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Another compression question thread...

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Old 09-13-2017, 06:29 PM   #1
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Default Another compression question thread...

I am in the process of rebuilding an engine right now, and am starting to panic about running pump gas with the DCR I'm getting from running the numbers I've got.

This is gonna be a 383, with AI heads and a 239/240 cam on a 108 LSA. I requested my builder build it to 12.25:1, based on what I was recommended from AI. My builder is an old school guy, and was trying to talk me into 10.5:1, so there's no help from him in that department.

When I run the numbers, I get a 9.7 DCR. I have no idea if this is any good, and I really don't wanna be stuck with an engine that I won't be able to run without knock on hot days.

Anyone with any experience with this?
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Old 09-13-2017, 07:00 PM   #2
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What does AI say about the cam and the compression in regards to pump gas?

FWIW, I run 9.35-1 dynamic with a 12.1-1 compression and I won't run pump gas.
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Old 09-13-2017, 07:19 PM   #3
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They told me to use between 12 and 12.25:1, which is what I passed on to my builder.
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Old 09-13-2017, 07:22 PM   #4
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I also have access to 93 where I live, and rarely travel in this car to anywhere with 91 or lower.
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Old 09-13-2017, 08:35 PM   #5
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Well, since you have a hydraulic roller you can run knock sensors and see if it is knocking on pump gas. I would make sure your numbers are correct on the DCR. Usually, the intake valve closes at ATDC around 70 degrees or so on a cam like you have. The cam card does not always show the number from base circle and usually gives you it at .050 so you have to guestimate and add about 15 degrees to your number.

I would give AI another call and see what they have to say to confirm but I would think they specified a cam the correctly meets what you wanted to do with the car.
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Old 09-13-2017, 08:39 PM   #6
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What's the intended setup you plan to run with this motor? Seems like your builder is trying to make the engine more versatile but AI is telling you how to set the motor up to max it out. 10 to 1 you can drive it around town 12 to 1 you just made it a track star. what I don't get is why your builder didn't compromise and suggest 11 to 1. You'll still get the higher CR benefits but you can still run 93 out the pump and Race fuel and benefit from it. BTW what are the specs on the heads you plan on running with the 239/240 cam? That's a 4th quarter cam isn't it (top end). I get why AI wants you to run higher compression you're going to need to breathe the "bleep" out of that motor. I haven't really seen anyone run a cam with those specs though other than LSX guys (and it works for them since their engines breathe differently). What exactly are your goals for that setup?

If it were me I would aim for 11 to 1 and pick a cam that was more comfortable to make power at that level. You're basically going to use a race cam and try to sneak in pump gas on it... I just don't see the engine liking that.

hehe btw this is how your valve springs will look when you peak in the cover

Last edited by Heatmaker; 09-13-2017 at 08:48 PM.
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Old 09-13-2017, 08:48 PM   #7
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What calculator are you using and are you putting the right data in? Sometimes they are unclear

Try this one: https://www.jeepstrokers.com/forum/calculator/
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Old 09-13-2017, 08:50 PM   #8
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Nostang: The cam card gives 44* @ .050". Also, 63* ABDC.

Heatmaker: They are the 200cc heads. Honestly, the cam isn't that huge, and it's a street car for weekends and the occasional work day. My builder has done LT's before, but honestly I don't think he's done any high end builds, if anything he's done stock rebuilds for guys that don't know what they have. He's stuck on old sbc tech, I figured that one out when he told me 9.5:1, or maybe even 10. However, he does have quite a reputation for his work on classic stuff and on LS stuff. The way I see it, as long as I supply the parts, clearance is clearance, and assembly is the same regardless of engines.

Last edited by 94TAisfast; 09-13-2017 at 09:01 PM.
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Old 09-14-2017, 05:35 AM   #9
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10.5:1 is ridiculous.
You'd need to drop down to about a 224/230 cam to make that work right.

To leave a nice tuning window to accommodate down to 91 octane fuel, I'd shoot for the 11.5 to 12.0:1 range. 12.25:1 is doable, but only if you have a COMPLETE tune, not just a WOT tune on a dyno, run knock sensors, 160 tstat,....etc.
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Old 09-14-2017, 08:54 AM   #10
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My plan was going to be to go with Solomon, and use his tool loan method. This seems like the best option to tune, since out where I am nobody tunes anything other than mustangs, LS, or Mopar. I have a 160* stat, and was gonna refresh all sensors. At this point I'm thinking about just sacrificing a half second in the quarter and tossing these on the 350 in my car now and putting in a mildish cam that will peak around 6300 or so.
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Old 09-14-2017, 09:28 AM   #11
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Quote:
Originally Posted by 94TAisfast View Post
My builder is an old school guy, and was trying to talk me into 10.5:1, so there's no help from him in that department.
You can do what AI tells you, but as suggested you need to find someone who is able to do a part throttle/WOT tune. None of the old schoolers can comprehend reverse cooling and aluminum heads.
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Old 09-14-2017, 09:33 AM   #12
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Quote:
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You can do what AI tells you, but as suggested you need to find someone who is able to do a part throttle/WOT tune. None of the old schoolers can comprehend reverse cooling and aluminum heads.
That's the plan. I figure Solomon is my best option, as it seems nobody else is still tuning remotely anymore. As long as I get all the right logs shouldn't it be just as good as a dyno tune?
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Old 09-14-2017, 09:39 AM   #13
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I don't know what mail order tuners offer. Personally I'd see what Ed Wright has to say before anyone else.

www.fastchip.com
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Old 09-15-2017, 12:05 AM   #14
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Quote:
Originally Posted by 94TAisfast View Post
.........This is gonna be a 383, with AI heads and a 239/240 cam on a 108 LSA. I requested my builder build it to 12.25:1, based on what I was recommended from AI. My builder is an old school guy, and was trying to talk me into 10.5:1, so there's no help from him in that department........
If a builder ain't gonna give me the product that I'm paying for.....I take my business elsewhere. Jus' sayin'.....

I think your Dynamic CR calculation is wrong. No way would AI spec a cam that would leave you with a 9.7:1 DCR....which is too high, BTW.

With a 12.25:1 SDR, my SWAG would put your combination about a half point lower.

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