Dynamic CR - how much is too much

[syncrogreg]

I know there is no hard and fast answer to this question, but I am hoping someone can steer me in the right direction for my situation.
The build is a wbx with a dpr 82mm crank, Qsc pistons and standard length 137mm rods. I am copying 10c's qsc idea only up'n the capacity(thanks 10c). As far as the numbers play out I think he is taking around 1.75mm of the qsc's. I will be taking around 2mm. My static compression will be in the low 10's. Cam choice is were I am stuck. I have a cb 2280 on the shelf for another job. The intake closes very early on this cam. If I use this it will put the dynamic in the mid 9's. High, I know, but is it too high? Here in Australia high octane fuel is everywhere (95 and 98). Also the digifant won't let the revs go into the 6k's. Am I mad for considering this, should I just fit a 2254? Its for my syncro and needs to be reliable as well as torquey. As a side issue does anyone know where I can buy barrel seals with a larger I.D. (96-97). Thanks for your time. Greg

[tencentlife]

General rule of thumb is that a dynamic CR of between 7 and 8:1 is safe for US pump gas, meaning US AKI of, say, 86 to 91. But you are probably on the RON-only rating system (our rating averages Motor Octane Number and Research Octane Number) so your octane number will be 3-4 points above ours, e.g. your 98 would be 94-95 in the US.

But as you acknowledged, there are a huge number of factors. Squish dynamics in particular, renders that 7-to-8 rule of thumb fairly irrelevant, and the wbx design has excellent squish. I turn off my piston crowns to have squish clearance tighter than stock (which is .040") and my production engines have DCR of about 9:1, but do hard duty in Vanagons needing only mid-grade fuel (US AKI 88-90) at sea level and happy with regular (85-87) once they're a few thousand feet above seal level.

Really what you want to look at is in-cylinder compression pressure, CR expressed as a ratio is more of a proxy measurement for the pressure the charge will actually be exposed to before combustion is well-established. Here's a link to an online calc I often use to compare in-cylinder pressures:

http://www.rbracing-rsr.com/comprAdvHD.htm

(you should check out the many other useful calcs there too)

I think that pressures under about 200psi are generally going to be safe for pump gas when you have good squish dynamics. When you turn your piston tops cut only enough to bring them down to zero deck, i.e. even with the cylinder top, and then the metal sealing ring will set squish clearance at .040". I have actually found it safe to tighten squish to as much as .008-.010" above deck with a stock 76mm crank.

To figure the wbx piston dish volume at various cut depths I made up this table mathematically. It's not verified by actual fluid measurements, but should be very close and will certainly help at the calculation stage of planning:

QSC wbx piston volumes

Reduction in total dish volume (including valve pockets)
per 0.005" removed from crown.

Column 1: Total cut
from crown (inches)

Column 2: Volume
reduction
(cc's)

Column 3: Dish vol.
remaining from 47cc (cc's)

0.005" 0.59 46.41
0.010" 1.17 45.83
0.015"" 1.74 45.26
0.020" 2.32 44.68
0.025" 2.88 44.12
0.030" 3.45 43.55
0.035" 4.02 42.98
0.040" 4.58 42.42
0.045" 5.13 41.87
0.050" 5.69 41.31
0.055" 6.24 40.76
0.060" 6.80 40.20
0.065" 7.35 39.65
0.070" 7.89 39.11
0.075" 8.44 38.56
0.080" 8.99 38.01

Have fun!

[syncrogreg]

Thanks Chris, I always wondered why your octane ratings were so low. So glad you mentioned the zero deck height thing. I was just about to mimic the original set up, mostly because I cant find barrel sealing rings that have a greater id than the piston and I am worried about contact. I just have to keep looking. Thanks for the calc link. It puts my cylinder pressure right on 200psi. I guess I will give this 2280 a go unless you have a hot tip on a cam that would suit this build.

[tencentlife]

With oversize bore you will want to use combustion seals that have the same nominal ID as your cyls. The piston will extend .010" or more above the cold deck height when the engine is running and hot. Rocky Jennings is making some coppers in 96mm, I believe, and GoWesty has them as well.

