I haven't come across this on the site yet, (unless I'm just not looking in the right place) But, is there a relationship posted somewhere as to what octane rating works well with what DCR. Just wanting to see what octane rating will work well with different head/head gasket/cam combo.

It's often discussed in the performance forum. There are more factors involved than just the DCR but if you search that forum, you'll see what people are running with what octane gas. Look for Pat Kelley's posts and check out his DCR calculator on his website.

Tim.

The DCR for lower octane is still pretty much up in the air. I'd say below 7.5 should run on 89 or maybe 87. There are some other factors that enter into it, too, that are not addressed in the DCR number (VE, chamber/piston shape, cam flank shape). DLoad the DCR calculator linked in my sig. Give it a go and post any questions. There are a number of people here that have an excellent handle on it.

I really like the DCR calculator. It gives an excellent quantitative way to compare cams. It is by no means perfect, but it is the best we have without building a million engine combos. I highly recommend learning to use it.

As for the octane rating, my car is running 9.6:1 static compression, 8:1 dynamic compression and I am using 89 octane. This is in a 3500lb 3.31 geared 4 speed Nova. My dad's 383 in his work truck has 9.8:1 static compression, and 8.1:1 dynamic. The truck weighs 6200lb on a daily basis and more when towing. So far we have been as heavy as 10,500lb. Amazingly we have been using 89 octane gas in it also. It has taken very carefull ignition curve tuning, but its worth it IMO. Something I wonder about, is how cam lobes will effect the way the engine actually runs. We are using camswith gentle lobes(Crane Energizers) which is probably contriduting to the ability to run lower octane on a given dynamic compression ratio. Something like a UD cam or Extreme Energy is probably going to knock the octane required up a notch I think.

Well, If I can afford to get some better heads or not, It shows with my stock heads and the XE262 cam static to be 8.5 and DCR to be 7.28 and with the World products S/R torquers to be static = 7.9 and DCR 9.3. It seems that maybe If I can afford a little upgrade with those heads (I know they are only a little bit better than my stock heads, but anything is better) I'll probably have to run at least mid grade fuel. I had already downloaded Pat's DCR calculator but was wondering what #s corresponded to what octane. Thanks Rad Racer for posting your real world #s.

Originally posted by Brian Z:
with the World products S/R torquers to be static = 7.9 and DCR 9.3. Looks like you mixed these up. Should be 9.3 static and 7.9 DCR. I think you'll find a noticable increase in power. The SRT's are just like putting fresh, new fuelly heads on. The increase in DCR will really pick the bottom end torque. Probably need 90+ octane but try lower, it might work.

Yes, I did reverse them. Any comment on which cam might work better in the real world?

I suggest using the cam which get you the closest to 8.25 DCR (7.9 is an excellent DCR and would run great). Provided you are willing to run premium gas. If you want to run lower octane, go for about 7.5 or so.

I guess If I wanted the better heads, I could try playing around with different head gasket thicknesses to get the dcr down, but it would probably mess with the correct quench distance. Maybe I'll check out some other aftermarket heads with 76cc chambers. The vortecs will definately bump my ratios up too much for trying to live with lower octane pump gas. Thanks for the replies.

The SRT's are available with 76cc chambers. You need to be aware that most all cast replacement pistons are shorter than stock Chevy pistons by .020". This will put the piston top down the bore an extra .020". If you have cast pistons this is likely the case. Hypers can go either way. Many forged pistons are .010" short for use with a 3.50" stroke. You might want to hold off on getting head gaskets until you can measure the deck.

That is a good point. I did put in flat top cast pistons in with 4 valve reliefs. I never measured how far they were down the bore when I put it together about 2 years ago. I guess I forgot that those heads came in 76cc's also. I was thinking I needed to bump the compression a little bit until now. Now I don't think that the extra compression will be worth having to go to premium gas in my daily driver. I already only get 11mpg w/ my now 2.73 gears and it probably wont get any better w/ 3.36 gears and the bigger cam. Right now I only have the crane energizer 260h(.440 lift) which is just too little. But I can run the low to mid grade fuel.

I have the 76cc SRTs and cast pistons. I used a Fel-Pro .018" shim head gasket (7733-SH1) mainly because I can get them for about $6.50 each. The quench comes out at .063". A bit much but there are not any options short of replacing the pistons or decking the block. The cam I have is smaller than your's (204/214) but it works very well. The DCR is pretty low (probably under 7) but I don't know what it actually is due to the cam specs not making much sense (ADV is 278º, which is very long for the .050"). However, the engine makes good power and tows my bracket car with ease. My static CR is about 8.7. I run 89 octane but can run 87, however it seems to need more throttle to maintain the same speed.

I run 16º initial with manifold vacuum (makes it 26º at idle) and 36º total. The vacuum advance has 10º. I use to get 10 mpg but leaning the cruise mixture, bumping the timing to 16/36, and installing a 200-4R trans pushed it up to about 14 mpg. I have 3.36 gears and 26" tires.

I understand that with Au heads you can get by with more static comp because of better heat dispensation than the iron heads???