Talk me into a centrifugal

[engineermike]

@Angrey intercooling would have to be pretty terrible for this to be an issue. If we take the OP’s example of a gen2, he’d be dropping from 11 to 9.5/1, which would take about 2 psi boost to offset the loss and break even on power. I’ve personally run and thoroughly logged as low as 9 and as high as 16 psi boost and never saw any appreciable change in charge temp after the Whipple intercooler. Even if the centrif intercoolers weren’t nearly as good as Whipple, I still wouldn’t expect to see enough difference in charge temp for it to negate the 1.5 drop in compression from a knock perspective.

Ford gave us a great example for comparison in the gt500. Stock is 12 psi boost ant 9.5/1 and they are able to run well over 20 deg timing safely on pump gas. They’re cranking boost up to 16-18 psi and making 900 rwhp on 93. So in this example, raising the boost at low compression didn’t lead to enough charge temp increase to negate the gains.

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[Angrey]

@Angrey intercooling would have to be pretty terrible for this to be an issue. If we take the OP’s example of a gen2, he’d be dropping from 11 to 9.5/1, which would take about 2 psi boost to offset the loss and break even on power. I’ve personally run and thoroughly logged as low as 9 and as high as 16 psi boost and never saw any appreciable change in charge temp after the Whipple intercooler. Even if the centrif intercoolers weren’t nearly as good as Whipple, I still wouldn’t expect to see enough difference in charge temp for it to negate the 1.5 drop in compression from a knock perspective.

Ford gave us a great example for comparison in the gt500. Stock is 12 psi boost ant 9.5/1 and they are able to run well over 20 deg timing safely on pump gas. They’re cranking boost up to 16-18 psi and making 900 rwhp on 93. So in this example, raising the boost at low compression didn’t lead to enough charge temp increase to negate the gains.

This is all true, but the discussion was very broad. I'm writing more about the low displacement guys who are running 40 psi. With sufficient prep of the charge, it's a better tradeoff to go low compression high boost. But you can charge the air so much that you run into knock sooner if you don't have sufficient intercooling.

 

[beefcake]

Car is 40% street, 50% autocross, and 10% track.

I have to rebuild (or part out) this Gen2 motor, so I need to decide on boost and choose a 9.5:1 compression motor or a 12:1 NA optimized motor.

I've had great results with this as an NA build, but I want more power/torque, and for reasons I want to lower the diff ratio to get a higher top speed in 2nd gear, which implies going from 3.73s to 3.31s and therefore losing torque in the process.

I had a smaller 1.8l turbo sports car in the past and loved the turbocharger's personality, but it looks like the spool time for these V8s from a lower RPM is too long for my autocross habits. Plus the weight of system. This car turns corners so weight is the enemy. I wish there was a turbo kit optimized for hyper fast spooling at the expense of top end power. ~600-650 at the wheels is my goal, I don't need 800 to race around parking lots with cones.. even big parking lots.

So I'm considering the ESS or P1X.

While it sounds like a bad idea for a track car. I'm willing to duct and upgrade the radiator to help with heat and I think its probably ok to get just a couple hot laps before cooldown laps in between. I like to get on track a few times a year, but I don't need to run 30 minute sessions or anything to enjoy myself.

I'm still on stock MT86 and axles, so I hope those can hold up for a while. I'm kind to things when launching but we do run hard.

we love the centri kits, paxton and procharger are our best sellers, the paxton being the best bank for the buck out there, we have kits starting at 4799 with a 1000hp intercooler.

we do more blowers on the coyotes than anyone in the country, feel free to ring me or dm with any questions you have and we can help steer you in the right direction

Feel Free to DM me or call with any questions.
Terry "Beefcake" Reeves
Office - 855-TBR-RACE
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Deal Directly with the boss, no pushy sales guys!!!!!!!
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[furdfan2018]

Not exactly. High boost and low compression has lower peak cylinder pressure, lower stress, and lower peak in-cylinder temps than low boost and high compression at the same power level. It’s an interesting thermodynamic kinematic relationship.

If ignition timing is the same between the two setups, yes-- lower cylinder pressure on the lower compression motor.

However, you'll need more timing in the lower compression setup to achieve the same HP as the higher compression motor thus creating the same cylinder pressure.

 

[engineermike]

…it's a better tradeoff to go low compression high boost. But you can charge the air so much that you run into knock sooner if you don't have sufficient intercooling.

This is true. Intercooling offers a huge advantage when knock limited. It’s basically two stage compression and the air temps in cylinder can be the same as na but at much higher pressures.

 

[GregO]

So no Rotrex users on this site ?
I see this style also used in Powersports SxS and motorcycles. Maybe it's more popular on the other side of the big pond.

