900 HP on a mainly street driven motor that needs to live on pump fuel.

PS I dont want to run this turbo out of it efficiency range.

 

I don't believe you will be able to make that much power on pump gas with that turbo. I think a GT4788 would be much closer to what you're looking for.

- Brian

 

:withstu: yep, I agree, that gt42 would be too small.

 

it really is fucking amazing that all of a sudden a Garret Gt42 is to small for a 400" engine, when I first started building my kit nobody on this board had any thing but positive about using this turbo in my application. Will it do 800hp, I believe so. Judging by how much power mine makes at only 12psi I'd say 800hp would be easy. Is it Ideal, no, but it would work. I need to hurry up and get my converter fixed and turn up the wick so I can be everyones guinea pig. Seems like there are a bunch of people on here who got excited about the Gt42 and then as soon as we get done everyone thinks its not enough turbo.

Edit* I see he said big block, but is that really any different.

 

Considering that Brian works/worked for Precision Turbo, I'd say he knows a bit of what he's talking about. I agree with him. The deal breaker in this request is the pump gas requirement. For that reason, I'd use the GT47-88 also.

 

Any idea on how quick these would spool? Like what kind of rpm would I need before i see any boost and where abouts would I see full boost?

Thanks

 

That's such an open ended question without any details. It depends on the motor and the torque converter. But.......... with a 342ci race motor we saw a GT47-88 go from vacuum into boost at 3500 rpm and would see 12psi at 4500.

 

Well the motor is going to be a 8.5:1 400 big block dodge with either ported iron heads or a set of Indy aluminum EZ heads, a single plane intake and a CSU carb. I am not sure on cam yet or the torque converter. the motor will be going into a 87 Dodge Dakota. I will be running a 727 transmission with a manual valve body. what would you suggest for a torque converter?

 

You want to flash the converter to the point where you go from vacuum to positive manifold pressure. I assume you'll have a transbrake? I'd suggest 3000rpm to start with. Once the converter flashes to that rpm, it'll start to build boost. Companies like Neal Chance and PTC both have done a lot of R&D on converters for turbo cars over the last 3 or 4 years.

 

looking at some maps for a 400" @ 6000 rpm and between 15 and 25 psi of boost, its a little on the small side, but still over 70% efficient, id say if you got it, then get it on there. i am still up in the air of running a bigger single or twins, right now i have a 60-1 on mine and its too small but im going to run it to get used to it before i switch to something else as its just amazing how hard and fast boost comes in and im just not used to it over a n/a motor.

i have been eyeballing the gt42 as its a popular turbo we use at work (detroit diesel) and id prob only have to pay core value for one, if i can find a decent one thats not totally beat.

 

Okay I understand the pumpgas part

 

I'm just a little curious what map you are looking at? The GT42 map I found for a 102mm version has around 750 hp capacity at 15psi and it's efficiency is probably 65% or less since it is on the very outer ring. At 30 psi it's look to be good for around 900hp...

Any 400" engine and a good set of heads and cam is going to eclipse both those number by 4000rpm easy....

 

this is the one i was looking at

 

Subscribing?

 

That map is actually for the smaller gt42. This map includes the one for the larger one, as it comes from garrett:




Now keep in mind, both these are the ones with the 74mm inducer. Precision sells a version with a 76mm inducer, for which there is no published map.

 

With this big of a turbo am I going to need to run two waste gates to keep the boost down around 10 pounds on the street? Also do you think I would see any boost before 3000RPM with the right cam and converter?

PS I am also thinking about using a set of 3.23's or 3.55 rear end gears. If that makes any difference.

 

I talked to precision Today for my 383ci SBF and they told me the standard gt42-76 would be ideal for my application to make 800rwhp shifting at 6200rpms. I dont see 17ci making that much different.

 

How different is the gt42-76 from the standard GT-42's we are referencing up above?

 

The GT42 will work fine for you app. Its not ideal, but will work.

The standard GT42 and GT42-76 are identical except for the inducer size. The standard Garrett units only use a 70 and 74mm units. The PTE modded version has a 76mm wheel in it.

BTW, you can get the GT42 in either a T4 or T6 flange. The flange size at this point isn't going to make alot of difference since both housings still flow through the same wheel.


Jose

 

JZ 97 SS 1500: would I be better off stepping up to the GT4788? would it be more ideal for my application? Also what kind of affect would it have if I stepped up to a 451CI a few years down the road?

I dont have any turbos right now so I can buy what will fit my application best.