I am considering boosting my car down the road. I can't decided what route to take. One question I had is right now I am at 11 to 1 compression. If I was to add boost and upgrade my fuel system to E85 would I still be able to run boost at this high of compression? Does anyone know of more info on how to determine when your compression ratio is to high for a boosted application? In general id like to keep the compression up so its not a dog out of boost. Also if this was setup to be flex fuel, could i still run this on 91/93 for worse case scenario and have a it tuned to where it will run low boost when I don't have E85 available?

Thanks for the help.

 

https://ls1tech.com/forums/forced-in...turbo-e85.html

 

thanks bud!

 

How much horse power do you think you would be losing at say 2500 for the short period of time you would be out of boost if you were to run 8.5 to 1 compression for instance? I am assuming an automatic and waiting for the converter or from a rolling start and having to down shift.

 

If I were to throw out a guess, I'd say maybe 20 hp or so.. going from 11 to 8.5 is a huge drop. Probably depends a lot on your cam too.

 

It is a big drop. All depends on what you want. I will give up 20 hp for a fraction of a second it takes to make at least 200 hp more. That has always been my philosophy. I love this quote from Forcefed86:

"I agree, keep compression as high as you can get away with on the fuel you plan to use.

On the other end of the spectrum, I don't see what people are whining about with low compression engines being dogs. If the turbo and hot side are sized correctly, with realistic power goals, it doesn't really matter. You'll be into boost quickly enough that the loss in power isn't noticed. Its roughly a 4% NA loss in power per full compression point. This is easily counter balanced with 1 additional pound of boost. Most DIY home build guys are killing themselves hot-side wise too. If your larger than 2" OD piping and not making over 900 hp... your piping i too large! If your planning on more power than that you're a race car IMO and shouldn't be looking at pump gas options anyway.

My dished piston 5.3 with LS9 gaskets and 317 heads calculated out to like 8.2:1 (though most claim 8.5:1) Either way It made 26lbs on the trans brake pretty dang easy, could basically pick my HP level with the boost. So who cares about NA performance if you're in boost before the wheels turn? Personally I'd say error on the side of caution and go lower when in doubt and building a pump gas motor. On a mild 6.0 your talking a loss of 15-20 hp dropping a full point from "stock". It's not a game changer. And for every point of compression you drop you can usually run an additional 4-5psi of boost. Pretty easy choice IMO."

 

OK so you won't always gain the ability to make 200 hp more but as long as everyone else is throwing out generalities as if they were absolutes I thought I would join the party. BTW, in your first post you asked if 11:1 is too high for boost. You cannot answer that question without knowing what boost level you are talking about.

 

I like when people use the "4% HP per point of compression".
I'm glad to know I'll still make 600 HP with no compression.

 

Taking things out of context again. It works both ways! There are points of diminishing returns with raising the SCR as well. But when dealing with the 8-12:1 ranges the typical gains/losses are around 4-5% per point. Its been tested a million times (on typical engines) and is proven. But yes, it is a range and there can be a ton of variables. (engine VE being a huge one) Just like jumping from 13:1 to 16:1 will result in a smaller gain per Compression point than going from 10:1 to 13:1. Boost doesn't have this problem. So if I can sacrifice 4-5% power for the potential to make 15% more power... It may be a better approach depending on the fuel and goals.

Assuming your power adder can handle it, you are trading response for greater power potential. Like anything, a balance between the two for your goals is best.

 

Im at almost 11 to 1 on my 6 liter with Forged Pistons on E85. The car is much more responsive out of boost than my other stock compression setups. Car made 830rwhp on 23lbs on a BW cast S480 with 16 degrees on E85 100% of the time.

 

But the numbers don't play out. At all. Everyone is just throwing numbers around without any actual math behind it. Nobody has a clue. Lots of "Rule of thumb" and never any "Here is why".
Sure, the percentage of power between 8:1 and 12:1 is typically found to be 5%. On your average turbo LS that makes 650whp, 5% is ~32 HP. Doesn't seem like much, but you also get the benefit of faster spool.

