oh..oh Dynamic Compression too high? 327/300hp

**Duke W.** · February 19, 2007, 03:43 PM

Re: oh..oh Dynamic Compression too high? 327/300hp

Every DCR calculator yields different answers as you have demonstated. The one on the Engine Analyzer yields lower numbers than the one on the KB piston site, but the EA documentation does not describe the exact algorithm.

I believe the KB site DCR calculator uses the .050" lifter rise closing point plus 15 degrees as the "inlet valve closing point", which is 41 degrees ABDC with OE indexing for the 929 camshaft assembly.

I am not familiar with "Pat Kelley's site", but obviously he uses a different algorithm than KB or EA!

As I'm sure you are aware, I have posted an online link to a SCR calculator a zillion times.

So what is the calculated SCR???

You are not "engineering" your configuration unless you are using a GOOD engine simulator, and IMO the only one out there at a reasonable cost is Engine Analyzer. DeskTop Dyno and its various clones marketed under different names like Dynosim are just toys ,not serious engine system engineering tools.

Duke

**Patrick H.** · February 19, 2007, 05:08 PM

Re: oh..oh Dynamic Compression too high? 327/300hp

Jim,

I used a few cam grinds once to run various scenarios regarding dynamic compression ratio similar to what you are doing. The base motor/300hp cam always had the highest dynamic compression ratio!

I would not be afraid to use it at all. The GM engineers knew what they were doing, and a version of that cam is still in use today in crate motors. Or, for more power, consider contacting Duke for a version of his "updated" cam.

Patrick

**Jim V.** · February 19, 2007, 05:54 PM

Re: oh..oh Dynamic Compression too high? 327/300hp

Duke...my SCR computes to 9.93 on your suggested SCR calculator (cgsnet...).

I used the 41deg ABC for the KB DCR calc. This yields a scary 9.1 DCR. Retarding the cam +4 (entered 45 IC angle into the KB calc) yielded a slightly lower 8.9:1 DCR.

Pat Kelleys calc can be downloaded at http://www.empirenet.com/pkelley2/DynamicCR.html Many other posters like this program. It is a visual basic runtime program which requires SAE IC angle NOT .050 IC. Here I used 54 and 58 with 8.52 and 8.30 DCRs respectively. Do you know the SAE IC angle for the 14088839?

Big difference between KB and Kelley..9.1 vs 8.52 straight up..
and 8.9 vs 8.3 at 4 deg retarded.

So...I either lower my SCR or go for a later IC angle cam...right? I may be paranoid about predetonation, but I am concerned about pushing quench up past 50.

Thanks
JimV

**Joe L.** · February 19, 2007, 07:01 PM

Re: oh..oh Dynamic Compression too high? 327/300hp

Jim-----

I can give you this bit of empirical information: the '929' cammed engines have a very great propensity for pre-ignition problems. I fought this problem from DAY ONE and for YEARS thereafter on my original owner 1969 300/350. I had the exact same problem on an identical GM replacement short block assembly (GM #3970655) I installed in the car in 1975. With the highest available octane pump gas available it would seriously pre-ignite. Timing changes (within a reasonable limit) made no difference, at all.

I did finally remedy this problem, though. I replaced the heads with GM #336746 cylinder heads. These heads use casting GM #3991492 with 2.02/1.60 valve size. However, the combustion chamber volume is exactly the same as my original '186' casting heads. The big difference in the heads is the relocation of the spark plug which causes the "angled plug" configuration.

After the installation of these heads and with no other changes, my pre-ignition problems disappeared 100% COMPLETELY. In fact, I could run 87 octane fuel with my original compression ratio of 10.25:1, nominal, with absolutely NO pre-ignition problems, whatsoever.

**Duke W.** · February 19, 2007, 07:04 PM

Re: oh..oh Dynamic Compression too high? 327/300hp

A few points to consider.

First, many OE 300 HP engines run just fine on unleaded premium, and they were rated at 10.5:1 SCR, but it was dropped to 10.25 in '67 or '68 IIRC. The actual as build SCR of these engines was probably about 9.75:1. Like SAE gross HP ratings, compression ratios of the era were way overrated.

