millardmt

Just a quick question. I'm still thinking of going F/I with my '09 xB, but worries relating to emissions and - mostly - reliability have stopped me. As an example, I've read here where head studs are prone to pulling out of our aluminum blocks, largely as a function - apparently - of heat. That kind of thing scares hell out of me. I'm not a rich man!

So I want to proceed very conservatively with this, and I was therefore alarmed to read the following text within a Wikipedia entry recently:

[http://en.wikipedia.org/wiki/Toyota_AZ_engine#2AZ-FE]

"2007+ versions of the 2AZ-FE engine were upgraded with 9.8:1 compression ratio, a slightly more aggressive intake cam profile, 6500 rpm red line, and piston oil squirters."

I don't WANT a higher (9.8 vs 9.5) compression ratio. I want a lower one if I go turbo. So my question is, why am I not reading of people replacing their head gaskets with thicker ones? Wouldn't doing so obviate the risk of detonation and lessen the need for an intercooler? I'm missing something.

Thanks, Marc

FromTheOld

People don't replace their headgaskets because as far as I know, our headgasket is capable of handling the general power that is made on turbo, and because there is no point in replacing something that is perfectly fine. The headgasket doesn't need to be changed....you can just run an intercooler and have a good tune. Intercoolers and thicker headgaskets are similar in price (unless you are getting a ripoff 600 dollar intercooler or something) and will offer the same benefit. You may not be rich, but it is smarter to run a ~100 dollar intercooler than go through all the cost/labor/time (and additional cost of labor if you can't do it yourself) to change the headgasket.

Also, some people enjoy high compression forced induction cars....the higher compression allows quick spooling, which is nice when you've got a big turbo in a car with a not-so-big engine.

millardmt

FromtheOld:

I think you're right; I just hadn't thought of it that way. I'm fairly old - and therefore old school - and I've pulled countless cylinder heads in my time. I think of it as no big deal. I guess my point was to wonder why an obvious variable - compression ratio - is so seldom considered as something to be manipulated: To the contrary, it seems to be treated as a constant.

Anyway, educate me about something if you will. How is compression ratio related to spool time? Is it just that - all things being equal - an engine with a relatively lower CR is by definition a less efficient heat pump (through which unburnt and burnt gases must travel at a slower pace)?

Thanks, Marc

AAG

iTrader: (1)

The size of your turbo is what determines how fast it will spool. A lower cr tends to let you run more boost safely, However now with direct injection motors that seems to be a thing of the past (requiring low cr to run boost)

FromTheOld

I wish I could educate you, but I'm just speaking from experience. Supra with a standard 7MGTE + CT26 vs a 7MGE converted to turbo with a CT26, the 7MGE (with higher compression) spooled faster. I'm not sure of the actual physics behind it.

I'm old school myself...and using thicker headgaskets is one of the most common ways that MKIV N/A supra guys go turbo in a "safer" manner. Honestly as I'm considering going turbo now, I'm trying to pick and choose my options carefully.

TrevorS

iTrader: (1)

This may seem a seriously weird comment, but I'm personally a fan of lower compression with F/I, it allows use of lower octane fuels. The 2AZ-FE with N/A is happy with 89 octane unleaded, whereas change it to F/I and you need 93 octane -- yuck, 89 is expensive enough in my opinion !

EDIT -- From what others are saying, it would appear I'm thinking the octane ratings are two higher than actual. Oh well -- what can I say. Sorry about that !

draxcaliber

you only need 87 octane gas when you're n/a. 89 is throwing money away.

FromTheOld

Well you either run high compression and low boost or high boost and low compression....and most would obviously say that low comp + high boost is better for power and safety.

draxcaliber

okay, with turbo engines, the enemy is engine knocking when the air and fuel pre-ignite before it should be ignited by the spark plugs. this is because there is a certain point of compression where air and fuel and heat will self-ignite.

lower compression pistons reduce this by allowing you to cram in more air and fuel with out worrying of pre-ignition. but out of boost, you'll have a considerable loss of power and it will contribute to turbo lag (or a higher boost threshold)

with higher compression engines, they're already making more power out of boost, and the greater power of the explosions made by high compression engines means they can spool the turbo faster and easier, but then you run a greater risk of pre-ignition.

