Low compression-high boost, or high compression-low boost?

[Splitdog]

Which is better?

I am running 7.1:1 @15 lbs boost, and one calculator puts me at about 14.8 effective compression. If I raise the compression to 8.3:1, I could lower the boost to 10lbs for the same effective compression ratio. Which scenario would be better? Would top hp rating remain the same? Then, I stumbled across this:

What's better, low compression and more boost or high compression and less boost?"

There are certainly reasons to try to raise compression ratio, namely when off-boost performance matters, like on a street car, or when using a very small displacement motor. but when talking purely about on-boost power potential, compression just doesn't make any sense.

People have tested the power effects of raising compression for decades, and the most optimistic results are about 3% more power with an additional point of compression (going from 9:1 to 10:1, for example). All combinations will be limited by detonation at some boost and timing threshold, regardless of the fuel used. The decrease in compression allows you to run more boost, which introduces more oxygen into the cylinder. Raising the boost from 14psi to 15psi (just a 1psi increase) adds an additional 3.4% of oxygen. So right there, you are already past the break even mark of losing a point of compression. And obviously, lowering the compression a full point allows you to run much more than 1 additional psi of boost. In other words, you always pick up more power by adding boost and lowering compression, because power potential is based primarily on your ability to burn fuel, and that is directly proportional to the amount of oxygen that you have in the cylinder. Raising compression doesn't change the amount of oxygen/fuel in the cylinder, it just squeezes it a bit more.

So the big question becomes, how much boost do we gain for X amount of compression? The best method we have found is to calculate the effective compression ratio (ECR) with boost. The problem is that most people use an incorrect formula that says that 14.7psi of boost on a 8.5:1 motor is a 17:1 ECR. So how in the world do people get away with this combination on pump gas? You can't even idle down the street on pump gas on a true 17:1 compression motor. Here's the real formula to use:

sqrt((boost+14.7)/14.7) * CR = ECR

sqrt = square root
boost = psi of boost
CR = static compression ratio of the motor
ECR = effective compression ratio

So our above example gives an ECR of 12.0:1. This makes perfect sense, because 12:1 is considered to be the max safe limit with aluminum heads on pump gas, and 15psi is about as much boost as you can safely run before you at least start losing a significant amount of timing to knock. Of course every motor is different, and no formula is going to be perfect for all combinations, but this one is vastly better than the standard formula (which leaves out the square root).

So now we can target a certain ECR, say 12.0:1. We see that at 8.5:1 CR we can run 14.7psi of boost. But at 7.5:1 we can run 23psi of boost (and still maintain the 12.0:1 ECR). We only gave up 1 point of compression (3% max power) and yet we gained 28% more oxygen (28% more power potential). Suddenly it's quite obvious why top fuel is running 5:1 compression, that's where all the power is!!

8.5:1 turns out to be a real good all around number for on and off boost performance. Many "performance" NA motors are only 9.0:1 so we're not far off of that, and yet we're low enough to run 30+ psi without problems (provided that a proper fuel is used).



So basically, more boost is increasing DISPLACEMENT, whereas increasing CR is not. More fuel/air= more power.

[rs58rag]

I'm at 8:1 comp. ratio on 1914cc engine and tried a Garrett t3 .42/.48 @ 10 lbs boost...nice scoot..... now using rajay e comp. side with b40 exhaust side @14 lbs boost. The t3 boost started sooner than the rajay...... rajay boost strikes around 3700 rpm and weeeee. So basically, off boost with good power till you step on it and powerband comes on.

[Dan Dryden]

Interesting....

I run 21psi with 7.5:1 CR and have often wondered if I should raise the compression and lower the boost to get snappier off boost performance. (Which I feel it sometimes needs)

Using your equation, this would give me an ECR of 11.7:1

If I raised compression to 8.5:1 and keep the same ECR, I would need to run boost at around 13psi, which would stand to lose me 27% of the oxygen I currently get in my cylinders.

