9:1 compression vs. 8.5:1 on a big 16G Pros/Cons?

[Turbo Shogun]

Hey guys.

As the title states, I'm in the middle of a ground-up rebuild of my motor (cylinders out of round ).

I've spoken to a very knowledgable DSM machinist who recommended a compression ratio of at least 9:1 on my motor for the rebuild. Now, I'm not the one to knock a professional's opinion, but I would have thought that 8.5:1 would be better, reliability wise.

My opinion, of course, was dismissed, and I was told that 9:1 was the way to go.

So what are the pros/cons of running 9:1 over 8.5:1? I know that more compression = power and response, but at the same time, it creates excess heat. I'd also like to note that I was told that the stock 1G big rods could take the abuse of such high compression. Would it be better for me to invest in a set of Eagle H-beams?

The car's not a 1/4 mile car at all, by the way. I would say that I am emphasizing response and all-around flexibility than all-out top-end power. It's an auto-x/circuit car. I'm only shooting for about 350 AWHP MAX.

Any ideas/opinions are greatly appreciated.

 

[groundPork]

Hey guys.

As the title states, I'm in the middle of a ground-up rebuild of my motor (cylinders out of round ).

I've spoken to a very knowledgable DSM machinist who recommended a compression ratio of at least 9:1 on my motor for the rebuild. Now, I'm not the one to knock a professional's opinion, but I would have thought that 8.5:1 would be better, reliability wise.

My opinion, of course, was dismissed, and I was told that 9:1 was the way to go.

So what are the pros/cons of running 9:1 over 8.5:1? I know that more compression = power and response, but at the same time, it creates excess heat. I'd also like to note that I was told that the stock 1G big rods could take the abuse of such high compression. Would it be better for me to invest in a set of Eagle H-beams?

The car's not a 1/4 mile car at all, by the way. I would say that I am emphasizing response and all-around flexibility than all-out top-end power. It's an auto-x/circuit car. I'm only shooting for about 350 AWHP MAX.

Any ideas/opinions are greatly appreciated.

Right there with you man! I want a super snappy and resposive engine for pure EXIT SPEED! When I am ready to go, I'll be planning on a 9:1 for sure. Just not sure if a 2.4 or 2.1L... mostly cause of the gas (daily driven poor guy). 8.5:1 is more forgiving and can make more peak horses for a given amount of boost, but you must consider that with a 16G you wanna make that power quick and at low RPM so the turbo don't have to spin to bejesus and heat soak the intercooler. it'll give you EXCELLENT throttle responce and great bottom end, with near instant spool up. When it comes to the engine in autocross, responce is key. It's not necessarily how much HP you make as where in the RPM range...

Also regardless of static compression, dynamic pumping compression can be made higher with more boost, so a half a point is really negligable with your power needs.

 

[Spoolin4Ever]

I would be highly skeptical of my engine builder if he tries to push me into something, and does not listen to or address my concerns. I have had experiences like that in the past; and the outcome was not good.

9.1 compression means you will not be able to run as high of boost without getting knock. Your engine will be fussier and more difficult to tune. And if you make a mistake, then it is more likely to cause damage, where with 8.5 you have more room for leeway in all those areas.

I have a 1G with 7.8:1 compression (that's stock for a 1G). I have never driven a 2G or any other turbo dsm, so I myself can't compare them.

However, the concensus here seems to be that 8.5:1 works pretty good. You get good off the line torque, but satill able to run higher boost levels then you can with 9.1 comp.

As always, we must ask: What's the goal for the car? HP? Daily driver, track? Drag racing?

EDIT: Actually though, if you want it to be fast out of the corners, and only going fot 350, 9:1 would probably be good. I forgot to read the last of your post the first time.

 

[andymoraitis]

9:1 will be fine for your motor, but as mentioned, your ECR (effective compression ratio) will rise thereby bringing on the onset of knock that much sooner on pump gas. For your goals, the motor will have more snap for autocross and pull you out the turns very well. Just for kicks, I would invest in a strong set of rods and pistons so that the bottom end can take the additional abuse in case you run into knock although stock internals can easily handle 350 WHP. If you have the money to invest in a set of Eagles or Manleys I would do it for the extra insurance.

Keep in mind that when moving from a 7.8:1 comp to 9:1 on a Big 16G, the heat generation will be higher. Running lower timing will allow you to run close to the same amount of boost that you were before. Also, keep the turbo in it's best efficiency ranges and it'll help. Just keep an eye on knock and never go above 2-3 counts and you should be fine.

Good luck,

Andy

 

[Turbo Shogun]

Well, I should say that the car only ran with the 16G for 50 miles before the water pump went ... One thing led to another, and ........... I needed a new head.

