Cylinder into head depth

[spoolin70]

Hi all

I'm hoping this will be an easy enough question and everyone will agree so here goes :

What's an exceptable minimum depth for a cylinder to seat into a head chamber ? See pic below.

[ Attachment: You are not allowed to view attachments ]

I'm trying to loose a lot of compression and cylinder shims/spacers are part of the answer
but how deep is deep enough

I've got 11.5mm chamber wall depth.
Heads are 4" MS250, 6 stud with Raceware studs. Engine will see about 30psi max of boost.

Any advice gratefully received as this is driving me mad.

Thanks
Darren

[Shag55]

Shallower is better. If you are using long cylinders 1cut the head side so that the 1st fin is 1.5mm or so from the head. Remember the cylinder that goes in the head bore is the weaker point and sees the most cylinder pressure.

[spoolin70]

Hi Shag
When you say shallower is better I'm guessing your referring to the spacer ?

If my wall depth is 11.5mm, how thick a spacer can I safely use ?
Something around 5mm would be great but I'm worried the cylinder won't be in the chamber enough.

I'm not even thinking about flame travel, quench, squish etc yet just trying to get the comp down.

Thanks
Darren

[K-Roc]

I would minimise the shimming and dish the piston. keep the quench area nice and tight for best combustion.

cheers

[spoolin70]

New pistons are a possibility but to get the comp down to where I'd want it, I'd need a dish of around 40cc.
This sounds massive to me !

Would it not ruin the quench area you mention K-Roc ?

I'm at 13.5:1 with a 12cc dish already with zero deck and need to be at around 7.75:1

A 20cc piston dish and 5mm deck hight gives me 7.61:1

What effect would a 5mm deck hight have ? Again it sounds massive (and wrong)

Thanks
Darren

[Erlend / bug66]

5mm would ruin the combustion..

Keep the NA spec on the heads, and order a custom set 

[Shag55]

I am referring to your depth as indicated in the pic with the calipers. As for the shim. I stick to 1mm thick copper shims on my build but not all that necessary. Use 1.5mm and deck what is needed to get the compression where you want it. A turbo motor doesn't rely on  tight deck for combustion like a NA motor. High cylinder pressures insure cumbustion as long as spark is adequate. No need to dish the pistons either. This is old school thinking and best for a pent roof chamber anyways.

[JLaw]

Shag55, have you had experience with running as much as 5mm negative deck? Following on from what you said earlier the weakest point of the barrel is at the head and I have heard of increased risk of cracking barrels when running large negative deck. I cant see this being an issue if you chose to use dished pistons with a more conventional deck height of around 1mm.

40cc does seem a large dish though..

Cheers.

[turbovan]

New pistons are a possibility but to get the comp down to where I'd want it, I'd need a dish of around 40cc.
This sounds massive to me !

Would it not ruin the quench area you mention K-Roc ?

I'm at 13.5:1 with a 12cc dish already with zero deck and need to be at around 7.75:1

A 20cc piston dish and 5mm deck hight gives me 7.61:1

What effect would a 5mm deck hight have ? Again it sounds massive (and wrong)

Thanks
Darren

Darren a 20cc dish in them pistons makes the crown to thin,i went there!!!!!
maybe a set of one off pistons JE will make them,, and pauter made me one off
barrels my measurements,, were good value also,, far better for a turbo motor..

chris

[spoolin70]

Hi Chris
I didn't know you'd had that happen.
Have you got 20cc JE pistons now ?

Looks like I might be going for somewhere around 35cc dish.

Thanks
Darren

[Shag55]

Shag55, have you had experience with running as much as 5mm negative deck? Following on from what you said earlier the weakest point of the barrel is at the head and I have heard of increased risk of cracking barrels when running large negative deck. I cant see this being an issue if you chose to use dished pistons with a more conventional deck height of around 1mm.

40cc does seem a large dish though..

Cheers.

Yes, though it is not ideal it works. He already has the pistons so use them. When using long stroke cranks it's hard to set up a tight deck. Yes the barrel is more prone to cracking but with a proper tune you should be fine.
Good barrels like Pauters or AC hold up better. 

[Fiatdude]

Are you planning on running alcohol? If so, your static compresssion can be up around 10.5 to 11.5 with no issues and that will allow you to run at lower boost to get the same HP....

Have you cut any valve pockets yet into your pistons??? these are 35cc dished 4" ,and the 4.030 I have in there now are 42cc

[spoolin70]

I plan on drinking alcohol when it's done 
Just gasoline being burnt in this engine, with a little water/methanol too.

I thought the usual for a turbo engine was 7.5:1 to 8.5:1 ?
It's a street engine primarily with hopefully around 30psi boost.

No valve pockets in my pistons just the dishes. There isn't enough in them to open
them up to the required CC though so I will be looking for some replacements soon.

What make are those in the picture ? JE

Thanks
Darren

[BeetleBug]

I plan on drinking alcohol when it's done 
Just gasoline being burnt in this engine, with a little water/methanol too.

I thought the usual for a turbo engine was 7.5:1 to 8.5:1 ?
It's a street engine primarily with hopefully around 30psi boost.

No valve pockets in my pistons just the dishes. There isn't enough in them to open
them up to the required CC though so I will be looking for some replacements soon.

What make are those in the picture ? JE

Thanks
Darren

30 psi on ordinary gasoline will be interesting. Even with methanol injection

[spoolin70]

Hole in a piston interesting 

Is this not achievable then ?

I'm not set on 30psi and while I like the idea of E85 or methanol it's just not
practical enough for me.

Thanks
Darren

[Fiatdude]

pump gas --- maybe 10-12 psi -- -- depends on final compression -- don't know but alcohol is way cheaper to run than race gas -- and is cooler for the streets --

[spoolin70]

I'm at 18psi at the moment on my small engine with 7.5:1 comp. Went to 20psi no problem but injectors were close to maxing out so I left it safe at 18psi.

So I figured 25-30psi was achievable but it's not a figure I have to run. I'll try and spec everything to 30psi, then if I only need 22psi, I know I'm safe 

Thanks
Darren