Relationship Between Compression Ratio and Octane Value Seems Inaccurate

I’ve been building a fleet of custom cars starting in the early eighties, and I’ve managed to get their performance comparable to cars of the era. Barring maybe fuel economy, there is one big disparity I’ve noticed between the engines I’ve created and the real ones I drew inspiration from, and that is the compression ratios. All of my engines have needed much lower compression ratios verses their real-life counterparts in order to run comparably.

I considered this could just be an issue of my design methods, so I decided to put the engine designer to the test. I decided to replicate the infamous Pontiac Iron Duke Tech IV (specifically, the post-83 version with a 9:1 compression ratio), an engine which was not exactly a precision-built work of mechanical art. The engine designer, however, completely choked. In order to get the in-game engine to match the real engine’s power outputs, I had to set top end and fuel delivery system quality levels to +10 and had to retard the timing all the way to 0 in order to keep the engine from knocking, among other things. Once I turned the compression ratio down to around 7.4:1, I was able to get comparable performance to the real Tech IV while keeping the various engine parameters realistic.

I understand there’s a lot of factors at play here, but everything I’ve been seeing designing my own engines and replicating real ones seems to point to this relationship between compression ratio and fuel octane being off.

Its all down to the fact that automation is not a simulation of real life. There is technology and innovations that just cant be replicated in automation. This is just a limitation of the game more than anything else, and a decision the devs chose, because if they tried to make a direct imitation of real life engine design, you’ll end up with a 100 different sliders, buttons and choices that just swamp down gameplay and make the whole experience boring. For some that seems interesting, but just remember that the intention of this game is to be a tycoon game where you’ll potentially end up making hundreds of cars during one game. So certain aspects had to be streamlined for the sake of everyones sanity :stuck_out_tongue:


100 sliders? I want that!!! :joy:


It is right on the name… “tycoon game” not “engine and car designer simulator” :stuck_out_tongue:

In all seriousness, if you get the design with similar power curve and numbers, what is the deal with using less compresion tho? :man_shrugging:

Fuel economy figures in automation look very different in real life because of how the economy cycle and calculations are run, however, that method is more “accurate” than real life methods using for companies who can “cheat” on testing.


I know it’s not a perfect simulation, but I would think that one of the intentions of this game is to get an engine creator that gets pretty close to real life behaviors. I hope the devs don’t share this attitude of “its good enough dude its just a game lmao”.

Besides, this doesn’t seem to come down to oversimplification of the engine creator. All the engines I’m making seem to come in at lower compression ratios than hey should.

Thats not the point. The devs have to allocate time and effort to certain aspects of the game. Its not that the engine designer is an oversimplification, its just that there is technology and ways that engineers get round issues that arent present in automation, that would include ways to increase the compression ratio. Besides which, the engine designer is getting overhauled in the future after the grand campaign is sorted.

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Well, the engine designer IS good enough, its not designed to replicate real world engines, the tech 4 had many engine technologies that the game does not really use.

Personally, i dont want 10 pages of box selections and sliders just so i can have a number on a screen the same as a real life engine.


This game seems to tend to underrate what OHV is capable of, particularly in conjunction with pre year ~2000. I made an engine shockingly close to the 964’s 3.6 liter (needed a bit more quality than I really wanted as well as forged pistons which I’m not sure is correct) but then needed +10 quality fuel and valves to get anywhere near the Boss 302 engine.

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Something something quality = n years ahead of the “average car”. +10 for a Porsche sounds reasonable (I’m no expert). “Boss 302” could be almost any year and any output, mind elaborating?

Always remember kids,NET vs GROSS power. That’s why don’t you get near those engines. And solid lifters which are not in the game.
EDIT: I mean, muscle cars engines like the one in the Mustang



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I mean I was trying to recreate the engine from the Mustang Boss 302.

From wiki:
Manufacturer Ford
Production1969–1970 and 2012–2013

Only one of them has OHV.

The regular windsor D code (sort of middle performance) from 1964 took much less extra quality (I guess +3 valves are ok for a premium engine), so it seems like the game is just incredulous at OHV’s going much higher than 5K rpm.

