- Banned
- #46
We also have raised our Gt500 shortblock packages to 11.1
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Ford went lower compression because they have to warranty the car against Billy Bob moron and sketchy fuel.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
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.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.
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.On the flip side I believe Lunds Whipple gen 3 car went 7s….and compression If I recall correctly is over 12:1
The Cobra Jet has no warranty.Ford went lower compression because they have to warranty the car against Billy Bob moron and sketchy fuel.
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”.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
Bingo! He say pump gas in post #3. He asked what compression ratio is best, not what fuel to use.there's just no reason to intentionally setup your motor with a handicap unless you A) Don't want or can't run E85….
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.Dynamic compression gets a vote
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.….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.
Luckily these s550 intercoolers are very efficient at higher boost levels.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
Who said any different? This is a straw man argument. The discussion, as specified by the thread starter, was around pump gas.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.
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.If you want high HP and 93, then do what they did in 1995 and get a bigger motor.
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?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?
Ask away. The market data gives information as to the upper limit when it comes to compression.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.
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.Ask away. The market data gives information as to the upper limit when it comes to compression.
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?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.
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.....GT500 ...they could have increased compression to yield more results. ..
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.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.
Yes Darton sleeves.are you sleeved?
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.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.
Asked and answered.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.
20 psi and 10/1 will walk all over 10 psi and 12/1, to the tune of 30% more power.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.
Then you’re choosing the route that results in higher cylinder temps, lower knock limitations, and higher engine stresses.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.
Thanks for giving us your permission. I’m already doing it your way and I’m not happy with the results.That's just me. You guys do what you want.