modaddict

I am surprised quite often on the forums when I hear about guys getting pump gas tunes with big boost. I have always been a big advocate on not skimping on octane and the relation boost has to compression.

You have to understand that there is no such thing as 'it's safe because it has low timing'. Timing or not, boost raises your compression rate and even with little timing, you cannot expect your motor last on 14+ psi levels on pump gas.

For example, take a look at the chart I attached and you will see that on a 9:1 compression motor, introducing just 10 psi of boost, you are now running at about 15.1:1 compression which is no way safe for pump gas. That is just 10 psi! Even on an 8.5:1 motor - introduce 14 psi and guess what - you are running 16.6:1!..[] Yes, this is on carberated pushrod motors and ours our different.

Granted, all motors and systems (like ours) are different but the numbers should be close regardless and this is a good guideline to show you the importance octane plays with your boost levels. Again - ours are different, but boost raises compression is a fact.

Just remember that when you hear you got a 16 psi 'pump gas' tune - you are running around with a hell of a lot more compression than you think...

[IMG]local://upfiles/85500/8773A405DF3E490584F954864D3A6630.jpg[/IMG]

Brian_Zinchuk

so whats the safe road to take then if the car is your DD, you want 500+ rwhp, and really dont want to put racing fuel in every time you fill up? I mean I use the highest octane possible, but I guess your not saying thats good enough. Will fuel additives help?

Riptide

The compression ratio of a stock GT is 9.8:1

Long story short. If I'm using 91 octane pump gasoline then I will blow up my engine if I throw a supercharger on there with 8 pounds of boost?

Brian_Zinchuk

I'm pretty sure thats not what he is saying. He saying it puts a lot more stress on the engine but 8 lbs isn't going to blow your engine up. I dont think the supercharger manufactures would be doing to great if there were engines blowing up with S/Cers only putting out 8 lbs of boost. Hes talking about higher numbers like 12+. But I don't know about the chart. According to that chart, if you have a S/cer pushing anything more the 4 lbs of boost you need race gas. How many people on here do you think would agree with that, its just numbers.

modaddict

I'm not saying you are going to have detonation. I am illustrating you are running a lot higher compression than you think when you add boost in the mix.

Even without detonation (which your tuner can determine on the dyno) - running around at a very high compression rate will not help the longevity of your motor at the very least.

Personally, I think you can get 500 safely on pump gas but I would not run pump only for 12+ psi IMO.

The chart is not meant to scare anyone - simply inform them and and help you decide how much octane you need for your boost levels. In my case at 22 psi, I can't use anything under C16 for my motor safely - but that's my particular case.

Tuning and timing due play a roll but you also can't argue the physics. The more you want to turn up your car, the more you need to invest in your gas...

Riptide

I guess this is over my head then. The attachment in his post states clearly that 15:1 compression is bad.

If the motor is 9.8:1 stock and you are using pump gasoline with 8 pounds of boost you are right around that level of compression. Hence why it seems - bad?

modaddict

These numbers are not exact cause they are not based on our drive by wire cars. However, they are close regardless. The computer really comes into play pulling timing and other stuff that keeps the boost in check. Also - you could be driving around at 15:1 but remember that's only at full boost and for how long? It is very possible - (many including myself have done) to drive around on pump gas with as much as 18:1 compression at peak. But, just because it can be done, doesn't mean it's good for your car.

As I said, taking into account our specific cars, I don't see any reason why you can't run around at 10-12 psi on pump gas with a good tune. You will be running higher compression than you think but I think through our drive by wire - it can be done and not effect the motor adversely.

However, for guys pushing 14+ boost levels, they are crazy if they think they are not hurting their motor (detonation or not) running around on pump gas or even 100 octane in some cases.

chadlet

Then I guess i will be getting methanol injection when I get my whipple installed. Eben just at 12 psi it will help not only by raising octane levels but also reducing temp.

modaddict

Again guys remember that this is based on carbureted pushrod motors. Our motors with drive by wire works differently.

The attachment is not meant to 'scare' people. It's simply meant to give you something to think about when adding boost into the mix. Our actual numbers are much lower, but boost still raises your ratio - that's the message.

I wish someone had a definitive chart for our cars - it would be nice.

hammeron

an important byproduct of the chart,
is to show why, when you build an
engine for boost, you should choose
your pistons (dished for low CR) very
carefully, based on the boost levels and
octane you plan to run