Man just run it on e85 it will be off tap.

To measure static C/R you need 2 things:

Swept volume : the area of the bore when piston is at bott of stroke to get that. Bore x bore x .7854(pie) x stroke devide 1000 = sv in cc's

 

Clearance volume : volume of head chamber, compressed head gasket and piston chamber added together.

 

Then it's just: sv + cv devide by cv = static comp

 

stu.

Edited February 17, 201115 yr by kickn5k

The first question you will get asked is "how thick do you want that spacer?"

 

So you need to know your compression ratio now, as kickn5K said, and then you need to decide what you want it to run at when you're finished, and then you can work out the thickness of that spacer to drop the CR that much.

 

The last gasket I took off my car is sitting in the garage and its 1.5mm thick. Maybe it compresses a little, but that is close to what you would lift the head by if you used two gaskets.

 

When I measured the head volume I got 31ml, the gasket gives 7.3ml, and the cylinder is 75 by 73, so 322cc. So it sucks air into 31+7.3+322=360.3ml and compresses that into just the head and gasket, 31+7.3=38.3 So my compression ratio is 360.3/38.3 which is 9.4 to one.

 

If I ran two gaskets I would have 31+7.3+7.3+322=367.6 compressing to 31+7.3+7.3=45.6 giving 8 to 1.

 

Go measure yours and tell us what you get. You will have to measure the volume of the piston dish as well.

...and I love pointing all this out to young guys who always wagged maths at school 'cos they hated it and said 'they'd never need maths in real life..' :laff:

Thanks for all that

 

Now what comp ratio is desired?

 

Based on my measurements I'm getting around 13.9 : 1...

 

Bore: 81 stroke: 74 and 21cc for the head plus adding for gasket and dish in piston

 

I am taking the head to an engine builder so he will confirm. But I will get him to work out what I need to lower the comp with a figure i give him. Just need to know what I should be aiming for?

Between 10 and 11 I'd say, but you can run more or slightly less.

 

The higher it goes, the more power you get, the shorter the time the rings last, the harder time you give the gasket and the higher the octane fuel you have to use.

 

The lower you go, the less power you get and everything lasts longer. Stock is 9.4 or so. The 10% alcohol fuel that is 95octane (E10) will handle 10:1 OK, I'm not sure when you'd need to use 98 octane only. I use E10 as its the cheapest for as long as the Govt is stupid enough to subsidize it, and I don't think the motor will run on 91 anymore.

 

Two gaskets will give you about 10.3 to 1 going on your figures and assuming 5ml in the piston dish. So, cheap fix two gaskets, better fix a 1.5mm plate under one gasket. See what your head man reckons.

Thanks for all thatNow what comp ratio is desired?Based on my measurements I'm getting around 13.9 : 1

 

Too high. Read this webpage and then download the Dynamic CR calculator. You'll need cam specs as well, to target a Dynamic comp ratio. (the 13.9 you have measured is Static compression, the website explains the difference)

 

http://www.empirenet.com/pkelley2/DynamicCR.html

 

I suspect you wont get your DCR down low enough, even with a very large grind.

 

Between 10 and 11 I'd say, but you can run more or slightly less.The higher it goes, the more power you get, the shorter the time the rings last, the harder time you give the gasket and the higher the octane fuel you have to use.The lower you go, the less power you get and everything lasts longer. Stock is 9.4 or so. The 10% alcohol fuel that is 95octane (E10) will handle 10:1 OK, I'm not sure when you'd need to use 98 octane only. I use E10 as its the cheapest for as long as the Govt is stupid enough to subsidize it, and I don't think the motor will run on 91 anymore.Two gaskets will give you about 10.3 to 1 going on your figures and assuming 5ml in the piston dish. So, cheap fix two gaskets, better fix a 1.5mm plate under one gasket. See what your head man reckons.

 

Honestly, 2 head gaskets is a bullshit K-sev full-sick drift-hack fix. Twice the gaskets = twice the chance of failure.

 

My engine builder advised me that .038-.040 inches is the value they work with for a compressed gasket thickness on a K motor. That's around the 1mm mark.

 

You are taking the engine to a builder, let him look at your compression ratio's. The best thing to do in this case is to increase the volume in the head, its fairly straightforward as there is plenty of meat there for them to remove. Do it once, do it properly.

I'll let you know what the engine builder says. I dropped it of this morning, so hopefully he gets back to me by this afternoon. He said just by looking at it, the head has been shaved alot. He will work it out and also the difference with the 1.5mm spacer that can be put on.

 

Note taken 2 gaskets = twice the chance of failure. So whats your opinion with the 1.5mm spacer?

 

Other way out is 3 or 4K head? or increase the voloume in this head to suit.

 

I will look at the difference between static and dynamic, thanks.

there is plenty of meat there for them to remove

 

I thought of that Mr P, but someone has already been over the head and unshrouded the valves right out to the gasket and around the plugs. Do you chop out the squish area or try not to hit a water jacket around the plugs? It looks like someone was aware of this problem and has already done everything they could to enlarge the head volume.

 

I figure now its all up to the quality of the head engineer you find. A bad one could wreck it for you, a good one give you something that will work fine. That's why I think the spacer is best, but two gaskets are the cheap option.

ok soo.. current comp is 12.65:1. Head has 20.6cc, does anyone know what is standard? Apparetly it has been shaved to the sht house.. Standard gasket won't last with that comp as seen. and can't get a multilayer steel gasket for the 5k.

 

.8mm shim would reduce it to 11.3:1 which they are not advising but not totally against..

 

1.5mm which are out of stock will get 10.35:1 which would be safer but again they do not really advise of shims at all.

 

Looks like 4k or 3K head to get me out of this one..

Are your pistons out of the engine? I could knock enough out of the bowls of the pistons for you to get it right. Would end up as a round bowl not a d shape which I could do but I doubt I'd get enough out of em. If your interested let me know.

Also you could try ridgecrest engineering in Vic they do a 2mm copper gasket.

On the spacer + gasket I wouldn't be keen to do it to my engine but doing what I do I don't need to worry I just fix it properlly.

Spacer is marginally better than 2 head gaskets but still dodgy.

 

Altezzaclub I'd have no qualms about opening the compression chamber to the full periphery of the cylinder instead of the kidney shape it is currently. I reckon taking out the little bulge in between valves would have minimal impact and gain some volume.

 

Failing that source a head which hasn't been skimmed to excess.

 

Flycutting pistons or opening out the bowl would be another good option as kickn5k mentioned.

Pistons are still in the block. Prefer not to touch the block if I can, but thanks.

 

I'll think about that 2mm copper gasket, thanks.

 

I've got my hands on a 3k head (apparently its a big port - will need to confirm the welse plug othe rear of it). Would this head suit? I guess I will need to do the same tests on it.

I'll cc my bigport tomoz as it's at work and I'll post it up for you. It looks like it's about half the difference between my flat top piston 4k head and my 10.2:1 with the big dish piston 5k head(by eye). I also have a big chamber head at work so while I've got the burrete out I'll check that too.

 

Stu.

I reckon taking out the little bulge in between valves would have minimal impact and gain some volume.

 

Well, I suppose its really meant to be a bowl-in-piston combustion chamber, so the squish area isn't so vital. It won't take much to work out how many cc to machine out of each chamber.

 

Looking at the head work that's been done, and I assume its been ported and polished, you'll be able to sell this head on John if you get another one. There must be someone with deep dish pistons who would want it.

Yeh I might be interested....