Comp. Ratio w/ regard to FI

Since i have this 530 head sitting in my garage, i've cleaned it out and ported some stuff but seeing the pictures of the pro's job really put me to shame....

i'm thinking maybe i should mill the head down, raise te CR up to 9:1.......

now my question is, when running same amount of boost do i need to add more fuel or is the current level fine?

example

i'm running 15 psi and my FI is at its limit, that is, the injectors, if i raise the compression ratio to 9 from 8.7 at the same boost level, will i need to add fuel or is the current level enough?

thx

The higher the CR you run, the less boost you will be able to run.

Boost in a way raises running compression (sort of), I have an early 230ET motor that runs a CR of about 9.2, and really by the time I'm hitting 10-12psi, things are getting prity lean, where as on the FTs most people can run 14psi without problems.

So yes you can run a higher CR, but you won't be able to use as much boost.
Race cars run real low CR, and lots and lots of boost, but then they drive around 99% of the time in the boost, on a street car it's a much nicer option to run a higher CR ratio and less boost, you don't make quite as much power, but it's a lot nicer to drive on the road.

[quote:25252382f4]now my question is, when running same amount of boost do i need to add more fuel or is the current level fine?[/quote:25252382f4]

The direct answer is ...yes. There has been much talk of additional injector controllers and Megasquirt systems. This is the way to add extra fuel that the stock system can't provide. Extra fuel allows you to run more boost at current your c/r or the same level of boost at a higher c/r.

Okay, this may seem over simplified, but changing your compression ratio in no way affects your mixture. The only way to change that is by changing the amount of fuel and/or air sucked into the cylinder. Adjusting your compression ratio changes what happens to that same air/fuel mix AFTER the valves close.

So, by increasing your CR, you will not be running lean, but you will be super heating the charge even more. This will eventually lead to the fuel self igniting, which is bad, just like running lean for too long.

So, you CAN run the same boost level, but you must compensate by running higher octane gas, adjusting your spark timing, or yes, you can add some more fuel to keep the cylinder a bit cooler. Most likely a combination of all of the above. As Dale said, most easily controlled via a programmable system. Also, as discussed in other threads, squish, quench and other combustion chamber variables can greatly effect how much boost you can run at a given CR, on a given octane, etc etc....

Thx guys..........that really helped


so heres the million dollar question........would YOU raise the compression or leave it the same?

I would refer you to the squish vs swirl topic....

...improve the squish...and you don't do that by planing the head...you do that by changing the deck height...aka decking the block...

...read that topic; do some followup reading....you'll see why you can get more by improving the squish...

Thanks for chiming in Stealth. Are you (still?) planning on compiling that thread into an article? I know I'll be filing through it quite a bit in the near future as my engine comes together. Uglyduck and I started mucking around with a practice head last night, seeing where we could try and improve swirl and whatnot without removing too much quench and squish area. Next step is to dry assemble my block to see how much I can take off the deck...

"would YOU raise the compression or leave it the same?"

hehehehehe.... Stealth beat me to it, check that thread out and check out what we established as "the goal", that is a key point imo.

Cylinder pressure is torque, torque when mixed with revs is power. And power is fun.

A major factor in the production of cylinder pressure->torque->power->fun, is compression ratio. What the engine "sees" as the effective (dynamic compression ratio) depends on a few things though, not just the static "mechanical" compression ratio.

The key though is that if you cannot develop that cylinder pressure in a controlled, chemically and dynamically correct fashion, You'll just detonate like a bugger without rocket fuel as Mike A would say.

Would I increase the compression ratio? Absolutely. Dale's car must just go like stink. AFter driving the B21ft and my built engine side by side there was no comparison. Even with the port work and K cam the engine was much more tracable and have a much broder powerband than the B21ft, and I think it was largely the (even though only slightly) higher compression ratio.

I think 9.0:1 compression is a great compromise, but ignition control, cam overlap and timing, boost, ebr, etc will all effect things as well- so since you're on the subject of compression ratios it will probably be very useful to you to check out the quench squish thread.

i'm on it!!

making it an article? yeah...but after I do a couple of motors....plans are now to get 3 blocks on the stands in about 3 weeks and start....

....I am planning on leaving the head(s) basically stock...either 160 or 398...for comparison simplicity...will play with A cams and possibly V cams...initially...not modding the head just to keep the analysis simpler...make too many changes at once and it gets tough to figure out what did what...will be dialin in the bottom end first for max squish/CR....then the head...

re determining deck height, you can go the dry assembly route, or add up the numbers....get my email from the 'Duck and I'll send you some of the numbers for what you're working with...might assist your efforts...

"Would I increase the compression ratio? Absolutely. Dale's car must just go like stink. AFter driving the B21ft and my built engine side by side there was no comparison. Even with the port work and K cam the engine was much more tracable and have a much broder powerband than the B21ft, and I think it was largely the (even though only slightly) higher compression ratio."

Yeah, helps get rid of some of the dreaded turbo lag (car has more power before the turbo even starts making noticable boost), makes the transition from off boost to boost much smoother and in the long run, while it may not seem importent, you'll save money on fuel because when your city crusing it's easier to drive off boost.

Has anyone else here driven a Subaru Turbo?, nice cars, but they are guttless down low untill you get some serious boost and lots of revs, the joy of a CR of 8.

My car came with the A cam, it sure likes to rev, bogs down off the line though, not sure I'm that keen on it, I'm thinking of either changing the timing of it or just going to the VX cam which I keep reading has good bottom and mid range power.

So raise the Compression ratio

by how much should i mill the block down? to get the 9:1 ratio....or should i even go higher?

What about changing pistons or rods rather than decking the block?

Yeah, yeah, I know, it's probably cheaper to shave the deck since we ain't talking about V-8s here, and there's the issues with changing the rod to stroke length and valve clearance and whatnot...I just wanted to throw it out there out of perverse curiosity.

Agh! GTlover, DONT do that......

milling the deck is the simplest thing......

anybody know the cost?

Thanks for not totally flaming me, I figger if I can get everybody's two cents on the technical topics, the rebuild will pay for itself.

Yeah, I can see how it would be much easier to have the machine shop mill the deck while the machine work is being done, and use stock pistons.

redliner....think squish clearance, not CR....

...get the squish right...don't worry about CR...

why? because static CR is not what counts....correct squish and dynamic CR are what count...static CR is a secondary factor....[and if the squish is right, static CR becomes a basically irrelevant number...]

...get the squish clearances right...and what the static CR is is not so important....good squish makes higher CR [static and dynamic] possible without detonation...