Compression Ratio Choices for a 2.3

[9 T GSX]

I guess as technology, build techniques/processes, and better materials come out as time goes by… the way we think about compression ratio on these engines might need to change too, or maybe I’m just confusing myself.

For years I always thought lower compression was the “safe” DSM move:

• Wider, more forgiving tuning window
• More timing headroom before knock
• Smoother operation and less stress on parts
• Lower chance of random knock events
• With the tradeoff of slower response and needing more boost to hit a given power number

Since I’m not building this car for drag racing, the “needs more boost” part doesn’t bother me. But wanting to make a modest 700HP " seems kind of silly with everyone chasing 4 digit HP numbers today" I care more about drivability, reliability, and consistency — especially on pump gas.

On the other hand, higher compression gives:

• Better response
• Faster spool
• More torque everywhere
• But a much tighter knock threshold, especially on 93 in hot weather
• Will get to my HP number with less boost required

Here’s where things get tricky for my setup:

I’ll be running all types of fuel depending on the situation —93 pump, E85, and 110+ leaded race fuel.

The engine will more than likely be a 2.3 stroker, paired with a BorgWarner EFR 8474, in Florida. I’m currently talking with Buschur about building the motor, and they’re recommending 10.5:1 compression, which seems like what the car manufactures are doing to turbocharged cars nowadays??

That seems high for a car that will see a lot of 93 and mixed‑fuel use. My gut says no more than 9:1, especially with a big turbo that already makes a ton of cylinder pressure.

So I’m trying to figure out if the old DSM logic — “keep compression low if you want reliability” — is outdated now that we have better tuning, better materials, better fuels, and a turbo like the 8474 that’s extremely efficient.

Is 10.5:1 actually reasonable for a mixed‑fuel 2.3 with an 8474? Or is something in the 8.8–9.0 range still the smarter choice for a street‑biased build that needs to survive on 93?

Then you have videos from Jay that says this.

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Thoughts??

 

[GIB]

Just an example; I'm running 8.5:1 with a 2.4 stroker and FP 68HTA and stock cams. I start to hit boost at about 2800 RPM. I'm running 93 Oct. I chose that ratio because I'd hit boost so soon. I didn't need much off boost response.
Maybe with a larger turbo and cams and hitting boost higher in the RPM range, more response off boost is needed.
What about running something close to 9.5:1?

 

[diambo4life]

What Jay said there is dead on. For a DSM that will see pump 91 or 93, keep it at 9:1.

 

[9 T GSX]

Thanks for replies, I still want to research more as I haven't committed to buying the pistons yet.

 

[Brandon Mau]

Buschur is stuck in his ways when it comes to what he sells. Not putting him down in anyway he is the O.G who started the aftermarket stuff for us. Out of all the engine builders I’d probably say his are the ones you’ll least likely hear any horror stories. I don’t think you can go wrong with 9:1 or his recommended 10:1 ratio.

The big question is and I’m just going to assume this is a manual car that will not be a dogbox? That will be your biggest limiting factor shooting for that 6-700HP on the stroker more so than the 1 point of compression. The tune will need to be soft to limit the torque and avoid shredding the teeth off the gears. As long as you have a good reputable tuner or you yourself have a good grip on tuning 10:1 comp will be just fine. Switching between all those fuels you’ll either have a flex tune or multiple maps no idea what ecu you’re running. Just my 2 cents

 

[Canadian_CD9A]

Higher compression does not speed up spool, it just makes more torque off boost, as mentioned. If it’s going to be run on pump gas for any appreciable amount of time, just play it safe at 8.5:1 to 9.0:1 and let the turbo handle the compression.

And yeah, this thing should never see factory Mitsubishi transmission gears. They would be vaporized in very short order with the 8474 on a 2.3. This is a recipe for an easy 500 lb-ft.

