Short block…. Compression ratio?

[TKM Conbread]

We also have raised our Gt500 shortblock packages to 11.1

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[Angrey]

It’s not my place to say why Ford chose to go lower compression. They have their reasons. Maybe they chose to go overly concervative being that it’s a flagship racecar thats big money that they don’t want bad feedback on. On the flip side I believe Lunds Whipple gen 3 car went 7s….and compression If I recall correctly is over 12:1

Ford went lower compression because they have to warranty the car against Billy Bob moron and sketchy fuel.

What Mike fails to always grasp is that there's no reason anymore to run low compression, high boost with quality fuels.

Maybe in 1995 when few(er) people could get race fuel and E10 blends were the most alcohol you could find.

With prevalent E85, there's just no reason to intentionally setup your motor with a handicap unless you A) Don't want or can't run E85 and B) Insist on having it with a firm safety factor in place.

There's PLENTY of high hp high compression cars doing very well. Dynamic compression gets a vote and you don't HAVE to run the same timing anymore. It's not 1995 where you set the timing and that's it. These cars can and do adjust (with limits).

In the end, the tuner is going to push the timing up to the edge of knock based upon how much boost at temp is going into the motor.

The other aspect Mike is discounting is that low, tractor motor compression is great for questionable fuel, up to the point that you run out of cooling. There comes a point where you either run out of intercooler or you have to do what all the crazy high boost cars do, add meth injection and/or shots of nitrous to cool down the air charge.

This debate will continue to go on, I just think there's enough data out there to say that you can run 12:1 compression and E85 and make 1000 wheel pretty easily and reliably.

If you want high HP and 93, then do what they did in 1995 and get a bigger motor.

 

[engineermike]

It’s not my place to say why Ford chose to go lower compression. They have their reasons. Maybe they chose to go overly concervative being that it’s a flagship racecar thats big money that they don’t want bad feedback on.

I know why. First, unlike the OP’s situation the concern is not knock, due to the octane available. Plotting cylinder pressure and temperature makes the reason obvious. The engineers at Ford determined a maximum cylinder pressure to ensure reliability of all the internal bits and pieces, then chose the compression ratio and boost combination that produced the power goal while keeping PMax under the pressure limit. As I’ve said many times before, lower compression ratio will produce a lower PMax for the same BMEP. PMax is what causes engine failures and knock. The CJ is small production run of cars designed to class-race week after week consistently with no failures.

On the flip side I believe Lunds Whipple gen 3 car went 7s….and compression If I recall correctly is over 12:1

Barring knock, higher compression improves efficiency and power at the cost of engine stresses, everything else being equal. No one is arguing any different. If a one-off shop car chasing records breaks, it’s not surprising and perhaps even expected.

 

[engineermike]

Ford went lower compression because they have to warranty the car against Billy Bob moron and sketchy fuel.

The Cobra Jet has no warranty.

What Mike fails to always grasp is that there's no reason anymore to run low compression, high boost with quality fuels….…With prevalent E85

And here we are having the exact conversation I predicted in post #41. My responses regarding compression ratio are are to answer the OP’s pump gas specification. I don’t know why he wants to use pump gas and it doesn’t matter because that’s his choice. You E85 guys always specify a compression way higher than optimal for pump then say “oh yea, you’ll need E85”.

there's just no reason to intentionally setup your motor with a handicap unless you A) Don't want or can't run E85….

Bingo! He say pump gas in post #3. He asked what compression ratio is best, not what fuel to use.

Dynamic compression gets a vote

Not really. Lowering dynamic compression ratio only works if you don’t have a positive displacement supercharger that doesn’t allow reversal of flow in the port. Happy to discuss. The only way to lower PD-blown cylinder pressure with cam timing is to add overlap, which allows blow-through and consequently increases DCR. Roush does this and it does allow more spark timing and boost and there is a benefit but only at low and mid-rpm. Up top, you sacrifice.

….you don't HAVE to run the same timing anymore. It's not 1995 where you set the timing and that's it. These cars can and do adjust (with limits)… In the end, the tuner is going to push the timing up to the edge of knock based upon how much boost at temp is going into the motor.

No kidding…I’ve set up my own calibration to adjust spark timing as a function of load, lambda, cam timing, and manifold charge temp, which actually is NOT how many tuners do it because it takes a lot of time. But I still hit that same boost/timing power limit, due to the compression ratio, that I described earlier.

The other aspect Mike is discounting is that low, tractor motor compression is great for questionable fuel, up to the point that you run out of cooling. There comes a point where you either run out of intercooler

Luckily these s550 intercoolers are very efficient at higher boost levels.

This debate will continue to go on, I just think there's enough data out there to say that you can run 12:1 compression and E85 and make 1000 wheel pretty easily and reliably.