But the thing with the 16G was it didn't spool until about 3,500-3,800 RPM. The cams helped a little before that ($600 pair of HKS's going in the scrap bin at work ), but it wasn't as responsive as my 14B at 15 PSI.

And no, it's not like he discouraged my ideas. This guy is building a 4G63 for his wife's Mitsubishi Expo and he said he was gonna run 9.5:1! WTF Tuning 9:1 shouldn't be an issue if the tuner was proficient, I was also told.

I was planning on a set of H-beams and 9:1 Wiseco's in a .020 oversize since the block needs to be bored out.

Well, thanks for the opinion, guys. That'd been bugging my brain for awhile.

 

[Enraged78]

Well, I should say that the car only ran with the 16G for 50 miles before the water pump went ... One thing led to another, and ........... I needed a new head.

But the thing with the 16G was it didn't spool until about 3,500-3,800 RPM. The cams helped a little before that ($600 pair of HKS's going in the scrap bin at work ), but it wasn't as responsive as my 14B at 15 PSI.

And no, it's not like he discouraged my ideas. This guy is building a 4G63 for his wife's Mitsubishi Expo and he said he was gonna run 9.5:1! WTF Tuning 9:1 shouldn't be an issue if the tuner was proficient, I was also told.

I was planning on a set of H-beams and 9:1 Wiseco's in a .020 oversize since the block needs to be bored out.

Well, thanks for the opinion, guys. That'd been bugging my brain for awhile.

I have a 2G with Ross .020 over 8.5:1 pistons, Eagle Rods, and a fresh 1G head. With Water/Alch injection, a stock sidemout, and upgraded intercooler piping, I see 16PSI of boost on my Evo III 16G by 2,700RPM. If you pick your parts right, you CAN get quick spool and still have detonation reistance with 8.5:1. However, if I had to do all this again, I would have gone with 9:1. The off boost response and spool would have been even better if I had gone with that setup. Make sure you have DSMLink or some other method to control your timing, as you WILL have to lower it to account for the higher static compression, especially in the upper RPM ranges.

Matt.

 

[2deep]

I'm running 9:1CR pistons, and boost 25psi daily without knock on my FP RED with 94 octane pump and meth injection. As long as you have a means of controlling the timing you'll be ok with 9:1CR.

 

[steel_3d]

Have you considered going with a stroker? It sounds ####in cliche by now I know. But the extra stroke will make a much bigger difference in response and spool than compression alone. The Wiseco stroker pistons for example are 8.8:1 - there, you get your compression too. Your only additional cost is about $200 for a 2.4 crank. Personally that's what I'm building and I plan to autox my car more than drag.

As for the compression issue, I'd go 9.0 rather than 8.5, for what it's worth. Honestly I don't know how much of a difference it would really make.

 

[shleppy]

9:1 is fine, and really isn't that fussy. I'm running 27psi on 93 and 50/50 mix of h20/meth. I've gotten as much as 56.5 lbs of airflow with 16 degrees of timing and 0 knock. It does make for a really peppy motor. When I had a evo3 I ran 24 psi on 93 with no injection at all with 0 knock. Its all in your combination, and dsmlink helps alot with the tuning.

 

[Pennywise]

9:1 with a good tune and you will be fine.

 

[blackGSX2g]

I've got 9:1 compression as well and love it. I have great throttle response and quick spoolup even with a 50-trim, plus I get 24mpg city with a heavy foot. I am running 25psi flowing about 52.8lb/min with no knock with the mods in my profile.

Only downside I had was that I needed fuel upgrades to run 13psi on an 16g.

 

[Tom N]

more compression = more power.
You just have to tune it properly.
I run 9:1 with a 16g.
I wouldnt go down in compression for nothing in the world.
As a matter of fact I will be building a new motor with even higher compression.

 

[Jimmy Jr.]

I'm a fan of higher compression but when it comes down to it there is no such thing as opinon in the mechanical operation of a motor. The engine is governed by the laws of physics. There is no "i think". It is or it isn't. You can take the time to learn and find out what is truly best for your setup.

On a side note. Loss of timing advance is not only inevitable with higher compression but it's ideal. The flame front will travel quicker as a result of greater air density and thus REQUIRE less timing to pull off optimal combustion.

 

[tstkl]

Just get a mitsubishi multi layer head gasket, 9:1 pistons, and eagle h beams, and you'll be set for anything. With a goal like 350 awhp, you shouldn't have to worry about the effects of higher compression ratios being negative. The new evo's use 8.8:1 stock, so using 9:1 with a mitsubishi multi layer head gasket will be somewhere near there, and with a goal of 350 awhp (depending on which turbo your going with) you won't have to run too high of boost to get there. You'll be fine with this set up, just make sure you tune out all of the knock and run a somewhat conservative afr/timing combination.

