Story is this...

I am having Ron from Axis put together a fully built 2.5L engine for me with modified heads and the works. This has been ongoing since last october, but i think work will finish on the engine this week sometime. I am thinking of having the guys at UR tune the setup for me. Talking to them today, they suggest a CR of 9.0:1 for the engine. Ron on the other hand suggests the 8.5:1 compression ratio for the engine as default.

Now, the car is going to be run on a diet of 94 octane pump gas and methanol or water injection in case that has any bearing on things.

Now, I emailed Ron, and while he technically can change the CR of the engine, it will involve tearing it down, rehoning, and new pistons which will add around a month to the build he says.

The guys at UR mention insist that the 9.0:1 works wonders for spooling larger turbos up which i understand, but is there a noticeable difference in response between an engine running a CR of 8.5:1 versus 9.0:1??

Would you suggest that i get the CR changed on the engine (it is late now, but still can be done since he still has the engine at Axis)? Yes, it will add a month to the build...

Is the change from 8.5:1 to 9.0:1 enough for me to lose sleep over and insist on changing it?

Since I want to do this build right the first time and at great cost also, i'm all ears really...

I think part of the reason why this is proving so difficult is that I know that both guys are extremely knowledgable, and I respect them, and with both telling me something different, it is hard to know which way to go...

Thanks in advance for your time :)

Thats quite a conundrum. If you're on 94 + meth you can definitely run 9.0:1 compression on that motor, but whether its worth the extra expense/time to do it is the question. It should make a noticeably difference in off-boost power, but I'm interested to hear what others have to say on this.

Yeh i've just been trying to think about what exactly would be the thing to do really.....The engine build has been on since october of last year, and it is now the home stretch and now this comes up :\

I know about the advantages of a higher CR, but Ron also tells me that it makes the engine much harder to tune and very sensitive to quality of gas and the like... UR on the other hand tell me that it makes for a very responsive engine... especially with the 2.0L engines they have put together in the past.... They reckon that with 2.5 i might not feel bad at all due to the increased displacement, but also mention that a CR of 9.0:1 on the 2.5 would be what they would go with if they were in my shoes.

ahhh :(

I am interested in this as well. But My build is going with the 2.2 block. ANd in WYo we only have 91 octane so I am highly considering water/meth injection.

Got an email from Ron regarding the CR of 9.0:1 specifically...

He says that the engine will in fact let go if one does not run an octane rating of around 97 all the time, one will also be relegated to having to run with alcohol/water injection for running boost.. This closes the door to having an engine that will be happy to run on pump gas...

So i intend to just leave the CR at 8.5:1 and just be happy with it. I think the lower CR will allow the engine to take more boost before running into detonation anyhow, which i think should make it easier to run the turbo I am running within its efficiency island.

8.5:1 he says is the absolute highest he would recommend running for a street driven car..

Ron has a lot of happy customers, no doubt. As with any 'expert in the field' he has his opinions and is set in his ways, with reason.
However I've noticed that his philosophies are often different than the norm, and after gathering a lot of my own info from many knowledgeable guys in the field and drawing my own conclusions i have to say that I don't always agree with what ron.
But at the end of the day, there is no best way to build a motor... its a guessing game and everyone goes about it a slightly different way.

For you, it wont make or break your motor either way so if your builder and you are confident with what you have going now then run with it... I'm sure you'll love the new setup. Just for arguments sake, 8.5:1 should be fine, but I just think you could do a little better (if you want the extra time and expense). The lower compression for higher boost philosophy is based off of 30 year old technology... everything about our motors is better and more efficient now. You can safely run higher compression especially with a better quench pad, and really especially with alcohol injection, and that doesn't mean you're going to have to run low boost to do it safely.

Whats most important is that you are seeing eye to eye with your builder and especially your tuner, and it sounds like you are so I think things should turn out great for you.

Got an email from Ron regarding the CR of 9.0:1 specifically...

He says that the engine will in fact let go if one does not run an octane rating of around 97 all the time, one will also be relegated to having to run with alcohol/water injection for running boost.. This closes the door to having an engine that will be happy to run on pump gas...

hahahaha!! :lol:

Ron's too smart to actually believe that. Methinks he just doesn't want to spend so many hours meticulously re-honing, and rebuilding, the engine if he would prefer it the way it is ... and wants to sell you on that idea too. ;) Can't say I blame him, but it's not a very nice way to get out of a little unwanted work. :lol:

Pretty much all of the fastest Saabs I know of have either 9:1 or higher. All of the Trionic 2.3L were 9.25-9.30:1 compression and they make TONS of power, yes, on pumpgas. 9000 Aero's are a good example. Some people were making like 375 lb-ft at the wheels with the stock block, head, exhaust manifold AND stock turbo (with stock-like crappy intercooler that sits behind the A/C condensor) on California 91 octane with just WI. :eek:

Since the thing octane really limits is torque, rather than power, and you're going to be running on 94 octane AND water injection, there's no way in h3ll you won't be able to make AT LEAST 400whp on pumpgas, with any decently-sized turbo. (IE: GT35R or equivalent)

