Not trying to open old wounds for anyone but I've gone through and read the old posts here and on a couple other boards regarding DCR and octane requirements. Nothing seems to be etched in stone but a slight majority seem to hold that anything above about 8 to 1 has to have 92-93 octane fuel. I ran the numbers on my current 350 and the calculators all seem to place it at 8.13 with an estimated cranking compression right around 162 PSI ( those that calculate CCR anyway ), my actual CCR is 160 and factory static CR rating is 8.5 which agrees with the calculators and based on my 160 PSI must be close. I run on 89 and have used 87 with absolutely no problem. I'm planning a 355 with 58 cc CI heads, 22cc D-cup pistons and a slightly bigger but mild cam. Calculated SCR is 8.9 to 1, DCR is 8.16 with a CCR of 168. What I've read tells me this won't work with 87-89 Octane, my experience with my current engine tells me I'll be fine especially since I'll be replacing the TH350 and 2.73 with a 200-4R and 3.73 when I swap engines. I've run the numbers through several different calculators and run them several times, maybe I screwed up somehwere. Opinions?

I'm a bit surprised that you can get 8.16 dynamic compression with only 8.9 static compression. Perhaps I'm not so familiar with the calculators.

Are you using a very mild cam?

Howzabout posting all the specs that you enter in the calculators.

Thats the part of DCR that I don't understand, you can build an engine that runs fine without detonation, IF you are using lower gears, and don't nail the throttle while cruising at 15mph in drive. Increase the load, it detonates, keep the rpm's up and it won't, wind it up as you run thru the gears and it won't ping, lug it around and it will. If you here it ping at anytime, run good gas, you cannot always hear detonation. Mike Lewis doesnt totally agree with the DCR idea, I tend to lean that way, running over 9-9.5-1, you better run better gas. Thats my humble opinion :)

There's a lot more to octane requirement than just a single number (DCR). Aluminum heads let you run lower octane with the same DCR. A well designed chamber with tumble & swirl (fast burn) lets you run less octane. Programmable EFI lets you more precisely control fuel & spark and lets you run lower octane.

I see DCR as a suggestion or maybe a hint on what's going on. It's not the end all, be all.

/Markus

I'm a bit surprised that you can get 8.16 dynamic compression with only 8.9 static compression. Perhaps I'm not so familiar with the calculators.

Are you using a very mild cam?

Howzabout posting all the specs that you enter in the calculators.

I used the calculator at the Silv-O-Lite website initially, my DCR with it comes out @ 8.107.............

http://kb-silvolite.com/calc.php?action=comp

Bore = 4.03
Stroke = 3.48
Rod Length = 5.7
Gasket Bore = 4.06
Gasket thickness = .041
Head CC 58 Piston cup = 22cc
Intake closing @ .050" = 29 (Plus 15 degrees) = 44
Guesstimated deck height = .015 ( Probably actually more )

Cam used will be a Summit K-1102, VERY mild. 262/272 Adv Dur; 204/214 Dur @ .050"; .420"/.442" Lift 112 LSA; 106 Int CL. This is an idle-5000 motor that will never see 5100.

The highest I got using other calculators was 8.16.

As stated above, there are a lot of factors involved in what you can run on pump gas. You may have 8.16 DCR, but under certain conditions your engine can and most likely will see the 8.9 mark. I run 9.5 static, with my Cam I'm down around 8.1 or so, and I can run on 89 with no problem at all. This is with a total mechanical of 36 degrees total. The cam is not radical and is street friendly, as yours seems to be. Some of the factors are Cam, Vacuum advance canister used, springs in the dist (if used), total mech timing, total timing, type of heads used, type of cam used (spec's), type of plug (to some degree), and the list can go on. For the street, (not racing), I feel that 9.5 is the limit for 89 octane (depending on the engine). Anything above needs 91 plus, but that is again really controlled by your engine setup, hence no standard set of rules.

Fred.

