Raising The Compression

[ifcho 0]

Hi,

 

So I have now started to built my engine...and I need to raise the compression ratio from 10.2 (xu5ja) to around 11.5.

AFAIK there are several ways to do this: the best one - to buy custom pistons (I already have pistons so this is out for now) the second to grind the head and the third one to grind the block/liners.

 

I spoke to a race car builder here and he said that we should better grind the block/liners and pistons, instead of the head, is this the way to go?

 

Also if somebody has made any calculations for this engine, I would be happy of an figure about how much should be grinded

[brianthemagical 1]

Skimming the liners increases squish, which can be good.

[whizzer71 0]

Skimming the liners increases squish, which can be good.

 

 

Forgive me if Im wrong but does that not cause probs with protrusion and hence sealing on the head gasket ?

 

[Batfink 201]

dont you deck the block and liners the same amount so keeping the same sealing effect?

[brianthemagical 1]

Forgive me if Im wrong but does that not cause probs with protrusion and hence sealing on the head gasket ?

 

 

dont you deck the block and liners the same amount so keeping the same sealing effect?

 

Depends what the liner protrusion was like to start with. Common sense would say do the liners first then calculate the desired block skim.

Edited April 4, 2008 by brianthemagical

[Bally 4]

Doesnt using 1.6 pistons or the flat top 405 pistons raise the compression substantially on a 1.9?

[ifcho 0]

It is a 1.6 engine as I'm entering sub 1600 class...

but I think that maybe I'll skim the head (it's easier), cause I don't have much time till the first hillclimb I want to visit

[Sandy 189]

Skimming the liners sounds alot easier than it is!

[ifcho 0]

And if I had someone to do it properly, would you recommend to go this way Sandy?

 

Thanks.

[ifcho 0]

so after doing some calculations without measuring I take it that to get the compression up to 11,5 I'll need to skim 1.2mm of the head.

 

Is this too much? Should I really consider skimming the liners/pistons instead?

 

Also, does anyone has any data about the volume of a xu9 head and a xu5j piston?

[DamirGTI 327]

Hi !

 

I'm not a fan of heavy head skimming , and i really don't know how much you need to take off the head to reach 11.5 CR but it'll must be a lot .. which isn't good cos with skimming the head down you're retarding the cam timing also ..

 

On the other hand , i would take block/liners skim as a option only if you can find someone who will do that , who know how to do that properly/professionally/quality/precisely ...

 

Volume of the XU5 piston (dish volume) - 9cc

Volume of the XU9 head (combustion chamber) - 34cc

(please correct me if i'm wrong )

 

Regards !

Damir

Edited April 6, 2008 by DamirGTI

[Sandy 189]

Clamping a liner without radial distortion or being to free to flex is extremely difficult. The net result could be endless head gasket problems. I wouldn't attempt it and I'm pretty adventurous!

[ifcho 0]

Sany and Damir, thanks for your replies, highly appreciated.

 

based on the information from Damir I'm now calclulating the desired compression ratio and how much must be skimmed..but I musst be making a mistake:

dish - 9cc

head chmbr - 34ccc

swept volume - 395,1cc

1mm of swept volume - 5,41cc

So for the current compression ratio I have:

(395,1 + 5, 41 + 34 + 9)/5,41+34+9 = 9,16

and according to peugeot data, this engine must be with 10,2:1

 

Are the xu5ja and xu9 head actually different in terms of head chamber volume? or the head gasket tickness is not calculated in official data

[DamirGTI 327]

Well as far as i'm aware the XU5/9 heads are the same bar the camshafts and the valve size (on very early XU5 engines the valves are smaller dia. ) .. so me thinks that the chamber volume is also the same , although i didn't measure the chamber volume of the XU5 heads ..

 

You must calculate compressed thickness of the head gasket , but which formula did you use anyway ?

 

Also why are you aiming on 11.5 CR ? I'm sure that 10.5 will be just enough for todays unleaded pump petrol , unless you planing to run this engine on race fuel and fully mappable ignition ? ... and difference in between well prepared/adjusted 10.5 and 11.5 CR engine will be , i dare to say , minor .. not so much different in power output ..

The available fuel type/quality , and engine management is the most important factor to consider when building high CR engine .

 

Rgs !

Damir

Edited April 6, 2008 by DamirGTI

[ifcho 0]

well its for my track/hillclimb car with a lairy camshaft and throttle bodies on MSII with ignition controlled directly by the ecu..

 

I took that the head gasket thickness is 1mm

[DamirGTI 327]

.. volume above the piston at TDC is missing

 

Damir

[ifcho 0]

do you happen to know it

[DamirGTI 327]

nope , sorry but it's not so hard to measure if you have dismantled XU5 engine (with removed head) , buy an burette and one square piece of fiberglass , drill one hole in the fiberglass for burette , bring the piston up to the TDC (ideally measure true TDC with dial gauge if you have one , but me thinks that it must not be so precisely) , seal the gaps between the piston/bore wall with some grease , pour some liquid (paraffin , diesel ..) up to the top of the liner , fit the fiberglass on top of the liner surface , top up with liquid if needed trough the hole and measure the volume with the burette ..

 

Rgs !

Damir

[ifcho 0]

yes, I'm familiar with the method.....

but I'm sitting at the office extremely bored and wondering what to do till the end of the shift

[DamirGTI 327]

..ah ok. .. anyway didn't have XU5 engine yet , just the XU9 ones so i've been working on XU9 ones mostly

 

But someone from here will know that value for XU5 engine..

 

Rgs !

Damir

[Dom9 2]

As Bally has said... Have you looked at getting a set of 405 SRi 8v flat top pistons? They remove the 'bowl' in the head of the GTi pistons so increasing cR without needing to skim anything...

 

I always wanted to build a high CR engine with SRi pistons and the liners decked so that (if it didn't already) the piston sat flush with the top of the block. Then check to see if the piston needed machining for high lift cams...

 

This would give more squish etc... Not sure anyone has really built or raced one in anger though...

[Sandy 189]

A friend of mine has used the SRi pistons successfully in 1.9s, but later erred towards Mi16 ones for the benefits of floating pins. For the 1.6 the floating pins aren't an option without custom rods, the standard rods don't have enough "meat" to be bushed.

[28CRAIG 2]

Have a look at THIS may help you when working out CR's.

[adrian_gti 0]

pardon my ignorance.. what benefit does higher CR have over lower CR? Been meaning to ask this question for a while

[petert 554]

Decking the block and liners achieves two things. Because the piston will be pushed above the deck and into the gasket, the CR is increased and the squish distance is decreased. Correct me if I'm wrong, but doesn't the XU5JA piston have a 37.50mm compression height? ie the top of the piston is already flush with the deck?

 

As Sandy said, decking the liners is not for the faint hearted. The risk of failure is very high if you're not tooled up for the job. I have a friend who decks them for me, but he has special mandrel to hold the liners, then faces them to length in a lathe. I suggest finding someone who has done the job before if you take that route.

 

An easier method to achieve both of the above objectives is to fit a thinner gasket. I've successfully used copper gaskets on alloy/liner engines, using 0.5mm and 0.7mm thicknesses. They work very well providing liner protrusion is good to begin with. So long as you're aware of their limitations and have the skills to fit them, they're a cheap and effective upgrade. You can buy direct from www.ridgrecrest.com.au.