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The OHV “deficit” has been addressed a number of times. It comes down to this:

OHV is so simple to engineer that even generous quality spam doesn’t put it into the same territory as basic OHC setups in terms of cost, production time, or engineering time. And furthermore, engineering familiarity which is intended to be in the campaings and which is NOT included in sandbox, would either eliminate quality spam or significantly decrease its need. In the 1940s and 1950s, OHV in game is actually pretty comparable to what was then available and achievable. But while many manufactrurers transitioned into OHC designs in the 1960s, some (particularly American ones) held onto said designs well into the 1980s and beyond.

Even in the 1960s and 1970s when most people complain about OHV’s inaccuracy, they are usually comparing to the memorable cars of those eras. What about the forgettable ones? Chrysler Slant-6s and detuned LA V8s that you will find in any base Mopar redline at about 4000 RPM. Same with Ford Thriftpower-6s and detuned Ford Windsor and Cleveland V8s in base Fords. Same with detuned GM straight-6s and low-end V8s. Automation OHV actually models those engines quite well with only minimal – if any – quality spam to top end. I recently made an AMC 232 straight-6 and had to NEGATIVE quality spam an OHV setup to get that engine to behave like its real life counterpart.

As for compression ratios, in general I tend to agree that compression seems way down across the board until the mid 2000s. But then I have to say, it don’t really care because it rarely makes for any real problem getting the important figures to where they should be i.e. power and torque.


One thing to add to all those responses is that if you are dealing with a complex system of parameters (count how many design parameters an engine has in Automation) trying to model reality, getting the majority somewhat right is pretty easy. Out of 40 or so parameters getting all but something like 3 right is still doable, all but 2 is damn hard, and all but one nigh impossible. You can think of it like one variable being used as a buffer for all mistakes in other calculations. So I think it is a lot more difficult than you’d expect to get that parameter right too! :smiley:

“its good enough dude its just a game lmao”

Have you ever worked on a big project? :slight_smile: In case you have not but intend to do at some point I highly recommend not being a perfectionist. 95% correct often is more than good enough, because those last 5% take 10x as much time than everything else.


This occurred to me last night when I was working up a 1965-1970 full size Pontiac for hooning around in BeamNG. The compression ratios might not be as inaccurate as you think due to Automationeers’ tendency to use very conservative and unrealistic fuel system and in particular carburetor tuning.

Not to boast, but I made a rather lengthy post about this in my American car realism thread. Basically, a carburetor with an AFR leaner than about 13.7:1-ish is pretty much never seen in road car applications. I found when I was building a faithful Pontiac 400 V8 that if you push the carburetor tuning into its realistic territory (I was using 13.0:1 on the high compression 325 hp 400 4-barrel), the disparity is a lot less or even eliminated. I managed 9.7:1 compression on my 325 hp 400 4-barrel which is reasonably close to the 10.2:1 compression of its real life counterpart.

Ignition timings on stock engines are also similarly not quite as people probably think. At idle, many engines will fire at or only a few degrees before top-dead-center (TDC). Comparatively, many Automation players have learned that more advanced ignition timing is virtually always better so I would guess many players’ engines are firing more like 10-20 degrees before TDC at idle.

Combine those two things and I would wager that compression disparity is a byproduct of min-maxing strategies in a video game rather than anything real.


Also remember our AFRs are kinda inaccurate and weird, and more accurately just represent general richness of fueling strategy.


The Boss 302 that matters was a Trans-Am homologation Ford Mustang with a 5.0 liter OHV V8 (not the same as the DOHC unit found in modern Mustangs), producing around 290 bhp (horsepower ratings were a bit imprecise back then, but the Boss 302 was rated conservatively for insurance reasons). It was sold only for model years 1969-70.

As for “10 years in the future”, 10 years later the related WIndsor V8 would produce 140 horsepower thanks to emissions regulations. The Boss 302 had some pretty trick stuff in it for 1969 (solid lifters, for instance) but nothing that was radical or unknown to Detroit engineers.

I tried to recreate a 1974 Opel Kadett 1200S, which has a 1,2 litre 58 hp OHV four and one single barrel carb. Using the same compression ratio as the real engine, even on the premium leaded gas the IRL equivalent was using, I found out that I pretty much had to drown it in fuel to avoid it from pinging to death. That made for a very thirsty car while the real counterpart was a fuel sipping economy car. If that means that the engine designer is bad? No, it just shows that when everything just is a crude representation of reality, with just a few parameters to change, it’s turning out this way sometimes, so it’s no complaints, just a reflection.

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