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yet another deck height question.......

ive looked in the archives about deck height questions, theres lots there but im still lost...heres some pics of my deck height measurement process. please note that the top of the barrel is not clean and will be getting flame ringed (along with the rest) so im aware that ill be doing this all over again but i had a couple of spare hours so....the ist pic is of the location of the solder. seems pretty ok to me. the second pic is the result of the test. the only area that has any visible compression is on the very tip of the solder. im not running any cylinder gasket here, although i plan to. my engine is a 3.3 turbo. the pistons are 8.5-1 je's. i was looking to shim them down to 8(ish)-1, perhaps a little lower. the heads are 3.2 carrera with a chamfer, as they were going to be used on a 3.2 with 98mm pistons. the only measurement that i can take is from the portion of the solder that is now deformed from origional. it measures at 1.48mm. what are peoples views? am i doing this right? what measurement should i be expecting to achieve?

thanks
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1980 SC soon to be big hp 3.3t powered 73RSR Replica (well, I'm keeping the engine but everything else is going )
Old 08-27-2005, 06:30 AM
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You are on the right track here, but the measurement will not even be in the ball park until you have the flame ring and gaskets in place.

Use calipers to measure the compressed portion of the solder.
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Old 08-27-2005, 07:06 AM
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thanks ashphalt fair point on the flame ringing.will there be THAT much removed from the heads/barrels to greatly affect the measurement? p.s. i measured using calipers.
is there any pre-determined deck heights available? what am i working to?

Andy
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1980 SC soon to be big hp 3.3t powered 73RSR Replica (well, I'm keeping the engine but everything else is going )
Old 08-27-2005, 08:15 AM
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When you have have all the components, parts, gaskets and final configuration in place you do a 'mock' assembly to test.

Basically you are trying to achieve the minimum clearance possible between the piston / cylinder head squish for your application.

The reason is the less 'dead space' you have behind /side of the flame travel; you will have more effiecient combustion by forcing the combustion process back towards the center of the piston. This also aids in creating turbulence which further promotes more complete combustion

Remember that this specification has nothing to do with bore, stroke, compression ratio, piston dome height and combustion chamber shape. However all the above will affect the squish area measurement.


There many different opinions on how little the clearance should be. M/O. On any street performance engine with steel rods where the max RPM is under 7K I set up for .020".

In the case of the Porsche where the whole engine is aluminum, I shoot for .015". This is because aluminum case and top end will 'grow' as the engine heats up; increasing this clearance slightly.
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Old 08-28-2005, 10:22 AM
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ashphalt am i missing something here? surely a low comp engine (3.3 turbo) will have a largere deck height measurement that a high comp race lump?
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Old 08-28-2005, 11:56 AM
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Quote:
Originally posted by adomakin
ashphalt am i missing something here? surely a low comp engine (3.3 turbo) will have a largere deck height measurement that a high comp race lump?
The desk height measured at the edge of the piston and the edge of the combustion chamber. The compression ratio is a function of the combustion chamber volume and that is (mostly) a function of the piston dome volume. I.e. a flat top piston would have a lower compression than a high dome piston all other things being equal.

-Chris
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Old 08-28-2005, 12:45 PM
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As for my flame ring job, there was no adjustment needed in deck height as the cylinder and head mate perfectly - the flame ring is like a 0 clearance gasket (I know there is no such thing) or similar to an o-ring.

Chris is right - deck height measures the squish clearance as the piston comes to top dead center. The idea is to compress the mixture from the outside, to the center of the combustion chamber.

My turbo piston was made to deliver an 8.0:1 CR, and has a slight dome to it, used to increase CR.
Old 08-28-2005, 01:33 PM
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right.... I understand! cheers Chris, i wasn't differentiating between deck height and compression. (or at least I couldn't see how the two were different) Its so obvious now! but there is still something a bit blury in my head- I have 8.5-1 je's and i planned on shimming them down to just under 8-1. if i run 1.25mm of shim then doesn't that wreck my deck height? ive heard of people shimming barrels so are the people that shim their barrels wrong? should you machine the barrels or is shimming ok (even if its 1.25mm) and while im talking about shimming, is there any truth in the old '1mm is equal to half a compression point' saying?
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Old 08-28-2005, 02:53 PM
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Quote:
Originally posted by adomakin
right.... I understand! cheers Chris, i wasn't differentiating between deck height and compression. (or at least I couldn't see how the two were different) Its so obvious now! but there is still something a bit blury in my head- I have 8.5-1 je's and i planned on shimming them down to just under 8-1. if i run 1.25mm of shim then doesn't that wreck my deck height? ive heard of people shimming barrels so are the people that shim their barrels wrong? should you machine the barrels or is shimming ok (even if its 1.25mm) and while im talking about shimming, is there any truth in the old '1mm is equal to half a compression point' saying?
Yes, shimming the barrels to reduce compression wrecks your deck height. If you want to reduce the compression I'd just remove some material from the dome of the JE's. BTDT.
By doing so you can optimize your deck height and have the compression you want.
-Chris
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Old 08-28-2005, 03:54 PM
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Something to keep in mind is to not mix up the terminology. Deck height is not the same as piston to head clearance. What you are measuring and talking about here is piston to head clearance. The sources I've read say to have a minimum of .035 inches of piston to head height. There is no maximum spec. I think all this talk about squish area and flame propogation is overly hyped. I would worry about meeting the minimum and getting the compression ratio you want and not lose any sleep over having too much squish area.

