Building the motor

[sigintel]

This whole thread de-rail compression argument is irrelevant. Static compression means next to nothing anymore. Cylinder pressure is greatly affected by the tune and the cam timing. High compression lower boost and junk tune=low output. Lower compression higher boost and junk tune=low output.

Even non cam timing adjustable engines can control the cylinder pressure with custom cams and a grinder who knows what they are doing. Cylinder pressure at the combustion event is directly what affects timing advance capacity, and the resulting power output. Lowering compression just for the sake of adding boost is 30 years ago thinking. Oem's lower compression on most boosted applications just for the sake of reasons stated above like the New Balance wearing CEO who loves getting into the throttle on sunday afternoon without downshifting at 2,400 rpm, and makes 7 figures a year but is too cheap to buy higher octane fuel.

OP you have lofty expectations from 93 pump gas without octane booster. If any way possible find and use E85, or add meth injection.

Think you missed the discussion on cam timing/late intake closing:
TLDR: Attempting to late close on a FI setup to decrease CR unfortunately results in pumping back into the intake where that rejected mass increases the intake pressure (and temperature(doh!)).
You just add to the mass pumped into the intake: except instead of it getting cooled by IC, it got heated by cylinder (sux).

TLDR#2: Ford lowered the compression for the GT500 cause God said so and they have direct line.

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[engineermike]

No...you’ve used a small percent of the car population that’s limited (though not really) by availability of E85 at local stations combined with the need of big HP to fuel the argument that low compression with a ton of boost and pump chocolate milk is a good idea and makes tons of power.

I never said it was the most popular or most powerful option. But I’ve studied making power on pump gas A LOT so if that’s your goal then I can probably help you get there. The OP stated pump gas in the very first post so I can help him. I never tried to convince you or anyone that they shouldn’t run E85. It’s just not for everyone.

Lots of straw man in these arguments.

FWIW one inescapable FACT is that for any given Pmax, a lower compression ratio will have a higher BMEP. BMEP makes power; Pmax breaks parts.

 

[SolarFlare]

Guy said he was building the motor and then some time later getting a fuel system which means E85 is available.

 

[Jackson1320]

This is rather exaggerated. Lower your compression to 8/1, run 30 psi, and enjoy 1000+ hp on 93 if that’s your prerogative and you have everything else needed. The Mercedes m139 engine does this and makes over 200 hp/liter. It does it at 9/1 compression and 30 psi on junk gas all day long, so this route has been proven in something more reliable than your typical supercharged mustang.

Or we could be more reasonable and talk about dropping compression from 11/1 to 10/1. Adding 1-1.5 psi boost will recoup anything lost from the compression, but be much safer. Add another 2-3 psi and you’re still safer but making 40-60 more hp on pump gas. IIRC the PBD gt500 (9.5/1) made almost 900 rwhp on 93.lp BBC v m Yes,


they had to raise the boost to something like 17 psi but 17 psi and 900 rwhp is not possible at
11 or 12/1 on 93 no meth or booster.

900 on pump 93 is absolutely possible and been done. If the engine is having problems with detonation odds are it’s not the compression ratio. I’ll agree that the engine would not last long. He has the wrong supercharger to make 900whp on pump gas. He would have to lower compression to somewhere around 8.0 and he would need to spin the hell out of his supercharger to get enough boost. the car would be worthless off the line.

 

[Jackson1320]

Correct on cid and turbo losses. But as compared to the turbo LT1, our cylinder heads and engine controls are vastly superior, we have 25% more rpm available, and the 18+ has DI. The old LT1 didn’t make power past 6000 rpm. 900 rwhp is a tall order on pump gas but very far from impossible. Palm Beach Dyno made 890 rwhp on 93 with a GT500 so I would suggest learning and applying as much as can be from that. I can predict what compression it would take to achieve 900 rwhp on 93 and be OEM-reliable but you guys wouldn’t like that answer. But consider for a second that a current Mercury Racing inboard V8 runs on 93 with port injection and has a BMEP of over 24 bar. The motor will run reliably at WOT for hundreds of hours. It has 7.8/1 compression. 24 bar BMEP in a coyote would put about 810 hp/640 ftlb to the wheels and be rock-solid reliable on 93.

Probably but with a centrifugal the car would be worthless until he started making boost. From a dig with only 7.8:1 The race will be over by the time he gets enough boost to start making power

 

[Angrey]

Everyone is free to do what they want.

