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Discussion Starter · #1 ·
Say, your at 8.5:1, and you want to bump your C/R to 9.5:1, will you have to tune for more fuel?
 

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As long as your efi system is capable of delivering the extra fuel needed for the power bump that change will generate, it will deliver the proper fuel. The mass air will measure the extra air going into the engine and adjust fuel accordingly. You may have to look at your timing - especially with boost. With more compression, you won't be able to run as much timing without risk of detonation.
 

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Please tell me you're not asking for yourself... rule #1 with a blower or turbo: lower compression with higher boost is better.

but, yes, if you take a 8.5:1 motor built up and boosted or not swap it with a 9.5:1 motor with the same heads/intake/cam/power adder(s)/etc you will need more fuel because you will be making more power. The computer will compensate and add fuel during normal driving, but once you go to WOT it's all about the computer mapped fuel curve.
 

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Discussion Starter · #4 ·
slo5oh said:
Please tell me you're not asking for yourself... rule #1 with a blower or turbo: lower compression with higher boost is better.

but, yes, if you take a 8.5:1 motor built up and boosted or not swap it with a 9.5:1 motor with the same heads/intake/cam/power adder(s)/etc you will need more fuel because you will be making more power. The computer will compensate and add fuel during normal driving, but once you go to WOT it's all about the computer mapped fuel curve.
I am looking at the volume being compressed. Its still the same, just compacted in a tigher area. I guess I am looking at it wrong. If you turn up the boost, you are actually increasing the volume of A/F in the cylinder, so that is why its plan to see the need for more fuel.

As for the 1st rule, it all depends on what you are trying to achieve for the engine. I am wanting a better part trottle response for normal driving, a better power curve if you will.

Thanks for the input guys.
 

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i dont know that more fuel is needed , but better octane is needed .
 

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More fuel isn't needed if you just raise the CR with no other changes. The CR does not affect the AF ratio; it just compacts the mixture more which results in more power. It's like a spring, the more you compress the spring, the more violently it decompresses when you release it.
 

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Discussion Starter · #7 ·
So, then my thinking was correct? I do see a higher octane will be needed.
 

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Guys I think it's more complex than the simple one-liners that are being offered. Raising the compression ratio a point may or may not affect whether more octane will be required to keep detonation away. If at the lower c/r, you're running right on the ragged edge of detonation with your current octane and timing choices, then it's probable that increasing the c/r is gonna cause detonation problems. They can usually be solved by either reducing ignition advance, or by increasing octane. If you're not running right on the ragged edge currently, then you may find that you can raise the ratio without changing anything. I don't think the answer is predictable without knowing a lot more about your current state of tune.

Changing the static compression ratio should impact power/torque production in a positive way. It may slightly shift the rpm at which those numbers are produced. It may also allow for slightly higher revs. If more air is moving through the engine at the same a/f ratio, then more fuel will be needed. But we're not talking a major change in power output with a one point raise.
 

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I think Michael is right. A one point increase is not that great, just experiment with the timing and go up in fuel octane to prevent detenation.

Any higher, I would go to a larger injector and a larger fuel regulator. When an engine is under boost, the injector is fooled into thinking it is larger, thus guys running 5-8psi can get away with 19lb injectors. In fact, they think they are 24's because of the increase in manifold pressure caused by s/c.

Stock injectors on an n/a motor are just that, 19lbs. That's because injectors are factored on atmospheric pressure, not manifold pressure. If I were going 10:1 c/r, I'd go to more fuel just for the peace of mind.
 

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Michael Yount, I agree... this is more then a one line question and answer.

A blower or turbo changes your static compression. If you put 10lbs of boost on a 9:1 motor that 9:1 motor will act like it's a 10:1 or even 11:1 motor (I don't know the math on it, maybe some1 else can help out with that)

And yes you will need more fuel, not necessary just because it's making more power, but there's a point where extra fuel is needed to cool the combustion chamber and stop detonation.

Full Tang, I see what you are saying and it sounds like it makes sense, but I don't agree.

Now, to the original question.... you want to make more power down low, right?
Something is wrong if you are not making over 400lbs of tq to the rear wheels. In your tag you put GT-40 alumin TS ported... what is a TS Throttle Spacer? If you have a GT-40 intake ported and stock heads... there's your bottleneck... but if you have gt-40 heads ported with a stock intake, it's time to buy an intake.
The thing a lot of people don't think about is that everything needs to be smooth, TB should match intake, intake should match heads, if you go from a big opening into a small one everything backs up, and if you go from a small into a big you lose velocity (and this loses TQ)... but with a blower or turbo the effect is made less drastic. Its not about "how much air can my engine suck?"... its about "how much air can I force feed my engine?".

