Will Forced Induction Potential be Limited for New Engine?
 

Hey guys,

Looks like the compression ratio for the new 2.4L is 13.5:1, which is crazy high. That is even higher compression than the ND Miata, which only has very low boost FI kits available. I think power mod potential will be pretty limited for the new twins unless people do lower compression mods. Any thoughts?

/insert theorycrafting

If I had to guess it will be a fueling issue more than anything.
E85 will probably be needed to run boost levels the old stock engines could handle on pump.

Meth injection, e85, race gas, etc. Some will find success, some won't. Everything new has a trial and error process and we won't know until some people toe the line and cross it repeatedly.

Data acquisition is time consuming, and takes a lot of testing- we don't even have 1 person who can say they own the car, so we are a long way from having answers for anything

Turboing a High compression car used to be sketchy. You had a fixed lift valve train, variable duration at best, pretty imprecise port injection and a slow 8bit ecu. We used to have to richen the mix to ensure we didn’t pre-ignite because injection systems just weren’t that precise.

Now we have super fast 64bit ECUs with literally over 10X more computing power than before. They can react ridiculously fast to changing engine conditions. We have variable lift AND duration which we can use to alter the CR on the fly. We have incredibly precise direct injection systems that can have the car run within half a percent of a stoichiometric reagent mix (14.7/1 air/gasoline).

It’s a whole different ballgame now. With modern ECUs, valvetrains, and injection systems you can force feed a high compression engine, as long as you’re not tuning them like we’re in the 90s.

Nothing wrong with moderate/light boost, I'm set to 9psi and that's about + 100whp, which is fine for me.

Who is reporting the compression ratio is 13.5:1?

Subaru - https://media.subaru.com/pressreleas...s-global-debut

I think low boost and e85 full time will be safe. I've been running 12-13 psi on 93 octane(310whp/255wtq) for 3 years and 4 months and I redline it daily with 0 problems since. I would think 7-9 psi with my turbo(tdo5-20g)plus e85 sounds doable.

The compression ratio on the turbo version of this issue is only 10.6:1.
Won't take much to over boost at 13.5:1.
I see a bunch of guys saying the rods in these engines are crap in the future!

The fa20 boosted up easily, so will the 24

It will be the muppets with poor tuning abilities or pushing to hard that will be crying foul

It is not a matter of can you, people will either turbo it or not

In the end you can turbocharge anything if you use common sense and good mechanical principles

The "purity" bandwagon will love telling people how they wrecked this car by adding more power though

So simply having high compression ratio doesn't make it a bad platform for forced induction. There is a couple factors to consider, but in general you simply have to make some changes to higher compression engines when turbocharging them. The basic principle here is that higher compression engines are more likely to experience knock at an earlier point than a lower compression engine. While this is obviously not a good thing, it simply means that you have to take a different approach to doing things.

For instance, the ND Miata has a 13.0:1 compression ratio. Because of this, aftermarket turbochargers need to be sized accordingly. The higher compression ratio allows you to run a much larger turbocharger than you otherwise could. The reason that you want to do this, is that running a larger turbocharger on less* boost is more efficient (thermally) than a smaller turbocharger. Since a higher compression motor is more prone to knock/detonate, keeping the charge air cool is critical. Using this tactic, plus modern engine control strategies and ethanol (when possible) to raise knock resistance, I'm sure the new car will boost just fine.

https://www.ft86club.com/forums/show...6&postcount=56

My understanding is that larger turbos lead to greater turbo lag than smaller turbos because of the increased inertia they need to overcome.

Yeah that is the case. Luckily with higher compression you get better spool and transient response. This is why you can run larger turbochargers than you otherwise would be able to (given the same spool).

I’m sure inertia of the turbine is a factor, but I believe more of a factor is the housing diameter. In flow dynamics, the radius of the cross section is the most important factor, and it is why pressure increases and water/air shoots out faster when someone reduces the diameter of the tip of a hose. Similarly, it takes a lot of air to raise enough pressure in a big turbine to force air to move rapidly through the turbine. And like our example with the water hose, the small turbine reduces the diameter, which means it takes less air from the pump (engine) to accelerate air through the turbine.

I guess my point is that if you actually measure the weight of the turbines, you might find that they aren’t too different in weight. In fact, I bet you could improve transient response with some exotic, lightweight titanium turbine that weighed as much as a small turbine, and maybe you would shift the spool time slightly lower, but I bet the large turbine would still struggle to spool as fast as a small turbo.