It's certainly food for thought!

I always question how turbo size and cam profile would effect ECR though...

[Clonebug]

Here is the calculator I use and I am at 19.47:1 ECR.

7.8:1 static and 22 lbs boost at 110 ft ASL.

http://www.rbracing-rsr.com/compression.htm

[Splitdog]

Yes. I am running a FK41 in a A.J. Sims turbo T3/T4 78X90.5 motor. Hardly any overlap, so I'm building cylinder pressure early.

[stevepugh]

Interesting thread. So if I have a 2276 FK10 44x37 heads and a CR of 10.3:1, what turbo and boost would I run or would it be better to start again with a lower CR and a different cam?

[Splitdog]

Depends on how you want it to drive. If you're going to be driving it around for pleasure, you will want a smaller cam. Even a 120 cam in a turbo 2007 cc was completely unresponsive below 3000 rpm. And that's right where you drive. I'd go with less compression-more boost theory.

[Howie]

I run 7:1 CR, and have only run 12lbs of boost to date. A simple change of the springs in the waste gate will double the boost. I plan to do that this Spring.

But ever since I built this engine, I've never run anything but race gas. Mainly because I have "too much invested in this engine to take any chance of it "knocking" running pump gas. And second, I can get race gas two blocks from my home.

"Better safe than sorry".

[Splitdog]

water/meth injection is a Godsend for that.

[Clonebug]

I run 7:1 CR, and have only run 12lbs of boost to date. A simple change of the springs in the waste gate will double the boost. I plan to do that this Spring.

But ever since I built this engine, I've never run anything but race gas. Mainly because I have "too much invested in this engine to take any chance of it "knocking" running pump gas. And second, I can get race gas two blocks from my home.

"Better safe than sorry".

That's not feasible for my summer time daily driver. I have taken my buggy on 900 mile drives in four days across the mountains with my Wife along and can't have a tune like that.
All my boost settings are done with an EBC now. If I need to cut down the boost I can flip one switch to turn off the EBC limiting me to the stock WG of 6 lbs and also flip a second switch to turn off the WI.
If I happen to have the Asus Notepad I can set it anywhere.........
The idea of having EFI is being able to tune it to a finer edge to take advantage of the new technology......

Having to run race gas is not acceptable for me.

22 lbs on pump so far and hope to go higher!!!!!

I need to put in a 4.0 bar map daddy, an intercooler.......oh and bigger injectors.......roll bar.......... It never ends.....

[Howie]

I don't HAVE to run race gas at the boost level I drive it. But for me, it's just a toy. I don't even drive it very often, or very far. If it were my daily driver, I definitely would have built it differently.

One thing I like is the injection is adjustable from the driver seat. You adjust and drive, and repeat until it gives you the best performance.

I've been hot rodding VWs for 45 years, but this was my first with a turbo, or FI, so I wanted simple, and reliable, and not looking to squeeze the last bit of HP out of it.

I bought my first VW at 17, and have owned one ever since. Even drag raced a Ghia a few years. I love these cars, and the folks that keep the love of VWs alive.

Enjoy yours!

[Clonebug]

Ahhhh Sooooo..........

I should have figured that seeing that race gas would break a man's pocket book if a daily driver.

Crank the boost.........it is a blast.......

[Howie]

It scares the hell out of me now, but I love boost. It really is an addiction! I'm becoming a boost junkie!

[Splitdog]

Only those who know............know!

[Buggsy]

In 2001, when I first went turbo, I used semi-hemi cut combustion chambers and 7 to 1 compression. The problem with running compression that low is that it is a very slow and dirty burn. It will be OK at the track, but if you drive around town, your spark plugs will be black in a month. Then it will miss. If you don't mind changing your plugs every month, then go with it. It would be one of those rare occasions when you would have to run a timing curve that would be very similar to a locked distributor to make it run decent, or at all. 22 to 25 initial advance.