Back in the day I had a stock 5.0 with an A-Trim... One day playing with the timing I put up about 18 to 19 BTD... and can I tell you... HOLY $HIZNIT... it felt like I was making 500lbs of TQ, until it hit about 3500-4000rpm and it sounded like a grenade under the hood. With that Tweeker you can map your own timing curve.... perhaps it's time to add a degree or 10 down low. I don't see that you are running any sort of vacuum BTM so I'm guessing that you are using the Tweeker to pull out time up high?
 

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Discussion Starter · #11 ·
Very infomative post slo5oh. I see your point on the bottle neck. I have a VORTECH intake. I know the GT-40s are not the best, but when the engine was built back in '96, it was one of the better ones on the market.

Its been a while since I was deep into the Domestic side, what is a BTM?
 

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Everyone has touched on most of the points.
There needs to be some definition for a more correct answer.
Basicly with no other change than compression pressure, the engine becomes more efficient on the same amount of fuel.
The side effects are it makes the fuel ignition speed faster (lowers effective octane) of the same fuel.
Depending on what conditions prevailed before the change, higher octane fuel may be needed and /or the ignition timing may need to be retarded due to the faster burn time if the fuel octane has not been increased.
The reasons for all this gets into combustion chamber design, iron or alum head material and other things so the final answer is to try it and do what ever is needed to get the motor to run correctly.
Super charging causes the same things to happen by greatly increasing the total effective cylinder pressure at ignition time, making high octane fuel and timing much more critical.
This is also where water/alky injection slows down combustion, helps cool the charge thus reversing some of the undesirables that occurr.
 

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Discussion Starter · #13 ·
So, let me get this straight.

With an increase in C/R there will be a need for a little more fuel to detonation at bay, the the ECU should cope with this. I know the power increase will not be dramatic, but I want it to be more streetable. The is now N/A, the S-Trim is leaking oil, so I have taken it off for the time being, and I am not sure if I will be putting it back on, that is why I was think of the increase in C/R.

The question arose about the fuel charge just popped into my head, because I was weighing both sides, but leanning towards no increase in fuel.
 

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Boost Time Master (the last word could be meter...)
MSD makes a MSD6-BTM... it has a vacuum plug in the side of it and comes with a dial knob for inside your car.... the dial allows you to pull out 0, 1, 2, or 3 degrees of time for each 1lb of boost. This allows you to run good street-able timing down low with less chance of detonation up high.
It's an expensive MSD box, and you can always buy the BTM part seperate... I can't recall what it's called, you'll see it in Summit.. all the spark box companys sell one.
 

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Discussion Starter · #15 ·
I do have a Boost Retard installed, from MSD
 

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ohh....
I get it... Damn!! you should have said your blower was taking a dump.

My question to you is: How are you at 8.5:1? Is it with dished pistions or did someone open up the combustion chamber on the heads (although I don't think you can open them that far). If you have a good built lower end and you're thinking it would be cheaper to rebuild it with stock pistons, think again. Unless you plan on doing a junkyard rebuild (don't get me wrong, I've seen it done many times with great results) you're looking at least 5 to 6 hun for a new lower end... where rebuilding your S-Trim would probably be the same price. Call Vortech, they will rebuild it. I hear they will even let you grow it (if possible) such as from an A-trim to an S-Trim or maybe from your S to a T-Trim. You want low end TQ... strap on 20lbs of boost :)
 

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Discussion Starter · #17 ·
I just threw out 8.5 to 9.5. I should have used 9.5 to 10.5. Sorry, my bad.

The engine is a crate motor with the hypers from Ford from back in the 90s.

I was just gong to have the heads skimmed for the comp increase, is this a bad idea?

T-Trim? Maybe go that route, Hmmm...Damn options:)
 

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I think if I were in your shoes, what I'd consider instead that's less invasive would be dumping the B cam for something else. I believe there are cams that will not only help your bottom end but may work better with the supercharger as well. And then once the charger's healed, you can put it back on and still take boost advantage of the lower compression ratio. Just a thought...

Oh, and you don't have to worry about fuel at all......except maybe do you have too much capacity w/those 42's at low engine speeds. With the boost coming off, even with an increase in compression ratio, you're not gonna be using anywhere near as much fuel as you were before.
 
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