20 replies to this topic

[BaronVonchesto]

Switching to flat top pistons, so I need to recalculate my CR and do some adjustments as my head volume is too low (ie current CR is too high).

 

Block was previously decked so that the pistons come up flush with the block at TDC. With these flat top pistons however they appear to be ever so slightly shorter, average depth from the block face at TDC is 0.18mm (at least according to my vernier caliper).

 

Head was previously skimmed to increase the CR when i was using dished pistons. Just checked the volume with the water + syringe method --> averages to 20.1cc

 

I'm back to a standard 998 bore, so diameter = 64.15mm, depth (at BDC) = 76.67mm.

 

Swept Vol = 247.8cc (pi * 3.2^2 * 7.67)

Unswept Vol = 25.44

 

That means the CR is 10.74.

 

Now, considering that adjusting the timing hasn't helped with pinking even with RON 97 fuel and just the lightest touch of the throttle, I'd reckon the CR is closer to the high side of that range.

 

Suppose I target a CR of 9.6 (since thats about the most you can have if you want to use RON 95 fuel) that means I need to increase the Head's chamber vol from 20.1 to 23.5cc.

 

The big question is, how do I ask the machine shop to cut away at the chamber to achieve this? here's a picture of my head. The boundaries of the Head gasket are clearly visible, so there's quite some room to cut away. I suppose I can cut away at this lip here? Is it going to be a trial and error process where they keep shaving away at the metal and measuring the volume? or I suppose I could approximately calculate how far to cut that would give me my target volume?

 

 cylinder_head.jpg   38.95K   3 downloads

Edit: Updated measurements and CR after taking more accurate measurements

Edited by BaronVonchesto, 01 May 2023 - 12:39 PM.

[BaronVonchesto]

PS: For what it's worth i'm using a Minispares EVOLUTION001 cam timed to 107 degrees.

and Moke suggested that for my current CR my be suited to that cam in this thread

[DeadSquare]

People with a cherished, expensively worked head on a 998, used to fit two head gasket if they traded up to a 1098.

[Avtovaz]

Think you need to know what cam it has and the spec of the cam, when you work out the static compression it can give you a high number, but actual compression ratio with the cam opening times is different.

 

I have a volvo b230 engine, its had 2.5mm skimmed from the head and with a normal cam it would have massive compression, but it hasnt, its got a 300 deg cam in it, so some of the compression is lost in the stroke. hth

[Lplus]

Switching to flat top pistons, so I need to recalculate my CR and do some adjustments as my head volume is too low (ie current CR is too high).

 

Block was previously decked so that the pistons come up flush with the block at TDC. With these flat top pistons however they appear to be ever so slightly shorter, so theres <1mm depth from the block face at TDC (at least according to my vernier caliper).

 

Head was previously skimmed to increase the CR when i was using dished pistons. Just checked the volume with the water + syringe method --> averages to 20.1cc

 

I'm back to a standard 998 bore, so diameter = 64mm, depth (at BDC) = 77mm.

 

Swept Vol = 247.7cc (pi * 3.2^2 * 7.7)

Unswept Vol = 26.45 if I estimate the TDC height at 0.5mm, or 24.85cc if the piston is perfectly flush.

 

That means the CR is 10.36 - 10.97.

 

Now, considering that adjusting the timing hasn't helped with pinking even with RON 97 fuel and just the lightest touch of the throttle, I'd reckon the CR is closer to the high side of that range.

 

Suppose I target a CR of 9.6 (since thats about the most you can have if you want to use RON 95 fuel) that means I need to increase the Head's chamber vol from 20.1 to  22.5-24.2cc.

 

The big question is, how do I ask the machine shop to cut away at the chamber to achieve this? here's a picture of my head. The boundaries of the Head gasket are clearly visible, so there's quite some room to cut away. I suppose I can cut away at this lip here? Is it going to be a trial and error process where they keep shaving away at the metal and measuring the volume? or I suppose I could approximately calculate how far to cut that would give me my target volume?

 

cylinder_head.jpg

Find a photo of a 12g295 cylinder head combustion chamber and work towards that as far as necessary to achieve the chamber volume you want.

[BaronVonchesto]

Think you need to know what cam it has and the spec of the cam, when you work out the static compression it can give you a high number, but actual compression ratio with the cam opening times is different.

 

I have a volvo b230 engine, its had 2.5mm skimmed from the head and with a normal cam it would have massive compression, but it hasnt, its got a 300 deg cam in it, so some of the compression is lost in the stroke. hth

 

Think you need to know what cam it has and the spec of the cam, when you work out the static compression it can give you a high number, but actual compression ratio with the cam opening times is different.

 

I have a volvo b230 engine, its had 2.5mm skimmed from the head and with a normal cam it would have massive compression, but it hasnt, its got a 300 deg cam in it, so some of the compression is lost in the stroke. hth

I already specified my cam here :)

 

PS: For what it's worth i'm using a Minispares EVOLUTION001 cam timed to 107 degrees.

and Moke suggested that for my current CR my be suited to that cam in this thread

[Spider]

PS: For what it's worth i'm using a Minispares EVOLUTION001 cam timed to 107 degrees.

and Moke suggested that for my current CR my be suited to that cam in this thread

 

Just to clarify here, I didn't say it was suited, I said it may be suited. You need to do a DCR calculation to see what you need in terms of Static CR. I would suggest a DRC number of 8.4:1. This may equate to a SCR of anything from 8.8:1 to 13:1.