So 32 HP isn't worth the risk when you can just turn up the boost. Sure. The biggest problem is that people keep dropping their compression ratio out of fear and have absolutely no idea where the threshold is.
8:1 with E85 is like using a grenade to crack an egg. Overkill just for the sake of "being sure". Zero actual proof that you NEED compression ratio that low in any shape/form.
Literally giving up power out of fear of the unknown.

The heat of compression difference between 9:1 and 10:1 compression with 100*F (311 Kelvin) intake temps using the formula TempIn(K)*(PressureRatio^[0.4/1.4])=TempOut(K):
9:1 Compression - 100*F In/588*F Out
10:1 Compression - 100*F In/620*F Out

32* F temperature difference with a full point of compression with chamber temps increasing from 588 to 620 using 100*F inlet temps.

By Comparison using a non-intercooled setup from your very own words:
We'll go down a full point to 8:1.
8:1 Compression - 230*F In/787*F Out

An 8:1 non-intercooled setup has significantly more heat in the cylinder at TDC than a 10:1 compression engine with a good intercooler...
and is giving up 13% power from the 130 degree temperature difference (using the SAE J1349 formula) and apparently 10% power from the 2 points of compression.
23% loss with zero gain in knock reduction.

Just because you always run super low compression and it doesn't detonate doesn't instantly mean that you can't run more and still see zero detonation.

 

I don't think that was the argument? The topics here are generally should I run 9:1 or 10:1+. I haven't really heard of anyone running 8:1 ranges? Nor am I claiming that's ideal?

I ran ultra low compression when I first started in 2012 because everything was still untested back then. Everyone claimed you needed 317 heads for boost on your dished piston 5.3. It's just an extreme example I like to throw out because everyone claims "low compression" engines are such dogs. And they generally aren't talking anywhere near 8:1 ratios I ran. Yet the car still got up on the brake and made impressive power. It was also very forgiving tune up wise.

I''m in no way saying people need to run 8:1 or lower compression. I'm saying for a person starting out, lower SCR is more forgiving and you're not really giving up much.

The numbers will never "add up" and don't make great arguments because there are always a million variables that can skew the numbers a ton. The power gain/loss is 4-5% of the NA power. So if a cam only 5.3 is 400 crank at 9:1... 10:1 is gonna net a whopping 16 hp. Which will go from making 27ish to 28 ish hp per pound. But thats all round number bench racing.

Also not sure what formula you are using to calc CC heat? And cyl pressure... But I don't think they are correct.

The higher compression engine is more efficient, but it will generate more heat, not less? And your comparing apples to oranges. Compare like cyl temps with like setups and 1 variable changed. Inter-coolers and IC efficiency ranges muddy everything up. Same trend woudl follow on either engine if the same IC was added, so eliminate it.

Cylinder Temperature = (Manifold Temperature+460) X Static Compression Ratio^0.283 – 460

Cylinder Absolute Pressure = (Manifold Pressure+14.7) X Static Compression Ratio

I was getting 548 at 8:1 and 644 at 11:1. NA motors at 100 ambient

The fuel you run dictates how high you can go temp wise b4 detonation.

 

I'm using the same formula you are, but the generic Heat of Compression formula. Basically the exact same thing you posted without the automotive specific nomenclature.
Pi*Vi^K=Pf*Vf^K
TemperatureIn*PressureRatio^((AirGamma-PressureIn/AirGamma))=TemperatureOut.
TempIn*(PressureRatio^((1.4-1)/1.4))=TemperatureOut
TempIn*(PressureRatio^0.2857)=TemperatureOut

The +/- 460 in your formula is to convert from *F to *K and back again.

 

So by my math and using basic knock rules on say pump 91 fuel. (and of course this is making a million assumptions and very general)

If the knock threshold for CC heat is say 650*...