The alternative that lets you use stock steel comp seal rings ( with ~94mm ID ) is to machine a chamfer or rabbet around the piston crown to provide clearance when hot , since you'll have the piston on the lathe already this only takes a minute more to do. For a rabbet go in .050" on radius and down .020" from crown and Bob's yer uncle. If you cut the crowns for a negative deck (meaning crown is higher than cyl. top) then add whatever distance you are above zero to the .020" depth.

I do the rabbet because my philosophy is that whatever plans you make, someday someone will reseal the heads and they won't know that the oversize copper seals that were put in there are reusable and will use stock seals instead, and the piston will slam into them as soon as the engine warms up. Plus it saves the expense of buying special copper seals when I already have stock ones from the gasket set. My engines go out in the world where I don't have any control over who and how they might be serviced or repaired, so doing it this way avoids a predictable disaster that would otherwise await my customer years later when it needs a valve job.

[syncrogreg]

Thanks mate, will let you know how it turns out. Greg

[profe]

I run a WBX DJ pistons -measured by myself 10.3 static CR- with a CB 2280 cam -thanks Tencent for your advices-. No issues with detonation if igniion is carefully adjusted (as same stock cam)

I use european -Spain- 95 gas. No differences using 98 gas (except loosing money )

Using http://www.kb-silvolite.com/calc.php?action=comp2 I got following resoults:

- OEM DJ pistons & OEM VW cam: Dinamyc CR 8.755:1
- OEM DJ pistons & CB 2280 cam: Dinamyc CR 9.021

May be the calculatror from silvolite needs more paremeters to adjust better to reallity, but relative values gives you an idea.

I give yo a llink with these information -in spanish- of this point.

http://www.furgovw.org/index.php?topic=206719.45

http://www.furgovw.org/index.php?topic=206719.135

[syncrogreg]

Hi profe, glad to hear you like the 2280. I Guess the only difference is that I am going to run 82 stroke, so this will increase my cylinder pressure (assuming you are running 76mm crank).

[profe]

Hi profe, glad to hear you like the 2280. I Guess the only difference is that I am going to run 82 stroke, so this will increase my cylinder pressure (assuming you are running 76mm crank).

You will have tons of low rpm torque with that stroke.

[Piledriver]

Kinda OT, but perhaps not, I discovered years ago that a nice tight deck can cover up the sins of LOW dynamic CR to a great extent as well.

Built a ~stock 2L T4 with Bus 12cc pistons for my son, had cam/lifter woes.
(C20 cam, almost stock, ran great, normal DCR, decent combo as long as it lived)

To make a long story short, when the motor came apart, he wanted headwork and a wild cam, as it became obvious his wife wasn't going to drive it anyway.

After another round of flat cam with a different vendor, I had him order a 163/86b@104LC, 44x38 heads, but he would not spring for a new P&L set for decent compression, so the dynamic CR is far beyond horrible. (~5.5:1 DCR)

...but set up with a .030" deck, it was still tunable and rocked hard, much to my surprise.
I still want him to put a turbo on it. Just for fix the DCR. Really.

[syncrogreg]

An update. The engine has gone back together, unfortunatly it is detonating/knocking under load. I have used 98ron fuel, added a octane booster and knocked back the timing, still no joy. So I have ordered a 2254 from CB and will take some material out of the cyl heads to get the numbers down a bit. The big question I have is that I am working off the numbers of 36degrees abdc inlet close on the currently fitted 2280 and 51d on the 2254. Are these figures correct? Are they both measured at .050"? With one being a hydro grind I thought they might measure them differently.

[Stripped66]

I don't know about the CB cam, but many cams for VWs are ground with 4 degrees of advance. Have you considered retarding the cam timing to reduce the dynamic CR?

[syncrogreg]

Hi Stripped66, Yeah I did consider that. Looking at the cam card on the 2280 I am guessing it would take 2-4degrees retard. This still would not bring down my cylinder pressures down as much as using the 2254. I would be willing to try it if it were not such a pita to pull a cam on a wbx. If I was smart I would have fitted it 2 degrees retarded in the first place. Live and learn ah! Thanks

[profe]

This pic is from a CB 2280 I fitted it last year. May be it help you

[syncrogreg]

Thanks Profe, I have the same cam in mine at the moment. Interestingly my cam card has the same numbers just advanced one degree.

[syncrogreg]

Has anyone got a cam card for a 2254? Do they measure hydro cams at 0 and not at .050"?