Maximum DCR - Chevelle Tech
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post #1 of 89 (permalink) Old Nov 22nd, 04, 3:59 PM Thread Starter
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What's a good rule of thumb maximum DCR for Iron and Aluminum heads figuring premium 93 octane gas. Thanks
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post #2 of 89 (permalink) Old Nov 22nd, 04, 4:16 PM
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For the average motor, limit an iron headed motor to 8:1 and an aluminum headed motor to about 8.25:1. Those numbers are not set in stone though. Assuming you have good heads with better chambers and quench is properly set at or very near .040 and the cooling system properly does its job as well as the tuneup, 8.2 with iron and about 8.5 with aluminum is do-able. And, some people take it even further by thermal heat coating the piston tops and valves to even further reduce the sensitivity to detonation and take the numbers even further than that. It takes some good engineering, but if you picked up the issue of hot rod with the pump gas drags, many of those cars were in the nines and some in the eights supposedly on 93 octane fuel, so you can push the limits, but once again, for the average buildup in a heavy street car that will be driven in different situations and conditions, its probably safest to go with the first set of numbers(8:1 iron and about 8.25:1 for aluminum).
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post #3 of 89 (permalink) Old Nov 22nd, 04, 4:23 PM Thread Starter
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Thanks, that's the info I was looking for.
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post #4 of 89 (permalink) Old Nov 23rd, 04, 6:55 AM
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Empirical testing of the upper end of street DCR's:

8.6 is barely doable with great quench, old porous GM alm heads, and a motor that makes best power at 200 degrees. To wit, even with all the 'stars lined up' 8.5 is too high, not giving you any margin for the occasional bad batch of fuel.

Greg's numbers are good point estimates.

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post #5 of 89 (permalink) Old Nov 23rd, 04, 2:23 PM
 
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mine came out at 8.19 I have .043 quench and Canfield out of the box heads. I run 91 octane at 35 deg.

BTW, as for the pump gas drags, it says right in the article " SPECIALLY BLENDED 93", For the 20 guys that pulled it off, 1,000 more will explode their stuff trying.


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post #6 of 89 (permalink) Old Nov 23rd, 04, 2:31 PM
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how about for Iron-headed on 87 octane?

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post #7 of 89 (permalink) Old Nov 23rd, 04, 7:36 PM
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This gets into the 11+ to 1 CR. motors that can run 87 octane if you want to read up on it by searching under my name.

A Tech. School Class even did a build-up of a 502 with Mr. Jim Fuelings BB truck, Torque-Heads on it that was above 12 to 1!!!

BUT these build-ups are by no stretch race motors, but rather work horses!!

Mr. Ray Bohaz(Sp?) wrote two articles on the 502 build-up and I have talked to him personally several years ago on the subject.. Great Tech Writer imho..

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post #8 of 89 (permalink) Old Nov 23rd, 04, 7:43 PM
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Back again, his name is Ray T. Bohacz!!

I forget but figure he wrote the articles for PHR if they have an article archive at their web site??

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post #9 of 89 (permalink) Old Nov 23rd, 04, 8:07 PM
 
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post #10 of 89 (permalink) Old Nov 23rd, 04, 8:19 PM
 
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Quote:
Originally posted by Motor Martyr:
Old Gold's burnout
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post #11 of 89 (permalink) Old Nov 23rd, 04, 8:51 PM
 
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click the picture jason i was trying out the link.
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post #12 of 89 (permalink) Old Nov 23rd, 04, 10:16 PM
 
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Cool pic
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post #13 of 89 (permalink) Old Nov 23rd, 04, 10:35 PM
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deleted. They are right: ignorance = bliss.
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post #14 of 89 (permalink) Old Nov 23rd, 04, 11:51 PM
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Quote:
Originally posted by 66 283:
And my rant about DCR That's like trying to measure rpm by counting tire revolutions at the finish line. If it could be calculated it wouldn't be dynamic now, would it? LOL There are more important things that will determine the octane requirement - LOAD for instance. Don't be governed by these artificial feel-good laws of the universe!
Crap man you are starting to sound like me

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post #15 of 89 (permalink) Old Nov 24th, 04, 1:31 AM
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Uh, 66 283, I dont know if that was supposed to be a bash or what, but if you read my whole post, I used quite a few different numbers for some common DCR levels........ Kinda hinting at the fact that the numbers DONT say it all(actually I said it, not hinted at it), otherwise I woulda said "this is the only way to go". It kinda sounded like you were tying to say Im wrong by using an arguement that agrees with mine? As far as me saying supposedly 93 octane, I say that cause I wasnt there, nor did I send out an order for a gallon of that fuel so I can test it in hopes to bring up a case on the pump gas drags. So Im in no place to say, yes it was definitely 93 octane. My point was that many things, including DCR dictate how a motor responds to a given octane. If it was a totally useless number, I doubt any of us would concern ourselves with static compression numbers either, cause they are both forms of compression. One is theoretical, and the other is real world and does vary, but it works off of the theoretical number along with everything else. As far as your motor, if you give me your rod length, stroke lenght, and intake valve closing angle, I could figure out what the calculators say your DCR is with 11:1 compression, and Im guessing itll be a lower number. That number might not matter to you cause it sounds your plenty happy with the way things worked out reguardless, but I bet whatever number it is, it partially contributes to the fact that you can run so much nitrous through your motor on 93 fuel. If it dont matter, run a much smaller cam with no other changes, or just boost the static compression a couple points and see if it still works out as well as it does now. If I misinterpreted your post, than I apologize, but if not, than I am a little confused cause I though I made it pretty obvious that the numbers arent an exact science and can vary in every situation(like you said) depending on many things of which I listed just a few.
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