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Old 10-11-2006, 07:10 PM
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dynamic compression

i'm about to pull my motor and wack the tops off the pictons and install a bigger cam. i'm at 11.9 to 1 now. i was going to take it down to 11.1 and change the quench to .040. but here's my problem. when i check the dynamic compression with the crane 232@.050 hyd roller i come up with 9.43 dynamic compression (41 ABDC + 15= 56) ( rod 5.858). from what i have read i need to get it down to 8.5 to run pump gas.

to get down too 8.85 i'd have to go down to a cr of 10.4 to 1.


the calculator says i need a 10 to 1 cr to get a dynamic compression of 8.5

something doesn't seem right. 401 with indyhead, crane 232@.050 hyd roller with max 10 to 1 to run pump gas

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Old 10-11-2006, 09:14 PM
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A lot of work to lower this compression.How about getting a carb specially tuned for Ethanol and using the new E 85? There are 2 specialty carb shops in the USA able to do it.
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Old 10-12-2006, 09:14 AM
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Quote:
Originally Posted by 71javelin
i'm about to pull my motor and wack the tops off the pictons and install a bigger cam. i'm at 11.9 to 1 now. i was going to take it down to 11.1 and change the quench to .040. but here's my problem. when i check the dynamic compression with the crane 232@.050 hyd roller i come up with 9.43 dynamic compression (41 ABDC + 15= 56) ( rod 5.858). from what i have read i need to get it down to 8.5 to run pump gas.

to get down too 8.85 i'd have to go down to a cr of 10.4 to 1.


the calculator says i need a 10 to 1 cr to get a dynamic compression of 8.5

something doesn't seem right. 401 with indyhead, crane 232@.050 hyd roller with max 10 to 1 to run pump gas

How does it run now? You've got 11.9:1 compression with a cam less than 232?
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Old 10-12-2006, 11:34 AM
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apparently on race gas. . . .
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Old 10-12-2006, 01:17 PM
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Beware the problem with DCR, is it goes up with rpm untill the induction becomes a restriction.

You need to know what rpm your induction system will run out of power and tune the DCR to match the No. in mind to prevent of compressing and creating detaonation without pulling timing.

The same goes for those who over cam an engine so it'll live on pump gas at low rpm. It'll beat it self to oblivion at race rpm if the induction system can't contol the amount of air reaching the cylinder.
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Old 10-12-2006, 06:23 PM
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it blew the head gaskets on all 8 cylinders cylinder pressure was 240lbs when i did a compression test.

i had someone build this motor for me and was told it would be 10.8 to 1. i had told him i didn't want over 10.5 to 1

turned out the motor was 11.9 to 1 with a little 224@.050 cam
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Old 10-12-2006, 07:13 PM
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If in doubt go with Crane's compression recommendation.

Try the DCR calculator from this site:
http://members.uia.net/pkelley2/DynamicCR.html

You will find it more accurate than adding a generic 15 degrees to the .050 specs... and he has what I think are realistic "real world" recommendations.
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Old 10-12-2006, 08:40 PM
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thanks, i read that page the other day when i did a google search. but i didn't download the program.
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Old 10-13-2006, 09:00 AM
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This is an AMC engine? If so, you might want to go over to www.amcforums.com. There are lots of AMC racers over there who may have some better recommendations. Not slighting anyone here, and I know an auto engine is an engine for the most part and all respond pretty much to the same treatment, but we all know each manufacturer's engines have their own special quirks. Those AMC racers will be able to recommend specific cams and such that will work better in the AMC engines than "Big 3" guys. Same as you wouldn't go to the AMC guys for detailed Chevy info.
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Old 10-13-2006, 09:27 AM
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Quote:
Originally Posted by 71javelin
i had someone build this motor for me and was told it would be 10.8 to 1. i had told him i didn't want over 10.5 to 1...turned out the motor was 11.9 to 1 with a little 224@.050 cam
At least you know now that guy is an idiot (at best). Expensive lesson. Good luck with the engine.
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Old 10-15-2006, 10:35 AM
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Quote:
Originally Posted by 71C10
If in doubt go with Crane's compression recommendation.

Try the DCR calculator from this site:
http://members.uia.net/pkelley2/DynamicCR.html

You will find it more accurate than adding a generic 15 degrees to the .050 specs... and he has what I think are realistic "real world" recommendations.

that calculator gives me a lot lower D CR than the ones that add 15 to the .050 number.
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Old 10-15-2006, 07:54 PM
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Quote:
Originally Posted by 71javelin
that calculator gives me a lot lower D CR than the ones that add 15 to the .050 number.
That is exactly why I think it is more accurate! Compression can't begin (to build pressure) until the intake valve is closed.

FAQ from: http://members.uia.net/pkelley2/DCR_FAQ.html
7) My cam specs say the intake valve closes at 30?. Why does the program say it closes at 66??

The program uses advertised duration for the calculations. Most mfgs don't give advertised duration cam timings anymore and only supply .050" timing. .050" valve timing events are totally useless to both humans and engines. When you enter the advertised duration, LSA, and the ICL in the Valve Timing Calculator, the program will calculate the timing events. These calculations are accurate and are the values needed to accurately calculate the DCR. That is why the Valve Timing Calculator is included. Since overlap is easy to calculate from the results, it is included for your information. It is not used in calculating the DCR.
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Old 10-15-2006, 08:14 PM
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By the way, what is Crane's compression range recommendation for the cam you want to run?
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Old 10-15-2006, 10:23 PM
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i'll have to call them to find out. what they gave me was a grind # and the cam spec's. but there is nothing about a compression range on the spec sheet.

i think i might go a little bigger than the cam i was thinking of running. i need to call them again and talk cams
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