Ok here is my issue. Have a built 355 (everything is listed in my sig) that I am starting to second guess. The car is going to either be getting a ST80 turbo or PT-88, have yet to decide. Currently my combo is right at about 8.5-1 compression wise. The car will be mostly driven on the street but taken to the strip on the weekends when I am off work.

Lately I have heard alot of guys say its going to be a complete turd out of boost. Now of course if this is true I will have to change seveal things on my motor Oh well its only money right! FML! Anyway what has been everyones experience with a turbo LT1 or LS1 for that matter with 8.5-1 compression. I plan to run about 15-20psi to reach my power goals. Thanks

 

You will be fine with the 8.5 to1 compression. Most of the turbo charged cars made in the 80s and 90s had around 8.5 to 1. Now how much power are you gonna be running on the streets and how much do you want for the track? If its gonna be your daily you can run 18 psi on 93 oct and run 25 at the track with race gas. That t88 is nasty, i seen some in some supras. So in a built 355 you ll have less lag than a 3.0 2jz.That 38mm wastegate is not gonna make it bro you need at least a 50mm turbosmart wastegate.If you dont you ll get BOOST CREEP.. As far it being a terd out of boost think about this if a 3.0 2jz engine can spool it do you really think your 355 won't? Choosing a power goal is the key here.By the way i own 2 supras and a 97 ws6 and i love em all.

 

With that low of compression and that large of a turbo and your boost PSI goals, it WILL be a turd as a street driven car. However, on the track it will be a beast. If I were you, I would consider my options. More compression, smaller turbo and less boost to be streetable or the way you are predicting to go. How much street driving do you do? Do you have a primary daily vehicle other than this car?

 

It has been my experience that 8.5:1 is the best compression ratio for a turbocharged engine. Little 4 and 6 bangers manage to feel decently peppy off boost with this kind of compression so if your LT1 is a complete turd off boost, you did something wrong.

 

I would shoot for 9.1. And also get the 50mm wastegate.

What are your power goals??

 

I don't think it will be a turd on the street. It depends on your cam (is it right for a boosted application?) and the tuning. The PT-88 should have full boost just before 3500 RPM (a properly built engine and valve train should easily have a 7,000 RPM red line). The 3.0 supra engine is laggy because out of the boost it's only pumping 3.0L worth of air, your 5.8L V8 is just about double the air volume. That is a very important difference when it comes to spool time. The gearing and transmission is also a big factor. I have a manual and still have my 4:88s in from my old N/A set up, but I do have a set of 4:10s that will find their way back in. I don't know for sure yet, but the majority of turbo guys I talked to say that less gear is the way to go (the 4:88 is too much). Their point was something about loading the turbo and letting the boost do the work.
Ultimately I wouldn't change anything about your engine (assuming the cam is right for boost).
Lastly, I run a HKS GT-II 60mm WG. The price sucks, but the part is totally cool.

 

Tial has the best bang for the buck when it comes to wastegates and they make a 60mm as well.

 

Your compression ratio should be based on the maximum boost you want to achieve. The higher the maximum boost, the lower the cr should be.
The lower the compression, the lower the torque at low speeds, all other things being equal. I am running 9.5 cr b/c I will max my boost at 15 psi, or roughly 2-bar. And it was built from the ground up as a turbo engine and is not a "turd" on the street.
I run an auto with a 4k stall and 3.42 gear. It is very pleasant as a "normal" car and a beast when it hits boost. A turbo has to have a load in order to start building boost. Otherwise it just sits by idly and lets the engine do the work. This increases efficency and fuel economy, which is why a small engine can get good fuel economy when not under boost and lots of power under high boost conditions. When I nail it, the engine is working hard, exhaust energy is increased, and the turbo starts doing its thing. Look up turbo thermodynamics for more info on the subject.
Spool up time is based on two things: maximizing heat energy at the turbocharger, and A/R. "Smaller A/R housing has faster spool up. Bigger A/R housing has less back pressure, more flow for the top end." (source http://www.ztechz.net/id4.html).
You should also be aware that as you increase boost into 3-bar territory you need to consider pushing water and plan accordingly.

 

Thanks for the responses. I have a pretty solid understanding of turbos. My plans changed and so I am looking at my options. I thought it might be a good idea to look into a bit more compression as many say this is the ticket for a car that sees a good amount of street duty. My original turbo was a 76GTS. I am now looking at a ST80 or a PT-88. The ST80 being smaller than the 88 builds boost closer to the 76GTS, which might be a bit more fun to drive on the street. The cam is a turbo cam done by AI. I will have to look for the specs. The car is a manual at present. Might go to an auto at some point.

This car will never be a daily driver. Putting a wolfe six point in it this spring after I put the motor back in. It's a car I would like to take to speed world and run into the 9's someday and still be able to take out on weekends once in a while. It still has a/c ( you will die here without it). My original goal was to make about 850rwhp. The 76GTS is just about tapped at that level. Decided there was no reason I couldnt hit 900-950 with my setup and am now looking into the PT-88 or ST80. The big issue here is not very many people seem to be doing this in a 355, most are 370,383,402 blah blah. Guys with 370's and 402 seem to absolutely love the 88, but having smaller cubes worries me. I want to run about 20-25 psi boost which is the original reason for choosing 8.5-1. My builder seems to think its ok but he probably assumes I will hardly ever drive it on the street.

I also had planned on a larger waste gate, but am still trying to sell my 76GTS so I have not had the opportunity to do so.