Who said any different? This is a straw man argument. The discussion, as specified by the thread starter, was around pump gas.

If you want high HP and 93, then do what they did in 1995 and get a bigger motor.

A bigger motor is definitely an option regarding making more power on limited octane. Other options are airflow&rpm, gDi, temp control and/or charge chilling, and lowering of the compression ratio. I’d say increasing displacement is probably the absolute least practical of these options.

 

[Angrey]

If 10:1 is better (for 93) then why not 9:1. Hell, why not 8:1 or 5:1.

At what point are you claiming lowering compression yields negative returns?

Couldn't someone simply run 1:1 compression and a ginormous turbine to run intake charge at 12 bar of pressure?

 

[engineermike]

If 10:1 is better (for 93) then why not 9:1. Hell, why not 8:1 or 5:1.

At what point are you claiming lowering compression yields negative returns?

Couldn't someone simply run 1:1 compression and a ginormous turbine to run intake charge at 12 bar of pressure?

I could ask the inverse question to you: if 12/1 is so great on boost and 93, then why not 15/1? What about 22/1?

To answer your question, as I’ve explained before, lowering compression will always yield more power for the same peak cylinder pressure. But it is also less efficient at turning air and fuel into power. Dropping from 12/1 to 10/1 only loses about 6% efficiency, so in order to break even on power you’ll have to increase air and fuel flow by 6%. That’s not too hard with the size of most modern blowers and fuel system tweaks, and the result is less failures. However, by dropping to 5/1 it loses a whopping 25% efficiency, meaning the sc will need to be spun much faster, possibly losing significant efficiency or even hitting compressor choke-flow, and the fuel system will need a sizable enhancement as well. This is precisely what I was getting at in post #6, as it starts to get impractical if you go too low.

 

[80FoxCoupe]

@engineermike props to you for your diligence in addressing everyone with lower CR questions and concerns. You do it with solid info and tact, it is much appreciated!

 

[SolarFlare]

And I thank you for your responses towards me but I literally do not care.

 

[Angrey]

I could ask the inverse question to you: if 12/1 is so great on boost and 93, then why not 15/1? What about 22/1?

To answer your question, as I’ve explained before, lowering compression will always yield more power for the same peak cylinder pressure. But it is also less efficient at turning air and fuel into power. Dropping from 12/1 to 10/1 only loses about 6% efficiency, so in order to break even on power you’ll have to increase air and fuel flow by 6%. That’s not too hard with the size of most modern blowers and fuel system tweaks, and the result is less failures. However, by dropping to 5/1 it loses a whopping 25% efficiency, meaning the sc will need to be spun much faster, possibly losing significant efficiency or even hitting compressor choke-flow, and the fuel system will need a sizable enhancement as well. This is precisely what I was getting at in post #6, as it starts to get impractical if you go too low.

Ask away. The market data gives information as to the upper limit when it comes to compression.
We could go round in circles. The reason the OEM's go lower compression on boosted apps has less to do with efficiency and more to do with the fact that they have financial/risk/exposure via warranty claims for an unknown or at least a potentially variable component and that's fuel quality.

It's not speculative that if the engineers were told that every GT500 would ALWAYS receive high quality and consistent 93, across all locations and all times of year, they could have increased compression to yield more results. Furthermore, given the limitations with the transmission and torque over limit protection, the engineers probably intentionally shorted the predator motor. Why produce more power/torque than necessary when it can be done at lower compression/risk (especially given wide ranges of fuel).

My point is, the engineers at Ford didn't give the BEST product they could. They had other constraints to consider, so pointing at their lower comp motor as evidence of the preferred solution is dubious at best.

 

[K4fxd]

Engineer Mike is correct on this. Ford engineers had to make a power number and warrant it for 60,000 miles of hard abuse.

If they could have do e that with 11 or 12 to 1 they would have due to gas mileage for 1, less parasitic HP loss because they could have hit the number with less boost and turbine speed.

 

[engineermike]

Ask away. The market data gives information as to the upper limit when it comes to compression.

Correct, and the highest CR the market offers in a boosted motor is 11/1 in the low performance version of the BMW 3.0 (123 hp/liter). The high performance version is 9.3/1 and makes 168 hp/liter.

We could go round in circles. The reason the OEM's go lower compression on boosted apps has less to do with efficiency and more to do with the fact that they have financial/risk/exposure via warranty claims for an unknown or at least a potentially variable component and that's fuel quality.

This may be partly true, but if you can make the same power with greater safety margin (consider that the OP broke a piston at stock compression), then why wouldn't you? Or better, yet, what if you could make more power AND have a greater safety margin?