As stated above, there are a lot of factors involved in what you can run on pump gas. You may have 8.16 DCR, but under certain conditions your engine can and most likely will see the 8.9 mark. I run 9.5 static, with my Cam I'm down around 8.1 or so, and I can run on 89 with no problem at all. This is with a total mechanical of 36 degrees total. The cam is not radical and is street friendly, as yours seems to be. Some of the factors are Cam, Vacuum advance canister used, springs in the dist (if used), total mech timing, total timing, type of heads used, type of cam used (spec's), type of plug (to some degree), and the list can go on. For the street, (not racing), I feel that 9.5 is the limit for 89 octane (depending on the engine). Anything above needs 91 plus, but that is again really controlled by your engine setup, hence no standard set of rules.

Fred.

I have a 4200 Lb. '76 Monte Carlo, when I swap motors I will also install a 200-4R with a stock lockup converter and a 3.73 posi. I'll be running HEI with an adjustable vacuum canister, as far as the mech advance curve I'll use whatever makes it run the best without pinging ;). My biggest concern has been the head and piston combo because I'm pretty well locked in on that. The heads are 305 HO 416's that will be pocket ported and cut for 1.94 intakes which will increase the chamber size a little but not a bunch. To keep static compression in the reasonable zone I need at least a 22cc D-cup. I have 3 different intakes I can try, but I will start with an old Streetmaster with a plenum divider and 1 inch 4 hole spacer. It's old tech and most people use them for door stops but I have seen it do wonders for performance and mileage on a '69 Chevy 1/2 ton PU with a 327 automatic and they were designed for the RPM range I'm building for, plus it's the correct vintage for my car. Not that it's important, I just like the idea of having it period correct. I've had the intake for 15 years and don't have a dime in it except for the spacer. I'll use a Holley 600 to break the motor in but will switch to a Quadra Jet and dial it in. I'm feeling more confident that it will work. Thanks to everyone for your input, it ALL helps. :thumbsup:

At low engine speeds the engine can have a higher VE. This increased VE (volumetric effciency) will increase the dynamic compression.
Before Nascar set the compression ratio rule, some of the restrictor plate engines had 18-1 static compression. At 6600 rpms at WOT the VE was low enough to run the engine on the Unocal fuel. Once in a while a driver would exit the pits and nail the throttle and smoke a piston.

The problem is advertised duration.
I do not agree with this method:
"Intake Closing Point (degrees)
ABDC @ 0.050 lift plus 15 degrees"

If you use the true advertised duration closing point of your camshaft in the calculator (59 degrees) the DCR is now 7.455-1.

7.4-1 is fine for most engines with 89 octane fuel.

If you use the true advertised duration closing point of your camshaft in the calculator (59 degrees) the DCR is now 7.455-1.

7.4-1 is fine for most engines with 89 octane fuel.

Thanks, it's looking more and more like I'll be OK.

Using my calculator, I get 8.86 SCR and 7.5 DCR. This is using the advertised duration (57º Int closing). BTW, what head gasket are you using. I've not seen or heard of a 4.06 bore. For a 350 4.100" or 4.166" are the usual. See the link in my sig for more info.

DCR is one of many calculations to be used as a guide. No one said its the Holy Grail.

Guideline. Estimate. Etc.

( alll I know is that 8.6:1 is too high for todays 93 oct when you have to run the motor hot ( 200F ). So THEORECTICALLY I could cut the eye-run heads I'm planning on using down to a 113 cc chamber to hit the same 10.5:1 SCR. BUUUUUT, I could likely get away with a 160 thermostat not 195 and a much lower operating temp. THEREFORE, in this specific case, the eye-run heads would ping less than the old alums on there now at the same DCR due to a reduction in operating temp, to wit, its an estimate man, one calculation of many and extrapolating any universal, case-specific rules is spurious as heck).

DCR is one of many calculations to be used as a guide. No one said its the Holy Grail.

And that's all I'm using it for, the only accurate determination is to bolt it all together and fire it up I'm just trying to cover all my bases. ;)

Pat, Mr. Gasket has a head gasket with a 4.060" bore that is .040" thick. I'll probably use a std. Fel-Pro that's 4.166" X .041" though.

