2276 with fk10 cam compression ratio recommendation.

[vwracerdave]

Non of you guys talking about high compression are including what octane gas you need to run to support those CR. If you want to run 10 or 11 CR that fine. You must run the octane to support it.

If your gonna run more than 9.5 CR you need more than 91 octane pump gas.

ACN's CR suggestions DO NOT mention anything about what octane you need to run.

My drag race engine with a FK-10 and 10.5 CR I mix 1 gallon of 110 race gas with 4 gallons of 91 pump gas.
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[bugguy1967]

10.3:1 with a tiny cam on 91 octane in So-Cal. 🙋🏽‍♂️ Stop already. You're embarrassing yourself.

Building two more engines - a 100x71 at ~11.5:1 and a 105x66 at ~12.0:1. Both will run 91. The 105 might be flex fuel compatible just to have some versatility.
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"A petrol engine can start readily, run smoothly and give every appearance of being in good order, without necessarily being in good tune." - Colin Campbell, "The Sportscar Engine"

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[bugguy1967]

Over 190 psi can still use 91 octane. A lot of Raby's higher end builds hover around 190 psi.

I have an Engle V26 cam with 1.4 rockers on a 2374cc. 44 IDFs and a 1 5/8" exhaust. I'm somewhere around 170 psi.
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"A petrol engine can start readily, run smoothly and give every appearance of being in good order, without necessarily being in good tune." - Colin Campbell, "The Sportscar Engine"

[jpaull]

[jpaull]

[Alstrup]

While the Wallace calculator can give you a good (conservative) estimate it is not really a good guideline if you wanna build an engine on the edge (or just under) The reason is, that the most crittical point in the rpm range is actually just below to just over peak torque. The DCR @ cranking can vary too much IMHO.
Next, ignition timing plays a role. The more efficient the burn is (good chambers, good deck height and good mix) the less timing is needed. Less timing have a great impact on power. Luckily most of the ACVW cylinderheads of today have fair to good combution chambers, so for the most part is is up to the builder to get the rest right. A good chamber along with a tight deck and a good fuel mix/distribution versus a sloppy deckheight and a less than desireable chamber shaps with a lesser quality fuel distribution can soon be the difference of 3- up to 4 degrees of timing. That´s equal to more power and less heat soak into the rest of the engine.

Naturally octane has an impact on how high static CR you can run with a given set up, along with ambient temps and humity to an extent. But the quality and attention to the build matters maybe 30% or so. A slap together engine in a given combo may have trouble living on 87 octane and 9-1 static while the next detailed build with the same parts will live well, make 10% better power and run cooler at 9,3 static.

[jeff68]

Question for discussion to help explain this for those who have spent lots of time and research with this, not trying to be a wise guy. If Compression is calculated dynamically.......does it vary at different rpm? At what rpm does it reach 10:5:1?

[bugguy1967]

[sled]

accurate dynamic compression calculations are extremely complicated. The calculators do not factor in how "good" your heads and intake system are, or what kind of exhaust you have and how "good" it is, or the cams actual lift characteristics, among many other things.. They can be used to input different things like cam numbers, rod lengths, etc for COMPARISON, but they are not super accurate for a running engine. You could have two identical engines aside from exhaust, one with an off the shelf header, and one with a custom tuned header, and the tuned engine will have a higher VE. Same goes for heads, two engines, one with out of the box heads, and one with really nice hand worked heads. The custom engine will have a higher VE regardless of the inputs into the DCR calculator. Then theres the whole aspect of cam lobe design...which is another rabbit hole.

the engines ability to fill the cylinder (or overfill in the case of VE above 100%) dictates how much DCR it will have at any given rpm.
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[Alstrup]

Correct
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[UK Luke 72]

I'm not sure were likely to see a decent back to back between two heads like that. Unless I buy some Super pros and head back to VW speedshop
I am running the 'less optimal' as cast Mofo head. Softened the edges and ceramic coated and it's survived a good few road miles and some heavy dyno sessions at 11:1 on UK 98 ~ US 91 at sea level.
0.040" deck, 50cc chambers.

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[motomwo]

On the two side by side pictures above the combustion chamber on the right I believe is what is referred to a quick burn chamber. This design of chamber from what I know was designed in NASCAR racing back in the eighties. It produces a quicker burn which require less ignition timing advance and reduces detonation and allows for higher compression compared to the chamber on the left. I know there is a lot more to it than that like turbulence/and air flow. People a lot smarter than me can explain it better. Much more efficient than the "bath tub" style chamber.

Marty
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1961 Bug, 2276 cb wedge ports 44x37 11.1 comp fk10 1.5 rockers 48idf's 1 3/4" header Rancho pro street trans SAW axles Airkewld 4# narrowed beam, Koni shocks, Airkewld dis brake all around. 5.5/4.5 BRMs

1966 Bug, 2127, panchito heads, fk8, 1.3 rockers, dual 44idfs, 1 5/8 merge header, Rancho pro strt flyer, empi axles, Airkewld's 4" beam and discs all around and adjustable shocks. 4.5 & 5.5 BRMs

[UK Luke 72]

[Brian_e]

[Alstrup]

Yeah.
If we look at the two chambers above and look at ONLY the burn quality and flame propagandation, there is actually not that much difference between the two. The CNC chamber has a small advantage due to the relocated spark plug.

If you look at a given (street or super street N/A engine with 0,060" deck height versus the same engine with 0,040 - 0,045" DH, and we look at the ignition and burn aloine, it is normal to experience that the timing can be reduced by 1 to 1,5 degree and at the same time get a more complete burn. All things equal, performance wise, you will typically see a 2 hp gain per 1000cc. Sometimes more.

The closest thing I have done in recent times was a 1776 bus engine which I did the heads for and the customer and his mechanic friend assembelled themselves. (W100 cam, Kadrons and a VS bus muffler.) I recommended 8,8 static with his vehicle and set up, with a 1,1 m,m deck height. His mechanic friend claimed that it would run hot and ping, and that there was no to very little difference in engine behaviure by just assembling it with whatever deck it had. - Which I later found out was 1,6 mm (0,063" and 8,3 CR) Anyway, tyhe engine was dynoed somewhere else to 90 hp and 135 Nm torque. But once on the streets it was not very fuel efficient, had a tendency to run hot and unfortunately it developed an oil leak 5 months in. The oil leak was a "shit happens" thing. I got the job of fixing this. It was here I discovered the deck height issue. I agreed with the customer to fix it and get it dialed in properly again. Once back on the road it now had 8,8 CR and once back on the chassis dyno it now wanted less fuel and less timing, and now pulls 98 hp and 152 Nm torque, it also runs 15 degrees cooler, but thats partly because I replaced the 009 with an SVDA. Also, the fuel consumption went from 20 mpg to 24,5 mpg on cruise.
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[Paul.H]

[UK Luke 72]

Wedgies about 20 then do you reckon?

You're probably right. (as usual :p)
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