Many internet bloggers make a big deal and harp over "quench clearance". According to Taylor, the increased detonation resistance of tight quench clearance is lost once it exceeds .005 time bore diameter, which is much less than the .035" recommended by Chevrolet.

I'll take Taylor any day over a bunch of backyard hotrodders. Most OE engines were built with way more than the minimum specified quench clearance, and there is no issue running it up to over .060", which is typical of production engines.

Remember that when Chevrolet started double gasketing SHP/FI engines in mid-'62 to address customer detonation complaints, the actual CR dropped about 0.5 point along with increasing the quench clearance by the thickness of a second gasket, and the detonation complaints were addressed. So much for quench clearance being the determining detonation factor.

Notwithstanding the above, 9.93 is a little high, and in fact, probably higher than as built because the OE shim gasket was thicker than .015". Shim type gaskets should only be used if measurements indicate that the deck and head are dead level with no warp, but regardless of what your situation is, you need a thicker head gasket.

Consider the Victor Reinz 5746 (4.100" x 026"). This will drop the SCR to about 9.66, which should be fine with either OE cam indexing or a few degrees retard, and since it is a composition type, it will handle a few thou of warp.

There's a LT-1 cammed '65 L-76 running around using this gasket and the OE replacement pistons at a true 10.5:1, which I felt was the ragged edge of detonation, but the owner was willing to push it to the limit. The block was right at nominal deck clearnace, so the quench clearance is about .050". No big deal. On 92 PON unleaded premium the initial timing has to be set at 10 deg. BTDC, which yields a total of 34 degrees total WOT timing at 2350 revs. With a little avgas he can advance the initial to 14. These limits were established during hot summer weather testing, which is the worst case due to high inlet air temperature.

The spec I have for '65 300 HP max centrigual advance is 26@4100. For '66 it was slowed to 30@5000, so initial timing could be reduced to ward off low rev detontion. So start with the OE curve and 10 degrees initial. If it detonates grind out the slot to 30 and reduce the initial timing to 6. Either way once you have a baseline that doens't detonate, you can experiment around with a quicker curve to bring it to the detonation limit.

BTW, I ONLY recommend retarding the 929 cam four degrees if the heads are pocket ported, port matched, and chamber relieved with multiangle valve seats and a top cut off the valve seating surfaces to eliminate the unused seating surface. Otherwise, the early opening exhaust valve with OE indexing is best for the poor E/I flow ratio of the OE machined heads.

I believe the SAE standard for timing points is .006" valve lift, which would be .004" lobe lift assuming a true rocker ratio of 1.5:1. It's less than that, but let's just neglect that and use the .004" lifter rise point, which is about 50 deg. ABDC for the 929/839 cam with OE 108 ATDC inlet POML indexing.

Duke

**Duke W.** · February 19, 2007, 07:29 PM

Correction

I mistakingly took the .004" lifter rise timing point from the opening side of the 929/839 lobe which is 50 CAM degrees before POML. On the closing side it is about 68 cam degrees after POML, which gives you an idea of the lobe's asymmmetry.

So the actual .004" lifter rise closing timing point at OE indexing is 64 ABDC, which should drop the computed DCR a fair amount.

I downloaded the big file with the runtime library on it, but my system doesn't recognize the file type and won't open it up, so I must be missing some necessary component.

Duke

**Duke W.** · February 19, 2007, 07:35 PM

Re: oh..oh Dynamic Compression too high? 327/300hp

But your '69 engine was emission controlled, likely with ported vacuum advance. This heats up the chamber boundaries considerably, especially in low speed driving, which increases the propensity to detonate relative to pre-emission engines that have full vacuum advance at idle.

Locally there are a couple of original '66 327/300s (with the lazy OE centrifugal advance curve), one even with K-19 and the owners report no significant detonation on our 91 PON premium at sea level in Southern California.

Duke

**Duke W.** · February 19, 2007, 07:40 PM

Another thought

Did the engine detonate prior to taking it down for overhaul?

What was the measured gasket thickness? Was it the OE gasket? What is the computed SCR?

If it didn't detonate before teardown and you assemble it with no more than the original actual SCR, it likely will not detonate, and any minor detonation can likely be dealt with by spark advance map adjustements.

