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Wed Sep 10, 2014 3:58 am

Hi all I have a pair of heads that only have 46cc combustion cambers and a deck height of 1.25mm but my compression ratio is a little high so basically what I need to know is if I fit cylinder head shims do they add to the deck height valve or do they add to the combustion camber cc?
Cheers

Wed Sep 10, 2014 6:18 am

advinnie wrote: Hi all I have a pair of heads that only have 46cc combustion cambers and a deck height of 1.25mm but my compression ratio is a little high so basically what I need to know is if I fit cylinder head shims do they add to the deck height valve or do they add to the combustion camber cc?
Cheers

I use a shim underneath my cylinders on my 1835cc engine to reduce compression ratio (between the case and the heads).

Is this what you describe? I understand that real English isn't your first language !!!

Wed Sep 10, 2014 7:36 am

No mate I'm British but this bloody phones predictive text is pissing me off :-)
I need to reduce my compression ratio but im already running a 1.25mm / 0.049" deck height so i was wondering if fitting a shim between the cylinder and heads would count as increasing the deck height value or would it be considered as increasing the combustion cambers cc. If that makes sense?

Wed Sep 10, 2014 8:59 am

The combustion chamber is the volume of the "pocket" in the head. You typically measure this by filling it with fluid (oil). This is an awkward shape so cannot easily be calculated on paper.

The deck height volume is a simple calculation of the surface area of the cylinder opening x the deck height. You don't normally measure the volume here, you calculate it.

Same for the piston displacement. You don't measure the volume, you just measure the radius and stroke then calculate the volume that the piston displaces.

Using the diagram as a guide, if you add a shim between the top of the cylinder and the head you will be increasing the height of the "head gasket". The stock cylinder to head mating surface doesn't actually have a "head gasket" so you would be adding one. Or, you could look at it as increasing the "deck height".

To be honest, you don't want to add the spacer between the cylinder and head... that is a sealing area that must deal with the pressure of the combustion chamber. Why would you want to add another point for a leak?

If you do as Cusser suggests and add the spacer between the case and the bottom of the cylinder you get the same effect, increased "deck height", but the spacer is added to a less critical sealing area. The bottom of the cylinder must be oil tight to prevent oil leaks, but there is no pressure there other than the low case pressure caused by the pistons moving. That is much lower than the combustion chamber pressures. Since the heat at the bottom of the cylinder is also much lower, you can add a bead of sealer to this junction to further prevent leaks. You can't do that in the head.

So by adding the spacer below the cylinder you get your increased "deck height" (which means lower compression ratio) and you don't add another cylinder leak point at the head.

Don't get me wrong, they DO make copper head sealing rings. These are thin and designed to improve the seal between the cylinder and the head. They do slightly decrease the CR, but that is not their purpose.

Wed Sep 10, 2014 11:50 am

Basically when you add shims underneath the cylinders, you increase your deck height and increase the combustion chamber volume. As Ashman said, this is a much better way (for longevity) of reducing your compression versus copper head shims.. However, the best way is to open up the combustion chamber (grinding). This does two things. It keeps your deck height small, so you have less over heating. Also when you remove the material around the valve (un-shrouding) it allows the intake charge and the exhaust gasses to exit the head more rapidly. Of course there is only so much material that you can remove from the combustion chamber, so in most cases it is a balancing act between grinding and barrel shims. Personally I never have and never will use shims between the cylinder and the head In fact VW came out with a bulletin to not use the copper head shims in the T4 motor, because it decreased the longevity of the engine.

However, I have now opened up myself for an argument that will never have an answer. Just like is it better to use aluminum or copper for a computers heat sink.

Wed Sep 10, 2014 12:02 pm

Multi69s wrote: Basically when you add shims underneath the cylinders, you decrease you deck height and increase the combustion chamber volume. As Ashman said, this is a much better way (for longevity) of reducing your compression versus copper head shims.. However, the best way is to open up the combustion chamber (grinding). This does two things. It keeps your deck height small, so you have less over heating. Also when you remove the material around the valve (un-shrouding) it allows the intake charge and the exhaust gasses to exit the head more rapidly. Of course there is only so much material that you can remove from the combustion chamber, so in most cases it is a balancing act between grinding and barrel shims. Personally I never have and never will use shims between the cylinder and the head In fact VW came out with a bulletin to not use the copper head shims in the T4 motor, because it decreased the longevity of the engine.