-Andy
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Old 08-28-2005, 08:48 PM
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nice one fellas, every days a school day!
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Old 08-28-2005, 11:47 PM
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Piston to head clearance (squish) is essential only as far as you don't want your pistons to bounce off your cylinder heads. Absolute minimum is .035, .040 is better (1mm). Squish is designed into motors to speed up combustion in slow burning chambers, however a turbo engine is supposed to burn slower and build pressure over a longer period of time. Aside from all this there is nowhere near enough squish area available in this particular combustion chamber with these pistons. Other than changing the compression ratio setting a tighter deck clearance will accomplish nothing for performance. Chris does have the right idea about adjusting the compression ratio by machining the piston dome as long as you don't make the piston any thinner than about .225in or they will self destruct with high boost and accompanying high temperatures. Also changing the deck height dramatically will change your cam chain slack. Push the heads out too far and you won't be able to get the chain adjusters into place.

Oliver
Old 09-03-2005, 06:39 PM
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thanks blokes, this place is great
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1980 SC soon to be big hp 3.3t powered 73RSR Replica (well, I'm keeping the engine but everything else is going )
Old 09-04-2005, 01:10 AM
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Another view..

Squish is a Good Thing...

It reduces the effective area of the combustion chamber, and helps avoid detonation.

This is especially important if you are running single, not twin plug, high boost, low octane, no water or alcohol injection, or hottish charge temperature.

If you ask around ( try a search on this site), race engine builders generally try to run the tightest squish clearance possible, minimising little pockets of charge which can overheat and detonate.

I have run 0.25mm with no problems in N/A motors..but attention to detail is important obviously, and ex valve clearance needs to be checked..

If you strip race bikes, they normally have evidence of piston/head contact at max rpm..this is not a problem..
It's valve contact which matters much more..

Kind regards
David
Old 09-04-2005, 02:22 AM
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David, I agree with you that in many racing applications squish is necessary, just look at the 962 cylinder head with its D-shaped combustion chamber. But we are talking about an engine that was running extremely high boost while running relatively lean to conserve fuel and using a very slow burning fuel, while not beeing allowed to use a second spark plug(IMSA racing if memory serves me right). But in adomakins combustion chamber there is no real squish area to consider. He has approximately a 2mm area around the piston dome in a 95 or 98 mm bore. That amounts to exactly nothing.
As for having your pistons touching the cylinder heads, that is one of the worst mistakes any engine builder can make. If the piston makes contact it will lift the head away from the cylinder and cause a head gasket leak, destroy the connecting rod bearings and wrist pins and in a very short while close up the clearance on the top ring and make it bind in the groove so there will be no ring sealing.
I also have to tell you that I used to build racing engines for a living and have never heard of anybody running .25mm squish. The tightest of any engine I have ever experienced was .026in on a Honda CBR 600. On my personal record winning Kawasaki ZX-11 I needed .032in with a standard crank and .036in with a 5mm stroker crank. This was an engine that got torn down every 1000-2000mi. If this would have been an engine that would have stayed together for long periods of time, I would have had to give it more clearance as, with wear the bores enlarge and the pistons get smaller and therefore allow the piston to "rock over" @ TDC and make contact with the head.
A lot of different variables have to be considered when establishing piston to head clearance, like piston weight, max rpm, rod stretch(different for factory steel, CARRILLO, Titanium, etc.), crank flex, case flex, but if you assemble any engine over 50cc with .25mm(.010in) clearance, you will have a pile of junk the first time you roll into the throttle. If you don't believe me that for a turbocharged 3.0 or 3.2 Porsche engine .040 is a good safe limit, please contact any of the reputable Porsche race shops like ANDIAL, Dennis at AASE Bros, Henry at Supertec or Wayne at Pelican to name a few.

Oliver
Old 09-04-2005, 08:32 AM
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Thanks for the correction..my stupid error

Its 25 thou's, not 0.25mm. I have run happily.. ie about 0.62mm

Sorry for that rather important mistype!

I'll do a little search round here if I get time...I recall a thread on this very topiv generating quite a few differnt responses from pro builders..

As you suggest, I'm pretty sure the bikes and karts I'm describing having stripped personally were not designed to touch!..they just did happen to touch, a little bit, when over revved etc..

And lived..but this would only be marginally acceptable even in a 20-minute motor..

The point I'm making is that, provided that SOME clearance can be assured, there seem to be many who believe the less the better.

And the 911 head/pistons are not well designed for squish, anti detonation, or combustion generally..

So anything we can do to help might be good..

Kind regards
david
Old 09-04-2005, 09:05 AM
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