What would be best all-around SCR/DCR range to consider running in a heavy street BBC 489 or 496 Chevelle with GM cast iron heads? I understand that a 8.1 - 8.2 DCR has been mentioned in many cases to shoot for, but SCR's have been all over the place from 9.1 to 10.5.

My Fall 489 engine project "on paper" will have a 9.5 SCR and a 7.8 DCR (using a small Lunati solid cam). Is this range too conservative or do I need to go to a higher SCR, DCR, or both? Again, this will be a pure street car and needs to run pump gas.

Thanks, Charles

This always sort of confuses me as well. What octane do you want to run since you say pump? 93? Or lower 87ish? Guy who sold me my engine rebuild kit says I should be running 9.5:1-ish compression on my 350 (76 cc chambers, flat-top pistons...running Crane 272HE cam), and I run 87 with no trouble, but then I think I see some people say they need to run 91ish with 9.5:1? Then there is my friends 406 who is supposedly 11.5:1 compression that has to run 93 but is fine on that, even at drags...but then again, these are all SCRs, and DCR is kind of where I get lost..

Charles, as you know, this is a very complicated subject.

For a reference, I have a DCR of 8.3 (12.16 static), have iron heads, .049 quench, .5cc dome pistons. I must run over 92 octane, usually 100.

Here we go. The best DCR to shoot for depends on your engine specs. Head material, quench, piston type are major players that you can focus on that determine your best DCR.

My opinion is, if you want to play it safe, go for 8.1-8.2 with Al heads and 7.8-7.9 for iron heads. For a pure street machine, you want to have a reliable machine...which means keeping the DCR on the conservative side, like 7.5-7.8, higher for Al heads.

Some people will complain about 7.5 being a low DCR. Are you going to notice the difference between 7.5 and 8.0? Probably not, and you can be rest assured that you aren't beating up your bearings from detonation and ping. You loose about 10HP (on a 500HP motor), and that's not a big deal.

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In my experience there are too many variables to make an accurate prediction of an engines fuel requirements based on DCR alone. I had a 350 with 305 heads, flat-top pistons, .058 quench, and a DCR of 8.35 that would rattle like a Cummins on anything less than 100 octane. The engine I ran last summer was a 350 with Dart Iron Eagle heads, flat-top pistons, .045 quench, and a DCR of 8.28. I ran it all summer on 91 octane without a hint of detonation. This summer I'm running a 383 with the same Dart heads, flat-top pistons, .038 quench, and a DCR of 8.88. So far I've been running 94 octane, and so far so good, but due to starter issues I've had the timing pulled back to 25 degrees. I plan to get everything fixed this weekend and get the timing bumped up to 34 degrees. Then we'll see what happens. I'll be real surprised if this thing will run detonation-free on 94 octane.

Having said all this, I think that piston dome shape, combustion chamber shape, and quench distance are all major contributing factors to an engines fuel requirements. I believe the reason the 305 headed engine needed 100 octane was due in part to the large quench distance and design of the combustion chambers. Another factor in my favor is my Dart heads only need 34-36 degrees timimg. I know some guys running 38-42. Based on my experince it seems that keeping the quench distance under .050 and shooting for a DCR between 7.5 and 8 should keep you safe with iron heads and 91 octane. Hope all this rambling helps.

Stuff like this is why the heads combustion chambers call for different octane requirements due to quench and shape.

AND this should only be considered for true flat-top pistons b/c domeed pistons really stir the pot so to say with changing flame front eff's!!!

A trapezoidal shaped large open chambered smog head is probably the worst, next, the double hump heads like the old -462's, -186's, -041's and -291's, next the old -461's and -461x's, next the Vortec's and finally the fast burns but I figure none of this is set in stone!!!

As always, jmho...

pdq67

Thanks for the good responses. I am feeling a little better now about building my pure street 489 BBC with a target range of 9.5 SCR and 7.8 DCR. That's a "safe" place to stay at I believe.
If anything, I was a little uncertain about the low 9.5 SCR on a large dispacement BBC.

Oh, some additional info:

67 Chevelle (3750 lbs.)
Flat top forged pistons (-3cc's)
Closed 063 iron ovals with 2.19/1.72
.005" deck
.044" qwench
UD/Lunati solid LS6 Replacement cam 272/272 and 238/238 @ .050", 112LSA).
TH400
3.31 gears
2400 B&M Converter

I just hope that the power level of the 489 will be "good" with these numbers.

Pat Kelly, PDQ67, 427L88, Mr. 4 Speed, BBMike, or anyone else have comments or advice?

Thanks, Charles

With small chambers and good quench, you could probably schooch it a bit higher, but the 7.8 seems conservative. It's not really complicated, but confounded by each individual setup. Quench, etc. It's really a balancing act between torque ( cylinder pressure as given by DCR as a estimate)and pump gas reliability. LArger ci mills don;t need as much DCR to make the grunt. However, in a case like mine, a short stroke big port revver, you can't get enough DCR! It's what makes my mill very streetable. Yours on the other hand will have PLENTY of low end grunt. Suspect it will run out of steam quite early though. Maybe too early even with mild 3.31's. Particularly if you stab it in +4 +6 as Harold would prefer.

If anything Charles, your 7.8:1 target should allow you to run mid grade fuel, and with the price of gas these days, thats a nice option, an option I don't have. But again, in my case I had to push the DCR envelope to give the engine enough cylinder pressure to make substantial grunt and overcome the large ports. It has and does both, due to cylinder pressure.

But at 8.6:1, a bad batch of premium fuel and it pings. Too close to the margin. With cubes, you don't need to push the envelope. But again, IMHO, more DCR=more TORQUE.

With your gears and converter I think you are right on with your dcr/scr. smile.gif

What octane do you guys think my combo would require? I have been worrying lately that those Victor Reinz head gaskets my builder was using looked awfully thick - and may have lowered the compression furthur(?)

Imho, you are RIGHT in THERE!!!

The 272 cam should compliment your 9.5 to 1CR. just fine!!

I have a CC 282S solid in my about 9.8 to 1CR., 496 b/c I ran a whole bunch of cams through D2K and finally settled on 5500 and 4500 rpm's for peak hp and t for my motor so, to me she would produce max. midrange grunt!! (My First Gen. car has an M-20 and 3.31 gears)... And I got my supposed 9.8 to 1 CR., (up from 9.5 ROSS pistons), by switching from .038" composition headgaskets to .022 steel shims so you may think about doing that if you want to??

Your's should be some lower in the rpm ranges but should produce more grunt!!! (Imho...)...

And I'm a solid cam kinda guy too..

pdq67

Im running mid grade 89 octane ethanol in my little 11:1 350 and my 36* total timing is all in before 3,000 rpm, it sure is nice with gas prices.