I'm really feelin' all warm and fuzzy now. :thumbsup:

Cam-copier,

Are you sure on this, at what RPM?

"At low engine speeds the engine can have a higher VE. This increased VE (volumetric effciency) will increase the dynamic compression."

I was always under the impression that VE would normally increase as RPM increase, then drop off. All the charts that I have ever seen show a increase starting around 1.5 to 2 K rpm and then, based on the breathing ability of the engine, it drops off. I am just talking normally aspirated engine, without N.O., etc. Just here to learn....not attacking.

Fred.

Low rpm VE is why E-brock's old SP-2P dual plane intake HAD to be matched with the rest of the motor!

An SP-2P, with the right little-bitty, smaller then stock mileage making cam and a set of little-bitty, 1-1/2" four tube "looooonnnnng" TORQUE headers, it could make an 8 to 1 CR. smog motor detonate on GOOD gas b/c it sucked so goooood!!

AND this is one reasons why E-brock quit selling it b/c I e-mailed and asked them directly about it!!!

It's all in the combination!!

pdq67

PS., and fwiw, I have a BB, SP-2P up on the shelf to play with onna these days.. I WANT a SB, SP-2P too if anybody wants to part with one??

A 7.5 DCR corresponds to 167 cranking psi:

(7.5 ^ 1.25 x 14.7) - 14.7 = 167

Low rpm VE is why E-brock's old SP-2P dual plane intake HAD to be matched with the rest of the motor!

An SP-2P, with the right little-bitty, smaller then stock mileage making cam and a set of little-bitty, 1-1/2" four tube "looooonnnnng" TORQUE headers, it could make an 8 to 1 CR. smog motor detonate on GOOD gas b/c it sucked so goooood!!

AND this is one reasons why E-brock quit selling it b/c I e-mailed and asked them directly about it!!!

It's all in the combination!!

pdq67

PS., and fwiw, I have a BB, SP-2P up on the shelf to play with onna these days.. I WANT a SB, SP-2P too if anybody wants to part with one??

I have an SP2P 4 Bbl. intake on the current 350, it was on there when I got the car and an SP2P 2 Bbl. SBC intake up in the rafters, somebody gave that one to me. I added a 1" four hole spacer to the 4 Bbl. last week because the carb was getting really hot. It seemed to like it, I can get a little more RPM on the top and there was no change on the bottom according to the Butt Dyno. Watch ebay for one they show up fairly often listed as "Performer" intakes and usually sell cheap if at all. I may play with it on the new motor just to see what happens. Or not. A friend of mine has a 267 he wants to get rid of and I was thinking that might be fun to play with. Have to wait until he gets back from Iraq though.

Some engines are built to perform in rpm ranges where the VE is allready falling off. It might be port size or carb sizes restricting the airflow etc. These engines can benefit from increased compression ratios. These engine would not be operated below peak torque rpm.
SS4speed, this might not answer your question.

I was watching a dyno pull on a 434 SBC. They made about 15 pulls from 4500 and up. Nice running engine I think it made about 680 hp. The dyno operator whispered something to his friend like "watch this". The next pull he started it at 2500 rpms and by the time the engine got to 5000 rpms it had lifted a head gasket.

I will check ebay..

I wish E-brock made the SP-2P bigger ported b/c I will bet a dollar to a donut hole that the long separated ports the SP-2P have if big enough would produce some dandy torque up higher then either the Performer or RPM!!

Kinda like the Z-28, flat ram manifolds long ports, but without all the low speed driving and tuning hassles they have.

pdq67

PS., if I recall right, the name SP-2P means "single plane-two/dual plane" b/c each ports mouth is located right at the plenum and not back in the ports off other runners like the Performer and RPM have. It's actually made like each plane of the dual planes are like single plane intake ported...

Looks like this one's about wrapped up. Just want to say thanks to everybody, I got something out of every single reply and you guys have been a huge help. :cool: :thumbsup: