Increasing Compression
 

As I understand it there are a number of ways to increase the compression,
Shave the heads to reduce the area around the valves,
Use different pistons that have a higher or larger top,

My question is can I also use longer rods to acheive the same thing,

If so then by how much would they need to be longer by?
Tony

Re: Increasing Compression
 

No mate there is only 0.60" between the piston and the head, so you can't push them up any more. Shaving the heads is not the go as it will need adjustable cam pulleys to keep the timing right.
Higher comp pistons would be the best.

Harvey.

Re: Increasing Compression
 

Or a slightly larger diameter bore which would increase both displacement and compression ratio.
It still requires new pistons, but hey, boring is relatively cheap!
Its the pistons that are expencive.:rolleyes:

Re: Increasing Compression
 

Are you saying .6 of a inch?
Tony

Re: Increasing Compression
 

Harvey's right Tony...I shaved my heads and am having difficulty getting the timing right. Didn't realise that the poofteenth I took off would impact the timing as much as it has.

If you're interested in pistons, and you need some extras to get going, then I'll be interested in a set. Mike offered me a set a few years back, but I just wasn't in a position to do anything then. He may have the specs and contacts that could help?

What ratio are you thinking? And how did you work it out? Are you thinking E85 as well? So 13:1?

M

Re: Increasing Compression
 

I was think of trying 14 to 14.5,
pretty sure the pistons I have in the motor from Mike were 13, I want to go higher.

Re: Increasing Compression
 

I did not realize that you were speaking of LANs built engine.
If your HP was 350, (for example) at 13 to 1 compression, and you bumped it up to 14.5 to 1, the increase is only to 358 HP.
IMHO not worth the expence!

Bowling's Compression Ratio -> HP Calculator
Computation Results:
Engine Horsepower (peak) is 350
Old Compression Ratio is 13.0
New Compression Ratio is 14.5
Computation Results:
Computed New Engine HP is 358, a 2 percent change

Re: Increasing Compression
 

So Harvey what does your calculator say about bumping up the compression.

I don't get the 2% gain only,
If thats all that is in it why have a number of race rules forced the compression ratio down. You would think for 2 % it would make stuff all difference.
Tony

Re: Increasing Compression
 

There is a diminishing results effect as the compression goes up.:(
The same NA engine going from, say 7 to 1 to 9.5 to 1 has a MUCH different outcome.



Bowling's Compression Ratio -> HP Calculator
Computation Results:
Engine Horsepower (peak) is 350
Old Compression Ratio is 7.0
New Compression Ratio is 9.5
Computation Results:
Computed New Engine HP is 384, a 10 percent change

Re: Increasing Compression
 

No sorry, I ment 0.06"

I think Mikes pistons were 11:1, so an increase of 3 points.
Whats that say Tom?

Harvey.

Re: Increasing Compression
 

Bowling's Compression Ratio -> HP Calculator
Computation Results:
Engine Horsepower (peak) is 350
Old Compression Ratio is 11.0
New Compression Ratio is 14.0
Computation Results:
Computed New Engine HP is 370, a 6 percent change
:)

Re: Increasing Compression
 

svxfiles, your right they were 11 to 1.

Matt what did you bring your compression up to by the head mod?
Tony

Re: Increasing Compression
 

Tony it is not the actual compression that makes the difference, as Tom says it has diminishing returns. It is the way it controls other things, like reduced engine speeds, and the type of fuel that they can use.

The overall factor that we work against is the actual combustion pressure that is developed, to expand the air, to push the piston down, and this needs the maximum pressure to happen at around 15* After Top Dead Center.

If you work back to why increase the ratio? The simple answer is the speed of the gas/air burn in the combustion chamber. If we are to run the engine at a faster speed, the burn has to be faster, we get this by running the highest comp pressure that the fuel can stand till detonation limits it. So we tend to go to higher-octane fuels to achieve this.

In the quest for more power we have to go to alcohol fuels, these have a much slower burn rate than petrol, so to speed the burn up, up goes the compression ratio, to increase the combustion pressure, to get the burn done in the necessary time. Of course it has other benefits that help us up the next step to increase the power.:)

Harvey.

Re: Increasing Compression
 

bazza seems to agree with Matt that we can use E85 and get around a heap of problems.

My question is can I weld to the inside of the head and increase the compression ratio. I understand that it is band by some rules but could it be done. If we weld the head then CNC the metal away to get the right ratio.

