should i run a 9:1 compression on a daily driven sr20??? help!!

[Yousefelk]

hey guys i getting ready to build my sr20det s13 black top, im going to be using a gt2871r turbo and looking to make 400whp, my question is it ok to be dailying an my car with the 9:1 compression or should stay stock?? thanks

[Kingtal0n]

hey guys i getting ready to build my sr20det s13 black top, im going to be using a gt2871r turbo and looking to make 400whp, my question is it ok to be dailying an my car with the 9:1 compression or should stay stock?? thanks

most people will recommend it, if you are going into the engine anyways, might as well give it a slight bump. Just be safe, run more fuel than necessary, and use an EGT gauge for final timing numbers.

[jacobs13]

most people will recommend it, if you are going into the engine anyways, might as well give it a slight bump. Just be safe, run more fuel than necessary, and use an EGT gauge for final timing numbers.

Run more fuel than necessary?

OP youll be fine. I as alot of others have higher comp ratios on our dd's. Mine is 9:5:1 and havent had a problem, just made sure you have a appropriate tune.

[Yousefelk]

so your saying its ok to run 9:1 compression as a daily, like a legit daily, i drive about 70 miles a day and would like to get on it. and race a few mustangs and corvette on the highways.

[slider2828]

Depends on your turbo setup. I don't condone street racing but if you are going high end to top speed power your engine ain't going last long.

[rbs14kouki]

Tune it right with the right fuel and it will last forever

[Kingtal0n]

Run more fuel than necessary?

OP youll be fine. I as alot of others have higher comp ratios on our dd's. Mine is 9:5:1 and havent had a problem, just made sure you have a appropriate tune.

lol I guess that was a bit ambiguous. I was thinking that most cookie cutter tunes turn out around 11.8:1 and I was thinking he should be around 11.2:1 on 93 octane at 18~psi and 380rwhp-400rwhp under 7000rpm with 12* or less of ignition timing peak rpm, and as little as 9* around peak torque. Was that better?

The truth is, higher compression always means more power, more fuel economy, better response, etc... EVERYTHING gets better. the trouble is, as you add boost, the safety net on pump gas dissipates exponentially, and it cannot be compensated by ignition timing alone, there is a point at which you simply need to increase the octane of the fuel to be on the safe side.

The fine line I am talking about may be situational; the engine may run fine most days, but say one day you are going uphill, and it is exceptionally hot outside... if you have a bulletproof headgasket, you might be pitting up your pistons without realizing it. And you wont see the damage for months, perhaps years later, when it finally comes apart. That fine line is often crossed without any indication.

[bardabe]

^^^ and that happens because of a bad tune. as long as the tune is good you will never have worries. I've built a good amount of 9:1 daily driven Sr20's with both GT30 and GT28 turbos without a problem.

[Yousefelk]

Ok so as long as I have a good professional tune I should be fine. I have an apexi power fc and have some decent tuners out here in Memphis tn. The main thing is that I don't want to go back and spend more money fixing a blown motor. So what do you guys think?? Should I go for it??? Oh keep In mind I have to daily this car 60 to 70 miles a day!!

[fliprayzin240sx]

Think of it this way, higher comp piston, less boost you'd need to make the same amount of power as stock compression with more boost.

[Crondy]

Think of it this way, higher comp piston, less boost you'd need to make the same amount of power as stock compression with more boost.

then what's the point of lowering compression on a turbo motor? safety?

[Kingtal0n]

Think of it this way, higher comp piston, less boost you'd need to make the same amount of power as stock compression with more boost.

But at the same time, with less compression he can make more power with more boost on the same octane. for peak numbers, 8.5:1 > 9:1 @ 93

then what's the point of lowering compression on a turbo motor? safety?

Its part of tuning. Your ignition profile, desired power level, brake mean effective cylinder pressures, peak cylinder pressures, the design of the combustion chamber, the size of the ports and chambers and valves, even the valve lifts and shapes, spark plug location, piston surface design, the space between the head and the block, the shape of everything the design of everything and of course... the desired fuel to be used... it all comes into an equation that requires testing and repeated modification to get the most from an engine.

For us, since we are generally not doing that testing/shaping- thats the engineer's jobs- the decision to go with this or that camshaft, this or that turbo, this or that intake, this or that piston... is arbitrary. We buy the parts, assemble it, and pay someone else that really has no clue how the engine was designed or tested to tune it. That's how accidents happen.

[Kingtal0n]

then what's the point of lowering compression on a turbo motor? safety?

Ill throw you an example of a situation in which lower compression would be desirable.

I have an engine here, 2.0L @ 7000rpm. It is 11:1 compression, and non turbo design. It makes 150 horsepower as it sits.

