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View Full Version : Will a 1.6mm HG change compression ratio?


Darkist 240sx
10-20-2009, 11:15 AM
I actually have 2 questions that Im sure some of you more knowledgeable zilvians can answer for me.

Motor is a s13 redtop SR
setup is as follows:
BC 264 In/ex cams
BC springs and retainers
ISIS Rocker arm stoppers
HKS cam gears
Peakboost ehxaust mani
Peakboost downpipe
tomei 740cc Injectors (possibly going to sell for DW 810's
Walbro 255 fuel pump
N62 MAF
apexi SAFC II
tial 38mm WG w/ 1 bar spring
cosworth 1.6mm HG
Lightweight flywheel
ACT 6 puck Sprung rated to 450ft/lb
Garret Gt35r .63 AR with anti surge compressor housing
Megan racing 3in exhaust
FMIC 12x24x3
ISIS Turbo lines
Stock IM
Stock TB
And apparantly a built bottom end (Read lower)

I had the head built and was going to hold off on the bottom end as I ran out of funds.

1. While getting my motor rebuilt, My builder calls me and tells me that I already have forged pistons (not sure about the rods yet) and I didnt know this. While Im not complaning by any means Im not sure of the brand or Compression ration of the pistons. How do I figure out the brand and CR of the pistons? Are there any markings on the pistons themselves?

2. Also I opted to go with a Cosworth 1.6mm HG instead of a cometic as i've heard mixed reviews about them. So hypothetically if the pistons are 8.5:1's how much will the 1.6mm HG lower the over compression? Will it be a negligble amount or will it be worth noting when I send enthalpy my list to have a rom tune made?

3. Is a nistune setup worth getting if I have no tuning experience over an enthalpy setup?(i have a safc to fine tune). however there will be a shop about 35-40 mins away from me that tunes nistune. However when i factor in dyno time and all that the enthalpy is a cheaper route. Any input/advice would be appreciated.

SuperiorS14
10-20-2009, 11:21 AM
yes the 1.6mm gasket will drop the compression

and dont build a motor and then try and save money on tuning your car. get it done right and have someone tune it with a standalone(dont know much about nistune)

Darkist 240sx
10-20-2009, 12:10 PM
I appreciate the quick response. However I was looking for more of a figure as to how much It would lower the CR? Any ideas?

And Its not that I'm trying to skimp on the tuning, I was under the impression that a enthalpy rom tune was adequate, but maybe I will just lean more towards the standalone/nistune options.

anyone else have any ideas as to the actual figure that It might lower the CR?
Or how I could identifty the pistons brand and CR?

ericb382
10-20-2009, 03:23 PM
If you get an enthalpy just tell him that you have a 1.6 HG. He'll know how to tune for that. I say get nistune though and have it tuned by the shop near you.

Darkist 240sx
10-20-2009, 03:53 PM
thanks alot.

jspaeth
10-20-2009, 08:30 PM
thanks alot.


I did the calculation before.....it really isn't hard.

- 2.0 L displacement, with a compression ratio of 8.5:1
- This is 0.5 L per cylinder.
- 8.5 "corresponds" to total volume, 1 corresponds to compressed volume, so set 0.5L = (8.5-1).....this means that 0.5L = 7.5*compressed volume.

- This means compressed volume = 0.06666667 L and total volume of the cylinder is 0.56666667 L.


- Now, add volume to the cylinder.....this basically increases the total volume, but not the displacement.

- volume added is the increase in HG thickness * area of the cylinder.

- So, a 1.6 mm is 0.5 thicker than stock, so the added volume is:

Added volume = pi*(43 mm)^2*0.5mm (43 mm is the cylinder radius)

Added volume = 2903 mm^3 = 0.002903 L.

Now, the compressed volume is 0.066666667L + 0.002903L = 0.069571L


The new total volume is 0.5L + 0.069571L = 0.569571L.


The NEW compression ratio is the ratio of the new total volume to the new compressed volume

New CR = 0.569571L/0.069571L = 8.186


So this will decrease you compression ratio from 8.5 to 8.186



Bingo!

Devil Man
10-21-2009, 03:31 PM
so are you implying if i read correctly that this motor will now be a 2.5 liter? with a compression ratio of 8.1 to 1?

jspaeth
10-21-2009, 03:58 PM
so are you implying if i read correctly that this motor will now be a 2.5 liter? with a compression ratio of 8.1 to 1?


No....you must have misread what I wrote.

The "displacement" is just that....the volume displaced as the cylinders move up and down through a full stroke.


Changing the headgasket thickness does not change your displacement, only your compression ratio.

Darkist 240sx
10-22-2009, 02:48 PM
I did the calculation before.....it really isn't hard.

- 2.0 L displacement, with a compression ratio of 8.5:1
- This is 0.5 L per cylinder.
- 8.5 "corresponds" to total volume, 1 corresponds to compressed volume, so set 0.5L = (8.5-1).....this means that 0.5L = 7.5*compressed volume.

- This means compressed volume = 0.06666667 L and total volume of the cylinder is 0.56666667 L.


- Now, add volume to the cylinder.....this basically increases the total volume, but not the displacement.

- volume added is the increase in HG thickness * area of the cylinder.

