Increasing Compression

[Dessertrunner]

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

[oab_au]

Originally Posted by Dessertrunner

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

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.

[svxfiles]

Originally Posted by oab_au

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.

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.

[Dessertrunner]

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.

Are you saying .6 of a inch?
Tony

[dynomatt]

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

[Dessertrunner]

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.

[svxfiles]

Originally Posted by Dessertrunner

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.

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

[Dessertrunner]

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

[svxfiles]

Originally Posted by Dessertrunner

.

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

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

[oab_au]

Originally Posted by Dessertrunner

Are you saying .6 of a inch?
Tony

No sorry, I ment 0.06"

Originally Posted by Dessertrunner

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.

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

Harvey.

[svxfiles]

Originally Posted by oab_au

No sorry, I ment 0.06"



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

Harvey.

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

[Dessertrunner]

svxfiles, your right they were 11 to 1.

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

[oab_au]

Originally Posted by Dessertrunner

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

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.

[Dessertrunner]

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

[bazza]

Originally Posted by Dessertrunner

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

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 image search for E85 vs 98RON in NA applications - this is a random V8 I found: