ChevyNo1

I'm going to dyno tune the car this weekend now that I have my boost loss issue figured out.

What I'm trying to figure out is how you guys decide how much you can run on pump gas. Do you make an educated guess? Or do you keep bumping it up until you hear knock, then back it off a bit? Reason I say 'hear' is because my knock sensors or PCM has decided that they won't work. I've replaced the harness and both sensors, no dice.

When I cracked a ringland I was running 12psi, 91 octane, stock motor, + alky injection ( a fair bit too! ). The alky pump seized and boom!

Now I've got a 370ci iron block, fully forged internals, 8.7:1 CR. I'm wondering how much boost to run.

I think my tuner is scared to push it on 91, I just want to get it as high as possible then back it off a bit from there. He's thinking only 10psi, but dammit I built this engine to handle it didn't I??

Let's hear how you guys figure out the max to run, regardless of type of gas.

- Dug

jsniper

Even 10psi is pushing it. I think 8psi at 91 octane would be ok. Here in Florida we have 93 octane and I would't go past 9 psi.

Camaro477

I saw a guy with a 427 and he had 13psi.

ChevyNo1

But do you have 8.7:1 CR, or stock?

F8L Z71

Is there even a hard formula? CR is only part of it. Cam and head design is going to change things alot. You may have ran 13psi on a stock motor but then you build something with a bigger bore/stroke, add a specific cam and set up the valvetrain to be more efficient you may only get to use 9psi but you'll make a TON more power and still have reached the limits of your fuel system or whatever.

I know you know this but sometimes we forget that just because we ran XX amount of boost on 1 motor doesn't mean we can run the same amount on the new more efficient motor right?

Got Me SOM

iTrader: (2)

I'd keep it at 10-12 psi, since you have more displacement and the cr is lower the turbo's will have to push more to make the same power you have. I'd run about 12 psi and see how the engine likes it. That should be ok. The determining factor is the fuel you have , the engine can handle more but you'll need a higher octane to slow down the burn.

Mark C

This is how I would do it.

Assume that a stock tune and CR will run on 91. You know the stock mass air flow. You can figure out cylinder pressure. Thats probably close to the most pressure you can have with stock timing. Change the values for your 8.7 CR and find out what the mass flow would be for your new set-up assuming the same pressure. Find out the amount of boost you need for that mass flow and you got it. Not exactly simple.

In the end, there's a lot of dynamic things going on, and you'll have to find it experimentally, but I bet this would get you close. If you have the smaller turbos, you might not get there.

ChevyNo1

Yea, I know that . . . I didn't mean it in terms of numbers, I guess what I'm really getting at is on the dyno, HOW do you tune? What do you see on the dyno to say, woh, I better pull back the boost a bit, or the timing. Do you wait to see detonation? How do you know you're within the safe limits by 100 or 50 HP? You could be missing out on an extra 50 and still be well within safe limits. How do you determine when enough is enough simply by observing what happens on the dyno?

- Dug

F8L Z71

I gotcha,

I think most start off with small amounts of boost and a medium level of timing and start ramping up till they notice any detonation. Lots of ways to go about it really but since I'm not a tuner I wont advise. I don't wanna give bad advice.

LSs1Power

iTrader: (5)

With that C/R i would run 13-15psi on 91 octane. Keep timing low and start increasing until u get see slight knock and back down 2-3 degrees to keep things safe. Good luck.

BIGBOS

I would run around 12psi, no more than that.......

Especially on 91....if your compression was 8:1, then it might be a different story....

MECHAM

iTrader: (4)

i have about the same compression ratio

364 cu iron block

i run 12 psi on 93 octane. i can run the same boost no problem with 91 octane....BUT then again...im wayyy up here in the sky....so we can run a little more boost with less octane

ChevyNo1

Thanks a lot guys. I was thinking that 12 would be OK on 91. I mean, I read all these Hot Rod Articles where they hop something up. How do they tune? They usually play around with timing and bump it up lets say 5 degrees all over, and see how it responds ( power-wise ). As long as they don't get detonation and they keep getting more power, they keep going. That's what I'm looking for.

- Dug

NicD

iTrader: (7)

Dug don't worry about it so much, WOT is the easy stuff. Boost and timing is all we have to play with assuming a constant A/F. It won't be hard or take long to figure out what it will take. Watching the scanner and looking at the dyno graphs tell a lot. I am guessing 10-12 psi will be the most on 91 oct with a decent timing map. Our 91 oct gas is ****, and it's quality varies quite a bit from tank to tank. I am not going to let you blow it up, yet. Bring some 110 with ya so we can really turn it up.

nitrorocket

iTrader: (12)

On an engine dyno, A 427 has been run to 18 psi with 22 degrees of timing on 93 octane. So 12-14 work probably be close to your limit with 91 with a GOOD tune. Good luck!

ChevyNo1

Thanks a lot guys for the advice . . . and Nic, I won't have 110, but I'm getting a few gallons of 100 today in prep for tomorrow

I know, you don't want to blow it up . . neither do I! And I can understand your hesitation. I just see people throwing out #'s, when REALLY that's just a guideline, you have to see what it does on the dyno.

- Dug

jsniper

My setup is a 382 stroker LS6 block with stock LS1 heads, stock cam, stock intake, stock exhaust. I have a complete aftermarket fuel system with 60lb injectors and 8.85:1 pistons. I do not have an intercooler either. I have the STS kit and was told by several performance shops as well as STS themselves not to push more that 9 psi without using the methanol. Maybe they assume I will keep the timing stock. You can always raise your boost higher than what's recommended but you will most likely have to retard your timing as you do so once you find that spot where it detonates. If you are able to log your ECM results with your dyno runs I would start off with lowering the timing a few degrees and then start bumping up your boost from 8 psi on up until you see detonation in your ECM's logging software or whatever you are using to view the ECM's data.

If you can make, let's say 600rwhp @ 10 psi and 26 degrees timing and you also find that you can make 600wrhp @ 12psi and 23 degrees of timing then the obvious choice would be to keep it at 10 psi with 26 degrees of timing so that you don't have to up the boost. Or try to find the median of 11 psi with 24 degrees of timing. I'm sure you get what I'm saying. Any time you can find power without upping the boost is always a plus.

NicD

iTrader: (7)

I have a 5 gallon can also that you can fill up at saads gas station. It is on chandler right near the freeway near dyno chip. Don't fill it up with 100 before we get there. We want to nail the 91 oct tune on the dyno, then put in the race gas and go from there. Torco works well too and is cheaper than 100 unl. Every motor is different and likes different things, we just need to throw it back on the dyno now that it is fixed and turn the wick up.

ChevyNo1

Yea, I figure about 1/4 tank should be good to finish up the pump gas tune, we'll run 'er dry and dump in the 100.

I have used Torco, the stuff did work good but sure discolored the plugs!

- Dug