But the days of running lower compression and making 1000hp are gone. The guys that run low compression now are making 2, 3, 4 thousand horsepower, with everything from full on race fuel to nitrometh, etc.

What's changed in the past 10 years is engine management and 1000 hp cars aren't just for hardcore trailer queen racers anymore.

The reason Ford lowered compression isn't because it's "optimal" it's because it's "NECESSARY" to ensure the average consumer doesn't fill up with questionable pump gas and lug the motor uphill on a hot day and fry it (and then turn around and demand a warranty replacement). Using Ford's approach and comparing it to what's preferable to aftermarket/enthusiast approach is misguided in this respect. Ford engineers didn't do it because it was the best or easiest way to make power, they did it because they wanted to make reasonable power AND have the car stomach crappy fuel.

If the OP insists on pump gas, then I would agree the ONLY sensible path to making BIG power is to lower the static compression. Yes, the tuner can adjust the dynamic peak pressure with timing of the ignition, but that as limitations and eventually drawbacks.

However, as stated numerous times, with better fuel comes BETTER solutions, like increased static compression ratios.

Again, if that weren't true, then Ford could simply just lower the compression further and run junk 87 and more boost. Which speaks to WHY they don't do that. Because "just lower compression and add boost" has not just detrimental limitations on the comp/motor side, it has REAL WORLD limitations on the intake side.

"big boost" requires a couple of approaches. 1) You can run a giant blower, which is heavy and contributes to overall inefficiency OR you can run a small(er) blower and spin it hard. Spinning it hard also has limits. You just can't pulley down enough before you have to resort to OVERDRIVING the rest of the components (and a top down redesign of those accessories which Ford wasn't obviously interested in doing). Then, in order to cool the boost, you're now adding more intercooler required. This is obviously easier with a turbo setup, but with a PD blower, that becomes a limiting factor as well.

Given ALL the inputs and outputs, it's BETTER to run better fuel (aka E-85 or race fuel) and a higher compression ratio to make street car power (i.e. the 900-1200 hp range).

Basically, a LOT of the final outcome depends on what fuel you choose and how much you can safely, reliably make power taking into account all the real world possibilities (like bad batch of fuel, Arizona in the summer vs Alaska in the Winter, etc).

I would rather run 12:1 compression and 12-14 psi to make 1000 hp (on E-85) than run 9:1 compression and 21 psi on 93. I think if Ford Engineer's were given the realistic option, they would agree. But I'm not the average consumer market (nor most of the guys reading these threads). They don't HAVE that option, so low compression higher boost is the path they chose.

 

[engineermike]

@Angrey I don’t disagree with the majority of your post but I would like to point out an interesting benchmark. The 2018 Cobra Jet, per the owners manual, REQUIRES VP C16 race fuel. They aren’t octane-limited, nor are they concerned about inadvertently lugging the engine. They ARE concerned about power and reliability. It uses the same gen5 Whipple that any of us can buy, so there’s nothing special about its forced induction. The interesting bit is in spite of not being octane-limited and the intended use being race-track only, Ford decided to build it with only 9.7/1 compression. Looks like they’re spinning the Whipple up to 20-21 psi and making over 1100 hp. Why would Ford elect to run such low compression when not octane-limited? It uses custom pistons by Mahle, designed specific to the application so they could have used whatever compression ratio they wanted. I believe it’s to achieve the power goal (high BMEP) while retaining reliability (limited PMax). Running lower compression and more boost for any given power level actually reduces stress on the block, pistons, rods, crank, bearings, and even the heads, valves, and head gaskets.

The reason Ford lowered compression isn't because it's "optimal" it's because it's "NECESSARY"..

While Ford does do this partly for reliability, I would argue that it actually is better for power production on pump gas. My example from earlier still holds true; that when adding boost to the 9.5/1 GT500 it doesn’t hit its optimum boost/spark timing combination until ~100 hp higher than supercharged 12/1 GT350’s. Again, if a 9.5/1 engine and a 12/1 engine are both limited to the same PMax, then the 9.5/1 engine will have a higher BMEP, this more power and torque. The temperature of the compressed charge prior to spark is significantly higher in the 12/1 engine. This is irrefutable.

I really wish you guys would take a few minutes to understand what’s going on temperature and pressure-wise in the cylinder during the compression process, then compare different combinations of boost and compression ratio. It was only then that it all made complete sense for me, and matched everything I had observed in practice and benchmarking.

