Will Forced Induction Potential be Limited for New Engine?

[sdot]

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?

[HKz]

/insert theorycrafting

[RToyo86]

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.

[DarkPira7e]

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

[Yoshoobaroo]

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.

[HaXx]

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

[R2]

Originally Posted by sdot

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?

Who is reporting the compression ratio is 13.5:1?

[Ohio Enthusiast]

Originally Posted by R2

Who is reporting the compression ratio is 13.5:1?

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

[SUB-FT86]

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.

[Tcoat]

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!

[86MLR]

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

[Goingnowherefast]

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.

[Irace86.2.0]

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

I don't know what new, anti-knock tech might be utilized on this new engine, but the 13.5:1 CR is pretty high for boost. Boost limits will likely be lower than the limits for the FA20D, but this engine might have new tech and the benefits of the extra 400cc of displacement and the higher compression to make the extra power, so perhaps there will be modest power gains or neutral gains on pump gas. Even if the static compression is high, the dynamic compression can be lower.


On E85, people are running really high boost levels for the FA20D without issue, and I am not aware of what the boost limit is on the FA20D or what it could be on the FA24D. I think anyone looking for 500whp or less will find that the higher compression isn't an issue, and that it is an advantage for faster spooling, so more power across the rev range, along with better fuel economy over a conventional booster car. The downside will likely be much more heat from the higher compression, so tracking a boosted car might require more cooling issues, and there will likely be more variance between weather conditions, as timing gets pulled on hot days.


The popularity of modifications typically follow an expense bell curve, in which, the most basic and least expensive mods are the most popular and the most expensive mods are the least popular. Diving deeper, the majority of people will do a catback then there are the ones doing full exhaust and a tune with maybe a CAI then there are the ones adding E85 to that setup then FI on pump gas then FI on E85 then FI on a built motor w/ or w/o E85, but typically it is with E85, and in that subset, there is a spectrum to the depths of the builds from piston/rod motors to built heads, closed deck, large stud setups. With that said, adding exhaust mods and E85 should yield decent gains with the high compression. FI on boost gas with the FA24D will have modest gains over the FA20D, if any. FI on a stock block with E85 should net 400-450whp with the higher compression and larger displacement and proportionally stronger motor unless they went cheap like using the same size rods as the FA20D (unlikely). This is going to be the sweet spot because the extra 50-100whp over the prior limit means the car is now in a new power bracket capable of competing with much more powerful cars. Anyone going for a built motor will likely want 500+whp (unless they just want more reliability or better pump gas gains), and they will likely want to drop compression to 10.5:1 on E85 (or something like that), so they can raise boost, while significantly lowering heat, whether that is to hit a power level or reliably track the car.

[Spuds]

Originally Posted by Goingnowherefast

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.

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