But you are correct, that such a problem is one of the reasons why compression is dropped and a smaller turbo is used. Goingnowherefast is correct that a higher compression motor will spool the turbo sooner and provide enough low end torque on its own, but this is not as desirable in the industry from the factory. Look at sports cars like the Supra that have a small turbo for immediate torque, but that gases out up top, which gives high torque, but low, relative horsepower.

Top Gear did a comparison of the Evo vs Lamborghini to show how capable the Evo was, but to our topic of interest, they did a demonstration of the turbo lag. Actually, it was more a demonstration of boost threshold, but the point was that the Evo they were testing was a 400hp model with a large turbo and really low compression, so the delay was terrible. They recommended getting one of the less powerful models. See part 2:

https://youtu.be/Ees2aZcDUn8
https://youtu.be/VVt1IjIdLxY

The ideal thing is to use an electronic motor to torque fill down low along with a bigger turbo. Some have used an electronic motor to spool the turbo to help with boost threshold and turbo lag when a larger turbo is utilized in lieu of a supercharger and compound boosting.

The Muppets will prevail!

So if I can do 10lbs on the FA20 I should be able to do 20 on the 24 since it is larger.

Can't afford the turbo an the tune at the same time but I should be OK to run a little boost until I can tune.

There is no need for premium gas it is a scam!

I read on Facebook that the higher the compression ratio the better it takes boost so I am cranking it to 20.

My 1999 Civic need all sort of things so this car will as well.

I also thought of this. I think in order to stay cheap they just put the same cylinder heads from the old fa20 and this resulted in the compression this high:clap:

If they did that then my Harrop kit would fit on the FA24 swap or second gen car.

For what it's worth supposedly the FA24 has hit 500+ on a stock long block with ARP head studs. I think anything over 300 on a 2800lb car is already plenty quick.

https://forums.nasioc.com/forums/sho....php?t=2907007

I bet a bag of donuts it doesn't match the MPH or trap that a 500hp car of that weight should do

Copies of dyno runs to prove power gains is lame

Excuses as to why they cannot do a run are usually many, and lame

Are you saying they are lying about the power? I see no reason why they would lie or why the motor couldn’t hit that power level.

I am not a tuner, so I can't say this for certain, but if E85 can protect the car then there may be more gains at the same psi for power because compression is higher. I have been told by a tuner, and I have read tuners write here that lowering compression is not necessary on E85 at 12.5:1 when people were asking if they should lower compression when they build their motor.


Obviously compression should be lowered if all someone has access to is pump gas, so they can maximize their potential. The lower the compression the more power the car can theoretically make, but there is obviously a trade off with low-end/off-boost power, especially as compression drops and as the turbo compressor gets larger to accommodate more boost.


For perspective though, the FA24F in the Ascent makes 277tq. The FA20D makes 150tq, so the Ascent makes almost double. The rods on the FA20D hold to around double the output, so 250-275wtq. I wouldn't be surprised for the torque to be close to double too, so around 500tq.

To be clear that is the FA24F they are talking about. Just because it is the same base engine code does not mean it is the SAME engine. Who knows what they changed for internals.
The FA24F has a compression ratio of 10.6:1. That is a huge difference that can not be ignored.
We will know what they can do eventually but saying they did this that or the other thing to the F is almost meaningless.

Ah I didn't realize it was different. My point was just that even if that dyno run was exaggerated, it seems reasonable that the engine could be hitting 3-400HP on E85, which in my mind would be enough. I guess we'll just have to wait like 2 years to find out

The FA24F is a low compression, direct injection only, turbo engine. It brings it torque and power on way down low in the rev range and is for all intents and purposes a truck engine.

The FA24? is a high compression, direct and port injection NA engine. It brings it's torque and power in at high revs and should be a very nice sports car engine.

The FA24? should work really well on E85 but looking for 300-400HP out of it with a tune and some bolt on's is very optimistic.

What they can do to the one doesn't mean much when compared to the other.

OH and they were running 22PSI on that heavily modified FA24F to get that 500HP. Try doing that on an engine with a 13.5:1 compression ratio and you will be finding parts of the block and rods for several blocks around you!

Yeah you're right. I didn't think that the engine would have such a different compression ratio. Although I guess that means I won't be as tempted to drop big money on a supercharger so yay?

some engine pics from Motortrend

https://www.motortrend.com/uploads/s...aru-BRZ-45.jpg
https://www.motortrend.com/uploads/s...aru-BRZ-46.jpg
https://www.motortrend.com/uploads/s...aru-BRZ-47.jpg
https://www.motortrend.com/uploads/s...aru-BRZ-48.jpg
https://www.motortrend.com/uploads/s...aru-BRZ-49.jpg

Yep. Looks just like the top of a Subaru engine with the Toyota fuel system.

thanks for not quoting all ten pics like everyone else here :bellyroll:

I could never understand why people do that.

