SS RRR

iTrader: (2)

He's a fuckign Mo. I have zero decked block on mine with less than half that relief in my pistons and am sitting at 11:1. What is most likely happening is he's using some internet calculator where that requires using a "-" sign before the value, or ones that just assume entering a relief value will mean it is a negative number and he's not doing it and that value is most likely meaning the piston is domed.

PS- Engineers have zero common sense. FACT!

KW Baraka

iTrader: (7)

Build double checked.....you're good.

No need to continue with that guy, or even with us for that matter!

Get a good tuner.....and then have fun!

KW

ChopperDoc

Oh he likes math? So do I. What a hack. lol.


ECR boost compression ratio formula:


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

Example - Sqrt((14PSI+14.7) / 14.7 * 8:1 = 11.1:1 ECR <<< leaves room for more boost lol.

For this, 12:1 is the limit for pump gas. Let me plug your numbers in here...

sqrt((8PSI + 14.7) / 14.7 * 9.25

8 + 14.7 = 22.7 / 14.7 = 1.544...

Sqrt 1.544 = 1.24266... * 9.25 = >>>>>> 11.49 <<<<

Yup... Cleared hot for at least 8 PSI.

Obviously this will vary a bit depending on cam IVC etc, but it's a good start point for determining boost level. Also take into account the 28 rule for timing. Also pretty generalized but a great little trick for determining spark at any given PSI level. Spark should be half the number, the other half is the boost. So at 8 PSI, you could run 20* or so. This again is different engine to engine, but a decent start point that will keep it from detonating too bad. If you had 15 PSI, then timing would be 13*, etc. You get the idea I hope.

PS I do math as a hobby, mostly on here. Seems like every day I am posting some formula somewhere. I actually posted this in another thread weeks ago and had to find it lol.

Polyalphaolefin

I like that equation, but it assumes a lot. From my experience, boost is a completely different animal than naturally aspirated and you can't cross reference the two to a given effective compression ratio. There's far too many variables. A good example is with a 2.3L Duratec 4 banger I built and tuned a while back. On naturally aspirated builds, around 12.2:1 compression is the absolute max compression for those engines on 93 octane. No matter the coolant temp, plug range, or spark timing, it's at it's limit right there. However, that same engine was rebuilt with 10.1:1 compression, smaller cams that gave within 0.2 dynamic compression of the 12.2:1 NA build, and added a GTX2871R turbo and Garrett intercooler. Same intake manifold, same head, same .038" quench distance, 1 step colder recessed tip plugs, tubular turbo manifold, and 15 psi. Using that equation, I get an effective compression ratio around 14.35:1, but that engine took 18* of timing at 6500 rpm and 15 psi on 93 pump gas without missing a lick. This engine was actually an oil experiment for film stress NA vs boosted, but the air/fuel ratio and spark timing limit was fun to watch as well.

Why does this occur? Because temperature is a far greater concern to pre-ignition than pressure. Yes, pressure builds heat, but it's a matter of where you start. With good cooling of the intake charge, you begin to skew the ECR results. Another factor is the fuel in the cylinder. Gasoline absorbs heat as it vaporizes in the cylinder while the piston is compressing the mixture. While the pressure is higher, the volume is not, yet you're putting more fuel in the cylinder to match the higher concentration of oxygen for the same air/fuel ratio. That higher amount of fuel in the same volume, regardless of pressure, will draw more heat out of the surrounding air in the cylinder. This keeps cylinder temperatures lower at the time of ignition for a given ECR.

This is also how E85 makes as much or more power than C16 despite having 10+ points lower octane. Ethanol vaporizes at a lower temperature than gasoline and therefore is able to absorb more heat in that process. Methanol pushes this effect to the extreme.

ChopperDoc

The equation does indeed assume a lot. It does not include heat, cam timing, spark, and so forth. I get that. It is merely a starting point to plan a build. Literally everything is is controlled basically by the tune, even the cam timing in newer cars. That is strictly a pressure formula, and 12:1 is the "assumed" and/or recommended limit. Just the same as the assumed limit for SBE gen IV engines is 1000 bhp, which is where the pistons snap, normally, without tuning intervention.

Here's how it works: Because pressure under boost literally pressurizes the entire cylinder, valve closed or not, we use the entire stroke (SCR) for the formula. We want to know how much that total volume will compress, and this is measured at 2 points, essentially. TDC and BDC, or overall volume. So, that's why we develop an atmospheric multiplier to apply to the volume of the cylinder by adding boost to 1 atmosphere, dividing the result by 1 atmosphere, then dividing that on itself to determine the boost multiplier, which accounts for the volume in the cylinder between TDC and BDC, because that sucker is gonna be full lol. Multiply that by SCR and you have a decent point of reference for deciding parts, and build plan, or how much boost to push initially before trying for more. In reality, some of the volume will be pushed out as the piston is coming up, but this cannot and should not be assumed. So using actual ECR (IVC to TDC) would be possibly unsafe, depending on engine, cam, so on, because you can't really say how much will be pushed back into the intake through reversion.

Tuning it past 12:1 is absolutely possible. Just like tuning a SBE Gen IV to 1400 HP is also possible, and has been done. There are always exceptions to the rule. You are correct in everything you said here. Yes, fuel is used to cool, not for power. That is one of the reasons boosted engines target a 11.5:1 AFR and not 12.5:1 like N/A, mostly to account for the extra heat from the added pressure pressure.

Pressure and heat are funny things aren't they? They are linked... More heat = more pressure. Also, more pressure = more heat. It's a vicious cycle lol.

Also, for N/A engines, the IVC event is what determines the real "max" limit for pump. It's said to be at 8.61:1 for 93. Anything above that and it will detonate. Retarding or increasing the cam duration will change this considerably, but at a loss of low end torque. I'm at 12:1 on a 427 stroker, but my ECR is only 8.3:1. My spark is at 16*-18* WOT. I could deck it even more with those numbers lol. Still got some room there, especially if I run a bigger cam. Moral of that story is every engine is different at the end of the day.

I've also seen mostly stock hondas and neons running 20-30 PSI on pump too, and not exploding... unfortunately.