ScionFred

iTrader: (1)

Not necessarily. Given the dynamic ignition timing by knock sensor method employed by Toyota it can easily be argued that the use of 89 octane permits more optimal ignition timing advance. The ECU very likely pulls more timing with 87 than 89. As I understand it, the ECU uses as much advance as possible and then pulls timing when spark knock is heard. The question then is whether the oem timing tables allow enough timing advance to benefit from 89 vs 87. I think they do but have no solid proof.

Of course this is all just speculation without proof or more insight into the actual oem ign timing tables. I do know that I've seen tangible gains from 89 octane with other engines after a few mods designed to add ign timing such as IAT relocation/mod, TB coolant bypass, colder thermostat, CAI or adjustable ign timing module.

To the OP:

A thicker head gasket sounds like a simple solution to safer boosting but now that you've lowered the CR you're adding it back with boost and now have a potential weak spot with that thicker head gasket. With all the people pounding the snot out of their 2AZ's pushing 300+ whp on stock blocks with mostly favorable results, my personal boost plan is to stay at or below 250 whp and tune for reliability rather than max hp.

ScionFred

iTrader: (1)

Great explanation!

FromTheOld

I always use 87 octane, which is Regular around here

johnhawkins

iTrader: (1)

*The headgasket is perfectly fine people pushing 500whp on the Stock MLS gasket. The head studs are what fail first (14-16psi of pressure)

millardmt

Thanks to all of you, especially Drax and Fred. I have a pretty thorough (if lay) understanding of the physics and thermodynamics involved in the internal combustion process, but I still find the myriad implications of F/I in actual application a little difficult to entirely comprehend.

Anyway - and remember I said in my OP "all other things being equal" - I understand that a lower CR "per se" implies a less forceful power stroke (i.e., torque) and that, with F/I, one is compensating for that loss by more fully filling the combustion chamber with a fresh intake charge. So, you're right, Drax, my car could become a slug when not under boost if I'm not careful. (Also, I must assume that Toyota spent a great deal of money on the shape of the 2AZ's combustion chamber; reducing the CR could theoretically alter the CC's "swirl" pattern and play havoc with both power and efficiency.)

To return to my original worry, you and many other SL posters continue to preach that a turbo (probably even more so than a S/C) is going to deliver a HOT mixture to the engine - that is, vis a vis a N/A "all motor" build - and it is with that extra heat that I am particularly concerned. I have to say that I'm still not convinced that an intercooler - no matter how big (and provided it's not running inside a chest freezer) - can effect the same immunity from detonation that a .5 point decrement in the compression ration could achieve. But I'll have to read more on this ...

All I really want is about 200-220whp and I'm willing to pay for it IF the result is in every respect as reliable (or nearly so) as stock. It was in this context that I decided to invite comments about the seeming scarcity of people willing to pull down their compression ratio (and believe me, I know how tedious pulling, an especially reassembling, a DOHC head can be!).

Based on your comments, I've decided to "hurry up and wait," and continue lurking and soaking up your collective wisdom until I feel I've learned enough about the idiosyncrasies of our cars (and also of the aftermarket products available to us) to proceed.

Marc

(PS: If I pull my head, I'm going to inquire about sleeving around the studs. M)

(PPS: I must admit I'm in full agreement with a recent poster - Trevor? - who doesn't care for the appearance of a front-mounted intercooler! M)

ScionFred

iTrader: (1)

It sounds like you know more about this than you're giving yourself credit for. Of course in addition to the IC you'll also be running colder plugs, a colder thermostat and a richer AF mixture for some additional cooling when in boost. Have you considered water/meth injection to really beat the heat?

millardmt

THAT, Mr. Fred, is exactly the option I'm considering.

Marc

ScionFred

iTrader: (1)

Good option. I bought a WMI kit but never installed it because of the hassle (installation and maintenance). It's for sale but I may end up installing it if no one buys it.

BTW, in my previous post I forgot to mention the extra detonation control provided by switching from 87 to 93 octane but you already know that.