 

[engineermike]

If ignition timing is the same between the two setups, yes-- lower cylinder pressure on the lower compression motor.

However, you'll need more timing in the lower compression setup to achieve the same HP as the higher compression motor thus creating the same cylinder pressure.

If you set the hp as the same, then the peak cyl pressure will be lower with the lower compression. Example using basic thermo relationships:

Lets say we set Pmax (max pressure in the cylinder during the cycle) to the same in 11/1 and 9.5/1 scenarios at 1800 psi (roughly 120 bar) at TDC. This would be regardless of how you got there with a combination of boost and timing. The peak is the same, therefore the stresses are the same and the tendency to knock is the same (assuming the no intercooling). When you expand this pressure by the respective expansion ratios, you find the final pressure at BDC to be 63 psi in the 11/1 case and 77 psi in the 9.5/1 case. Therefore, even though the Pmax pressure at TDC started the same, the lower compression motor had a higher final pressure, which means the cylinder pressure was higher at all times during the power stroke which means higher MEP and higher output. The downside is that when the exhaust valve opens, it relieves a higher pressure and you lose more energy out the exhaust, which is why lower compression is less efficient and needs more air and fuel to make the same power.

If you were to arrange this another way to have equivalent hp (MEP) between the two compression ratios, and back calculate the PMax, you'd see the low compression combination has lower PMax for the same power output. Again, how you achieve the PMax notwithstanding.

I've modeled every part of this process using different compression ratios, boost levels, charge temps, and spark timings and come to the same conclusion every time.

 

[Angrey]

If you set the hp as the same, then the peak cyl pressure will be lower with the lower compression. Example using basic thermo relationships:

Lets say we set Pmax (max pressure in the cylinder during the cycle) to the same in 11/1 and 9.5/1 scenarios at 1800 psi (roughly 120 bar) at TDC. This would be regardless of how you got there with a combination of boost and timing. The peak is the same, therefore the stresses are the same and the tendency to knock is the same (assuming the no intercooling). When you expand this pressure by the respective expansion ratios, you find the final pressure at BDC to be 63 psi in the 11/1 case and 77 psi in the 9.5/1 case. Therefore, even though the Pmax pressure at TDC started the same, the lower compression motor had a higher final pressure, which means the cylinder pressure was higher at all times during the power stroke which means higher MEP and higher output. The downside is that when the exhaust valve opens, it relieves a higher pressure and you lose more energy out the exhaust, which is why lower compression is less efficient and needs more air and fuel to make the same power.

If you were to arrange this another way to have equivalent hp (MEP) between the two compression ratios, and back calculate the PMax, you'd see the low compression combination has lower PMax for the same power output. Again, how you achieve the PMax notwithstanding.

I've modeled every part of this process using different compression ratios, boost levels, charge temps, and spark timings and come to the same conclusion every time.

What's the major shift that fuel quality and octane rating play? After all, there has to be limits. You couldn't run 2:1 compression on a zillion psi of boost and practically make the same power. I'd imagine as fuel quality and knock resistance increases, the advantage of higher compression increases.

On pump 93, I absolutely would not run boosted 12:1 without some sorta booster. 93 is just too inconsistent. (but that's me). But on E85, 12:1 seems to fare well (or at the very least, much better).

A similar exercise would be to examine this difference with say 87 or straight trash pump. I'm betting you can achieve the same output on 87 boosted, but you have to drop the compression much further and ramp in charge pressure much much more.

 

[engineermike]

What's the major shift that fuel quality and octane rating play? After all, there has to be limits. You couldn't run 2:1 compression on a zillion psi of boost and practically make the same power. I'd imagine as fuel quality and knock resistance increases, the advantage of higher compression increases….

For sure, the case for lower compression isn’t as strong when knock is removed from the equation. At that point, the only downside to high compression is cylinder pressure and the stresses caused by it. Ford has internal standards for things like max rod loading and piston stress and such, and in the spark timing logic one of the spark calculations is a max timing based on cylinder pressure limits. Its always doing a low-select for spark timing. For the gen2 coyote, this is what it looks like:

For comparison, this is what it looks like for a GT500:

Obviously, its much higher on a GT500, likely due to both component strength and compression ratio. This is also my theory as to why the CJ is only 10/1 even though it has basically unlimited octane.

It's hard to make a strong case that we should limit timing on 12/1 and E85 when so many are running 20+ deg which is way over what Ford would allow. However, Ford's standards require a lot more safety factor than your typical racer. I also tend to believe that the 22 or so deg everyone runs on E85 is a de facto cylinder pressure limited timing to achieve the reliability that most enthusiasts expect.

A similar exercise would be to examine this difference with say 87 or straight trash pump. I'm betting you can achieve the same output on 87 boosted, but you have to drop the compression much further and ramp in charge pressure much much more.

Yes, very true. Mercury Racing offers switchable octane calibrations in their 9 liter QC4, making 1100 hp on 89 octane and 1350 on 91. This motor is under 8/1 compression ratio. I wonder what it could make on 87....

One cool thing Ford builds into the Gen3 and Ecoboost PCMs is the ability to float the max load/torque as a function of learned octane, learned ethanol, and commanded lambda. The ecoboosts are set up from the factory to run more boost when it learns higher octane. You could actually set up a gen3 to run more boost as it learns higher ethanol in a flex tune, or even run higher boost only when cat protect kicks in and it goes rich.

 

[furdfan2018]

If you set the hp as the same, then the peak cyl pressure will be lower with the lower compression. Example using basic thermo relationships:

Lets say we set Pmax (max pressure in the cylinder during the cycle) to the same in 11/1 and 9.5/1 scenarios at 1800 psi (roughly 120 bar) at TDC. This would be regardless of how you got there with a combination of boost and timing. The peak is the same, therefore the stresses are the same and the tendency to knock is the same (assuming the no intercooling). When you expand this pressure by the respective expansion ratios, you find the final pressure at BDC to be 63 psi in the 11/1 case and 77 psi in the 9.5/1 case. Therefore, even though the Pmax pressure at TDC started the same, the lower compression motor had a higher final pressure, which means the cylinder pressure was higher at all times during the power stroke which means higher MEP and higher output. The downside is that when the exhaust valve opens, it relieves a higher pressure and you lose more energy out the exhaust, which is why lower compression is less efficient and needs more air and fuel to make the same power.

If you were to arrange this another way to have equivalent hp (MEP) between the two compression ratios, and back calculate the PMax, you'd see the low compression combination has lower PMax for the same power output. Again, how you achieve the PMax notwithstanding.

I've modeled every part of this process using different compression ratios, boost levels, charge temps, and spark timings and come to the same conclusion every time.

You've obviously modeled and researched this to the theoretical level which is cool and very interesting.

In practice -- it doesn't matter if you can "cheat" with e85. Higher compression allows for more power sooner in the RPM range which, in turn, delivers a better driving experience. Cylinder pressure considerations are a moot point at the 1000 hp level in these motors because the hardware has proven to be very durable.

Now --- If you have no, or limited access to e85 and must run pump 91 or 93, then you need to be very careful in your parts section when building a motor. No way I'd run 12:1 in that scenario.

 

[engineermike]

... Cylinder pressure considerations are a moot point at the 1000 hp level in these motors because the hardware has proven to be very durable...

Its hard to argue that the super-high cylinder pressures are hard on parts when so many are out there making this power on them. But keep in mind from Ford engineering's perspective that long-term reliability is paramount in even the most extreme operating contexts. These are the same people who weren't happy with the deflection they were seeing on regular Coyote con rods in the DarkHorse so they upgraded to Predator rods. I imagine you've have to pound on a DH for many hundreds of thousands of miles for this to ever show an issue. And look how overbuilt the Predator is to make "only" about 650 rwhp. I guess my point is that Ford would never allow 12/1 compression and 20 deg timing at 1.6 load due to the high cylinder pressure and the stresses that causes, but their standards for reliability are much more stringent than ours.

 

[furdfan2018]

Its hard to argue that the super-high cylinder pressures are hard on parts when so many are out there making this power on them. But keep in mind from Ford engineering's perspective that long-term reliability is paramount in even the most extreme operating contexts. These are the same people who weren't happy with the deflection they were seeing on regular Coyote con rods in the DarkHorse so they upgraded to Predator rods. I imagine you've have to pound on a DH for many hundreds of thousands of miles for this to ever show an issue. And look how overbuilt the Predator is to make "only" about 650 rwhp. I guess my point is that Ford would never allow 12/1 compression and 20 deg timing at 1.6 load due to the high cylinder pressure and the stresses that causes, but their standards for reliability are much more stringent than ours.

All so true.

 

[K4fxd]

Ford builds them to last through warranty even if the owner fires up the car, backs out of the driveway and floors it.

 

[80FoxCoupe]

Ford builds them to last through warranty even if the owner fires up the car, backs out of the driveway and floors it.

I worked with a guy at one point that bought a Suzuki Hayabusa. For the first 6mo of ownership he would bring people out to the parking lot to show off the bike. He would start it cold and put it immediately on the limiter. I'd laugh my ass off everytime he did that.

 

[engineermike]

… to show off the bike. He would start it cold and put it immediately on the limiter. ..

Seen it too. For the life of me I can’t figure out what this proves.

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