Thoughts on Built Engine Compression Ratios?

[Gpcalero]

I'm thinking of building my motor soon, so I can really kick up the boost. I know who I want to build my motor, but I don't know what compression ratio to go with...I'm torn.

What I know is, with less static compression (engine compression) and more boost compression, the combined dynamic compression makes a bit more power than an engine with more static compression and less boost compression with the same overall dynamic compression. I also know that it is safer and easier to tune this way with, but the lower static compression makes the motor less responsive until the boost kicks in.

What I see and read are, people who argue that 11:1 is better, or wished that they went 11:1 instead (two of my local buds). I don't really see why when, the higher static compression you have, the harder it is to tune, and more important the octane rating of the fuel needed.

I can see that heat soak would be an issue with lower static and the added boost levels, but I'd like to hear what other people think because like I said....I'm torn.

 

[CobraRed_96_GT]

Avoiding compression is kind of the old school way of building for boost. With modern efficiencies, variable valvetrain timing, tuning capability and the prevelance of e85 - going with lower compression is leaving something on the table a bit, and this is compounded when considering turbo's as they live off exhaust gasses and thus doubly benefit from compression.

It's not a huge gap, but I think the days of 8.5, 9:1 may be in the past. Especially with direct injection growing. It used to be you would design for low static (like the last engine I built, at 9.3) and choose your cams down the line based on your boost levels to augment dynamic (running) compression to increase if desired. But the necessity from cams is fading IMO.

 

[Scott8583]

I see your currently running a VMP TVS. Keep in mind you can only crank up the boost so much. I'm running 11.25:1 comp with 14psi of boost on E85... I could only see running lower compression with a large turbo making crazy boost.

 

[oldstv]

I'm running 11.5 at 15psi. The car is making mid 700hp and my tuner is telling me that should only run race gas at the track even with meth.

 

[03Steve]

Sig. Brother in law has a copy of the engine with a TVS in the 8s.

 

[Gpcalero]

Avoiding compression is kind of the old school way of building for boost. With modern efficiencies, variable valvetrain timing, tuning capability and the prevelance of e85 - going with lower compression is leaving something on the table a bit, and this is compounded when considering turbo's as they live off exhaust gasses and thus doubly benefit from compression.

It's not a huge gap, but I think the days of 8.5, 9:1 may be in the past. Especially with direct injection growing. It used to be you would design for low static (like the last engine I built, at 9.3) and choose your cams down the line based on your boost levels to augment dynamic (running) compression to increase if desired. But the necessity from cams is fading IMO.

Even so, lowering compression works as you've experienced, although with the coyote I wouldn't go anywhere lower than a 9.5:1 CR. I've even seen a coyote motor with a 9.0:1 CR, albeit with a large turbo. I can see the benefits though, less boost needed for higher static CR's.

I see your currently running a VMP TVS. Keep in mind you can only crank up the boost so much. I'm running 11.25:1 comp with 14psi of boost on E85... I could only see running lower compression with a large turbo making crazy boost.

You're right. I realize I'm limited by my small displacement blower but the little 2.3 still has plenty left in her. Once the motor is built I'll end up either with a larger blower, or....I might even go **gasp** turbo. Currently where I live E85 is scarce so that isn't an option for me right now

I'm running 11.5 at 15psi. The car is making mid 700hp and my tuner is telling me that should only run race gas at the track even with meth.

See, that's exactly the sort of pains I don't want to deal with. If I can safely get away with big numbers on 93 octane, then that's what I'm going to do. Maybe even go with a 10.5:1 CR

Sig. Brother in law has a copy of the engine with a TVS in the 8s.

Wow, a TVS in the 8s is no small feat!

 

[CobraRed_96_GT]

See, that's exactly the sort of pains I don't want to deal with. If I can safely get away with big numbers on 93 octane, then that's what I'm going to do. Maybe even go with a 10.5:1 CR

Ahh, I didn't know you were considering 10-10.5 as low compression. IMO that would be a nice sweet spot on higher boost for pump gas.

 

[Gpcalero]

Ahh, I didn't know you were considering 10-10.5 as low compression. IMO that would be a nice sweet spot on higher boost for pump gas.

