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Old 10-08-2019, 12:09 PM   #1
RegalRocket
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Default SBC soft chamber heads and timing

I had my SBC 235 CC pro action heads CNC softened, and so far I love them. My question in regards to timing is it seems they want a good 5-7 degrees more timing than I would have used before the chamber work.

Does this seem like a common change? I know the point is to make the chambers a bit more “lazy”. Just making sure the timing requirement change is what others have seen as well.

Just making sure I’m not missing something. Thanks guys!
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Old 10-08-2019, 04:21 PM   #2
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Default Re: SBC soft chamber heads and timing

That doesn't sound unreasonable to me, 23* SBC heads don't have super efficient chambers anyway. They generally have to run 5-8* more advance than a comparable SBF or LS head. Either add timing or leave timing and add boost/fuel.
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Old 10-08-2019, 04:56 PM   #3
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Default Re: SBC soft chamber heads and timing

But you are talking about the same head with the same chamber just softened. Wow that seems like a lot. Was the size of the chamber significantly changed?
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Old 10-08-2019, 06:47 PM   #4
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Default Re: SBC soft chamber heads and timing

No the volume doesn’t change a whole lot, but the lack of quench makes for a slow flame travel across the piston face. This is what the softening does to keep detonation away, I just wanted to see if others had seen similar results.

I pull plugs and tune based off of them. I was reviewing data and I pulled timing before the last race and raised boost and the motor certainly got lazy. ET was the same at 6 lbs less boost. It seemed much happier with more timing and less boost.

So now I need to try more timing AND more boost!
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Old 10-09-2019, 09:05 AM   #5
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Default Re: SBC soft chamber heads and timing

Why would you want a slow flame travel? It just gives more time for a detonation event to take place elsewhere in the chamber.
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Old 10-09-2019, 10:17 AM   #6
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Default

Post up some pics of that chamber if you have any, on a small block with the valve seats so close to the deck, do the valves have to be sunk any?
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Old 10-09-2019, 11:40 AM   #7
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Default Re: SBC soft chamber heads and timing

Quote:
Originally Posted by Taff View Post
Why would you want a slow flame travel? It just gives more time for a detonation event to take place elsewhere in the chamber.
I believe the idea of softening eliminates the quench/squish areas of the chamber where detonation seems to occur most often. Quench/squish helps stir the mixture motion to help create efficient fast combustion. In high boost the mixture is already more dense and should burn fast as it is, sometimes too fast.you dont need additional squishing /swirl /motion effects. That rapid pressure and heat rise can cause detonation. So you soften the chambers to help control combustion abit.
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Old 10-09-2019, 09:29 PM   #8
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Default Re: SBC soft chamber heads and timing

One of the best softened wedge heads on the market designed by a man that has probably forgotten more about detonation than we will ever know. My AJPE SB heads.

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Old 10-09-2019, 09:38 PM   #9
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Default Re: SBC soft chamber heads and timing

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Originally Posted by Taff View Post
Why would you want a slow flame travel? It just gives more time for a detonation event to take place elsewhere in the chamber.
Sorry, it don't work that way. Why do you think the Hemi is still the king of extreme boosted engine designs?? Its not only because they out flow everything else. Flame front speed/propagation is the reason most engine builders in the know usually don't go any further than 14* valve angle if they can help it
The shallower valve angle chambers beyond 14* are great for NA combos but are generally very timing sensitive with a narrow tuning window.
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Old 10-11-2019, 06:24 AM   #10
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Default Re: SBC soft chamber heads and timing

Quote:
Originally Posted by TurboII View Post
Sorry, it don't work that way. Why do you think the Hemi is still the king of extreme boosted engine designs?? Its not only because they out flow everything else. Flame front speed/propagation is the reason most engine builders in the know usually don't go any further than 14* valve angle if they can help it
The shallower valve angle chambers beyond 14* are great for NA combos but are generally very timing sensitive with a narrow tuning window.
Sorry but I have to disagree with you again. The hemi may be king in extreme NITRO boosted applications, the large chamber being suited to the almost liquid fuel/air charge that goes into these engines. Discounting the twin plugs needed in these extreme applications, the single centrally mounted spark plug in an ordinary hemi is one of the main benefits of the design - this gives equal (and faster) flame travel to the edges of the chamber - so in fact the flame travel distance (and speed) is shorter than in a wedge or bathtub type chamber where the plug is off to one side.
Anybody in the know uses a flame travel enhancer ,(as did Formula 1 turbo cars-1500hp from 1.5 litres), because race fuel tends to slow and stabilise flame travel a bit. It's common knowledge that big chambers are usually more susceptible to detonation than smaller more efficient chambers. Proper race fuel stabilises flame travel,along with less ignition timing advance.
I see the 'softening' of chambers as not much more than what I would normally do in a performance engine build - smoothing off all the sharp edges in the chamber and on piston crowns to prevent a hot edge starting off a detonation or pre-ignition event. Having a smooth rounded chamber will actually INCREASE flame travel speed across the chamber as there are no sharp edges to impede flow!
Look at the import 4 and 6 pot cars for high hp to cu in , nearly 2000 hp from a 2 litre on race gas, or the Coyote pro mod engines from MMR -3500hp from 5 litres, makes the hemis look weak.