 

You need to size the turbo to the cubic displacement of the engine, boost level and the peak RPMs. That will tell you the maximum volume that your air pump (the engine) will require. Too big, spools up too late in the RPM range and the extra potential volume is wasted. Too small, runs out too quickly and will overheat. That is why the 383s run a different turbo from the 355s, and the LSxs could run a different size from the LTxs based on peak RPMs.
In a theoretically perfectly sized system, the turbo achieves peak performance at the same time the engine does. In the best of all worlds, the turbo resizes itself, like the Holset variable geometry turbo. But those are impractical for our intentions, due to their bulk and requirements for computer controls.
Also, vendors who sell the turbos also have lots of experience with their customers as to what works and what does not work.
I suggest checking several turbo sizing calculators; none are perfect, most are pretty close.
At that much boost, you will most certainly have to build to prevent pushing water. For that power range, you'll either need an aftermarket ECU for higher RPMs or else run higher boost levels at our PCM's limit of 7k RPMs. Also, for that much power you'll need to replace then entire rotating assembly, so why not move up in cubes at the same time? I am perfectly happy with my 355, but my goals were much more modest than yours.

 

Here is the thing, the 88 will work I have spoken to several well known turbo guys(some vendors here). Looking at compressor maps and the the like only takes you so far. Real world is more of what I am looking into. My engine will take 900rwhp, i have seen a few LT1's with similar internals do so. And there is no reason to go bigger in cubic inches to achieve this. Supras do it all day long.

I am running a EFI connection 24x so I now have a LS1 computer. Should be more than capable with my goals. I dont plan on spinning this to the moon so a FAST is not needed. I had a FAST system but sold it not wanting to deal with a standalone. I am running cometics so pushing water should not be an issue all things being equal. Have seen a few LT1's using cometics at 20psi without issue.

Jose over at forced inductions had a 383 LT1 that was very similar to mine and I remember him making over 1000rwhp and it seemed to hold up fine. Not comparing numbers just internals.

 

Perhaps you misunderstood my post. You don't have to increase cubic inches, you have to increase the amount of air being pumped. ALA your internals can take it, you can use the increased RPM available to the LS1 PCM (to the moon isn't that descriptive) and/or boost levels. The LT1 PCM is limited to 7k RPM, so you have already overcome that obstacle.
Running 20 pounds of boost should prove interesting. Since others have already made adjustments to prevent pushing water, following their lead should be a good idea, which is what I attempted to state.
Supra engines are air pumps also, so to compensate for fewer CI they increase boost. Just basic physics, they aren't doing anything special.
Building the LT1 to take that much HP isn't magic, it just takes careful consideration and decisions. Mine is built the same way, I just don't run it that hard so it is understressed, in the hope that it will last longer that way.
Best of luck.

 

I have taken steps to prevent pushing water. Cometics work perfectly fine when machining is done properly. My builder consulted cometic as he mostly builds desert trucks. They gave him the proper recommendations for surface finish. No need to o-ring which we had considered in the beginning. How is my boost goal vague in any way?? I set out to run 20psi. That may change.

The frustrating thing about forums is that alot of times people assume things. I am fairly familiar with how turbo motors work. Listing the supra was highlighting the fact that I want big hp numbers with less cubic inches. There are other ways to get to my goal and I have chosen this one. No offense but i never asked for "turbo 101". I want to see what works "real world" in smaller cubic inch v-8's. I already know the 88 will spool at about 3500 to 4000 in my app so the 76 would be more "fun" on the street. Thats not the issue, Im trying to go bigger yet have the car be repsonsive on the street, ie possibly ST80 as its in the middle for size and spool. Im not trying to be a dick but its frustrating how asking a simple opinion based question leads to "well you should read turbos for dummies".I would like to do this right the first time around and learn from the mistakes of others as what I am doing goes a bit beyond an sts turbo kit in terms of both power and dollars. Thanks

 

Your question about compression comes down to how much boost you will be able to run per motor octane point you put in the fuel tank. More compression is going to spool the turbo quicker but, will make it more prone to detonation. The power you are going to make will come from the turbo more than the engine, you want to make sure the turbo can operate at an efficient pressure ratio on the fuel you want to run (93 octane pump gas?).

 

I plan to run 91 as its all we have at the gas stations in AZ. I am running a custom A2W intercooler and plan to intall my meth kit as well. From what forced inductions is saying, now that the 80mm upgrade is no longer available the ST80 should be the best unit for my combination. Have just heard alot of negative about running a lower compression ratio on the street. I don't think detonation will be much of a worry with what I am planning.

 

Find a compressor map for the ST80 and go from there. If it isn't happy until high boost, I think 9:1 is going to be too much to get there on 91 octane. Some turbos don't care so much but, others make a huge power difference with a few psi, if it jumps from say 70% to 80% efficiency. Worst case is you are below the surge line with the boost you can run and then you will rip the turbo apart as well as make no power.

I never liked meth kits, to failure prone.

 

Yeah I need to contact jose since I will be buying another turbo from him and ask for a map for the ST80. We have discussed my setup in great detail and he thinks for what am attempting that the ST80 would be the best possible fit. I agree that meth kits are not the most reliable but Im kinda stuck with 91 and meth because I dont see filling it up with race gas every time out. At the track is one thing, although i plan to turn down the boost for the street. Thats another thing, Im not planning on beating the **** out of this poor thing at 20psi all the time. I have a guy here in phoenix that is going to tune it and we will see what it tops out at and where it will be reasonably safe to run it on the street.

Thanks for your insight. I think Im just gonna stick to the 8.5-1 and look further into the ST80. Should make for a good combo considering my cam and heads.