I decide to turbocharge it. Initially, I just run 7psi of boost, power output rises to 225 horsepower, but it blows it's head gasket quickly. I start asking around online, and people are telling me that it happened because of the ultra high compression.

Now, what they dont tell me, is that I could have raised the octane, or cooled the mixture somehow (methanol injection) or put an EGT gauge on it and dialed back the timing on my desired fuel octane until it started to spit unburnt fuel into the exhaust- possibly preventing head gasket failure, which occurred due to a pressure spike in this case. One of the jobs of the head gasket is as safety net for pressure spikes that would otherwise damage an engines components. Without being able to escape via the head gasket, those pressure spikes will just put holes in something else, such as a piston.

So what do the people recommend online? Lower your compression, they say. Indeed, by moving from 11:1 to 9:1 I am giving myself more headroom for mistakes and poor tuning, and by tuning I am referring not just to the fuel and ignition maps but to everything, including the design of the engine itself with regards to turbocharging. (Since this engine was not initially designed to be turbocharged, lowering the compression may not have the same benefit as it would on a similar displacement engine with components designed with turbocharging in mind.)

Now my engine makes only 200 horsepower at 7psi, and I lose 4mpg highway, and I lose some throttle response. But now I can also raise the boost to 14psi and make 300 horsepower where before that would have been impossible at 11:1 compression with 93 octane.

The lower compression allows me to run the engine safely now, despite it not being optimized... I might still be missing 10-25% of my horsepower that I could have had were I one of those engineers with all those cool test equipments, but at least it runs now without blowing up.
And I will never miss that power because, you never miss what you never had.

[nissanfreak317]

KingTal is right on point with his reasoning.

A lot of guys think that higher compression is the best solution but in reality it isn't always the answer. Exactly the reason that I'm running 8.5:1 on my GT2871R build.

[jk26]

Ill throw you an example of a situation in which lower compression would be desirable.

I have an engine here, 2.0L @ 7000rpm. It is 11:1 compression, and non turbo design. It makes 150 horsepower as it sits.

I decide to turbocharge it. Initially, I just run 7psi of boost, power output rises to 225 horsepower, but it blows it's head gasket quickly. I start asking around online, and people are telling me that it happened because of the ultra high compression.

Now, what they dont tell me, is that I could have raised the octane, or cooled the mixture somehow (methanol injection) or put an EGT gauge on it and dialed back the timing on my desired fuel octane until it started to spit unburnt fuel into the exhaust- possibly preventing head gasket failure, which occurred due to a pressure spike in this case. One of the jobs of the head gasket is as safety net for pressure spikes that would otherwise damage an engines components. Without being able to escape via the head gasket, those pressure spikes will just put holes in something else, such as a piston.

So what do the people recommend online? Lower your compression, they say. Indeed, by moving from 11:1 to 9:1 I am giving myself more headroom for mistakes and poor tuning, and by tuning I am referring not just to the fuel and ignition maps but to everything, including the design of the engine itself with regards to turbocharging. (Since this engine was not initially designed to be turbocharged, lowering the compression may not have the same benefit as it would on a similar displacement engine with components designed with turbocharging in mind.)

Now my engine makes only 200 horsepower at 7psi, and I lose 4mpg highway, and I lose some throttle response. But now I can also raise the boost to 14psi and make 300 horsepower where before that would have been impossible at 11:1 compression with 93 octane.

The lower compression allows me to run the engine safely now, despite it not being optimized... I might still be missing 10-25% of my horsepower that I could have had were I one of those engineers with all those cool test equipments, but at least it runs now without blowing up.
And I will never miss that power because, you never miss what you never had.


Kingtalon I assume drag international is no longer??? Are you still tuning?? Wanna get my rb tuned in acouple months after its all done

[Crondy]

thanks for the info and example, gave me some more insight to where i want to be.

[Kingtal0n]

Kingtalon I assume drag international is no longer??? Are you still tuning?? Wanna get my rb tuned in acouple months after its all done

Yes, I will gladly tune your engine, and yes I work solo these days. I am located in South Florida still, just shoot me a PM anytime, thanks.

[Def]

9:1 is fine for a street engine. I almost went for it on my track engine SR20 build, but I feel the engine is already knock limited in how much boost I can really run on track with the huge heat amounts, so I'd get more power by being able to run more boost vs. the higher compression ratio.


That said, the thermal efficiency difference between 8.5 to 9.0:1 is not that huge. A few percent max. A few percent more boost on top of say 1 bar is less than 1 PSI.

[S13 curtis]

9:1 is completely fine and is still a very low compression ratio.

On the evolutionm forums (i know its a different engine) but people build 10:1 motors with 28-29psi on 93oct. You just need a good tuner that knows what they are doing. Any motor I build for now on will be 9:5.1 or higher. Also when your out of boost the car feels alot more responsive.