Tony

Re: Increasing Compression
 

Indeed - E85 is great stuff, however I've only played with turbo applications and don't have anything proven apart from my own theory in regards to NA applications. Also I've done quite a bit of research via SAE / engineering and university publications and it would seem that in the professional world E85 is accepted as a faster burning fuel in comparison to 98RON for engine combustion applications which is very widely mistunderstood through online forums it would seem.

Also the theory of E85 making more power is good, high compression + E85 will make more power. 98RON is such a nasty fuel that its very hard to get near the engines true potential due to detonation. Stock Subaru tunes are unbelievably aggressive. A mate of mine couldn't run more than 14 psi on street 98RON for example with the stock ecu running timing - major detonation. We put in E85 and a microtech to control fuel only and it was clearing 30 psi without a hint of detonation - car was absolutely ballistic. Timing was around 25 degrees at peak power. I think we could push it even harder and get close to 30 degrees and make a bit more. It actually snapped a rod it had so much torque. Engine now rebuilt to 2.2L with forged rods and running 25 psi with 25 degrees timing... madness!!

So I think trying to bump up the compression is a great idea. My own engine has had the block and heads shaved, a cometic head gasket and stock pistons. I'd assume the compression is a touch higher than stock. One thing in turbo applications is to actually use E85 and reap proper benefits you need high compression - something I've noted is the low compression turbo engines don't get the gains as the high compression ones.

Also did a 2 second google image search for E85 vs 98RON in NA applications - this is a random V8 I found:
http://www.castlehillexhaust.com.au/files/samfalcon.jpg

Re: Increasing Compression
 

baza was I dreaming or did you tell me you have the standard cams in your motor?
Tony

Re: Increasing Compression
 

That's right mate, standard cams at this point in time. They turn rather asthmatic around 550-600 bhp which from what I recalling Ecutune Mike saying it's mainly the exhaust cam.

Re: Increasing Compression
 

OK, Tony,
On our stock 10 to 1 Compression Ratio engines we have 3318cc, or about 202.46 cubic inch displacement.
That should work out to 3.318 cc (or 20.246cid) above the piston at TDC.
You now have 11 to 1 CR on a 3.3L, or 202 CID engine, with a bore of 97mm, (roughly) or 3.815".
This has been done with different pistons. (As far as I know.)
So, the displacement, above the piston at TDC has to be about 301.636cc, (or 18.405cid.)
To get a 14 to 1 ratio the above the piston displacement now would have to be 237cc (or 14.461cid.)
Since there is only a reported 0.060" clearance,
and getting a thinner head gasket by say 0.030" would only increase the compression ratio up to about 12.384808 to 1 CR.
AND you still have the problem that Harvey mentioned that you have to "adjust" the cam sprockets to correct the timing.


No mate there is only 0.60" between the piston and the head, so you can't push them up any more. Shaving the heads is not the go as it will need adjustable cam pulleys to keep the timing right.
Higher comp pistons would be the best.

Harvey.

I have not heard of anyone welding the heads to increase compression, but it sure sounds more expensve than, and risky then a bigger bore resulting in both more displacement, and higher compression.

Re: Increasing Compression
 

The reason I mention it is that it has been banned in some rules,

have anyone else heard of it being done?

Re: Increasing Compression
 

Does anyone have any testing behind this with the EG33 NA motor and the results before and after dialing in the cams? Reason I ask is I don't think it makes any difference on our motors and if anything actually gains top end.

Reason why is I've done a fair bit successful AVCS tuning on the intake cam (02-07 STI heads). As most know it effectively advances the intake cam during low end RPM and you wind it back to zero as revs and air flow climbs.

The result is a massive improvement in low end torque. 0 advance gives the best top end (no bottom end) so theoretically a couple more degrees of retard will help with more top end. Also the exhaust AVCS works by retarding the cam as the rpm and airflow climb - retarding makes more top end power.

So in english, decking the head in an EG33 and the resulting timing belt effects which cause overall retardation of all cams should if anything make the car more powerful up top due.

Re: Increasing Compression
 

Matt tried shaving the heads and had problems part because you can't buy adjustiable pulleys for the cams on our engines.

To get 14:1 how much has to be shaved off the heads?
Tony

Re: Increasing Compression
 

I did correct the 0.60" typo.:)

Yours is out a bit, I make it 42.5 cc for a 14:1 ratio.
Comp ratio is, swept volume, plus the clearance volume, divided by the clearance volume.;)

Yes welding the heads would be a huge job for very little return. There would be a big chance, that you would change the shape, to the detriment of the combustion turbulence, which would cost you a lot more than the 2% gain.

Harvey.

Re: Increasing Compression
 

Yeah I saw that Matt had some issues with the timing, but I'm keen to understand what the problems were... as I said my heads have also been shaved and the block shaved with no issues however as I said the turbo masks issues which might be far more prevalent in the NA. Of course if he won't over the Subaru limits then I'd understand it.

Also for your interest Tony, adjustble pulleys available on eBay.co.uk ... 400 pounds including delivery, just checked, search "EG33 pulley" or "EG33" etc.

Re: Increasing Compression
 

Giday Bazza, good to see you over here. :cool:

E85 is a good fuel, if full advantage is taken of it, but it is a case of what is the engine going to be used for?
Like I can't imagine Tony using it, as he couldn't carry enough to get to the next pump.
For circuit racing, especially if the regs only allow, 'pump gas', you would use it. In your case it is almost essential to allow the boost that you use.
As I said before, it depends on what fuel you are going to use, and build the engine to suit. There is no real point in chasing a number, just to use it.

Harvey.

Re: Increasing Compression
 

Were is Matt, would be good to see if he thinks shaving the heads has any futur?
I sent a email to Mike about high compression pistons but did not hear a single word back.
Tony

Re: Increasing Compression
 

Sorry for the confusion, Harvey,
but I was speaking of the combined head volume,
for all six cylinders.

Re: Increasing Compression
 

Wouldn't it be retarded on the right bank and advanced on the left bank?
Or do you believe that the tensioner would equalize the timing?

Re: Increasing Compression
 

Prolly, I drew it all up before on paper but then confused myself again lol. Need to spend more time figuring it out.

Re: Increasing Compression
 

Actually I gave it a little more thought and I believe that the right bank would be a little retarded, and the left bank would have double the amount of retard because all of the slack,
(Due to a shorter distance from the crankshaft to the camshafts)
would be between/after the left camshaft and the crankshaft.
Left cam is 0.030" shorter,
Right cam also 0.030" shorter,
ALL belt slack is adjusted out after the left cam


SO, get custom pistons to up the compression ratio if you like,
cuz shavin heads won't do it.

Re: Increasing Compression
 

Two questions
If shaving the head is the way to go how much needs to be shave to increase it to 14:1.

Does any one know were to get the piston if thats the way to go.

Harvey the higher compression is for the race car.
Tony

Re: Increasing Compression
 

Sps pistons in melbs
Cp in USA

Will do whatever you wish after you send em your stock piston for base. $$$$ though

Re: Increasing Compression
 

CP pistons doesn't need a base any more, they have done enough EG33 pistons by now.

Re: Increasing Compression
 

Indeed, however you only get sub 10:1 compression pistons for their on file designs.

Re: Increasing Compression
 

baza,
You may not know but I am building a engine dyno so at the end of the day it will be easier for me to shave the head and that way I can play around with different compression ratios.

So guys how much do I need to shave off?

Tony

Re: Increasing Compression
 

What a top idea. Have you got a thread detailing this build, I'd be very interested :) :) :)

Re: Increasing Compression
 

Tony you need to remove 18.5cc from the clearence volume.
Removing the 0.060" head geaket will only drop it 1.87cc.

Harvey.

Re: Increasing Compression
 

here,
http://www.subaru-svx.net/forum/showthread.php?t=54640

I have been as "flat out as a lizard drinking" over the last year but plan to get back to it soon. I have worked out soluations in my head.
Tony

Re: Increasing Compression
 

Okay some rough numbers.

3,318 Litre
6 Cyclinder
97 mm Bore
7.3928 Area of piston

Target compression Final Volume Difference MM to Shave
Current 10 55
11 50 5 0.680
12 46 9 1.247
13 43 13 1.726
14 40 16 2.137
15 37 18 2.493


I think there maybe mistakes so if so please yell.
Tony

Re: Increasing Compression
 

Here is some more info,


It might make more sense in the JPG.
Spoke to the guys with the cam pullies and they allow plus & minus 12degree adjustment. According to my calcs if you were to run 15:1 the left bank would need 7.8 degree of adjustment.

Okay were am I at.

Shave heads is the only way to go because I have a lot of heads and I can experment with different compression ratio with little effort. I can just put them in my cnc mill and plan them off. The pullies will enable adjustments simply.

Have a great day all.
Tony

Re: Increasing Compression
 

detail of calcs