Just make sure you have plenty of quench/squish and you should be fine. :)

It's a tough call. I'd say go higher compression. I did, I would do it again. I love the low-end and spool. I run 93 pump, with no problems. I'd think it would work really well with 94 + meth, you'll have a bit of a "safety net" there. In the end it is going to be your choice though :alien:

hahahaha!! :lol:

Ron's too smart to actually believe that. Methinks he just doesn't want to spend so many hours meticulously re-honing, and rebuilding, the engine if he would prefer it the way it is ... and wants to sell you on that idea too. ;) Can't say I blame him, but it's not a very nice way to get out of a little unwanted work. :lol:

Pretty much all of the fastest Saabs I know of have either 9:1 or higher. All of the Trionic 2.3L were 9.25-9.30:1 compression and they make TONS of power, yes, on pumpgas. 9000 Aero's are a good example. Some people were making like 375 lb-ft at the wheels with the stock block, head, exhaust manifold AND stock turbo (with stock-like crappy intercooler that sits behind the A/C condensor) on California 91 octane with just WI. :eek:

Since the thing octane really limits is torque, rather than power, and you're going to be running on 94 octane AND water injection, there's no way in h3ll you won't be able to make AT LEAST 400whp on pumpgas, with any decently-sized turbo. (IE: GT35R or equivalent)

Just make sure you have plenty of quench/squish and you should be fine. :)


Yeh, I see your reasoning... As mentioned, I'm not really an expert on this CR stuff to be honest... Heck, another tuner on the board suggested that i even drop down to 8.0:1 CR to make the engine more boost friendly on pump gas.

As fas as power goals for the motor, I'm aiming a fair bit higher than most... a significant chunk more than 500whp anyhow. Would that change the CR that i want to go with?

The other thing i have noticed also in the subaru world is that things that work for other motors sometimes don't appear to work on these engines... Since they run a larger bore, they are not as resistant to detonation as other engines such as the EVO's, which I would think would lead one to not want to run a really high compression ratio...

I wonder what CR Cosworth runs with that sleeved race motor they used to produce....:confused:

i checked in on Cosworth's sleeved 2.5 race motor... and guess what CR that sucker runs? 8.25:1. Seems there might be reason behind the 30 year old madness afterall... at least Cosworth thinks so on their 18K race motor...

i checked in on Cosworth's sleeved 2.5 race motor... and guess what CR that sucker runs? 8.25:1. Seems there might be reason behind the 30 year old madness afterall... at least Cosworth thinks so on their 18K race motor...

Or it could be that they just didn't want to tamper with things too much. R&D is expensive and an all-out race engine may actually need lower compression due to octane restrictions, Water Injection disallowance, and high-load conditions at many GT racing events. How does Cosworth know this engine won't get thrown into a car competing in actual professional-level SCCA/GT racing events at Sebring or Road Atlanta? Putting full load on a race engine 50+% of the time will certainly demand lower compression ratio numbers than you'll ever need for a street, or drag, engine.

Subaru's pancake-like combustion space on the 2.5L certainly doesn't help things, but I doubt it exacerbates the compression-tolerance of the engine THAT badly.

Obviously, the people from UR do just fine with 9:1 compression, so it's certainly not going to "rule out pumpgas all together". :rolleyes:

Or it could be that they just didn't want to tamper with things too much. R&D is expensive and an all-out race engine may actually need lower compression due to octane restrictions, Water Injection disallowance, and high-load conditions at many GT racing events. How does Cosworth know this engine won't get thrown into a car competing in actual professional-level SCCA/GT racing events at Sebring or Road Atlanta? Putting full load on a race engine 50+% of the time will certainly demand lower compression ratio numbers than you'll ever need for a street, or drag, engine.

Subaru's pancake-like combustion space on the 2.5L certainly doesn't help things, but I doubt it exacerbates the compression-tolerance of the engine THAT badly.

Obviously, the people from UR do just fine with 9:1 compression, so it's certainly not going to "rule out pumpgas all together". :rolleyes:

That is definitely something to consider... the use of the engine... The guys from UR were actually talking specifically about their 2.0L buildups that they use the CR for. They did in fairness still say that they thought 8.5:1 was pretty good, but for their 2.0 engines, they use a 9.0:1 CR. No mention was made about any 2.5L engines.

Again though, I think I might be going a little further up the whp pole than what most of their customers come in looking for, so perhaps a drop in CR was in fact in order?

From what i read about the engine that Cosworth put together, it was supposed to allow whoever had the beans to pick one up the ability to compete in the world of drag racing against the likes of Easy Street and what not, and make a good 750+whp without flinching.

I guess what I am trying to say is that Cosworth could easily have left the CR of their full-blown race engine at 8.5:1 or even raised it...but they decided to drop it (which is even what one respected member of the forums suggested i do with the engine buildup).