One way to think about designing a combination is by determining what is fixed and what is variable between: displacement, boost, compression ratio, rpm, fuel, injection type (DI vs PI), reliability/duty cycle, and torque output. If you constrain all but two, I can calculate those two. For instance, the OP’s case is actually over-constrained:

displacement: 5.0
boost limit: not specified
compression ratio: 10/1
rpm: 7500
fuel: 93
injection (DI vs PI): PI
reliability/duty cycle: high
torque output: ~750 ftlb (crank)

I can say that the above combination is basically impossible. To reach the torque goal requires a BMEP of 25 bar. A BMEP of 25 bar is absolutely possible and proven reliable at high duty cycle on pump gas, but not at 10/1 compression. It would take 8/1 compression (and a lot of boost) to reach 25 bar BMEP on pump gas with high reliability. If you’re willing to sacrifice some reliability, you could get away with 8.5/1 most of the time. If it were 5.4 liters then it would only take 23 bar BMEP to get there, which would take 8.7/1 with high reliability. Or you could get away with 9.5/1 if you sacrifice some reliability. But no, it’s not going to make 900 rwhp at 5 liters, 93, port injection, and 10/1.

I personally know professional, career OEM engine calibrators and they target a Pmax as much as they do avoiding detonation. I believe limiting PMax is why the cobra jet only runs 9.7/1 compression. The mechanical designers dictate the maximum peak pressure based on what the parts can withstand reliably. They might limit Pmax to, say, 150 bar. Pmax occurs 10 or 20 deg ATDC, then decays rapidly as the volume increases during the power stroke. Lower compression ratio slows the rate of pressure decay, therefore retaining higher pressure for longer to push down on the piston. For example, by 45 deg ATDC on the power stroke, the 12/1 engine cylinder pressure might decay to 35 bar, but an 8/1 engine would only decay to 50 bar. Higher average cylinder pressure during the entire stroke (BMEP) puts more power to the crank at the same peak pressure (PMax). I hope this makes sense.

 

[ReapSow]

@Angrey I don’t disagree with the majority of your post but I would like to point out an interesting benchmark. The 2018 Cobra Jet, per the owners manual, REQUIRES VP C16 race fuel. They aren’t octane-limited, nor are they concerned about inadvertently lugging the engine. They ARE concerned about power and reliability. It uses the same gen5 Whipple that any of us can buy, so there’s nothing special about its forced induction. The interesting bit is in spite of not being octane-limited and the intended use being race-track only, Ford decided to build it with only 9.7/1 compression. Looks like they’re spinning the Whipple up to 20-21 psi and making over 1100 hp. Why would Ford elect to run such low compression when not octane-limited? It uses custom pistons by Mahle, designed specific to the application so they could have used whatever compression ratio they wanted. I believe it’s to achieve the power goal (high BMEP) while retaining reliability (limited PMax). Running lower compression and more boost for any given power level actually reduces stress on the block, pistons, rods, crank, bearings, and even the heads, valves, and head gaskets.



While Ford does do this partly for reliability, I would argue that it actually is better for power production on pump gas. My example from earlier still holds true; that when adding boost to the 9.5/1 GT500 it doesn’t hit its optimum boost/spark timing combination until ~100 hp higher than supercharged 12/1 GT350’s. Again, if a 9.5/1 engine and a 12/1 engine are both limited to the same PMax, then the 9.5/1 engine will have a higher BMEP, this more power and torque. The temperature of the compressed charge prior to spark is significantly higher in the 12/1 engine. This is irrefutable.

I really wish you guys would take a few minutes to understand what’s going on temperature and pressure-wise in the cylinder during the compression process, then compare different combinations of boost and compression ratio. It was only then that it all made complete sense for me, and matched everything I had observed in practice and benchmarking.

One way to think about designing a combination is by determining what is fixed and what is variable between: displacement, boost, compression ratio, rpm, fuel, injection type (DI vs PI), reliability/duty cycle, and torque output. If you constrain all but two, I can calculate those two. For instance, the OP’s case is actually over-constrained:

displacement: 5.0
boost limit: not specified
compression ratio: 10/1
rpm: 7500
fuel: 93
injection (DI vs PI): PI
reliability/duty cycle: high
torque output: ~750 ftlb (crank)

I can say that the above combination is basically impossible. To reach the torque goal requires a BMEP of 25 bar. A BMEP of 25 bar is absolutely possible and proven reliable at high duty cycle on pump gas, but not at 10/1 compression. It would take 8/1 compression (and a lot of boost) to reach 25 bar BMEP on pump gas with high reliability. If you’re willing to sacrifice some reliability, you could get away with 8.5/1 most of the time. If it were 5.4 liters then it would only take 23 bar BMEP to get there, which would take 8.7/1 with high reliability. Or you could get away with 9.5/1 if you sacrifice some reliability. But no, it’s not going to make 900 rwhp at 5 liters, 93, port injection, and 10/1.