 

[9 T GSX]

Yeah car is a manual and has a freshly rebuilt TRE transmission with all usual upgrades, I agree on the soft tune to help with transmission life. Think for now I will run this transmission and see how long it lasts, still have awhile before it will be up and running, "hoping I got that one transmission that defies all logic " dogbox is always a option later.

 

[We're on Boost]

I guess as technology, build techniques/processes, and better materials come out as time goes by… the way we think about compression ratio on these engines might need to change too, or maybe I’m just confusing myself.

If you are confused, it could be because so many of the newer model turbo cars are using much higher compression ratios (like 11:1) than they used to.
But that's mainly because they are using direct injection rather than port injection, and they've got some other optimizations that are designed to go along with the direct injection.

I did some checking though, to see what the higher performance current models have for compression ratio. Checked only on BMW because all their cars are turbo now.
Turns out the BMW models that use 11:1 are the "standard" models. Basically the non-M models. Still turbo. But not the highest performance.

But the M models are using 9.3:1 compression ratio.
And they are only doing around 20 psi of boost on the M cars.

Anyway aside from all that, I'm a fan of 9.0:1 when you are talking a wide range of fuel mixes, 9.0:1 is what my car is, and I've been having a blast with it for 10 years now.

 

[Tyeler18]

I've had a buschur 2.3 at 9:1 in my evo since I purchased it in 2013, i've put 60k on it and it was installed prior to me purchasing it. FP black car making north of 600hp and it's been dual map 91/e85 and now full flex on a standalone. It's been absolutely bullet proof and the reason I've swapped majority of my cars to 2.3s because of how awesome the powerband is. My mighty max has a 10.5:1 2.3L in it and the out of boost response is awesome. It's only on an evo fp green so it's not an apples to apples comparison but it makes good torque out of boost which for a street car is really nice to lug it around with. I do generally run it on ethanol but it's full flex and it just knocks it down to wastegate pressure with anything less than e20 in it. I do like the bump in compression for a street car setup I just wouldn't run a ton of boost on it with 93 or less in the system.

 

[9 T GSX]

If you are confused, it could be because so many of the newer model turbo cars are using much higher compression ratios (like 11:1) than they used to.
But that's mainly because they are using direct injection rather than port injection, and they've got some other optimizations that are designed to go along with the direct injection.

Good point I totally forgot about the direct injection aspect. From the comments so far it seems the wiser choice is stay around the 9:1 range.
Tyeler18: What is the Horsepower on the mighty max and what is the max pressure you have ran it on. any issues at that level?

 

[Tyeler18]

Good point I totally forgot about the direct injection aspect. From the comments so far it seems the wiser choice is stay around the 9:1 range.
Tyeler18: What is the Horsepower on the mighty max and what is the max pressure you have ran it on. any issues at that level?

Haven't had it on a dyno and likely wont, I also haven't looked at airflow numbers on it because I honestly don't really care. It blows the tires off with anything over 8lbs of boost so it doesn't load the tire the same way an awd car does, but my summit van with a dsm fp green at 24lbs flows around 47lbs/min, the evo green tends to flow a little better and I run the truck around 21lbs just because I'm out of injector there. It only makes 14lbs on wastegate which is where I run pump gas. I do plan to max the turbo out I just need to upgrade the pump, injectors, and tires

 

[bling5tatus]

My vote is that you do things to make the engine responsive down low while still having lower compression. Jay is right IMO (as he usually always is).

This means:
* tuned ignition timing table for YOUR SETUP, not someone else's timing table.
* small port cylinder head from a 2G with a manifold that has small ports.
* appropriate cams - not huge because you don't need huge cams
* appropriate turbo - not huge hotside because you don't need a huge hotside
* appropriate fuel - e85 works great and is available everywhere
* water to air intercooler to have short piping and small pressure drop across the core
* unrestricted exhaust setup - 3" or larger which most everyone has at this point

If you do those things you won't even really care that the CR of the pistons is lower than what is considered modern. It's not worth grenading an engine over this.