lowering compression for turbo

[danbowe84]

Hi, just wondering if someone could explain to me the pros and cons of the different methods of lowering compression?

[rodhog]

why lower it. ?

Well lets look at reason they did it so much in the 80's.

Simple - cars had to last 100,000km's or 60-80K warranty.

So majority of early turbo cars were dropped down from what most motors were 8.7 - 9.0 :1 comp down to 7.5 - 7.8 :1 comp.
Main reason was poor quality fuel. Unleaded was 91ROn and Super was 96ROn. the replacement was NOT equivalent it was 95RON. On top Leaded fuel was all less prone to Detonation.

But by the mid 90's Turbo or FI cars were bring up static comp to 9.0:1 and you N/A cars well even a simple Mazda 2 runs I think it's 11:1 or 10.5:1 comp. It handles the fuel not just due to advanced ECU's but also with Variable cam timing adding to the ability to control detonation and still make power on low octane fuels.

Now why decompress reliability is the main reason so you can use massive amounts of pressure. Or as others call it cylinder Filling. the other side of it is the more decompressed the more Cylinder Filling you have. or the more room to Fill a up with a combustion mix. The other side of it is Camshaft timing.

The types of decompression is mechanical and timing. You only have one variant available to you. mechanical.

Thicker head gasket
So you can use a thicker head gasket which can decrease if designed right up to 1 whole point. Others have claimed more never seen it successful on Alloy engines. cast iron block and head yes.
Pros are cheap compared with others.
Cons, not considered as strong - limited amount of boost - much easier to lift a alloy head when pumping large amount of boost.

Pistons.
Change the pistons to change the comp.
Pro - widely used - can change to better piston material on older engines using a lighter piston creates more free spinning engines. Along with stronger materials even some of the latest cast pistons using the latest tech are stronger them those of ten years ago.
Con - cost - pistons on it's own - rebuild cost - other associated costs. Custom ratios - machining etc etc. But most reliable.


Depending on the required power or outcome determines the type or I guess procedure you would go down.

It's of my experience and work with FI engines. decompressing a engine is a waste of time. Especially for just simple bang for buck.
For years we ddi things like take a import U12 or U13 Bluebird rip off all Sr20 DET gear and just Jump it onto a N13 or N14. didn't decompress it no custom tune or ECU. Just run octane boost and stock boost. Many of these cars lived for a longer time then most would expect. Those that did DIE. OR throw a Rod or two - OR even in one case crack a piston. WAS DUE TO BOOST LEVELS. Lets say challenge is put down and someone lost by a nose so they wind in 14psi. instead of pulling out the sub woofers or the fat bastard sitting next to them in the car.

Others have amazingly done better I know of a SS N14 original motor untouched 300,000km's on it now . turbo's but uses a custom tuned ECU up too 14psi on on 98ROn. the owner is so lazy sometimes goes 10,000km's between oil changes. But using a stock turbo off a 200Sx and a decent front mount it lives. Good safe tune and he drive it mostly on the low 10psi setting.

You can have Hi comp and easy 12psi of boost. You just have to put everything else together to be right. Correct turbo size - ECU and good fuel.

The only real advantage of a low comp engine is.
Hi boost - baby turbo - heap of timing off boost keep fuel use down. less effected by heat. Less heat transfer.

which is the only up side of a decamped motor is your cooling is less effected.

Unless your in 20 lap race it's not a issue.

[danbowe84]

So if I have a fs-de sr2 lase comp ratio of 9.7 - 1, want to get to about 150kws atw's, with a smallish turbo want low to mid range power, doesn't have to be a rocket, 8psi, in your experience you wouldn't lower comp ratio to 9.1/8.5 - 1? Will be doing work to bottom end new bearings and balance and would like to blueprint crank, groupthink this would be sufficient but also reliable?

[project.r.racing]

the higher the compression ratio, the better you low and mid power will be.

[Ice88]

Originally Posted by danbowe84

Hi, just wondering if someone could explain to me the pros and cons of the different methods of lowering compression?

My first question is. Why do you need to lower compression?

[NaughtyGT]

Originally Posted by danbowe84

So if I have a fs-de sr2 lase comp ratio of 9.7 - 1, want to get to about 150kws atw's, with a smallish turbo want low to mid range power, doesn't have to be a rocket, 8psi, in your experience you wouldn't lower comp ratio to 9.1/8.5 - 1? Will be doing work to bottom end new bearings and balance and would like to blueprint crank, groupthink this would be sufficient but also reliable?

OP. Please read info below (sorry if it's a little long-winded but, after reading this reply of yours a 2nd time, glad I wrote it) you have to be careful in choosing the size of turbo. Smaller turbos, while giving better low-end pep & yes, quicker spool time. The trade off is more heat up top-end = less performance & more importantly, higher risk of detonation & in extreme cases, engine damage. You will also need to raise the boost, not lower it. You run lower boost with larger turbos for similar power. You follow?

Concerning 9.7:1 comp ratio. This is ok if you plan to tune the thing properly & run 98RON with modest boost! By modest I mean no higher than say 14. Just my opinion but, pls DYODD! You need to understand why an engine comp is lowered & Rod touched on both the past reasons & current reasons.