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Old 10-11-2019, 11:32 AM   #11
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Default Re: SBC soft chamber heads and timing

If you don’t understand the difference between a quench pad head, and softened chamber I have to question your experience. They are vastly different in design. It’s not simply a matter of deburring and radiusing edges.

Hemi is also the king in just about any boosted setup, regardless of fuel. Promod, top alcohol, pro boost.... have hemi’s routinely in the top every race. The hemi is still to this day the motor all others are judged by.

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Old 10-11-2019, 12:08 PM   #12
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Default Re: SBC soft chamber heads and timing

It is abundantly clear that you know nothing about softened chambers and their effects on combustion speed or non-parallel surfaces in quench areas to prevent detonation. We're not talking about obstruction of flame travel by dome vs flat/dish top pistons. The OP stated with the same combo needed eight ( degrees more spark advance to make the same power. This is proof that flame travel has slowed down. There are quite a few well regarded head designers, engine builders etc that use this same method to reduce the chances of detonation occurring. Alan Johnson, Sonny Leonard, Curtis Boggs/Race Flow Development, Tony Bischoff/BES, Carl Foltz/CFE, Brodix and the list goes on. None of these people are designing or modifying these chambers to increase combustion speed on power adder applications. They are modifying the quench pad areas so they are angled or curved to give the high density air/fuel charge a way to be pushed back towards the center of the chamber without hydraulicing and setting off the mixture. These flat quench areas and flat top pistons have proven to be where most of the trouble begins on a power adder application. This becomes even more critical when the heads are steep valve angle and the cross section of the chamber is very thin which promotes very quick flame travel speeds making them very efficient as well as very timing sensitive. Even if the piston is lower in the bore by as much as 0.150" the detonation did not go away in the engine I have seen first hand. I have seen evidence the quench areas on a 13* billet head that actually pitted these areas from detonation and the pistons being hard anodized saw no evidence of damage due to the anodizing being so tough. Once the quench pad areas were machined to be angled/curved the detonation stopped. Over the last few years this design has evolved and the most common shape is now a parabolic dish which is about as close to a hemi as you can get with a wedge head design. As far as the 4-valve engines go, the majority of them are a pent roof chamber design which has little to no quench area and generally a centrally located plug location making it very similar to a hemi design using angles vs curved radius. Some of us are running A/F as rich as 3.0 on methanol so we're not all that far from the Fuel cars. Most of the Outlaw hemi engines are making well north of 5000hp from 526cuin or less so not all that far from your examples based on hp/cuin. There are a couple of Mod Motors making big power but at what cost, reliability etc. There are tons of hemi engines making 4000-5500hp every weekend around the country.
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Old 10-11-2019, 07:40 PM   #13
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Default Re: SBC soft chamber heads and timing

I have what you would call 'softened chambers' in my own turbo combo but I have machined Singh grooves into the radius'd quench pads to bring flame travel speed back up (I want my flame front to reach the outer edges of the chamber BEFORE a spontaneous detonation event can occur!) , I also have a large dish in the pistons that mirrors the chamber. You can't get more detonation sensitive an engine than the iron head, non intercooled Pontiac I use (on gas)-and I have none at 27* of timing.
Of course the OP's combo would need 8* more timing - it is slightly lower compression and has a slower flame travel! So I ask again, why would you WANT slower flame travel? Just so that you can add more ignition timing - to bring it back up to the SAME power level ! Less ignition timing advance equals less negative torque and less chance of detonation, no question on that. Much safer to make extra power with more compression (higher flame travel speed), more boost, and less timing advance. You need good fuel though.
And how do you know that the names you dropped are using 'softened chambers' to slow flame speed
We are seeing a trend over here with turbo pro mods switching to BBC based combos from the Hemis. Why? I couldn't tell you.
As for the 4 valve semi hemi pent roof heads, they have a quench band around the central chamber precisely to increase squish/mixture turbulence (read faster flame travel), just like a 2 stroke does. JMO though, I'm not 'pretending to be a guru'.
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Old 10-11-2019, 09:40 PM   #14
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Default Re: SBC soft chamber heads and timing

They are switching to BBC being willing to lose some power over the very high maintenance of a max effort hemi combo. The smart money is on swapping to an AJPE 481X over here if rules don't kill it. NHRA killed any chance of running a wedge head engine with their ignorant boost pressure rules. The hemi is the only way to be competitive as they out flow anything else. You are in your own little world with a iron Pontiac, you can't hardly pick a more irrelevant combo. Please don't be offended if some of us don't agree or take any advice and good luck with your pursuit of Pontiac performance. Cheers
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Old 10-12-2019, 05:54 AM   #15
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Default Re: SBC soft chamber heads and timing

I didn't give any advice, I leave that to the 'experts' like your self. I asked a question. Some of us are still waiting for the datalogs you said you had of turbo compressor speed changes relating to dump /bov's.
Yep, my own little 1400hp 420ci all factory iron Pontiac world. I'm one of these guys who actually builds his own cars, not a 'throw the check book at it' type of guy.
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