Brenspeed did an interesting dyno test of a Whippled GT350 vs a stock GT500. These engines are extremely similar with the exception that one is 12/1 compression and the other is 9.5/1. The cars made nearly identical power and torque numbers. Case closed right? Not really because the GT500 made it with knock margin and the Whipple GT350 made it at the knock limit. What does that mean? That means the GT500 makes the same power more reliably with less cylinder pressure OR can be turned up another 6 psi (as proven by PBD) and blow away the high-compression 350 if it were also pushed to the knock limit.

....GT500 ...they could have increased compression to yield more results. ..

I've compared the Ford GT engine specs and calibration to the workaday F-150 Gen2 3.5, top to bottom. The two engines and calibrations are actually extremely similar. They use the same bore, stroke, valve size, similar cam specs, same HPFP and injectors, etc, and the calibrations are actually quite similar as well. The biggest differences are that the 87 octane F150 is 10.5/1 while the premium-only supercar is 9/1, the supercar has bigger turbos, and the supercar calibration requests higher load. If your statement is true, then you could upgrade F150 turbos and tune, and actually make more than the 660 hp GT motor because the workaday truck motor has higher compression. However, this isn't the case. All the F150 ecoboost motors that come even close to 9/1 compression GT power numbers are running some Ethanol concoction for fuel since they are octane-limited due to the higher compression ratio. By the way, the stock F150 ecoboost at 10.5/1 compression runs miserably low spark advance less than 5 deg in many cases.

My point is, the engineers at Ford didn't give the BEST product they could. They had other constraints to consider, so pointing at their lower comp motor as evidence of the preferred solution is dubious at best.

Then you're not understanding what I'm saying. Refer to Whipple GT350 vs stock GT500 above. The low-compression motor is knock-limited at 100 rwhp higher than the high-compression motor. This is the "wall" I keep referring to, which you haven't addressed. I'm already running the boosted 12/1 93 octane combination that you think is ideal. I can trade timing for boost and vice versa all day long and still hit a power limit way below a low-compression equivalent.

 

[K4fxd]

Then there is the fuel issue, in an all out race engine would it be better to run less compression to allow higher boost which translates to more air, more air means more fuel, more fuel means more power, so long as you have time to react it.

An example is a TF engine they run low compression and run so much fuel as it is very close to hydro locking. If they say ran 10 to 1 it would be more efficient but they could not cram in as much air and fuel.

 

[Barrel]

are you sleeved?

Yes Darton sleeves.

 

[Angrey]

Then there is the fuel issue, in an all out race engine would it be better to run less compression to allow higher boost which translates to more air, more air means more fuel, more fuel means more power, so long as you have time to react it.

An example is a TF engine they run low compression and run so much fuel as it is very close to hydro locking. If they say ran 10 to 1 it would be more efficient but they could not cram in as much air and fuel.

So again, I ask the question, if lowering the compression and increasing the air charge is indeed the magical solution, then why not just lower the compression to 1:1 and have a giant compressor slamming 25 bar of pressure into the motor? If 10:1 (with more boost and fuel) is better than 12:1, then why not 9:1, or 8:1 or 5:1 or hell, let's just go 1:1.

I'll tell you why. Because "just add more boost and fuel" has cooling limitations. The idea that 20 degrees of timing on 12:1 and 10 lbs of boost is somehow more prone to knock than 20 degrees of timing at 10:1 with 20 lbs of boost is an interesting study and again, it comes down to quality of fuel.

In the end, I'dd personally rather make 1000 rear on 17 lbs of boost at 12:1 than 25 lbs of boost at 9.5:1. That's just me. You guys do what you want.

 

[engineermike]

So again, I ask the question, if lowering the compression and increasing the air charge is indeed the magical solution, then why not just lower the compression to 1:1 and have a giant compressor slamming 25 bar of pressure into the motor? If 10:1 (with more boost and fuel) is better than 12:1, then why not 9:1, or 8:1 or 5:1 or hell, let's just go 1:1.

Asked and answered.

I'll tell you why. Because "just add more boost and fuel" has cooling limitations. The idea that 20 degrees of timing on 12:1 and 10 lbs of boost is somehow more prone to knock than 20 degrees of timing at 10:1 with 20 lbs of boost is an interesting study and again, it comes down to quality of fuel.

20 psi and 10/1 will walk all over 10 psi and 12/1, to the tune of 30% more power.

In the end, I'dd personally rather make 1000 rear on 17 lbs of boost at 12:1 than 25 lbs of boost at 9.5:1.

Then you’re choosing the route that results in higher cylinder temps, lower knock limitations, and higher engine stresses.

That's just me. You guys do what you want.

Thanks for giving us your permission. I’m already doing it your way and I’m not happy with the results.

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