22 replies to this topic

[brad-the-bear]

yes... another topic with regards to compression ratio. its for a 1380 and am looking for around 10.5:1

 

just struggling with the maths really, help would be greatly appreciated. 

 

here is what ive measured:

 

-21.5cc           head chamber 

-9cc                dished pistons

-73.5mm         bore size

-81.28mm       stroke 

-BK450 head gasket 3.9cc??

 

now the weird bit, ive had the block decked by a reputable engine building firm and the top of the piston comes above the block by about 0.35mm is this ok?

 

cheers guys

 

[brad-the-bear]

also is 10.5:1 a good choice for a kent MD276?

[ACDodd]

Hmm you need to be careful here. The gasket will overhang the bores. If your pistons come proud you will very likely hit it. On big bore engine I aim for 0.006 to 0.010" down the bore at tdc to avoid this issue. Easy fix, pistons can be trimmed a little to rectify. I like 10:1 cr with the 276 cam.

Ac

[Turbo Phil]

V+C/C
V= Swept volume of one cylinder. So in this case 1380 divided by four. 345cc
C= Volume above the piston at TDC, so
21.5cc head chamber 

9cc dished pistons
BK450 head gasket 3.8cc

 

That would give 34.3cc
 

V=C/C= 11.05-1

 

But you have a negative volume as the piston comes above the block face at TDC, so this is going to raise the compression further.
Basically it's going to be far to high for that cam & for standard forecourt fuel. You need to gain some more volume somewhere, either from the head chamber, with a thicker head gasket or with a bigger piston dish.

Phil.

[carbon]

0.35mm of piston sticking out of the block sounds awfully high.

 

As well as the issue raised by AC Dodd about clipping the gasket I would be also worried about the pistons hitting the head at full chat...

[brad-the-bear]

hi, after a tense evening i calmed down and when back to the engine build and measured again how far the piston comes above the block with a straight edge and some feeler gauges, its actually around 0.2mm. spoke to the engineers today and they are going to be taking my cylinder head away to port out the chambers ive told them there needs to be around 24cc in each chamber does this sound about right?

 

according to my maths on this scrappy piece of paper it should be 10.5:1?

 

cheers,

bear.

Edited by brad-the-bear, 17 March 2014 - 08:41 PM.

[Cooperman]

That is still a piston protrusion into the gasket area of 0.008", which is too much for a 1380 sized bore with a BK450 gasket.

You need to get about 0.010" skimmed from the piston crown to be sure that the piston is not going to contact the head gasket, but bear in mind that this will reduce the dish volume whilst adding around 1 cc to the combustion volume. 

Get this right at this stage or risk a lot of unreliability later.

[brad-the-bear]

cheers guys 

[brad-the-bear]

so the only option being skimming the piston down 0.010"?

[coopdog]

Also there is a good app on the App Store called mini head which makes it a lot easier to work out :)

[Turbo Phil]

If the pistons came flush with the block & you had 24cc in the head the compression would be 10.37-1

Phil.

[Cooperman]

so the only option being skimming the piston down 0.010"?

Realistically yes, that is the only option.

I guess the pistons are on the rods, but did you use linished down gudgeon pins for the trial build so that the pistons can be easily removed from the rods. If not you'll need to find a company which can fly-cut the piston crowns with the rods attached rather than simply turning them down by 0.010".

That's why you always do a trial build.

[brad-the-bear]

Also there is a good app on the App Store called mini head which makes it a lot easier to work out :)

 

cheers thanks. its a great app!!!

 

 

 

so the only option being skimming the piston down 0.010"?

Realistically yes, that is the only option.

I guess the pistons are on the rods, but did you use linished down gudgeon pins for the trial build so that the pistons can be easily removed from the rods. If not you'll need to find a company which can fly-cut the piston crowns with the rods attached rather than simply turning them down by 0.010".

That's why you always do a trial build.

 

cheers pal, no the pistons are on the rods and i made the mistake of building everything up before calculating CR i work around the corner from an engineer that makes engines for RC planes, i'll speak to him.

 

thanks for the help guys.

[Cooperman]

The best way to build an engine is to do a trial-build and measure everything at that time, then strip and do any final machining. For the gudgeon pins, if you take an old set and put them in a lathe you can linish them down using emory cloth until they just slide into the new pistons using finger pressure. then you do the trial build, with the piston rings removed, and check piston crown to deck dimension. It is easy to strip right down, then machine block deck or piston crowns to get the magic 0.010" to 0.003" piston crown to deck figure. At the trial build the crank float can be checked & set to give, ideally, 0.015" to 0.004" (0.005" is max acceptable).

Trial build is when the cam timing is set correctly, primary gear end float is checked & set, piston ring gaps are checked/set and really everything else is confirmed as correct.

 

I hope this all helps.

[ACDodd]

if you have a problem with the pistons, I can machine and re-dish as required.

AC