Errr....lower than stock CR is what I meant. I know the Aluminator runs 9.5:1 CR and people make 800 rwhp out of them safely and I see others who build 11:1 cars who claim similar numbers or higher safely. I'm considering any CR that'll get me to my goal, I'm just looking for the optimum compression.

 

[corepwn]

Realistically you're going to have to turn down your setup to match the fuel no matter what CR you're using.

93 will only take you so far on a 302ci engine with boost or nitrous or NA. You can safely make pretty good power with it, but I'm not so sure that a 9.5 CR vs an 11.0 CR with the same setup isn't going to max out at around the same HP on the same fuel. I do know that 9.5 will take more boost to get to whatever that eventual number is though.

 

[CPRsm]

Unless you want to make more on pump gas, there is no reason to go lower.

 

[Aaron@JPCRacing]

Typically we stick with 11:1 compression engines.

It's free power, as long as you know how to tune them.
It's not that they are harder to tune, you just need to know how far is too far.

 

[Bgoins]

Its interesting to me to look at the compression ratio of factory cars with FI. I believe the latest GT500 was about 9:1, new M4 with twin turbos is 10.2:1 and a Mclaren P1 with twin turbos is about 8.6:1. Not too many running 11:1 especially without direct injection

 

[Blazer707@TBR]

If you are looking for all out power then the 11.1 stock comp is going to be the best bet.

 

[scholle]

Unless you want to make more on pump gas, there is no reason to go lower.

I'd like to see what could be done on pump 93. Seems that most folks just go with E85. I dont have that option unless I buy it by the drum.

 

[Gpcalero]

Typically we stick with 11:1 compression engines.

It's free power, as long as you know how to tune them.
It's not that they are harder to tune, you just need to know how far is too far.

If you are looking for all out power then the 11.1 stock comp is going to be the best bet.

Well I also want to factor in longevity. How long is that 11:1 going to last at 1000hp vs. 10:1 with 1000hp?

Sounds like 11:1 is the ticket though.

Its interesting to me to look at the compression ratio of factory cars with FI. I believe the latest GT500 was about 9:1, new M4 with twin turbos is 10.2:1 and a Mclaren P1 with twin turbos is about 8.6:1. Not too many running 11:1 especially without direct injection

Very interesting. Why do they go that route vs. upping the compression and using less boost?

Found a video Shaun made with an 11:1 motor making 1000rwhp, but on e85 though.

[video=youtube;QAl2qb7ZfvY]https://www.youtube.com/watch?v=QAl2qb7ZfvY[/video]

 

[techwerkz]

Haha I told you. I went 10:1 because I was running my/your TVS on pump gas extremely hard (72mm+5% OD = ~69mm). No other reason. The little less static allowed for a little larger tuning window on our shitty 93. Now that my plans have changed I wish I went 11:1 on the build. There is no longevity difference if the tune, fuel, and other factors stay in check. You're going to want a proper fuel setup if you build the motor and try to push it hard on 93. At the very least a GT500 fuel setup.

 

[Bgoins]

http://www.hotrod.com/events/coverage/0311em-power-squeeze/

Here's a cool article on this topic. Old-ish but still lots of good info

 

[Gpcalero]

Haha I told you. I went 10:1 because I was running my/your TVS on pump gas extremely hard (72mm+5% OD = ~69mm). No other reason. The little less static allowed for a little larger tuning window on our shitty 93. Now that my plans have changed I wish I went 11:1 on the build. There is no longevity difference if the tune, fuel, and other factors stay in check. You're going to want a proper fuel setup if you build the motor and try to push it hard on 93. At the very least a GT500 fuel setup.

Lol I thought you still lurked in on these forums. Last time I checked out the GT500 fuel system it couldn't work with the 5.0's for a couple different reasons. Maybe they've fixed in the year and a half Idk. Would be nice though

 

[Gpcalero]

http://www.hotrod.com/events/coverage/0311em-power-squeeze/

Here's a cool article on this topic. Old-ish but still lots of good info

Good read, thanks for the info

 

[downwardspiral]

If you run a lower compression piston is it possible to run a stock tune or are you married to the sct tuner at that point?