I personally know professional, career OEM engine calibrators and they target a Pmax as much as they do avoiding detonation. I believe limiting PMax is why the cobra jet only runs 9.7/1 compression. The mechanical designers dictate the maximum peak pressure based on what the parts can withstand reliably. They might limit Pmax to, say, 150 bar. Pmax occurs 10 or 20 deg ATDC, then decays rapidly as the volume increases during the power stroke. Lower compression ratio slows the rate of pressure decay, therefore retaining higher pressure for longer to push down on the piston. For example, by 45 deg ATDC on the power stroke, the 12/1 engine cylinder pressure might decay to 35 bar, but an 8/1 engine would only decay to 50 bar. Higher average cylinder pressure during the entire stroke (BMEP) puts more power to the crank at the same peak pressure (PMax). I hope this makes sense.

Excellent.

 

[SolarFlare]

Never heard anyone at the track say “damn the bmep on this car is just killing the competition” either. Weird. Must be one of those secret mods track guys use.

 

[Angrey]

@Angrey I don’t disagree with the majority of your post but I would like to point out an interesting benchmark. The 2018 Cobra Jet, per the owners manual, REQUIRES VP C16 race fuel. They aren’t octane-limited, nor are they concerned about inadvertently lugging the engine. They ARE concerned about power and reliability. It uses the same gen5 Whipple that any of us can buy, so there’s nothing special about its forced induction. The interesting bit is in spite of not being octane-limited and the intended use being race-track only, Ford decided to build it with only 9.7/1 compression. Looks like they’re spinning the Whipple up to 20-21 psi and making over 1100 hp. Why would Ford elect to run such low compression when not octane-limited? It uses custom pistons by Mahle, designed specific to the application so they could have used whatever compression ratio they wanted. I believe it’s to achieve the power goal (high BMEP) while retaining reliability (limited PMax). Running lower compression and more boost for any given power level actually reduces stress on the block, pistons, rods, crank, bearings, and even the heads, valves, and head gaskets.



While Ford does do this partly for reliability, I would argue that it actually is better for power production on pump gas. My example from earlier still holds true; that when adding boost to the 9.5/1 GT500 it doesn’t hit its optimum boost/spark timing combination until ~100 hp higher than supercharged 12/1 GT350’s. Again, if a 9.5/1 engine and a 12/1 engine are both limited to the same PMax, then the 9.5/1 engine will have a higher BMEP, this more power and torque. The temperature of the compressed charge prior to spark is significantly higher in the 12/1 engine. This is irrefutable.

I really wish you guys would take a few minutes to understand what’s going on temperature and pressure-wise in the cylinder during the compression process, then compare different combinations of boost and compression ratio. It was only then that it all made complete sense for me, and matched everything I had observed in practice and benchmarking.

One way to think about designing a combination is by determining what is fixed and what is variable between: displacement, boost, compression ratio, rpm, fuel, injection type (DI vs PI), reliability/duty cycle, and torque output. If you constrain all but two, I can calculate those two. For instance, the OP’s case is actually over-constrained:

displacement: 5.0
boost limit: not specified
compression ratio: 10/1
rpm: 7500
fuel: 93
injection (DI vs PI): PI
reliability/duty cycle: high
torque output: ~750 ftlb (crank)

I can say that the above combination is basically impossible. To reach the torque goal requires a BMEP of 25 bar. A BMEP of 25 bar is absolutely possible and proven reliable at high duty cycle on pump gas, but not at 10/1 compression. It would take 8/1 compression (and a lot of boost) to reach 25 bar BMEP on pump gas with high reliability. If you’re willing to sacrifice some reliability, you could get away with 8.5/1 most of the time. If it were 5.4 liters then it would only take 23 bar BMEP to get there, which would take 8.7/1 with high reliability. Or you could get away with 9.5/1 if you sacrifice some reliability. But no, it’s not going to make 900 rwhp at 5 liters, 93, port injection, and 10/1.

