Pro s and cons on small deckheigt

[Eddie DVK]

Goodday,

Don t want to stirr things up... just want your guys view on it.
Ok I am not ready what to choose on the deckheight for my new type 4 built.
If I read all the topics about it the sweet spot is between 1.5mm and 1.2mm.
Butt here is the thing the heads I have, have a combustion chamber of 50cc, the cam i have is a 86b webcam
which likes 10.5:1.
Well butt if I do the math with my combo 100mmx78mm, I will end up with a deckheight of 1,8mm.
Will be using 95/98 gas.
The strange part is I have built a 2.0 with the same heads and a 316 schleicher cam which likes 9.5:1 and used a 2.0mm deckheigt to get the right
compression ratio. that engine ran fine, had a max timing of 32gr with 3000rpms.
This was before I read stuff that you should not go further than 1.5 with your Deck height.
So I did a small rebuilt and change the deckheight on this engine to 1.25mm.
The engine started misfiring with the 32gr advance, so have adjusted this to 28gr at 3000 and the misfire is gone.

So what do you guys think I should do:
1.8mm with the right copression ratio or
1.5mm with the higher compression ratio (11:1)..

Thanks in advance  

[Dougy Dee]

What ignition set up are you using?

[richie]

Hi Eddie

did you adjust jetting at all after you lowered deckheight? guessing your compression went up? I try for 1.15mm to 1.25mm when ever possible, but do go up to 1.5mm if I have to, above that I would rather adjust chamber or add a dish to piston to get compression down

cheers Richei

[brian e]

I built a 2276 a few years back.  It was my second engine, and I was still new.  Standard build, w125, Tims S2 heads, 9.5, 44's etc...  The heads had a .055" step in them.  The chamber volume measurement included this step.  I then set the piston .045" below the top of the cylinder.  Once the engie was running, it always seemed slugigsh.  Made OK power, but not great.  It would run hot, and ping under load and slower speeds, even with the timing pulled back.  I had to tear the engine down to fix some rookie oil leaks.  While it was apart, I cut the step out of the heads, and cut a dish in the pistons.  I put it all back to gether with the same 9.5 as before, only now it had .045" between the pistons and the flats of the chabers. 

It was like a whole new engine.  Super crisp throttle response, no more pimging, easier tuning, and a smoother lower idle.  I also dropped some on the main jets.  I have since "cleaned up" a few other engines since using the same method, and I always have a very positive result.  Won't build an engine now with more than .050" deck.  I usually shoot for .045" when machining the case, and I have .035" on my own experimental engine.  Works great. 

Brian

[Eddie DVK]

What ignition set up are you using?

It is transistor hal unit, with strong spark.

[Eddie DVK]

Hi Eddie

did you adjust jetting at all after you lowered deckheight? guessing your compression went up? I try for 1.15mm to 1.25mm when ever possible, but do go up to 1.5mm if I have to, above that I would rather adjust chamber or add a dish to piston to get compression down

cheers Richei

Ok understand, Think I have to cut a dish in the pistons then, Not sure were I can do that around here...
Yes had the jetting a bit adjusted only one size if i remember correctly.

Thanks

[MeXX]

Hi...

To make the squish area work..... 1.5mm or less is needed....
If deck height is to big it is just dead area... causing poor efficiency

MeXX

[Eddie DVK]

Hi...

To make the squish area work..... 1.5mm or less is needed....
If deck height is to big it is just dead area... causing poor efficiency

MeXX

Ok thanks will aim for that.

[Martin S.]

I built a 2276 a few years back.  It was my second engine, and I was still new.  Standard build, w125, Tims S2 heads, 9.5, 44's etc...  The heads had a .055" step in them.  The chamber volume measurement included this step.  I then set the piston .045" below the top of the cylinder.  Once the engie was running, it always seemed slugigsh.  Made OK power, but not great.  It would run hot, and ping under load and slower speeds, even with the timing pulled back.  I had to tear the engine down to fix some rookie oil leaks.  While it was apart, I cut the step out of the heads, and cut a dish in the pistons.  I put it all back to gether with the same 9.5 as before, only now it had .045" between the pistons and the flats of the chabers. 

It was like a whole new engine.  Super crisp throttle response, no more pimging, easier tuning, and a smoother lower idle.  I also dropped some on the main jets.  I have since "cleaned up" a few other engines since using the same method, and I always have a very positive result.  Won't build an engine now with more than .050" deck.  I usually shoot for .045" when machining the case, and I have .035" on my own experimental engine.  Works great. 

Brian

.035" was what we figured would be ideal after my mild 1776 engine got .030" and you can hear the piston hit the head when started cold. After a few minutes the piston expands and goes straight in the bore and all is well. Very snappy response on that engine!

[mikko k]

Required distance from piston to head also depends on diameter of piston and how short piston is. Basically, the bigger and shorter piston will rock more in cylinder. And then hit to the head.

I had ones engine with short 103mm pistons and 1,1mm deck and pistons did hit to head.

1,2 to 1,3mm deck should be quite safe. I have never over 1,5mm.

[Martin S.]

Required distance from piston to head also depends on diameter of piston and how short piston is. Basically, the bigger and shorter piston will rock more in cylinder. And then hit to the head.

I had ones engine with short 103mm pistons and 1,1mm deck and pistons did hit to head.

1,2 to 1,3mm deck should be quite safe. I have never over 1,5mm.

Good point! I'm guessing that piston to cylinder clearance would make a difference too with the rocking. With my engine, I didn't bother fixing it and it's been running for years with hitting the head during warm up. The bearings are still original from 20 years ago and has good oil pressure. This is a daily type street car that has been on the road every year and driven a lot!
From this experience I recommend .040" (1mm).

For pros and cons, I can't think of any cons with my extra-low deck-height 1776, and the pros are more power, cooler, meaner sounding motor that wants to GO!

[Eddie DVK]

Ok thanks for the replies.
Aiming than below the 1.5mm, have to figure out where I can gain some extra cc's.

Regards

[Martin S.]

Here's where 7 of mine came from