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Comp Cam question

hunt2elk

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I am wondering if any of you that are knowledgable with 383 HP motors have ever used a Comp Cam #21-304-4. Gross vavle lift .464/.464, tappet lift 268/268. I had my motor professionaly rebuilt last year and am messing around with tuning. I told them I wanted a slightly lumpy sound(which it does) and a little more aggressive than stock profile(not sure about this). My question is that when I set the timing at 5 degrees initial with advance plugged(like my other Road Runner), it stumbles with no power. I should mention that I converted to the Mopar electronic ignition with orange box. I then set the timing per the electronic kit instructions, which has you advance the timing at higher rpm's. The car now runs excellent- 10X more power-easily smokes the tires in 2 gears with 323's in the rear. No pinging at all and starts good. Now when I check the initial timing it is set at 21 degrees. I have never heard of anybody with this much. Should I be worried it is so high? I am curious if the cam I have is causing this?
 
I just looked up the specs and this cam is nearly identical to the Isky I used to run. What's your compression ratio? Also check your cylinder pressure with a compression tester. You may have too low compression for that cam, but if it smokes the tires in two gears then it can't be all that bad. My guess is the amount of timing you have is making up for the lower compression. If you had an honest 10.5:1 it should require less timing. I used to run my 383 at 15 deg initial and 38 total.
 
I don't know the compression ratio. All I have is a list of parts that went in the motor. The pistons are kb162 030. I'm guessing Keith Black .030 over. The heads are stock - not ported. I do know they had a hard time getting the engine to run after assembly and blamed it on the carb. When I put it in my car they had the initial timing set at 5 degrees. It ran but didn't seem to have much power and would stall when I floored it.
 
If there is no pinging and she runs good, let her rip, some times the engines know more about what they want than we do. lol
 
I was hoping somebody would say that. Does anybody know of any books that pertain to final tuning techniques for a engine like mine? I have a couple of Mopar engine books, but they are building racing motors in the 500-800hp range. I would like something that addresses more of a stock build.
 
You need more info before you start tuning. Do a compression test. If you have 130 PSI or something low like that then running 15+ initial will be the only way to make it run. You might have a good candidate for closed chamber heads!
 
I don't own a comp. tester, but maybe I will buy one this weekend at Mopar Nationals. It probably wouldn't hurt for me to have one in my tool chest anyway. What should I get for readings? The motor has about 700 miles on it.
 
Yes, a good tool to have and not terribly expensive. Get the one with the hose that screws in and has a quick release fitting for the gauge.

It would be nice to see at least 150 LBS.

Some race only engines will lock the timing at full advance, minus a couple of degrees. My thought on your engine is the 22 deg initial is a band-aid for low cylinder pressure. But if it likes it what the heck.
 
I have a Comp cam in my 383 280/519 and I run 16 initial and 36 total.But sounds like yors is running pretty good where its at.
 
Got back from Ohio and finally did a compression test. All plugs looked perfect and the same color. The readings were taken after the engine had run for 10 minutes. They are 132,133,135,142,142,142,143,143. I should also mention that I requested that the engine would not have a problem running on pump gas. I use 91 octane because it is all that is available here without ethonal mixed in. Do you think that would have any affect on the timing setting. Thanks fugly for the website info.
 
I have almost the same came in a 383 and I think my compression might be 9.5:1 or 9.2:1 and I run my timing at about 19 degrees BTDC. Mine did the same thing yours did when not advanced enough, the engine would stumble. If plugs don't look bad, it should be good to go.
 
I am not an engine guru, so excuse me if this is a stupid question. How exactly does a person determine their compression ratio. Not that it matters much because the motor seems to be running strong the way I have it set up.
 
If the conversions are right from psi to compression ratio, you have 9:1 for the bottom number and 9.6:1 on the higher number. On the internet it said divide psi by 14.7.
 
So this would be considered a pump gas friendly engine? Would I be better off running 87octane gas? With this low ratio why does the car seem to have such good power?
 
If the conversions are right from psi to compression ratio, you have 9:1 for the bottom number and 9.6:1 on the higher number. On the internet it said divide psi by 14.7.

That may be ok for a rough guess but cam profile, particularly the intake valve closing point is going to have a big effect on cylinder pressure. There's no real way to determine the static compression ratio from a cranking compression test. 383's are notoriously hard to get decent compression ratios for street use using off-the-shelf pistons

So this would be considered a pump gas friendly engine? Would I be better off running 87octane gas? With this low ratio why does the car seem to have such good power?

I'd say it's a very pump gas friendly engine. As far as advance is concerned, the general rule of thumb is to throw as much timing at it as it will take without pinging or detonation. As long as that's not occuring, and the car starts when hot, who cares what the initial is. Just find out what the engine likes and write it down someplace. You can try running 87 but you may have to pull some timing out of it and may lose a little bit of power as a result. It runs good because for whatever reason, you have a good combination of parts.
 
X2 on what Satellite said.

I once owned a '64 383 2bll. topped off the 100,000 + mile engine with a set of 906 heads and a 460 -280 duration crowler cam, a set of headers and had that tank running high 13's... Don't underestimate a 383 !!!
 
I'll tell you why it runs good because its a 383 big block not small block ha-ha. I have about the same cam as Khryslerkid, and it will sh$@ and get even with I think 2.94 gears.
 
Thanks guys- I will leave well enough alone and have fun with it.
 
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