mitigating too high compressoin - Team Camaro Tech
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post #1 of 13 (permalink) Old Jan 14th, 19, 01:34 PM Thread Starter
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mitigating too high compressoin

Gentlemen - long time lurker, first time poster. I find my search for information often brings me to this forum... I'm dealing with an issue I need advice on. I'm running right around10.5:1 static compression on a 383 stroker, maybe slightly less. Total street motor... 64 CC aluminum Promaxx heads, block not decked, 4v relief flat tops. (Heads off, open to gasket suggestions). Switching to the GM HT383 cam - adv duration is 288/308 with LSA of 109... By my calculation I'm looking at some high overlap with this cam, but I'm out of my depth at this level of analysis. Objective: Looking to safely run on premium pump gas. Question: 1. Can I reduce timing a bit and make this happen. 2. Is this cam helping or hurting my situation? I'm calculating 80 degress of overlap, is this correct? 3. To help with torque and off the line power - increase crank advance, limit mechanical to 30 or 32 degress? Don't care to run this motor at higher RPMs 4. Anything else I'm missing? Thanks in 'advance.'
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post #2 of 13 (permalink) Old Jan 14th, 19, 04:17 PM
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Re: mitigating too high compressoin

What cam are you running? No way to know if you are helping the problem or hurting the problem wihtout knowing what you HAVE been running.

According to GM the cam you are considering is "great for additional bottom end torque" which usually means LESS overlap than the standard camshafts.

Also, according to GM the duration at .050 lift is only 196/206 (intake/exhaust).

Specs are here; https://paceperformance.com/i-625541...amjet-350.html

If you are wanting to bleed off some static compression, then you may be going the wrong direction.

At 10.5:1, you shouldn't have trouble running pump gas (assuming you can get 92) if the engine is set up correctly. Some locations only offer 91, which would be on the edge. IMHO you are fighting a losing battle backing off the timing to avoid detonation. You are leaving power and economy on the table. Get the engine right so you can run all the timing it really needs. Most SBC engines run pretty darn good with 36 degrees of initial and mechanical total. You can usually run another 10, sometimes even 15 degrees of vac advance if you are hooked directly to manifold vac.

As for head gaskets, first you need to carefully measure how far EACH piston is down in the hole. What you are looking for is the actual quench distance (i.e. the distance from the top of the piston to the head surface (the flat part of the head). You will get different opinions, but I like to see it really tight, like around .036. So, if you are .015 down in the hole, you want a head gasket with a compressed thickness of .021. If you are running forged pistons (hardly anyone is on a street motor) you may not be able to go that tight. Also, if your pistons are a little "loose" in the bores, you cannot run that tight of a quench. I built one very tight engine at .022 on the quench and it still runs great. However, much smaller bore on that one than your 4.030" bore.

The tight quench will help ward off detonation.

Lynn
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post #3 of 13 (permalink) Old Jan 14th, 19, 04:28 PM
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Re: mitigating too high compressoin

One temporary fix to any preceived detonation would be to stick in anti fouling spark plug adapters. The added volume to the combustion chamber will drop your static compression.

https://shop.advanceautoparts.com/p/...B&gclsrc=aw.ds

Can't remember volume inside (you could cc with an eye dropper) but last time I saw a set I wondered if it would allow that clapped out SBC to run on kerosene.

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post #4 of 13 (permalink) Old Jan 14th, 19, 05:01 PM
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mitigating too high compressoin

That seems like a really small cam for a 383. Below are the specs I got off of inter webs.

duration at .050" tappet lift (intake/exhaust) is 196/206 and maximum lift with 1.5:1 rocker ratio (intake/exhaust) is 431/451. Valve lash is zero/zero and lobe centerline is 109 degrees.

Would think you can do a 22x duration cam with 52x lift on a 112 or 114 LSA and you will still have very good low end and mid range for usable power under the curve.

I realize I didnít actually answer your question, but the guys above did. I was just thinking out loud. Good luck and hope you get it figured out. The guys on here wonít steer you wrong.

Derek

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Last edited by 97 CAIVIAROSS; Jan 14th, 19 at 05:16 PM.
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post #5 of 13 (permalink) Old Jan 14th, 19, 05:44 PM
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Re: mitigating too high compressoin

What makes you think you have a problem? Have you been experiencing detonation? Or are you just anticipating an issue?

Gasket thicker will not solve any potential detonation issues and increasing the quench may actually make things worse.

I would 1st determine exactly what the compression ratio is. We have run 10+ compression and bigger cams here in Vegas on 91 with no issues

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post #6 of 13 (permalink) Old Jan 14th, 19, 06:15 PM
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Re: mitigating too high compressoin

It makes me wonder what issues you are having as well, thatís not that much compression.
What is it doing? I would look elsewhere first if you have not already.