- So, a 1.6 mm is 0.5 thicker than stock, so the added volume is:

Added volume = pi*(43 mm)^2*0.5mm (43 mm is the cylinder radius)

Added volume = 2903 mm^3 = 0.002903 L.

Now, the compressed volume is 0.066666667L + 0.002903L = 0.069571L


The new total volume is 0.5L + 0.069571L = 0.569571L.


The NEW compression ratio is the ratio of the new total volume to the new compressed volume

New CR = 0.569571L/0.069571L = 8.186


So this will decrease you compression ratio from 8.5 to 8.186



Bingo!

Dude you are the man:snoop:. I was expecting about a .2-.3 drop in CR but now I actually see the math behind it. So besides being able to possibly run more boost with less chance of knock are there any Pro's or Con's to having a 8.186 vs 8.5 CR?

Also I decided to go with Nistune and am having the car tuned at Autovaugh Performance in TN. So at least I dont have to worry about getting a mail order tune and the effects the New CR might have whether it be more or less power. Now that it will be dyno tuned It will be optimized for whatever I have in the car anyway!

Once again thanks!

p.s. Since Im going with Nistune should I keep my SAFCII or sell it. Normally i would just sell it, but im not buying the software or consult cable with the nistune, I dont trust my tuning abilities and dont want to blow my motor:bite:. So i'm thinking it might be a good idea to keep it around for minor modifications like if temperature changes a fair amount. Advice is always welcomed!

jspaeth
10-22-2009, 03:06 PM
Basically, I am good at math and understand how to calculate that, but honestly, I don't know how C.R. will effect tuning/performance.

I know high performance N/A cars run high compression ratios, and I know that raising the compression ratio too much on a turbo car can make it go boom

But other than that, I don't really can't help ya

jr_ss
10-22-2009, 08:04 PM
I know that raising the comp ratio on a turbo motor will give you better off boost response, so I'd assuming the opposite in your case. Lowering comp will allow you to run more boost, but doesn't necessarily mean you'll net more power up top. I would so think that a lower comp ratio would lead to a bit more turbo lag.

ericb382
10-22-2009, 10:35 PM
Yes. Running higher compression ratio means better off boost response. It doesn't however mean less lag. On a given turbo the lag depends on displacement.(Obviously other factors come into play such as tune, volume of air from turbo to head, etc, but displacement being the main factor.) More air being displaced means more air turning into exhaust, turning the turbine. You do get through the lag quicker though because of the better power before the turbo comes in. This is how I understand it, someone please correct me if I'm wrong.

If you lower the compression ratio this allows you to run more boost without backing off the timing or adding higher octane.

So basically, you can get more power up top safer on pump gas, at the expense of your off boost response.

*edit*
After thinking about it a little bit, the higher compression ratio will cause the exhaust gas to exit a little more violently causing a little quicker spool, but I don't think it would be that major of a difference.

johngriff
10-22-2009, 11:48 PM
Yes. Running higher compression ratio means better off boost response. It doesn't however mean less lag. On a given turbo the lag depends on displacement.(Obviously other factors come into play such as tune, volume of air from turbo to head, etc, but displacement being the main factor.) More air being displaced means more air turning into exhaust, turning the turbine. You do get through the lag quicker though because of the better power before the turbo comes in. This is how I understand it, someone please correct me if I'm wrong.

If you lower the compression ratio this allows you to run more boost without backing off the timing or adding higher octane.

So basically, you can get more power up top safer on pump gas, at the expense of your off boost response.

*edit*
After thinking about it a little bit, the higher compression ratio will cause the exhaust gas to exit a little more violently causing a little quicker spool, but I don't think it would be that major of a difference.

The increase in compression will increase CFM. Making the turbo spool faster.

Lowering your C/R to run more boost is not suggested by me. That is the old way to do things, before you could fine tune ignition using advanced EMS systems. Now the way to go is run as high a CR as you can keep cool, and tune the ignition map.

ericb382
10-25-2009, 02:47 PM
The increase in compression will increase CFM. Making the turbo spool faster.


But increasing compression does not increase CFM. If the bore, stroke, number of cylinders (obviously), flow characteristics and boost pressure stay the same, the amount of air being displaced in the cylinder is the same.

The amount of air actually in the same cylinder with a higher CR would be less, because more volume is being used by the piston (assuming you raised the CR by using high compression pistons.) The displacement however is the same, since the bore, stroke, etc haven't changed, neither has the amount of air being displaced.

Not sure if I wrote that clearly, but basically if the diameter of the piston (bore) and the amount the piston moves (stroke) stays the same, the amount of air the piston has displaced has stayed the same.
Lowering your C/R to run more boost is not suggested by me. That is the old way to do things, before you could fine tune ignition using advanced EMS systems. Now the way to go is run as high a CR as you can keep cool, and tune the ignition map.
I agree with you on this point, except in a drag only/dyno queen setup. However, I've never built a setup like that nor done much research on it, my personal opinion is that any sr under 500hp should have a higher CR. Above that point I would THINK that the timing you could add by not raising the CR would help more than the benefit of the higher CR in the high RPM which is obviously the area you're concerned with in such a set-up. Like I said though, I have no experience in that type of build.