Sorry, rereading what I wrote, I can see how that is confusing. Sounds more like ramblings that a clear argument.

What I wanted to say is that the FA24F could reasonably hit the values they achieved without having to make dyno manipulations, and I say that based on comparisons to the FA20D's stock power and "reliable" power potential and the FA24F's stock power. I was also saying that power could be even higher on the FA24D when boosted with E85 because of the higher compression, assuming the internals could handle it of course, but I didn't state that.


We don't know what is different besides the D4S and turbo system. We can make a guess, but we don't know. The FA24F comes with more power and torque than the FA24D, but the FA24D comes with higher compression, which has its own risks. What we do know is that the FA20F and FA20D have similarly sized rods, where the WRX/FA20F's rods are slightly larger/reinforced than the 86/FA20D's, and yet, the WRX comes with 268hp and 258tq, which is a lot more than the 86. Interestingly, the specs on the FA20F is not too far off from the Ascent with the FA24F at 260hp and 277tq. It seems like the rods on the FA24F would be similar in size to the WRX's or maybe slightly more reinforced, but according to the link, the guys are saying that the rods are a lot beefier. Does this mean the FA24D's rods will also be a lot beefier? We don't know.

Again, we don't know what is in the FA24D, but it is likely the higher displacement and higher compression means the rods might be reinforced enough that the power potential will be raised compared to the FA20D. I wouldn't be surprised if the car will handle 400-450whp from the 350whp that the FA20D can reliably handle.


I've heard this phrase before, "for all intents and purposes a truck engine." What do you mean? Are you saying that because it has a fast spooling small turbo for low end torque? Does that mean the MKV Supra has a truck engine too because its torque comes on fast down low and fades up top? Does this mean the Accord 2.0T has a truck version of the CTR motor because of the smaller turbo, decrease in horsepower, but increase in low end torque?

Typically a truck motor is heavy like cast iron vs aluminum to keep reliability high and cost down. Typically a truck motor is a low stressed motor by having less aggressive compression, less aggressive cam profiles and less flow to the heads. Typically a truck motor is low revving and high torque, which it achieves through an undersquare design, but even if it is not undersquare, there is a long stroke on the motor.

Let's consider the FA24F now. It has an aluminum block. For a turbocharged car, it has relatively high compression. The engine is undersquare, and at 86mm, the stroke is relatively short for a "truck engine". The motor revs high enough--higher than a truck engine typically would. I wouldn't call it a truck motor. I would just call it a typical passenger car motor--not a sporty car motor or a truck motor. Just saying. Seems like anything that is not a sporty car engine is a truck engine to some.

Oh, and they weren't running a heavily modified motor. Essentially, it was a stock longblock with ARP head studs--no internal upgrades--just bolt-ons. Yes, it had several bolt-ons, but there whole point was to test the limits of the stock block and stock injectors, which is why the upgrades are headers, turbo, FMIC, exhaust, intake, etc. The high compression on the FA24D is not necessarily a deal breaker for turbocharging, which is what I said earlier, that E85 is very good at protecting the motor from pre-ignition. Many people with access to E85 don't drop compression when building the motor when going for 500whp+. Several tuners here say it is not necessary to drop compression, as E85 deals with the issues. This could be true for 13.5:1 too if Toyobaru cleverly engineered the motor or if the larger pistons act as a heat sink or increased the surface area to aid combustion or some wizardry. The big thing will be if they reinforced the rods enough to handle the power. They obviously increased the rod size per this quote below compared to the FA20. They didn't specify if they mean the FA20D or FA20F, but I will assume they mean the FA20F. Regardless, the rod size on the FA20F wasn't too different to the rod size on the FA20D, and the FA20F and FA24F make similar power, so we could see much beefier rods on the FA24D, if we are following the trends. So maybe the engine won't go kaboom.

It is an engine designed to move a large heavy vehicle. Close enough.
You can bet that the version that goes into the WRX/STI will not be exactly the same.


