Effective Compression Ratio for P80 Engines

Ryan — screen name callahanoffroad — did some math. What math? Why on this green Earth would someone do math outside of a school or a bank? Well, math to determine the maximum compression ratio for the three common 1993-2000 Volvo inline-5 cylinder engines.

What he came up with is a lookup table that takes two things: your boost and your engine and spits out your compression ratio. Alternatively, if you have any two you can get the the third. I take it you know which inline-5 you have, so this whole thing gets pretty simple pretty fast ?. (Until it doesn’t, when you start throwing altitude/AFR or octane in the mix.)

So why is ECR important?

It’s important because knowing what these numbers are means you can find your safe boost limit without going over, or if you are over, going to a tuner who will help you get back on the safe side.

Hello friends!

I was pondering what the safe turbo boost limits for each model were and it lead me to doing a little bit of math on everyone’s behalf so that you can have an actual reference chart to go by. 

I have calculated the Effective Compression Ratio (ECR) of the three available P80 Volvo Engines: 2.4L Naturally Aspirated, 2.4L Low Pressure Turbo, and 2.3L High Pressure Turbo. These numbers would be correct for any 5 cylinder Volvo engine.

This chart does not take into account tuning tricks like running lean to heat the mixture or running rich to cool the mixture. The optimum turbo efficiency or turbo heat output. It is just a general chart for quick reference.

If you’d like to do your own math here is the formula:

Square Root of: ((Boost Pressure in LBS + 14.7) / 14.7) * Compression Ratio = ECR

The Max Safe ECR for US Gas is as Follows:

91 Octane 12.1:1
93 Octane 12.5:1

2.4L Naturally Aspirated Turbo

Before I begin this one, I do want to say that it is mathematically possible to turbocharge a N/A car using a LPT setup. 
Stock Piston Compression Ratio and ECR = 10.5:1

4lbs of Boost = 11.84 ECR
5lbs of Boost = 12.15 ECR
6lbs of Boost = 12.46 ECR
7lbs of Boost = 12.75 ECR (not safe without tuning)

2.4L Low Pressure Turbo

Stock Piston Compression Ratio = 9.0:1
Stock Boost = 6 psi 
Stock ECR = 10.67
8lbs of Boost = 11.19 ECR
10lbs of Boost = 11.66 ECR
12lbs of Boost = 12.13 ECR
13lbs of Boost = 12.36 ECR
14lbs of Boost = 12.58 ECR (borderline unsafe/unsafe without tuning)

2.3L High Pressure Turbo/T5/R

Stock Piston Compression Ratio = 8.5:1
Stock Boost = 9.7lbs
Stock ECR = 11.33:1

12lbs = 11.45 ECR
13lbs = 11.67 ECR
14lbs = 11.87 ECR
15lbs = 12.09 ECR
16lbs = 12.29 ECR
17lbs = 12.48 ECR
18lbs = 12.68 ECR (not safe without tuning)

Volvo 850 5 Cyl 2 3 L - The 5-cylinder fits much better in the compact transverse engine compartments of modern cars than an inline-6, but provides more power than an inline-4.
The 5-cylinder fits much better in the compact transverse engine compartments of modern cars than an inline-6, but provides more power than an inline-4.

These are purely mathematical figures they assume a 14.7:1 air fuel ratio and as I stated before do not take into consideration temperature, altitude, superior intercooling, lower operating temperature, tuning for a rich environment to lower detonation temperatures, timing tricks, etc. Another interesting aspect that I have not calculated here is Effective Dynamic Compression Ratio, which can actually lower these numbers significantly. But again this is a really good back of the hand chart to go by. 

What’s most interesting is the definitive proof that lower compression ratio motors can handle extra boost and will inherently make more power at the higher boost levels as a result. Hope this helps anyone who is wondering how much boost they can safely run in their motor!

Forum thread: Effective Compression Ratio for P80 Engines

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