This is the same race engine setup that someone in proven power braging had tuned by Harman with a 35R and was pulling some spectacular numbers on 91 octane gas (actually, highest i have seen using 91 octane on a 35r), as well as 91+methanol injection.:eek:

Ron has a lot of happy customers, no doubt. As with any 'expert in the field' he has his opinions and is set in his ways, with reason.
However I've noticed that his philosophies are often different than the norm, and after gathering a lot of my own info from many knowledgeable guys in the field and drawing my own conclusions i have to say that I don't always agree with what ron.
But at the end of the day, there is no best way to build a motor... its a guessing game and everyone goes about it a slightly different way.

For you, it wont make or break your motor either way so if your builder and you are confident with what you have going now then run with it... I'm sure you'll love the new setup. Just for arguments sake, 8.5:1 should be fine, but I just think you could do a little better (if you want the extra time and expense). The lower compression for higher boost philosophy is based off of 30 year old technology... everything about our motors is better and more efficient now. You can safely run higher compression especially with a better quench pad, and really especially with alcohol injection, and that doesn't mean you're going to have to run low boost to do it safely.

Whats most important is that you are seeing eye to eye with your builder and especially your tuner, and it sounds like you are so I think things should turn out great for you.

Thanks for the words Wrinkleboi...

It has been a bit of a dilemma and still is now, but after checking out the Cosworth race motor (which I was actually going to purchase initially before I spoke to Axis), it makes me see a bit more reason into what he is saying.

And you are right.. everyone seems to have their own ideas as to what works and what doesn't work. I know the guys at AR Fab are pushing higher CR engines.. again, it works in other arenas with other engines. But again, I have seen that our subaru engines don't seem to follow the same rules that other engines do...Who would have thought that switching to larger hotsides would actually improve spool and whp output? Traditionally, going smaller was the way go it seems for spool and putting on increasingly bigger compressors was thought to be key.

I also know that Clark has mentioned in the past about our engines being much less det-resistant than evo engines which can get away with a higher CR ratio due to the relatively larger bore our engines run. This leads me to believe that we are less able to get away with elevated CR's also..

But at the end of the day though, since I myself don't build engines, I may as well go with what he tells me that works like you say.

I guess seeing the way Cosworth did it kinda makes me comfortable in leaving it as is... They may not be the end all and be all when it comes to motors, but from the start, I knew that if i did not get that Cosworth race engine, the one i got would mirror the specs of that engine where possible, and improve on it where possible also (i.e. through the use of JDM big port heads instead of working with USDM small port heads as Cosworth does.). At the end, I envision an engine that is quite comparable with the Cosworth setup.

Thanks for the words Wrinkleboi...

It has been a bit of a dilemma and still is now, but after checking out the Cosworth race motor (which I was actually going to purchase initially before I spoke to Axis), it makes me see a bit more reason into what he is saying.

And you are right.. everyone seems to have their own ideas as to what works and what doesn't work. I know the guys at AR Fab are pushing higher CR engines.. again, it works in other arenas with other engines. But again, I have seen that our subaru engines don't seem to follow the same rules that other engines do...Who would have thought that switching to larger hotsides would actually improve spool and whp output? Traditionally, going smaller was the way go it seems for spool and putting on increasingly bigger compressors was thought to be key.

I also know that Clark has mentioned in the past about our engines being much less det-resistant than evo engines which can get away with a higher CR ratio due to the relatively larger bore our engines run. This leads me to believe that we are less able to get away with elevated CR's also..

But at the end of the day though, since I myself don't build engines, I may as well go with what he tells me that works like you say.

I guess seeing the way Cosworth did it kinda makes me comfortable in leaving it as is... They may not be the end all and be all when it comes to motors, but from the start, I knew that if i did not get that Cosworth race engine, the one i got would mirror the specs of that engine where possible, and improve on it where possible also (i.e. through the use of JDM big port heads instead of working with USDM small port heads as Cosworth does.). At the end, I envision an engine that is quite comparable with the Cosworth setup.

AR Fab is building my engine and my CR will be 9.5 to 1 and tuned for 93 and meth/water injection on GT35R and AEM EMS. I'm on East Coast now and moving to Southern Cal in April where from what I understand I will have to settle for 91 octane fuel. Just like you I'm having my motor built to put out over 500whp and hopefully close to 600whp and will be daily driven. Not to sure how much a dramatic change the octane rating will change things but they feel that with my set up, 9.5 CR is the way to go.

I saw on AR Fab's car they were running 10.5 CR with the 2.71L stroker kit on a GT40 so I feel confident that my 9.5 CR is the way to go if that's the way they want to build it.

How much power did the 10.5 CR put down with that GT40? And what octane were they running?

91 octane is hard enough to tune for let along on a high CR block.

I personally prefer lower compression with the max being stock CR. My block is lower compression, no issues. I've tuned multiple customer blocks all at stock or lower CR with no issues. I've also tuned a couple of higher CR blocks, all required extra octane to be able to run the desired timing and hit the WHP goal.

I'm not saying higher CR is bad, I'm just saying that octane is the limiting factor to tuning a high compression setup.

The safer option is stock to lower compression when you are forced to run low octane fuel.

How much power did the 10.5 CR put down with that GT40? And what octane were they running?


http://www.iwsti.com/forums/showthread.php?t=70714

C16 I believe. it mentions it in the thread somewhere.