 

[groundPork]

I'm a fan of higher compression but when it comes down to it there is no such thing as opinon in the mechanical operation of a motor. The engine is governed by the laws of physics. There is no "i think". It is or it isn't. You can take the time to learn and find out what is truly best for your setup.

On a side note. Loss of timing advance is not only inevitable with higher compression but it's ideal. The flame front will travel quicker as a result of greater air density and thus REQUIRE less timing to pull off optimal combustion.

Right on... Advance timing=speed up flame travel, increase compression=speed up flame travel, tighten squish=speed up flame travel... the leaner you get the slower the flame front because there is less fuel to burn(fuel burns... air supports the burn, but does not itself burn). faster flame front w/out detonation and with a propper AFR means the combustion chamber is working efficiently which means for a given volume of air and fuel, you are exploiting maximum work, or effective pressure. If you think about it... your choices are 9:1@15 psi or 8.5:1 @ 18psi (an exageration). Thing is, you'll build your torque peak sooner, in an RPM range you can use for autocross and street racing, tune it right and youu won't need much timing (less abuse to the bottom end parts), and you'll spool that turbo like an SOB which will have great throttle responce... this is why I love the idea of good CR with moderate boost... ahhhh throttle responce....

That being said, the lower your CR the more actual psi you can cram into an engine of a given dynamic compression... thus more peak power. But You really don't need that, you need real usable power!. period.

 

[compression]

what about street\strip, i always run 93 octane on the street and usually put 2-3 gallons of c-16 or gt plus in it at the track, now would 9:1 be good for me being im running the evo 3 16g, i would like to make 500 whp on the e316g is this out of reach?, i would really like to only go as high as 9:1

 

[andymoraitis]

No one will ever make 500 WHP on an EVO III. 400+ has been done, but it takes the correct complement of supporting mods, tuning and an intimate knowledge of volumetric efficiency. Bigger isn't always better, but if you're chasing 500 WHP, you'll need to flow more air than the EVO can supply. I suppose there's always nitrous, but that's something I don't dabble in. As far as whether more compression is better for your application, take a look at groundPork's post. There's very good info in there that should be easy to pick out. Higher compression is discernibly different on the street. I love it and would never want to run anything below 8.8 or 9:1.

 

[groundPork]

No one will ever make 500 WHP on an EVO III. 400+ has been done, but it takes the correct complement of supporting mods, tuning and an intimate knowledge of volumetric efficiency. Bigger isn't always better, but if you're chasing 500 WHP, you'll need to flow more air than the EVO can supply. I suppose there's always nitrous, but that's something I don't dabble in. As far as whether more compression is better for your application, take a look at groundPork's post. There's very good info in there that should be easy to pick out. Higher compression is discernibly different on the street. I love it and would never want to run anything below 8.8 or 9:1.

Me myself, don' t feel I need 500 horses for the street. But I had felt the same thing about the EVO III. Is this just cause we would be heating things up too much in an effort to push volumetric performance beyond what that turbo can provide? I am on the same level as most in that I am trying to make the most horses per dollar as I can. Granted I often have fewer dollars than most. If I could and had the cash My ideal setup would be a KKK ball bearing turbo for it's throttle responce and efficiency range which (to me at least) seems well suited to street and short track road race stuff.

You've got a few major things working agaisnt you when it comes to tuning... knock, rpm, and displacement limited. So within these limits why not try to make the best toruqe possible. We don't have much displacement, we can't run race gas all the time, and we are usually trying to make an EVOIII turbo do the job. The sooner we reach the cylinder pressure we need for the power level we want, the less boost we'll need later, the less heat generated by the turbo as a byproduct, the smoother the power band, less timing advance needed, and we don't have to force the engine to rev to the moon. We just flat out have better acceleration on tap for the type of racing we do and I think we shouldn't be affraid of running higher CR's... I also think that strokers are good for the same basic reason, I just am hard headed about high specific output and prefer the 2.0L because it is more a tuning challenge and engine builder's challenge rather than an issue of just raw power. Kind of like injected nitro engine versus a nitrous fed promod Big block.

 

[Turbo Shogun]

Wow, I'm glad this topic really took off before I've had a chance to finalize things.

Well, I've had a few other questions that I came up with ...

1. What kind of fuel mods would I have to do? As of now I'm running a Walbro 255 lph and a set of AMS 580cc injectors. Would this be enough to support 9:1 with a Big 16G at, say ... 18PSI? I ordered Maximum Boost By Corky Bell, but I have to wait for it to get here. Plus I suck at math.