Duke

**Joe L.** · February 19, 2007, 08:14 PM

Re: oh..oh Dynamic Compression too high? 327/300hp

Duke----

Actually, I believe that ported vacuum advance signal goes back further than exhaust emission control days. I think that all 1966 carbs were ported vacuum advance. And, I think it goes back well before this time, too.

In any event, in my case the vacuum advance configuration did not change when I changed cylinder heads; it was ported before and ported after. Yet, after the cylinder head change, all problems with pre-ignition disappeared completely.

**Jim V.** · February 19, 2007, 08:39 PM

1408839 IC angle is.....

Duke...I found the lobe specs you emailed me a while back on the 1408839 cam. If I am interpreting these chart correctly, it appears the ADV ICA is quite a bit later than then we thought...at...64 ABDC. The ADV Dur comes to 258/270.

This late closing makes a huge difference...the Kelley calc puts DCR at 7.93:1 This is even with the .015 shim head gasketed 9.9SCR.

Please check your lobe data and confirm

Thanks

**Duke W.** · February 19, 2007, 08:49 PM

Re: oh..oh Dynamic Compression too high? 327/300hp

Ported vacuum advance was first generally used on '66 California exhaust emission controlled engines and vitually all '68 engines all of which were subject to either California or "49-state" exhaust emission standards.

It was also used on '63 FI engines, but FI went to full vacuum advance for '64, I expect because Chevrolet realized that FI exhibited better idle quality with full time vacuum advance as do nearly all engines.

That was the first year vacuum advance was used on any SHP/FI engines, and they didn't get it right. Even though the '63 SHP engine had full time vacuum advance, the engine generated less idle vacuum than was required to keep the VAC locked at full advance at idle, which created all kinds of idle stability problems. It took me a few year to fix that on my car, and the fix was to install the '64 SHP/FI vacuum can which required only 8" to lock at full vacuum advance, and the 8" VAC works for all the SB mechanical lifter cams, which generate 10-12" idle vacuum.

All '66 L-72s also have ported vacuum advance, and we've discussed this one before with no resolution.

I also recall someone coming up with and old early sixties AFB from a Buick that appeared to have ported vacuum advance. Maybe Buick engineers thought it was better with Dynaflow, and maybe that's why Dynaflow had a reputation for poor fuel economy in that era.

In any event, ported vacuum advance was, and still is, an emission control strategy. It increases EGT to keep early AIR systems and two way catalysts functioning properly by maintaining high EGT, and continues to be used for modern engines to keep TWCs and O2 sensors hot enough at idle to function properly.

The vast majority of pre-emission engines have full time vacuum advance, and virtually anyone who converts a ported vacuum advance engine to full time immediately notices less tendency to overheat and less tendency to detonate as long as the VAC is properly matched to the engine's idle vacuum characteristics, which in many cases requires a different VAC that was was used with the original ported vacuum advance design.

Duke

**Duke W.** · February 19, 2007, 08:59 PM

Re: 1408839 IC angle is.....

Yes, I think I typed in my "correction" post before yours.

If you want to push beyond what I think might be the limit of the envelope, you can stick with 9.93 static, but if it were me I would use the thicker Victor gasket and keep it under 9.75.

If you do stick with 9.93, modifying your centrifugal curve and initial timing to the less aggressive '66 spec will likely be necessary. The 0.2-0.3 difference in SCR will barely make a dent in high rev power, but the lower CR might actually make a little more low end torque because you can run a slightly more aggressive centrifugal curve.

Duke

**Jim V.** · February 19, 2007, 09:12 PM

Yep....got it...

I will also try a few other DCR prgms to see if there is any kind of consensus (wallace racing, smoke-m-up...). I know one thing...as echoed over on the CF C3 forum...the KB is way high on its DCR calc. Seems KB takes the CYA approach by assuming a very rapid and intense lobe closing rate. The Kelley program also has a more accurate but still conservative ADV LCA estimator which puts ICA at 57 deg ABDC.

thank you for your help!

oh..oh Dynamic Compression too high? 327/300hp

oh..oh Dynamic Compression too high? 327/300hp

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