Highest compression on stock ABF pistons?

Hi,

I have a spare ABF and want to raise the compression.

What is the maximum safe compression using stock pistons?
I'm trying to do it as an experiment without the expense of high compression pistons.

I can get any machining done, with little or no cost.

My plan/idea is to have a safe figure to aim for, then machine the block, head and piston valve cuts as required to reach the desired compression.
I would be interested on any opinions on compression, and the best way to achieve it.

I did think about offset skimming the head, as it would reduce the amount the piston valve cut's would need to be modified - idea from the Audi 16v thread

 

I found this thread with some searching.
It's a hotgolf built engine running around 11.5 compression on modified pistons, with a diesel crank.

http://www.clubgti.com/forum/showthread.php?t=230210&highlight=abf+diesel+crank

 

11.5 will be the highest you would want to run on std pistons.
And at that I would imagine you are into territory of fuel and spark control needing to be very accurate to prevent damage.
Mrhillclimber has also previously stated that figure as the max.
You have to be pretty accurate to measure the volumes when you get up to that sort of CR. It really has to be volume checked for accuracy.

 

Okay thanks.

Would this be the best way to do it?
Modify the head and then measure it's chamber volume.
Skim the block to gain the desired compression, and modify the pistons for clearance.

 

You can do it that way although most take the majority off of the head and then just recess valve pockets in the pistons to give required clearance for the valves. If you start decking the block you will also need to machine pistons otherwise you'll end up with too much potrusion of piston crown above block face.

Either way you'll need to measure chamber volumes accurately using a burette and some persplex, don't forget to consider the gasket bore and compressed thickness in your calculations as well

 

Thanks mitlom,

That sounds good to me, I'll have to try some calculations. And work out how much to remove

 

Just to clarify, when you say " too much potrusion of piston crown above block face", what actually is the problem? Is it simply clearance of valves and head chamber (i.e. the obvious!) or are there other reasons? I'm interested as I'm considering decking the block on my 16VG60 so that I in effect 'lose' the gasket thickness, because my custom pistons have produced a lower CR than I intended and I'd like to raise it.

 

If the block head gasket face is skimmed then the piston crown will effectively sit higher / increase it's protrusion by the amount skimmed from the block.
The head gasket is approx 1.6mm, there also needs to be a squish thickness large enough to cope with conrod stretch. Obviously there has to be enough clearance to prevent piston contact with the flat area of the head.

The other factor to consider is that as the inlet valves come into the combustion chamber from and angle, if you raise the piston crown height then the valve cut out radii on the piston crown will need altered to keep adequate clearance to the valves. Especially on cams with increased lift and overlap.

This needs taken in to account along with the actual adjustment of clearance volume when making compression ratio changes and calculations.

Of course one could fit the superior 8v head and not have such complex issues to deal with.

 

I'm probably going to bite the bullet with my project and get a set of high compression pistons.

I'm using a 9a though. How much clearance is there between ABF piston crowns and the top of the block not including the gasket? My pistons protrude 0.005" out of the block and 0.0165" on the raised part.

I'm wary of the duration of a set of Piper Rally Cams. I think trying to set them up with stock pistons might be more hassle than it's worth.

 

you could do but that's too easy surely. I have just finished my abf diesel crank machined pistons 11-1 compression kr head mad combo, goes like s*$% of a shovel , all the hassel was well worth it,

 

This would be good but for the money and work it would take to get my compression ratio increased on the 9a (with ABF head) I can go for Wossner pistons and raise the comp ratio to 12.3 and have better valve clearance.

Have you got any pics of the machined pistons?

 

The piston crown is -0.35mm , the edge radius is about 3mm concave , the lower flat area -1.35mm and the valve pockets are about -2mm i think. The kr head is not skimmed nor is the blocked decked.Standard 95.5 diesel crank 1y, standard abf 1.6mm steel head gasket split with a 1.1mm middle section from an old gasket fitted in resulting in a head gasket of 2.7mm, arp head studs. head is fitted with 278 deg. cams 11.2 lift. the compression is calculated to 11.03 , the actual compression measured was around 225psi.

Initially i built this engine without the 1.1 spacer in the head gasket as i forgot that the abf has as standard a positive deck height. The engine started but ran very lumpy, the measured cylinder pressure was above 250psi. When i removed the head there was evidence of valve piston contact, i could not catch the mark with my nail, but it was there.lucky escape.

 

^jamez, I do like your engine build, and the one's hotgolf has done.
Quite tempted to try a diesel crank in the future.

Have you had it on a rolling road?

 

I think I'll need to try a test fit and see what my clearances are like as I'll have larger cams than the build above. Piper 294 with 11.3 lift. Time to find my plasticine.

 

Hijacking Gareth's thread here a little still, I've got my block with my JE pistons bare and ready to take to the machine shop where I'm hoping I can have the block machined down a little to raise the CR. With above advice kept in mind, I'd like to measure the existing clearance myself prior to taking it to the 'shop, using the clay method.

Only issue is obviously a solid lifter (or two) is needed. I though of taking a hydraulic tappet and tacking it solid (as if pressurised) with the TIG. Then use clay, old compressed gasket and at least two engine revolutions. Feasible?

 

That is tricky Trev as the tappet has a few components, the chances of tacking the bits together at the correct place are slim.
However you could open up the guts of the tappet and play around with a fixed insert that would effectively create a solid lifter that gives a 10 thou valve clearance. This would be enough to allow you to check valve to piston clearance.

To get the insides out of a hydro tappet just whack it on a piece of wood sharply and eventually the centre section will come out. It takes a fair force to get them out sometimes.

 

Thanks Dan. That does sounds a hell of a lot less hassle, as I could simply insert a stack of washers or some loose change(!) or other shims to prop it up. Thinking about it, trying to weld it at just the right place would be almost impossible.

So basically choose something that provides about 0.01" clearance on the feeler gauge against the back of the lobe when the valve is fully shut.

Edit: Ah ok, thanks Cerips - will look for that Hotgolf thread. However as this is just for measurement purposes I just need to pack it with any old shims to get the height.

 

Plonk the bare head on the block with 1 tappet in place but with no spring/cap/collet etc Tighten it down as usual.
Set the piston to TDC with the valve held up to avoid any un-necessary contact.
Slow drop the valve onto the piston, then set up a DTI on top of the valve and lift it straight up with a long nose pliers. You've now got your total lift from seat to piston.
As a comparisson, my 9a JE pistons(186234) sit with the edge level to the block(edge not dome). Head skimmed about 30thou. Using 12.5mm lift cams I've had to pocket them pistons an extra 1mm due to the amount of recommended clearence they JE in the US confirmed when I rang them up.

As for making your own tappets. Nice easy number to be honest. I'll probably making my own as the Newmans ones, as good as they are don't like lift on my cams, and I've run out of adjustment on the locking nut.

 

Thanks Mart. Need to grab a DTI off eBay I think.

I attempted to do the clay measurement method today. I think it was a success of sorts. I'll cut up and measure and take pictures of the clay later. Initially it seems to indicate about 2mm of clearance to the head at the crown edge and absolutely stacks of room at the reliefs.

What confuses me though is the tappets. With a tappet free of oil and compressed, I expected to be able to measure some clearance to back of lobe. But with the thinnest of feeler guage (0.002" IIRC) I could not get the feeler in there. So I just did the test using tappets as they were, which I hope is okay.