2. I'm running e-Manage right now and I do plan on buying the ignition control and Injector control harnesses. I'm not going to tune it myself, rather, I'm taking a trip to Arlington Heights to have AMS tune it. How safe would it be to make a 20 mile trip on a loose n00b tune?

3. Would the valve/piston tolerances be closer? I'm kind of paranoid about that sort of thing since my timing belt slipped ...

4. I am running an MLS gasket. Should I mill it down or keep it so I can lower compression a pinch to keep things under control?

5. This may sound like a dumb question ... But would running higher compression make my top end suffer for the sake of lower RPM performance? Or would it make things a little riskier at that powerband?

I'm really more concerned about #1. I just got those 580's a few months ago. If running 9:1 would require bigger injectors that what I have now, I'd have to go with 8:5's ... I just wanna get the damn thing on the track!

 

[slipstream808]

1.) Most people go with 650's to be safe with an EVO 16g. Those 580's might be good for low-end boost, but you might have to deal with boost creep depending on your exhaust and wastegate setup. And you wouldn't want that. If you need to wait... then wait. Get the bigger injectors and higher compression both. It'll help on the track!!

2.) a "loose noob tune" really doesn't mean that much as far as specific goes. We would need to know what you are fully capable of with you and your equipment's tuning capabilities. For a 20 mile trip, though TRY to stay out of boost at least, and as long as your injectors are dialed in for closed loop operation, your fuel should be good. Do you have a baseline timing like the stock setup? Or will your new setup come needing full mapping of the timing. I ask this because timing is actually MORE crucial to how a car runs than fuel. If you are running timing close to stock with your new setup, than again, you should be OK.

3.) No, the pistons should actually be shaped with the same tolerances/clearances for your valves. But you could ask your builder or the piston manufacturer. In the end, though, it wont matter because if the belt slips... well... eh... you get the idea. Not good no matter what!

4.) If the gasket if fine... just leave it at that and don't mill it. As far as it goes... if it ain't broke don't fix it.

5.) Given 2 identical setups with the same tuning and boost and everything except for CR, the higher compression will make more power throughout the ENTIRE RPM band...

BUT!!!

More power comes from more torque, which comes from more explosive force on the pistons. So that also means that more heat is produced which means there is more a chance of knock. So you might have to pull timing up top to keep knock down. With the EVO III, though, I don't see you hurting up top really cause of the excess heat (ie, you'd be forced to run less boost or pull more timing due to the higher compression). Maybe if you had a bigger turbo... but that's for another day.

 

[2fast4]

I've got 9:1 compression as well and love it. I have great throttle response and quick spoolup even with a 50-trim, plus I get 24mpg city with a heavy foot. I am running 25psi flowing about 52.8lb/min with no knock with the mods in my profile.

Only downside I had was that I needed fuel upgrades to run 13psi on an 16g.

What kind WHP do you get or drag time with your current setup?

 

[steel_3d]

1. I think you'll be ok for 18psi with the 580's. You can also increase the fuel pressure to squeeze more out of the injectors (even a $40 B&M works for that). The increased compression doessn't require more fuel than stock compression, if that's what you're thinking.

2. You'll be fine. If you tune it to where it's drivable, no hickups, decent O2's, you'll be safe. Especially if you stay out of the boost.

4. Don't mill it for compression, but it's a good idea to resurface the head and block if you're gonna use an MLS anyway.

5. You should gain in the entire power band, assuming it's tuned right. Max power potential should be about the same as with lower compression, but that's where tuning a high compression engine gets really tricky. By the sound of it you're not going for max power right now so it shouldn't be too tricky. But as an example, Marco at Magnus runs 10.5:1 compression in his drag cars, and you know those are not built for wussing on the boost. There's a bunch of posts on the subject here, if you have access to NABR: http://www.teamnabr.com/cgi-bin/ultimatebb.cgi?ubb=get_topic;f=6;t=003452;p=2#000099

 

[Spoolin4Ever]

Kind of wierd, an experienced DSM engine builder sayin to run AT LEAST 9:1 . Most people would say that is the MOST you would ever want....

 

[TSisleeper]

I concur with spoolin4ever 9:1 being the LEAST you wanna run ? hmm? so is this a track only car (trailer'd there and back). running what fuel (c16, pump, alky?) need some more information

 

[Tom N]

Kind of wierd, an experienced DSM engine builder sayin to run AT LEAST 9:1 . Most people would say that is the MOST you would ever want....

Most people don`t know what they are talking about.