I would have done one for you, only I don't have the Opening and Closing Angles of the Cam, the only angles given are for total duration.

All the same, you'd be best off measuring those angles on what's fitted rather than relying on advertised numbers.

[Shooter63]

Would it not be easier to machine a dish in the pistons

Shooter

[BaronVonchesto]

Would it not be easier to machine a dish in the pistons

Shooter

the pistons were already fitted and put back into the car. the guy who built my engine didn't bother calculating the compression before assembling it. Now i'm trying to correct. hence why i need to mod the head.

Not interested in taking the whole engine out again.

 

Also its a lot easier to get 998 flat top pistons for press fit conrods. So i'm happy to stick with flat tops for the future

[Shooter63]

Would it not be easier to machine a dish in the pistons

Shooter

the pistons were already fitted and put back into the car. the guy who built my engine didn't bother calculating the compression before assembling it. Now i'm trying to correct. hence why i need to mod the head.
Not interested in taking the whole engine out again.
 
Also its a lot easier to get 998 flat top pistons for press fit conrods. So i'm happy to stick with flat tops for the future

In that case it's out with the die grinder mate and start grinding the cylinder walls around the inlet valve, but you must get an accurate measurement of how far down the bores the pistons are at TDC, it makes a massive difference in the calculation as shown when I posted the spread sheet on the other thread you started on a similar subject ( no need to thank me by the way)

Shooter

[BaronVonchesto]

I was able to get my digital vernier working again so Updated with more accurate measurements.

 

 

 

 

Would it not be easier to machine a dish in the pistons

Shooter

the pistons were already fitted and put back into the car. the guy who built my engine didn't bother calculating the compression before assembling it. Now i'm trying to correct. hence why i need to mod the head.
Not interested in taking the whole engine out again.
 
Also its a lot easier to get 998 flat top pistons for press fit conrods. So i'm happy to stick with flat tops for the future

In that case it's out with the die grinder mate and start grinding the cylinder walls around the inlet valve, but you must get an accurate measurement of how far down the bores the pistons are at TDC, it makes a massive difference in the calculation as shown when I posted the spread sheet on the other thread you started on a similar subject ( no need to thank me by the way)

Shooter

 

 

Yes I did see that table thanks. I notice you are using a gasket vol of 2.8 whereas Cooperman's guide estimates 4. Any reason for that? I'm using a victor reinz gasket which is a fair bit thicker than the standard copper gasket.

 

Anyway now I know what my target head vol is. I have a dremel, though I think I'd be more comfortable sending the head to the machine shop, if they are willing to do it.

 

Question, why grind only around the inlet valve? Why not grind off the lip between the intake and exhaust valves?

Here is a link to an image of a 12g295 head and it does appear that they grind more around the intake valve, though they also ground down the lip a fair bit

Edited by BaronVonchesto, 01 May 2023 - 12:47 PM.

[gazza82]

What does Mr Vizard say in his "bible"?? There might even be some template shapes to work to.

[Shooter63]

I was able to get my digital vernier working again so Updated with more accurate measurements.
 
 

Would it not be easier to machine a dish in the pistons

Shooter

the pistons were already fitted and put back into the car. the guy who built my engine didn't bother calculating the compression before assembling it. Now i'm trying to correct. hence why i need to mod the head.
Not interested in taking the whole engine out again.
 
Also its a lot easier to get 998 flat top pistons for press fit conrods. So i'm happy to stick with flat tops for the future

In that case it's out with the die grinder mate and start grinding the cylinder walls around the inlet valve, but you must get an accurate measurement of how far down the bores the pistons are at TDC, it makes a massive difference in the calculation as shown when I posted the spread sheet on the other thread you started on a similar subject ( no need to thank me by the way)

Shooter

 
Yes I did see that table thanks. I notice you are using a gasket vol of 2.8 whereas Cooperman's guide estimates 4. Any reason for that? I'm using a victor reinz gasket which is a fair bit thicker than the standard copper gasket.
 
Anyway now I know what my target head vol is. I have a dremel, though I think I'd be more comfortable sending the head to the machine shop, if they are willing to do it.
 
Question, why grind only around the inlet valve? Why not grind off the lip between the intake and exhaust valves?
Here is a link to an image of a 12g295 head and it does appear that they grind more around the intake valve, though they also ground down the lip a fair bit

If you grind on the short side radius you will naturally loose the beak, I've never known a small bore head gasket to have a capacity of 4.00cc. I happen to have the same gasket (Victor Reinz), so I'll measure it and report back.

Shooter

[BaronVonchesto]

What does Mr Vizard say in his "bible"?? There might even be some template shapes to work to.

 

does anyone have the yellow bible as a reference? I don't

[BaronVonchesto]

I used masking tape to make a copy of the shape of the head chamber, then photographed it as level as I could. then transferred that to my PC and opened it in GIMP. It matches to exactly 48 pixels per mm with a little bit of scaling, so i made a grid of 1/4 mm (12x12 pixels) and then drew out these shapes that gives me the approx. volume I want to increase (3.4cc)

 

Came up with 2 shapes.

Shape 1 (based on a simple oval) - obviously easier to grind.

 shape2.jpg   45.06K   0 downloads

 

Shape 1 - based on 2 scaled circles of the valves. looks closer to the 12g295 heads i've seen in pictures.

 shape1.jpg   44.83K   0 downloads

 

If either of these are good, I can make a template and give it to my local machine shop to cut out.

 

Thoughts?