Piston to head clearance

[Parksy]

Hi all

I've almost completed my v4 engine and attempted a start. While it does fire, it isn't self sustaining and I've attributed this to a low compression ratio.
I'm not familiar with what my compression ratio is as I don't know what capacity I've machined the chamber in the heads, but I did make the piston to head clearance at 2mm which I'm thinking is too much. I don't run a head gasket. When turning the engine by hand, there aren't any leaks and it does feel compression is there, but not enough.

My question is, what is the largest piston to head clearance someone has had success with a running engine, and what clearance should I be aiming for?

Thanks all

Andy

 

[Cogsy]

How much have you had it firing? The compression is likely to increase substantially as the rings bed in and the valves seal fully. I'd also play with the ignition timing and carb settings to try and get the 'best bang' you can and see what happens from there. It does sound like it wants to go in the video, it's my guess that with the right settings it'll run.

 

[Parksy]

Cheers cogsy

I've probably had it firing for maybe 30 minutes at a rough guess. This is while adjusting the timing and carb settings mind you.
And like you said, this did make an improvement to the feel and amount of compression. But whether it is enough, I'm not sure. I am curious if anyone has had running engines with a larger than 2mm clearance.
If so, then I may explore carb issues. The vapour carb setup I used had the engine running better than the RC carb, but after a while, due to the fuel mix having oil and other stuff in the mix, would use all the nitro/methanol and leave the oil behind. During this transition, the engine would stop firing at a certain point, which for troubleshooting purposes, is just horrible.
I'm not a fan of the RC carb I have, it may be too large.

Cheers

 

[Brian Rupnow]

Most of my engines run with 1/8" to 3/16" piston to head clearance. The compression ratio is best between 4:1 and 6.5:1 this is easy for me to calculate, as I design my engines with a 3D cad program. If you really want to know the cubic inch displacement of the head cavity, do like we used to with the dragster heads. Turn the head upside down like a dish and use a graduated beaker to fill a single head cavity with fluid, then convert the number of ounces to cubic inches.---and put a head gasket on that thing. It will make a remarkable difference.

 

[Parksy]

Thanks Brian.

1/8 to 3/16 is significantly larger than what I've got, so fair to say I shouldn't have any excuses.
The head and top of cylinders have been lapped and a smear of copper sealant added for sealing purposes. I'm fairly certain it's not leaking but I'll have to check now.

Might have to just keep playing with it...

 

[mayhugh1]

Parksy,
If you could post a dimensioned cross-sectional sketch of your cylinder/combustion chamber combination with a piston at TDC and at BDC, one of us could calculate your c.r. for you. Knowing only the head clearance isn't enough to make a meaningful estimate. Adding a head gasket will lower your c.r. - Terry

 

[Parksy]

Thanks mayhugh

I found an online calculator and was able to input my figures. The combustion chamber in the heads are only 0.4cc (according to the maths I just did) so this gives me a compression ratio of 7.8:1.
This is with a 26mm bore, 18mm stroke, 2mm deck clearance, flat Pistons and no gasket.

I suppose I should be troubleshooting elsewhere now...

 

[Parksy]

Ok, I fitted the carb i made for the last engine and by crazy coincidence the bolt holes lined up. Anyway, the engine fired consistently and a lot better with this setup, but something I noticed with this carb and the RC one was that I'm getting fuel spitting from the carb throat.
To me this would suggest that the camshafts aren't timed correctly?

 

[Parksy]

Or would a crankcase that can't breath cause fuel issues? I drained the oil and spun the engine by hand and saw bubbles come out of the the drain hole. Will be making a breather for this.