I personally know professional, career OEM engine calibrators and they target a Pmax as much as they do avoiding detonation. I believe limiting PMax is why the cobra jet only runs 9.7/1 compression. The mechanical designers dictate the maximum peak pressure based on what the parts can withstand reliably. They might limit Pmax to, say, 150 bar. Pmax occurs 10 or 20 deg ATDC, then decays rapidly as the volume increases during the power stroke. Lower compression ratio slows the rate of pressure decay, therefore retaining higher pressure for longer to push down on the piston. For example, by 45 deg ATDC on the power stroke, the 12/1 engine cylinder pressure might decay to 35 bar, but an 8/1 engine would only decay to 50 bar. Higher average cylinder pressure during the entire stroke (BMEP) puts more power to the crank at the same peak pressure (PMax). I hope this makes sense.

The proof is in the question. If 9.7:1 and high boost is better, then why isn't 8.0:1 and even higher boost preferable. Or 5:1 and 100 psi preferable? Why not have a giant bore and a tiny stroke and just send 200 psi into the motor? under your claims, that would be preferable.

 

[engineermike]

Never heard anyone at the track say “damn the bmep on this car is just killing the competition” either. Weird. Must be one of those secret mods track guys use.

You wanted to participate in a technical discussion, but when it gets technical you dumb it down to this?

Let me boil it down to some basics.

Quarter mile ET is a function of gearing, traction, and terminal speed

Terminal speed is a function of power/weight

Power is a function of torque and rpm

Torque is a function of BMEP and displacement.

So there you have it. When you see a really impressive car, it has one or all of the above.

The subject of this thread is an engine build with fixed displacement, fixed max rpm, and pump gas, which limits PMax. That leaves ONLY maximizing BMEP as the variable up for discussion.

 

[engineermike]

The proof is in the question. If 9.7:1 and high boost is better, then why isn't 8.0:1 and even higher boost preferable. Or 5:1 and 100 psi preferable? Why not have a giant bore and a tiny stroke and just send 200 psi into the motor? under your claims, that would be preferable.

We can definitely go through that modeling exercise, but I could ask the opposite: if high compression and low boost is so great then why not go to 14/1 and get rid of the blower? It would still be at the detonation limit (same Pmax but drastically lower BMEP). Still, I asked the question first: Why would Ford build the non-octane-limited 2018 cobra jet race car with only 9.7/1 compression? Why would ford build the Aluminator SC with 9.5/1? Why does a supercharged GT350 hit its detonation-induced power limit well below the GT500? Ive already answered this but I’m curious others’ explanation.

 

[SolarFlare]

I’m not really participating. This is the Internet, you fold and create truths almost at will. Especially when you handcuff one side to pump chocolate milk. If you notice the other side hasn’t mentioned limiting your boost. When you build your 9.5:1 motor and put the Whipple at 20psi on pump gas then maybe we can truly discuss it. But again, it’s the Internet and no one can guarantee you won’t use octane booster or race gas.

Sorry to the OP for hurting his motor....likely because he trusted the same Chocolate milk engineermike wants to limit us to. Gl on the build, can’t wait to see how it turns out.

 

[SolarFlare]

9.5:1 aluminator is trash lol. No good tuner/shop recommends it, they say stick to 11:1

 

[Angrey]

We can definitely go through that modeling exercise, but I could ask the opposite: if high compression and low boost is so great then why not go to 14/1 and get rid of the blower? It would still be at the detonation limit (same Pmax but drastically lower BMEP). Still, I asked the question first: Why would Ford build the non-octane-limited 2018 cobra jet race car with only 9.7/1 compression? Why would ford build the Aluminator SC with 9.5/1? Why does a supercharged GT350 hit its detonation-induced power limit well below the GT500? Ive already answered this but I’m curious others’ explanation.

They do! You want to skirt as close to the preignition limits as the fuel/combination will take you. That's my point. It's MORE preferable to running higher compression and better fuel.

And as far as Ford, Ford's safety factors and tolerance for max demands on parts and assemblies is different than the aftermarket.

That's why Ford doesn't recommend you do much with their systems (including torque capacity, drivetrain components, engine components) because they have a MUCH larger safety factor than we employ in the real/aftermarket world.

Again, everyone is free to do what they will.

There are some theoretical and academic discussions, but the market tends to gravitate (not always, but many times) toward those solutions that work and work well/better/best.

And the only engine builders who are recommending 9:1 compression are the guy's who are running THOUSANDS of HP.

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