Compression Ratio

[BlackMamba3]

So getting all my ideas ready for my built motor I want to do. Gonna have MMR do my build. Gonna stay N/A. In talking with them, they've said going to a 12:1 CR would be optimal.

Obviously these guys know what they're doing but I'd like to get some feedback from you guys. Going to a larger CR would decrease my cylinder walls. Having said that, I know the new N/A Cobra Jets are running a 12:5:1 CR and are doing just fine.

I'm just kind of at a stand still between going 12:1 or staying 11:1. Love to get some feedback from you motor experts. Honestly, CR, bore, stroke, etc just confuses me at times. Need a little noob help.

Thanks guys

 

[alex12gt]

could go right in the middle and do 11.5

 

[F8L SN8K]

Changing your compression ratio does not change your cylinder wall thickness. Only a change in bore changes your cylinder wall thickness. Compression ratio is the amount of volume the cylinder and combustion chamber posses at bottom dead center and then compresses into a space at top dead center. That amount is the compression ratio. 12 to 1 ratio. The higher compression ratio will raise cylinder pressure yes. It will also raise thermal efficiency in the cylinder. The higher compression that better your BSFC and efficiency will be generally(there are exceptions of course). The issue usually comes from excessive cylinder pressure causing fuel to pre-ignite or spontaneously combust from the excessive pressure. Higher octane is usually what solves that issue. The more octane the harder it is for the fuel to ignite. The harder it is to ignite the less risk you run of pre-ignition. Combustion chamber design has a lot to do with how much compression you can run in a engine on pump gasoline. The chamber design along with the variable cam timing goes a long way in the 5.0 modulars.

The naturally aspirated Cobra Jet does have 12.5:1 compression and it also uses a 3.7 bore.

 

[BDFabrizio]

CR or compression ratio has nothing to do with cylinder wall thickness. You're thinking of the cylinder bore which is the diameter of the cylinder.

IMO I'd go with what MMR recommends. Modern engines have come a long way and now 12:1 CR engines are completely streetable with 93 octane pump gas. A 12:1 engine is going to make more power and torque then a compairable 11:1 engine but it'll require race fuel for an aggressive tune for the track.

 

[itSSlow98]

I havent seen many 12:1 compression cars running 93 octane. If you are gonan run a race fuel mix or E85 then go with 12:1, if not then you better do 93.

 

[CshiznitzJ]

Go high compression and E85....

 

[BlackMamba3]

Ahh! See I was a little confused. Well that answered some of my confusion. I think I'll wind up going with the 12:1. In terms of fuel, MMR said I could run 93 but we do have E85 here in Iowa. I'll be running a BBR Jackel fuel system so I know that'll be able to support E85. My only thing is that, is E85 really all that beneficial?

 

[truebluedevil02]

13:1 and E85 IMO, if your gunna do it, really crank the compression up.

 

[Bud]

If 11 to 1 motors are surviving with 20+ pounds of boost, an N/A motor should be able to easily survive on 12 to 1 or higher with e85. If you have e85 take advantage of it and up the compression. Not sure how many high compression N/A motors are out there, somebody always has to be first though.

 

[BlackMamba3]

13:1 and E85 IMO, if your gunna do it, really crank the compression up.

13:1?? This is still gonna be a DD and see some road racing. You think a fully built 13:1 motor is streetable still?

 

[Ry_Trapp0]

13:1?? This is still gonna be a DD and see some road racing. You think a fully built 13:1 motor is streetable still?

absolutely, it's all in the tune! we aren't building hi-po engines with '70s technology anymore, with 12:1 carbureted 100 octane engines that have massive cams and can barely hold an idle at 1200RPM. electronic fuel injection and variable cam timing have opened up a whole new world that gives us both big performance and daily drivability so long as the tuner does his job(granted, an engine such as this would probably require a lot of attention to perfect the tune on).
and, damn, a 13:1 E85 coyote would be a screamer! rev it out to 8000 and let it sing!!!

 

[truebluedevil02]

13:1?? This is still gonna be a DD and see some road racing. You think a fully built 13:1 motor is streetable still?

Honestly, I would think you could go higher than 13:1 with the use of E85. But if I were building a N/A motor, that is what I would do. Like stated, these arent 302's or 351's. With the modern cam profiles and our adjustible cam timing, a 13:1 coyote motor would be no less streetable than a mostly stock one. Get a good tune from one of the well known shops(AED, Lund, BBR, JPC, HPP) and you probably wont even need to do much up keep on it. Data log it once a month to be sure everything is ok and walla. But if I were into road coursing and wanted to drive the car on the streets often/daily. Then you bet your ass a 13:1, e85, cam's, BEAST would be going in it. I would think something like that bored out a few more CI would be close to 540-570whp

Coming from a guy who had a 12.5:1, pump gas, huge cam, 408 stroker 91 notch:beer:

absolutely, it's all in the tune! we aren't building hi-po engines with '70s technology anymore, with 12:1 carbureted 100 octane engines that have massive cams and can barely hold an idle at 1200RPM. electronic fuel injection and variable cam timing have opened up a whole new world that gives us both big performance and daily drivability so long as the tuner does his job(granted, an engine such as this would probably require a lot of attention to perfect the tune on).
and, damn, a 13:1 E85 coyote would be a screamer! rev it out to 8000 and let it sing!!!