Also, I would look for another cam if it was me. Are your heads a vortec replacement? That cam is a vortec spec cam I believe and Iím pretty sure it leaves lots on the table to accommodate lift limits and computer controls.

Sean

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post #7 of 13 (permalink) Old Jan 15th, 19, 12:01 PM Thread Starter
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Re: mitigating too high compressoin

Hey guys - all good questions, and I should have specified a couple things. First, I am switching heads and as asked, I am moving away from vortecs. So really the head swap is the genesis for this whole thing. One head had a crack. I have been running approximately 34 degrees total timing and have not experienced detonation, but we have had a very tough time getting this thing tuned right, and it just seems to never really be spot on. The cam in the motor now is relatively tame. I don't know the specs unfortunately. My gut tells me there is an issue with the cam. It was custom designed to work with fuel injection, and it did actually work well with that.

Also of note is application - This motor is getting moved to either a boat or a truck, and that is why I'm interested more in grunt, driveability, and overall tame manners. Going to run 1.6 roller rockers, and dyno results with the GM 383 cam and those rockers has been very respectful, all with great idle and vacuum. Agreed that there may be some left on the table...

So as questioned above, I have not experience detonation. The obvious next question is 'why would you expect too now'? That is where the mild cam comes in. I'm wondering if that will be a problem. I had an engine builder locally who turned out to be somewhat questionable set this combo up. He was unable to get it perfectly tuned. A buddy and I got it much closer, but still not perfect. Absolutely driveable, and actually ran sweet, but not what you would call 'perfect'. The unknown to me is the current cam, and I'm thinking for the small cost of a new one, it is worth getting something that I know what it is.

97 Caiv - I appreciate the cam suggestion. That actually takes me to my next question. What would you guys suggest for a cam if driveability, grunt, liveability was the main goal... if I don't go with the GM cam?

I also agree with the head gasket suggestion, that tight quench assists with detonation, and will move in that direction. Need to figure out how far in the hole I am currently.

Again, thanks for the time put into this.
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post #8 of 13 (permalink) Old Jan 15th, 19, 12:32 PM
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Re: mitigating too high compressoin

I suggest you decide on what youíre going to put the engine in before you decide on a cam

Big difference how to set up a motor for a boat v truck

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post #9 of 13 (permalink) Old Jan 15th, 19, 01:41 PM Thread Starter
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Re: mitigating too high compressoin

I would say truck. Family truck, used to tow, long trips, etc. Hence the desire for problem free driving.
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post #10 of 13 (permalink) Old Jan 15th, 19, 02:56 PM
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Re: mitigating too high compressoin

Quote:
Originally Posted by ssmith396 View Post
I would say truck. Family truck, used to tow, long trips, etc. Hence the desire for problem free driving.

Why not just get a re-man or crate motor instead of messing with this 383? Not sure what you are putting it in, but these new crate motors put out serious power with very good reliability, run on low octane pump gas and will give you the torque you want at the rpm you are looking for.


Not to sound disrespectful, but you sound like you are opening up a can of worms your not sure or confident about, with regards to this 383 of yours. I think a crate motor would be your best choice.
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post #11 of 13 (permalink) Old Jan 15th, 19, 03:05 PM Thread Starter
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Re: mitigating too high compressoin

Bob - Maybe once this one blows up or dies, has about 7K on it now. I'm struggling a bit to make sense of the suggestion to scrap a healthy motor because I'm searching for the right cam/timing/tune setup. This is a solvable problem, of that I'm confident.
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post #12 of 13 (permalink) Old Jan 15th, 19, 03:33 PM
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Re: mitigating too high compressoin

Quote:
Originally Posted by ssmith396 View Post
Bob - Maybe once this one blows up or dies, has about 7K on it now. I'm struggling a bit to make sense of the suggestion to scrap a healthy motor because I'm searching for the right cam/timing/tune setup. This is a solvable problem, of that I'm confident.

Not saying to scrap it, but rather take a step back and look at what you have......

What is the history with the engine? Is this the HT383 GM crate motor?

Cracked head at 7k miles? Cracked where? What caused it to crack? Heads shouldn't crack.

What are the cam specs, besides custom, manufacturer? Why change it? Have you pulled it out and inspected it?

Why do you think it wasn't perfect running?


These are the cam numbers you are looking at? https://www.chevrolet.com/performanc...l-block-ht-383
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post #13 of 13 (permalink) Old Jan 15th, 19, 06:11 PM
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Re: mitigating too high compressoin

Have you already purchased the heads?

If not, I would take some very careful measurements and figure out exactly what you are looking at in compression.

Then choose the head gasket thickness, then choose the head.

There are options other than 64 cc chambers.

Like John said, DON'T go thicker head gaskets to bring down compression. Go with bigger chambers if you need to.

Having said all of that, you may not even need to go bigger to get the compression you want.

Lynn
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