02RedHawk

iTrader: (17)

Posted this in the 'internal' section, but got very minimal response. Would like to hear some additional thoughts about this subject...

Of these three options, which would you choose...and why?

DCR: 8.69
HG Thickness: .054"

DCR: 8.88
HG Thickness: .045"

DCR: 9.00
HG Thickness: .040"

The million-dollar question is.... Is it "better" (less prone to detonation) to have a lower DCR with a less-than-optimal quench, or a higher DCR and a "perfect" quench?

At what point does DCR become too high, regardless of HG thickness? In other words, is there a fairly known relationship (proportional / linear / exponential / ?) between quench and DCR, as its related to fuel quality & detonation resistance?

Fuel is 93-pump premium, non-oxygenated. Assume adequate cooling (170/180 t-stat, fans reprogrammed). AFR heads (66cc) which can tolerate higher DCR, ported intake, and headers - so cylinder-fill efficiency will likely be high. Year-round DD usage; summer temps normally 80s with a few (~10-15/year) days in the 90s.

Adrenaline_Z

Are you not able to get your required quench height and select the IVC to
tailor the DCR you desire?

I've read between 8.0 and 8.5 DCR is the target for 92-94 octane at sea level. You might get away with a touch more being an aluminum head and
a quality combustion chamber.

My thoughts are to run the tightest quench (keeping piston and valve
clearances in mind) which is good for reducing detonation. As VE changes
thoughout the engine RPM range, having quench at the more optimum level
would produce better results than IVC which is better suited for dialing in a
specific range of RPM for engine tuning.

Patrick G

iTrader: (14)

Personally, I'd go for the tightest quench. Basically, all the choices will be pretty close to each other in knock resistance, but the highest DCR will give you the most power under the curve.

FWIW, I think you may be running too small of a cam or one with too advanced of an ICL to have a legitimate 9.00 DCR. Can you disclose more details of your setup? Cam specs at .006 too.

02RedHawk

iTrader: (17)

Well, without writing a miniseries and risk sending my thread off onto other tangents not important to the original questions, I will try to keep this brief.

LS2 motor.
AFR205 heads, Unmilled.
Comp Cam, XFI Grind: 214/218 112+2
.006": 264/270 112-LSA 110-ICL

I originally had spec'ed out the cam on an XE high-lift grind (214/218 112+2 @.050", or 267/271 @ .006"), but the thought (lure) of taking full flow advantage of the AFR heads from .550-.600" enticed me to now lean heavily towards the XFI. Compared to the XE, the XFIs ought to provide a slightly cleaner idle, produce more power, and also still have excellent DD characteristics & spring longevity.... But, the change from XE to XFI lobes bumped up the DCR by ~0.1 at each HG-thickness listed. In other words, I could run the same cam on XE lobes & .040" HG thickness, and have a DCR of 8.9 (instead of 9.0).

The 214/218 at .050" is the biggest cam that I'm willing to run with the stock stall (which is staying for DD's sake, and possibly tow ~1500lbs in the winter only), and its also about the biggest cam that a reputable "builder" is recommending. It will have a power curve very similar to this one (see linky, same truck, 214/220 111.5+2, XE lobes, stock heads, LT headers ---> https://ls1tech.com/forums/generation-iii-internal-engine/548531-tbss-cammed-w-headers-etc-dyno-graph-sound-clip-inside.html ), with an emphasis placed on retaining low-end torque. I do not care nor want to spin the motor past 6200/6300 rpm; I'd rather have that power usable in the low and midrange where a truck needs it most.

The problem, tho', is what I am alluding to in the first post. To get what I want means a DCR of 9:1... And I'm thus then still on the fence as to what to do about it. Run with it as-is, or increase the quench, or change the cam to 112+1 on XFI lobes, or possibly just do it on XE lobes.

02RedHawk

iTrader: (17)

Interesting. I considered quench to be one of the factors to consider "after" spec'ing out the desired valve-events & resulting camshaft....Not so much as something that could "compensate" (?) for an IVC that was slightly "off". Is that what you were stating?

Adrenaline_Z

I don't think of quench height as compensation at all.

I believe quench should be as tight as tolerances allow for the application.

Having a good DCR value with a 0.070" quench height is not going to do any
good when the charge is sitting between the head and piston, instead of inside
the chamber.

1989GTA

I wonder would happen with the valve events if you retarded your cam a few degrees. Also would it be enough to lower the DCR to a level you could live with? By the way I will be running the XFI218 degree cam on my SBC build up. I retarded it one degree from the Comp Cams recommendation to get my DCR to 8.5 and add a little to the upper end.

Adrenaline_Z

Your theory looks good on paper and should technically help top end power.

Another method to achieve the end result is to vary SCR to place IVC timing
where the engine requires and have the quench as tight as possible.

I would assume the engine builder would add volume to the chamber area in
place of quench to dial in the SCR and create the most efficient design.

J-Rod

Don't give up quench, optimize it. Increasing quench excessively actually leads to detonation.

hammertime

iTrader: (1)

Since you are talking about a truck motor, I'd go as tight as possible with quench. I've seen articles on here where David Vizard recommended .030 (or less, IIRC) for small block chevys. The tighter tolerances, lower rpm and automatic trans should all allow for at least that or less in a truck. The closer the piston comes to the deck surface, the more mixture motion is created in the chamber.

I know you want torque, but low lift flow appears very similar between the AFR 205's and 225's. Wouldn't the consequences of the larger cross sectional area be small in comparison to the gains of a much better head (than stock). I'm thinkning starting with 72cc chambers and milling may get you the SCR and DCR you want.

Otherwise, you can look at milling in the chambers some, or sinking the valves.

Adrenaline_Z

Just wondering why there is a reference to truck motor when specifying
quench height?

hammertime

iTrader: (1)

Two reasons:
1 - original poster had already stated that there was no desire for power over 6200-6300 rpm
2 - since you can't (hardly) get a manual transmission in a truck, you don't have to worry about mechanical over-rev or missed shifts. Basically, the same reasons some builders leave extra PTV clearance for manual trans applications.

http://www.popularhotrodding.com/tec...on_ratio_tech/ as Featured in Popular Hot Rodding by David Vizard
And, as for what quench is worth in terms of power...

02RedHawk

iTrader: (17)

To me this seems to be just a little bit misleading. Its the added compression from the .010" reduction that is producing the 7 added hp....not the resulting tighter quench. Unless, the quote was paraphrased and with each .010" of quench reduction the combustion chamber was opened-up equally as much (volume-wise) - or dished pistons....?

Good comments guys, I appreciate the valuable feedback.

ringram

Quench also increases turbulance in the chamber which helps promote faster controlled combustion by minimising localised hotspots which cause detonation etc. So yes, tighter is nicer.

hammertime

iTrader: (1)

I agree - It would be interesting to remove a comparable volume from the chamber and run a back to back test. I think the author may be implying that the more complete combustion is also providing the power increase, not just the increase in compression.