DAMN STREIGHT!

 

[BlackMamba3]

Honestly, I would think you could go higher than 13:1 with the use of E85. But if I were building a N/A motor, that is what I would do. Like stated, these arent 302's or 351's. With the modern cam profiles and our adjustible cam timing, a 13:1 coyote motor would be no less streetable than a mostly stock one. Get a good tune from one of the well known shops(AED, Lund, BBR, JPC, HPP) and you probably wont even need to do much up keep on it. Data log it once a month to be sure everything is ok and walla. But if I were into road coursing and wanted to drive the car on the streets often/daily. Then you bet your ass a 13:1, e85, cam's, BEAST would be going in it. I would think something like that bored out a few more CI would be close to 540-570whp

Coming from a guy who had a 12.5:1, pump gas, huge cam, 408 stroker 91 notch:beer:



DAMN STREIGHT!

408 stroker?!?! Holy hell....

Damn, now you got me thinking more and more on going higher CR. My ultimate goal is to get as close to 600RWHP as possible while still having it be my DD and road race through out the year if possible. My tune would have to be spot on though. Also, I'm gonna leave the bore alone. I know some of the big drag guys opened up the bore a bit on these blocks but said it could be a bit dangerous cause of how thin the cylinder walls became. I've still got some time before I pull the trigger on this build. 13:1 + E85, ohh dear.....

 

[Bud]

Only concern I'd have would be if you ever have to run gas if you get in a bind and run low when not near an e85 station. I'd talk to a tuner about that and get their thoughts on limited tune for gas if that situation ever comes up and see if they think they could come up with something to get you by (lowered timing, limit TB opening, run it fat, etc)

Could be a cool build though, would be interesting to see what is possible with these things running a little higher compression.

 

[whatwhine]

My beef with e85 is availability. Like when you go somewhere for a week.... Now you're out in bum **** Egypt with only 93..... That could b a problem. I don't have any near me. That I know of.

 

[Driver500]

My beef with e85 is availability. Like when you go somewhere for a week.... Now you're out in bum **** Egypt with only 93..... That could b a problem. I don't have any near me. That I know of.

winner winner

Keep the compression at a level where it can live on 93 and run an E85 tune if you want to wring it out on the corn.

 

[BlackMamba3]

I see what you guys are saying. Here in Iowa, E85 is pretty easy to get but it's hard telling where my next duty station will be.

 

[alex12gt]

I see what you guys are saying. Here in Iowa, E85 is pretty easy to get but it's hard telling where my next duty station will be.

I live in moline il about 2 min from davenport ia and there on only 2 e85 station near me and buy near i mean 10-15 min drive....i dont get it im in the middle of corn land

 

[slagburn]

Here's a compression ratio case study. Two cars with the exact same 2.9L blower. One a stalled auto, 11:1 stock motor, E85, GT500 TB, JLT intake. The other, my M6, 10:1 motor with Boss exhaust cams and Boss springs, SCJ TB, E85.

Both with stock exhaust manifolds, one with an H pipe and one with a Y so no real difference there.

Comparing the two cars' best times with a DA calculator, the stock compression ratio car wins by 3 tenths and 3mph at the strip. And that's with approximately 3 less psi on the 3.125 pulley vs my 2.75, also take into account the much bigger SCJ TB. The same JLT 123mm intake tube was used on both cars and both used similar commanded AFR and ignition lead.

The only unknown is manual vs auto and both cars 60'd the same so should be relatively even in that regard.

 

[BlackMamba3]

Here's a compression ratio case study. Two cars with the exact same 2.9L blower. One a stalled auto, 11:1 stock motor, E85, GT500 TB, JLT intake. The other, my M6, 10:1 motor with Boss exhaust cams and Boss springs, SCJ TB, E85.

Both with stock exhaust manifolds, one with an H pipe and one with a Y so no real difference there.

Comparing the two cars' best times with a DA calculator, the stock compression ratio car wins by 3 tenths and 3mph at the strip. And that's with approximately 3 less psi on the 3.125 pulley vs my 2.75, also take into account the much bigger SCJ TB. The same JLT 123mm intake tube was used on both cars and both used similar commanded AFR and ignition lead.

The only unknown is manual vs auto and both cars 60'd the same so should be relatively even in that regard.

Hmmm that's interesting. My car won't be used for drag racing though but that's still pretty sweet info.