It's about fuel quality & octane & boost ability along with matching the size of turbo to application (if you wanna be technical). Always safer to run lower comp with poor fuel or less than optimum tune. Apart from engine internals composition, it's all in the tune. Ie: better piston quality the higher boost/heat they can handle. Conrod strength is more for the torque engines produce. Won't go there now.

If you were to run high boost in an engine with less than high quality pistons, then you wouldn't be running boost too high before there was damage. A high quality alum alloy piston such as you'll find in either a Volvo or Merc, will handle the heat generated by higher boost much better.

A real life scenario is as follows:

WRX STi's run relatively big boost compared to other makes, esp compared with years ago. A stock STi runs 18psi. Aus WRX runs 14psi. They also run 8.2-8.5:1 comp. Relatively low. Especially seeing as though the 2.0L NA Boxer engine, as used in these examples, run 12.0:1 comp nowadays. Boosted Subaru's though have always been about higher boost levels. Maximum volume of air into the cylinder chambers! The lower the comp ratio, the higher the boost capability. Evo's run 19psi (this is how they get their high kw figures) but that is on the top end of stock internals for reliability street longevity.

My B4 runs 9.0:1 comp & although engine internals are beefed up from the stock Aus delivered WRX (non-STi), it doesn't have the STi strength so, I run 16psi. I could easily run 18 or so but, not knowing the tune, I choose not to.

The GTR I'm putting together runs 8.2:1 comp with a turbo on the large size for this engine. If I were to choose comp ratio & was willing to give it a good tune, I would choose 9.2:1 or above. Simply so I get, as Project has pointed out, good low down & mid power basically. Especially due to larger sized turbo of the GTR's. But what this 8.2:1 GTR comp allows me, even with stock factory tune (using factory AFM/ECU etc) is room to move with boost. I plan on running 12psi & if that sucks, 14! Do you see why? I would also have chosen a slightly smaller turbo so I have quicker spool time & better low-end power.

I'll go a little more tech here. The higher your boost, the more you will get your turbo into the sweet spot where it can stretch it's legs. Not going tech here but so you have a better understanding of turbo size choice. A large turbo on a smaller engine affords you bigger power @ a lower boost level (small turbo has to work harder ie: must run higher boost) A smaller turbo on a larger engine will give you better low-end power but you will need to raise boost higher to get the mid to high-end power happening. This means the small turbo is worked harder & will be running hotter which results in less turbo life.

The turbine housing size & A/R ratio will also determine heat level that is generated. Ie: a turbo with a large comp housing that is mated to a smaller A/R turbine housing will produce better low-end power & quicker spool up time but when driven high in the power band, will produce more heat & less available power (kinda run outta puff so to speak)

All best with your choice.

[danbowe84]

I guess I should have explained what I want to gain from this build then wether it was necessary to lower comp. Still like to know the pros and cons.
Also shortening or lengthening the con rods had no mention or is it not possible?

[rodhog]

Conrods - that's an area used for stroking or increase capacity.
Has been many talks where people try to find out if they can cheat and use this piston of different design with different Conrods - with different head etc etc.

Waste of time IMO. Total joke because this is and usually should be left to be people who are from developing countries and have limited recourses.
Similar to original Hot rodding, when parts and availability is limited. but Real world it's a waste.

It's fine if you have all the parts and pieces and you paid nothing - own a workshop or are a mechanics it's fine. to mess around but you don't find it the Normal for a reason.

IMO

you need to work out your required outcome

Is it worth to you. both financially and are you dedicated enough to see it through.

From there you move forward to work out where to go.
If you going to try do it your self. Get someone else to do the work
Or go down a different path altogether.

Everything comes together once you decide on main 3

Money - Time - Goal.

[project.r.racing]

Originally Posted by danbowe84

I guess I should have explained what I want to gain from this build then wether it was necessary to lower comp. Still like to know the pros and cons.
Also shortening or lengthening the con rods had no mention or is it not possible?

lowering the compression would've been something you did as far back as the 80s to prevent pre ignition because they didn't have fine tuning ecus and knock sensors and the ability to advance and retard the timing on the fly.

[NaughtyGT]

Originally Posted by danbowe84

I guess I should have explained what I want to gain from this build then wether it was necessary to lower comp. Still like to know the pros and cons.
Also shortening or lengthening the con rods had no mention or is it not possible?

So, just what are you wanting to gain from the build. I think we've covered the pros & cons

[Ice88]

Originally Posted by danbowe84

I guess I should have explained what I want to gain from this build then wether it was necessary to lower comp. Still like to know the pros and cons.
Also shortening or lengthening the con rods had no mention or is it not possible?

It is certainly possible. I've increased the length of my conrod by 3mm.

This has numerous effects, such as better combustion as well as decreasing pumping losses, increasing combustion chamber temperatures and overall producing better mid to upper rpm torque. To name a few reasons.

The effect that a longer rod will have on your engine (i.e. FS) is that it will shift the torque band higher. Not to mention the add on benefits that you have access too.