Pump Fuel N/A Engine
100F ambient temperature
11:1 static compression ratio

Formulas above yield 644F & 162psi in the cylinder


Pump Fuel Turbo Engine
100F ambient temperature
8.5:1 static compression ratio
15psi boost intercooled
Assume 144F manifold temperature

Formulas above yield 647F & 252psi in the cylinder


So the N/A engine is near the detonation limit with pump fuel and no boost at a relatively low cyl pressure. We can't add much boost to this combination "safely",

Yet the low compression motor with a typical intercooled turbo setup can run 15lbs and still be below the knock CC temps. making 36% more cyl pressure. So does that mean detonation is more about CC temperature than pressure? If that's so, can we run all the boost we want as long as we control the peak cylinder temperature? Simply continue to dropping the SCR and raising boost as long as we stay below that knock threshold temperature of the fuel? This is assuming the bottom end can handle the power of course...

I'm sure it's not that simple... but the point that lower compression is more forgiving with higher power potential is clear. Whether or not you can utilize that power potential is another topic.

Also the peaky cylinder pressures of a higher compression engine near MBT are what tend to lift heads, break ring lands, bend rods, etc... The less peaky the cyl pressures are at lower RPM, the less likely you are to break things. You give up a lot of cyl pressure down low with less compression. Which isn't necessarily bad with high rpm, turbos, and SBE engines. So low compression *may* allow the higher average power across the band without lifting heads or causing damage.

If the fuel is not knock limited, and can handle 10-11:1, meet the power goal in boost, and not lift the head or destroy your rotating assy... There's clearly no reason to run less SCR. If a typical LS comes with 9 or 9.5, I see no reason to bend over backwards changing it either.

 

The heat in the cylinder calculated from Heat of Compression determines the likelihood of Pre-Ignition.
So yes, that all motor combo would be more prone to pre-ignition assuming inlet temps are the same.
E85 is significantly less prone to pre-ignition than gasoline. The auto-ignition temperature is much higher, and the increased cooling effect on the incoming air compounds the effect greatly.

Gasoline auto-ignition temperature is 477-536*F depending on octane, Ethanol is 689*F.
That is the temperature as tested in a heated crucible, and doesn't apply 1:1 to engines since duration and available oxygen are different. These will be higher in an engine.
E85's heat of vaporization is more than twice that of gasoline, so it cools the incoming air significantly more than gasoline.

Detonation is different, since cylinder pressures and heat of ignition are going to be much higher with the turbo setup.
Obviously it can't detonate before the spark happens (or it would be pre-ignition), so we reduce timing.
The calculated temperatures from the heat of compression will have a direct effect on detonation resistance, since it is the first variable in the formula (the starting point).

 

Actual TECH on TECH?! Blasphemy! Can you use the equations and a specific boost level to calculate what your timing should be?

 

Technically, you could determine the threshold for detonation from heat/pressure and work backwards, filling in variables like ignition advance, piston speeds, flame travel speed, and fuel type and come up with a basic formula that would get you in the ballpark just by plugging in the variables. Then a spreadsheet could be made, choose your fuel type and a couple other variables from a drop-down menu, and it basically generate you a timing table.

The only issue is that the timing table would have to be singular-oriented. You would have to aim for something like MBT or a percentage of it for every single cell, or whatever else you decide to base it on, and it might end up being a good timing table for either cruise or WOT, but not both.

 

I think most would be much more interested in WOT.....if you can make that excel it would be super interesting to start comparing what everyone on here has our engines set at.

 

Do you agree with/conceed the "higher power potential" point he is making?

 

I understand his point. Lower compression allows more boost.
My point is that with E85, you can get away with a hell of a lot of compression with no downside.
If there is zero advantage to running 8.5:1 compared to 11:1 as far as detonation threshold with E85, why bother going lower?