Depends on how you define "bolt ons" I guess.
I do not consider a completely changed intake system as "bolt on" even if that is how it is attached. Pretty much all that remains stock is the block and basic internals.
FA24DIT OEM Ascent long block w/ ARP head studs
STI drivetrain
FP XR Blue 73HTZ twin scroll ball bearing turbo
Killer B IWG twin scroll header
Invidia J pipe w/ 3" catback
Cobb intake
Cobb FMIC
IBR BRZ intake manifold conversion w/ complete TGV deletes
Stock WRX ecu w/ Cobb AP & SD tuning software
Cobb EBCS
Cobb Flex Fuel kit
Hardwired fuel pump
Nostrum HPFP
Stock injectors
IAG AOS

Forced induction is only limited by the wallet and financial sensibility. Anything is possible but is it worth it?

I'm just saying I wouldn't discredit it by saying it is a truck engine. Even if that meant something that mattered, it can be tuned and modified like any engine. I've seen turbo diesel trucks run 9's. The Ford Raptor has the Ford GT engine in it with different turbos and other things. The line between a truck motor and a passenger car motor and a sports car motor can be fine, but like I said previously, usually it follows a certain pattern like the below quote about the LS/LT truck motors, specifically the L8T:

In the case of the FA24F, it is in a crossover, which is a light duty SUV or heavy duty passenger car, depending on your perspective. The towing capacity is fairly low, and the instruction booklet warns about towing too fast up a grade causing overheating because it really isn't designed for much. The motor is inline with most passenger cars and crossovers in build more than trucks, but again, I'm making a small point that I hear this repeated without much context behind it, and it just doesn't make much sense to me.

Yeah, so bolt-ons are everything that a basic novice with a few tools can add tot he car, which is everything up to things like a turbo or supercharger system. One of the reasons the 2JZ is so famous is because it can handle so much power with just bolt ons; add a larger turbo, larger injectors, larger fuel pump, a boost controller and boost cut controller and a full exhaust and the engine was pumping out 700whp all day long. Stuff that goes beyond bolt ons would be reworking the heads by porting and polishing then flow bench testing the work, or machining the block before installing a stroker kit, etc. Typically when someone is cracking open the longblock, the knowledge set on everything goes up a lot from understanding cam timing, torquing things in sequence, milling things to certain tolerances, matching parts to meet specs, etc. It is an entirely different game.

As to Prime Motoring, what they did was simple bolt ons, and what they demonstrated so far seems to be promising from the perspective only of the FA24F, and hopefully, some of the good news applies to the FA24D. To keep the longblock unmodified is a testament to what this thing can do in the FA24F. I'm sure the FA24D will be impressive in its own way, and I agree that the new STI with their version of the FA24F will be a beast, but I favor the fueling of the FA24D, as I doubt the STI will have D4S and that is really nice for simplicity (adding larger port injectors), longevity/reliability (cleaning the valves) and performance (modulating low duty, while having enough overhead for heavy duty by running large PI and stock/low DI).

For what it’s worth, I was boosted on a stock block & pump gas up to 9psi, and when I decided to have my engine built, I had asked to maintain the 12.5 comp ratio to keep low end throttle response even if it meant a lower top end at the end. As DD reliability was a key goal for me The builders/tuners indicated they could do it, but strongly advised against it to keep some safety margin, so we ended up going for 10.0 CR. E85 is not readily available in my area, best I have is 94 octane. I think the build ended around 21/22 psi on HB. So it will interesting to see if the R&D on the new gen FA24 will find ways to reliably go beyond milder boost levels on a stock block when E85 is not available. nitro possibly ? However unless the tranny was also beefed up (I am not aware that it has), then that will remain a weaker point once you go beyond that anyway. My sense is that there will be more people that sit perfectly happy with a tune and some bolt ons without FI than on gen 1, and the bolder folks that do go all out, well they will just continue to do so... :-)

Though it may have been fairly useless, gen1 "beefed up" the tranny 6th gear for the "additional power gains" in the 2017+ models, which was all of 5 HP. I'd like to guess that with a pretty solid torque increase, they would have had to do some kind of modifications to the current trans.

It would be great news indeed if it was further beefed up...

No doubt they will have to do something to keep the margin for error about the same but the 2017 "improvements" were far from earth shaking!

I agree haha. That's why I have no high expectations yet. The only real conclusion I can make here is that they will definitely modify the current transmission based on the power increase. How? We will see. But the stock tranny was impressive enough for a 200 HP car. Not too unusual, though, as it was used in the MX-5, RX-8, and others that weren't high powered monsters.

Subaru doesn't go crazy with their WRX trans, which has limits at around 400 wtq from what I've read. This isn't significantly different from the limits of our trans, and the wrx has a LOT more torque. Now I know that isn't an apples to apples comparison, it just makes me think that to save on cost, they might have some minimal updates to our trans as it can handle plenty to begin with.