I will be installing a T67 turbo on my SC300, along with a MAP ECU for tuning. Here is a list of what's going to be installed:

T67 Turbo (.63 A/R)
Dave H Manifold
Downpipe, Midpipe
Tial 38mm wastegate
Wastegate dump pipe
Oil line kit
FMIC (intercooler, piping, couplers, etc)

ARP studs
Head gasket (to lower compression)

I have two options regarding the head gasket and compression:

Stock Supra TT head gasket ($140) ~9.0:1 compression ratio

or

BoostLogic 2.5mm head gasket ($280) ~8.5:1 compression ratio

What are the advantages/disadvantages in having 8.5:1 compression compared to 9.0:1, and vice versa? Which will yield more power? Which would be better for my setup, considering I will be tuning with a MAP ECU and driving the car daily?

Any comments or suggestions would be appreciated, thanks guys.

-Allen

 

ok there are two ways to this imo.

if you never plan on going all out high psi 400+whp then go with the tt headgasket. your off boost low end responce will still be good with this choice.

if you plan on keeping the tune and power on the high side id say choose the 2.5mm to get the compression at a true friendly 8.5:1 ratio. off boost responce will not be as good as the tt hg but you could throw high psi at the car all day.

if your localish to me near 818 let me know ill help ya install if your doing it yourself.

 

For street na-t vehicles, most folks like a safe tune, which entails lowering the compression to 8.5:1 (or slighly lower) and advancing the timing more so the off-boost engine performance is good. The lower compression allows you to advance the timing more and as we all know timing is power when it comes to tuning. The downside is, the lower compression means longer spool time compared to a higher compression boosted engine (via piggyback tuning). The upside is you can run more boost. It's a trade off that is compromised when your overall power & powerband goals are defined. (keep in mind there are a few tuning tricks with a full stand alone ems that can compensate for each of these characteristics at the sacrifice of burning more fuel )

With that said, your overall power-goal should determine which headgasket to use. If you're using a piggy-back fuel control (like map-ecu), I recommend not going lower than 9:1 engine compression so your low-end doesn't become too sluggish and you have better off-boost engine response overall. But, if you want the absolute highest peak power possible, lower compression (8.5:1 in this case) will allow you to safely push more air into the engine, thereby making more peak power at the sacrifice of spool-time and off-boost engine performance.

Anytime you only upgrade to a thicker headgasket on a 2JZ-GE engine, you are destroying the "squish" (aka quench) factor that was designed for the stock headgasket. It's basically an optimal distance the headgasket creates between the head and the top of the piston that allows an efficient-as-possible swirling of the combustion gases to minimize detonation. It's just a contributing factor that explains why some can make more peak power safely with engines that retain proper squish regardless of compression.

I like my 9:1 compression na-t and wouldn't consider any lower because my priorities are driveability and engine response instead of high-powered high-way monster vehicles. Motorheaddown has more experience with 9:1 than me, perhaps he can chime in...

FYI, Boostlogic's 2.5mm headgasket will lower your engine compression lower than 8.5:1. There is a guide explaining this on www.clubna-t.com in the tutorial / faq section allowing you to calculate exact compression ratios as a result of headgasket thickness values. Lastly, I can point you to a source to get that Supra TT headgasket for less than ~$90 shipped, just pm me .

 

I'm leaning toward the 9.0:1. I want to have a somewhat responsive car on the street- and not have to boost huge amounts all the time...

With 9.0:1 compression on a T67 (.63 A/R)- I could still crank up the boost on 100 octane, or really crank it up on C16 (117 octane leaded fuel), right?

I live in so-cal, so the best pump gas we have available all the time is 91 octane. Considering my car is a daily driver, maybe 9:1 is the wise thing to do.

Anyone have any thoughts about my plans on race gas, and high boost?

-Allen

 

you will be able to run higher boost levels with race gas. How high will be deteremined from many factors.

 

i know this is my first post here and i don't want to come off like a ****, but i would like to fix something in this thread. spool-up will be the same regardless of compression. higher compression produces better off boost response (more torque), and it also produces higher amounts of power given a set boost level.

 

Allen,

How much are all the parts together, just a rough estimate?

 

I'm getting a 2.5 mm head gasket, (Cometic)- so I'll be at around 8.4:1 compression

I'll let u know how it all goes

Allen

 

i run a 10:1 compression ration on 7 psi of boost. i get no lag and the hot side of my turbo is large so my boost is very gentle. ifd i had to do it i would run 2 na head gasket stacked with 15 psi.

 

Thats way cool if you can afford race gas for your daily driver. Where in ca are you? because lately Vp racing has been building gas stations that have 100 octane at the pump. i belive the price was like 6 bucks a gallon

 

Yep if you got money to put race gace in the tank everyday then you can doit. You said this is a daily driver you need 91 or better gas friendly compression and tune! For the track tune for 100 or better octane.

 

9:1 seems almost borderline dangerous on any daily driven turbo motor. (depending on the psi)

 

Yea thats way to close for me. But since i see alot of people are boosting the 2jsge motor i just might do the same with the spare motor i have So ket me know if you need any help or run into any problems.

 

im kinda new to this whole 1jz/2jz nat gte thing... i came from years of honda stuff

but higher compression always yeilded more power per psi of boost... but youre limited to the amount of boost you can run.

and lower compression yields less power but you can run more boost.

but the thing that gets me... is that i know so many b/k/h series honda motors that make 500+whp on 11:1 compression and they last without a problem...

y is it that every 2j na-t i see they said do not run more than 10psi on stock compression... is it the compression that blows the motor or just a weak head gasket that blows

 

So you're boosting again! LOL! Can't stay away from boost, huh.

I was planning on using a supra tt head gasket along with other upgrades to reach my goal of 350rwhp+. I don't want to lower the compression too much but all of this won't happen anytime soon.

Anyway, good luck on your project Allen.

~Josh