Originally Posted by rodhog

Conrods - that's an area used for stroking or increase capacity.
Has been many talks where people try to find out if they can cheat and use this piston of different design with different Conrods - with different head etc etc.

Increasing the length of the conrod does not alter your stroke at all.

Originally Posted by project.r.racing

lowering the compression would've been something you did as far back as the 80s to prevent pre ignition because they didn't have fine tuning ecus and knock sensors and the ability to advance and retard the timing on the fly.

Agreed.

What the people above haven't considered really is that the engines you are both mentioning are turbo engines which will all probably have 0 overlap or in some cases negative overlap. So the static compression ratio essentially becomes your dynamic compression before you add boost.

The FS cams will bleed off some of the compression. I haven't worked it out as stock compression is useless to me as an NA guy.

But the resulting compression ratio will be less then the stated compression.

The FS engine has overlap stock, so unless the OP is going to be getting aftermarket cams (which he does not need to make his desired power). Then stock compression ratio albeit with rods that will handle 11-12 psi will suit him fine.

Remembering lowering the compression will net the same result as an engine that's been overcammed. It will be really doughy and laggy to drive off cam or in your case off boost. The way around this is to run an engine with higher compression. Running higher compression will allow you to drive the car around town, etc without being laggy n lifeless.

And when the time calls for it and you hit boost the car will come alive as a boosted car would.

Based on your aims and the fact that you prolly don't want to try and achieve something too special (special = costly). I'd look at getting a set of forged pistons in the stock US FS Compression ratio.

Combine that with a 2871 perhaps (or an evo turbo if you so choose, will have other complications), along with a good aftermarket ecu, exhaust the car will easily exceed the aims you are wanting for the car.

You'd need to do the numbers yourself, that's just a guess regarding the turbo.

[rodhog]

Originally Posted by Ice88

Increasing the length of the conrod does not alter your stroke at all.




.

I agree. What I meant to say is it's really the only time I see the use of different Con rods of length is when doing stroker kits or de stroke kits.
And using different pistons. I still think it's short cut to using the RIGHT Piston in the first place.

[Ice88]

Originally Posted by rodhog

I agree. What I meant to say is it's really the only time I see the use of different Con rods of length is when doing stroker kits or de stroke kits.
And using different pistons. I still think it's short cut to using the RIGHT Piston in the first place.

Oh Then yeah that's typically the only time.

In the FS engines case the stock rod + stroke mean that at 7400 rpm the engine literally starts to spin itself apart. So by using a longer rod we can increase the red line that the engine can safely spin too.

Using a longer rod improves the rod ratio which allows us to push the limits that the engine can spin to. Hence why I increased the length of my rods by 3mm, can now rev the engine to ~7800 rpm if i so choose.

[TheMAN]

very simply, lowering the compression is due to the fact that at higher compression levels which you will see in a typical non-turbo engine will stuff itself under boost because of detonation and preignition... under turbo boost, the intake charge is hotter and high combustion temperatures tend to detonate or preignite spontaneously when above the threshold of a fuel's octane resistance to detonate or preignite.... advances in piston designs (shape of the piston and metallurgy), cylinder heads, intake, exhaust, cooling, and fueling systems have allowed turbo engines to go higher in compression over the years, same goes for non-turbo engines... the squish and quench areas of the piston is very important in its design in order to even out temperatures and reduce hot spots which are responsible for detonation/preignition... improved intake charges providing a swirl effect promotes better fuel atomisation and helps provide a more controlled cleaner burn... improved injection designs also allowed optimisation of fueling... better cooling designs in engine, cylinder head, and pistons also further help control detonation/preignition

of course let's not forget about faster computer management systems that have much quicker response than 25 years ago

the best safe reliable compression ratio in the FS is 9.1:1 if you are running beyond 0.5 bar


I'm really simplifying the explanation rather than going too technical


we all know you're trying to learn, but you really will learn a lot faster and better if you bought a book and read it... you really ought to buy Maximum Boost by Corky Bell... it is THE bible to turbocharging

[danbowe84]

Originally Posted by TheMAN

we all know you're trying to learn, but you really will learn a lot faster and better if you bought a book and read it... you really ought to buy Maximum Boost by Corky Bell... it is THE bible to turbocharging

Consider it done.

And silly statement time, I thought longering con rod was the same as getting a taller piston to increase comp ratio or reverse to lower haha because piston will always travel same distance(that is determined by crank?) but with longer rod air would be compressed more ie higher comp ratio.. Thoughts and corrections

[familiagtx]

Yes it's possible and yes reliability

You will have to produce more than 8psi to make 150kw atw and replace the exhaust cam to a Mazdaspeed and/or corksport item.

I use to run 9.7:1 for years with a stock unopened FS-DE
Now run 10.8:1 with a larger turbo and even more boost with no problems.

[Ice88]

Originally Posted by danbowe84

Consider it done.

And silly statement time, I thought longering con rod was the same as getting a taller piston to increase comp ratio or reverse to lower haha because piston will always travel same distance(that is determined by crank?) but with longer rod air would be compressed more ie higher comp ratio.. Thoughts and corrections

In terms of compression yes. But doing this has far far more impact then this alone.