However, I have now opened up myself for an argument that will never have an answer. Just like is it better to use aluminum or copper for a computers heat sink.

Sorry mate but surly if you place shims under the barrels your deck height increase not decrease?

Wed Sep 10, 2014 1:35 pm

Vinnie, thank you for catching that. Message edited and brain fart eradicated. :oops:

Wed Sep 10, 2014 1:44 pm

No problems mate :-).
I'm now starting to rethink this engine build cause I've now come into a 76mm crank giving me a 1849cc engine and at this size I think it would be s good idea to increase the valve size to something like 40mm inlets and 35mm exhaust and a 1 1/2" exhaust system but would the 36 dellorto carbs now be to small?

Thu Sep 11, 2014 1:19 pm

The 36s should still be fine. You can always to a bigger venture size if you need to.

Fri Sep 12, 2014 1:07 am

That's ture.

Fri Sep 12, 2014 4:36 am

.049 deck is almost perfect, I wouldnt change that. Add some volume to the chambers if you need to get the compression down.

Fri Sep 12, 2014 4:46 am

That's the thing do I need to get the compression ratio down? Is 9.4:1 really that high i will be using 99 ron unleaded fuel (UK).

Fri Sep 12, 2014 4:47 am

advinnie wrote: That's the thing do I need to get the compression ratio down? Is 9.4:1 really that high i will be using 99 ron unleaded fuel (UK).

Depends, what cam?

Fri Sep 12, 2014 5:15 am

have a choice of two cams a gene berg 307 cam and a bugpack 4062-10 cam
Gene berg 307 cam spec. Bugpack 4062-10 cam
Running duration. 276. 284
Gross Cam lift. 342. 377
Valve lift. 478 (1.4:1 rockers) 415 (1.1:1 rockers)
Lobe separation. 108. 106
Duration @ .050" 237. 284

Hope this makes sense?
The bugpack cam has more duration but less lift and the valve doesn't spend much time at its max flow lift height of 0.419".
Now the gene berg cam has more lift at valves and the valve spends more time at and above its max flow rate hight although the cam has less duration.

Fri Sep 12, 2014 6:06 am

What do you want out of the engine? What are you going to put it in?

Fri Sep 12, 2014 6:45 am

Engine will be going into a full weight bug and bolted to a gearbox from a 1200 beetle. What I want out of it is acceleration/off the line acceleration street racing:-) not to interested in top end speed 80mph is more than quick enough. The bug will be driven may in town and dual carriageways and one or twice a year on the motorway. Most of my trips are only 10 miles each way but once a year it goes on the motorway and must be able to do 70 mph for roughly one hour.

Fri Sep 12, 2014 6:59 am

The 1200 trans should be geared low enough to give all kinds of acceleration off the line and will probably keep the revs a bit high in all of the gears. If you don't want to mess with changing the compression ratio/deck height I'd probably use the Bugpack cam since it has the longest duration. I kind of like the specs on the Berg cam, particularly for a smaller street engine (but its' lift is spec'd with 1.4 to 1 rockers). But since you're talking about a stroker I think the Bugpack cam would be okay. JMHO Others with more experience might say different. :wink:

Fri Sep 12, 2014 7:35 am

The gene berg cam is in use in my current 1699cc engine so i all ready have the 1.4 ratio rockers witch can easy hit 5000rpm but then I think the heads start to restrict the engine cause the engine just stops accelerating at that point. Yes the rpm in all gears is a little high but the bug will still hit 95 ish mph at 5000rpm it will sit at 70mph all day. So you think the bugpack cam would be better in my new stroker?

Fri Sep 12, 2014 7:46 am

I think the longer duration of the Bugpack cam will bleed off some of the compression at lower RPMs which should help with running that ratio on pump gas. It will also move the power band higher in the rpm range. This should be fine because you have a stroked crank and a low geared trans, both of which should easily compensate for any loss of low rpm power/torque caused by the longer duration. There are probably other cams that would work better but, of the 2 you have to choose from, the Bugpack is what I would use. A set of 1.25 rockers would increase the lift closer to that of the Berg cam if you wanted to do that.

But there's others on the forum that know LOADS more about it and have a lot more experience than I do. :?

Fri Sep 12, 2014 9:11 am

Please tell me what after cams may be better im more than happy to get another one :-)