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Old 12-11-2011, 08:57 AM
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compressio ratio .040 w/ vortec heads and flat top pistons

A couple of threads that I read recently have me second guessing myself a little. I've done a search and couldn't find exactly the answer I was looking for to confirm I have a pump gas friendly combo . I put together a 350 .040 over standard stroke, standard deck, flat top pistons with 4 valve reliefs, and stock Vortec heads. I went pretty mild with the cam because this is supposed to be a cruiser. I went with a Lunati cam with 204/214 duration at .050 and low .420/.442 lift because of the limitations with stock Vortec heads. I think I used Mr. Gasket head gaskets that came in the cheapo engine builders gasket set; don't know thickness. I thought I was in the 9.5-9.75 compression ratio range determind by what some similar crate engines I was looking at advertised as their ratios. Now I'm wondering if my ratio isn't much higher than that and I should have added a bit more cam? I so far still have the tall stock 2.73 gears. After the break in and tuning I only drove it a few short times before getting serious with the body work. It ran really good and sounded awesome with open headers; didn't detect any detonation at all on 87 octane. I used a similar combo with only slightly bigger cam and better gears in a project I sold back a couple years ago, and it was good to go as well. Any good estimations of what my compression ratio actually might be? Am I pushing it running on 87 octane? Would I be alright if I just switched to 93 octane? What could make this combo better for cruising and occassionally tearing up the streets? I'm thinking I like this car and it might be a keeper . Thanks, Chuck
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Old 12-11-2011, 09:04 AM
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LSA is 112 degrees
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Old 12-11-2011, 09:06 AM
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the only true way of knowing your actual C/R is filling the combustion chamber with a burette to find out the voulume and then going trough the math, since you engine is already installed it would be very hard now, but if you like it why worrying about it?

measure the performance with low octane gas then switch to higher octane and measure again, if you find an improvement then evaulate if it's worth using it, most of the times the increase in performance is not worth the extra fuel cost.

now if you increase the timing you may gain some extra horses but now you will have to use higher octane always, if you car is a daily driver I wouldn't touch it as is now.

somewhere I read about happyness being a mild cam.
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Old 12-11-2011, 09:17 AM
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Thanks that's usually my attitude as well. I guess I just am seeking confirmation that I don't have a mismatched bunch of iron, or worse that
I built something that isn't going to survive very long. It's defnitely not going to be a daily driver in this climate
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Old 12-11-2011, 12:18 PM
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Depending on which Vortec's you have, assuming 64cc version, .040 over, 3.48 stroke, .041 gasket, .025 in the hole and 4 valve reliefs, I come up with 9.72, so you're close.

You plan on "tearing up the streets"?
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Old 12-11-2011, 01:36 PM
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Great thanks. Yes they are the 64cc chamber heads. I love blowing by overgeared, overcammed street rods that are rapped out at 75 mph. Better yet I like to be able to pull out in front of my brother's expensive new Camaro that is limited to 130mph
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Old 12-11-2011, 08:43 PM
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My post was meant to say we don't support street racing or other dangerous antics.
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Old 12-11-2011, 09:23 PM
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oh, I meant I like to know that I could pull out in front of my brother's new camaro.
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Old 12-11-2011, 09:51 PM
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Might be a good time to familiarize yourself w/"dynamic compression ratio". You might find something as simple as retarding the cam a few degrees will give you enough leeway.
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Old 12-12-2011, 12:15 PM
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Quote:
Originally Posted by 68NovaSS
My post was meant to say we don't support street racing or other dangerous antics.
you reminded me of this time, long ago in Tampa Fl, I was at a stop light in my '70-1/2 camaro, when this Turbo thunderbird pulled besides me I looked at him and he made some hand sign so I dropped the pedal to the metal an took off as hard as my 327 vette engine could, I lifted off as soon as the car reached 45 which was the speed limit, the Thunderbird took some time to reach me and when he did I saw in the rearviw mirror this blue light over his top, dam' he was an undercover cop, he approached me and said why the hell you did that? I was warning you not to, I gave you a hand sign, well I said I thought you were challenging me to a stop light grand prix, 'cmon you were drivin a hot rod car, he laughed and just gave me a warning, for "unnecesary demonstration of horsepower", he was a cool guy, and I learned the lesson.
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Old 12-16-2011, 08:39 PM
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Quote:
Originally Posted by cobalt327
Might be a good time to familiarize yourself w/"dynamic compression ratio". You might find something as simple as retarding the cam a few degrees will give you enough leeway.
I know retarding the timing can reduce detonation but it doesn't do anything to change compression right? Can anyone help calculate dynamic compression ratio with the information I have provided? I think I'm going to run a compression test too. I read in a magazine that 170-185 is a good amount of compression for running 93 octane pump gas. Can anyone either confirm or debunk that claim. I'd appreciate it. I also wanted to add that I didn't exactly pick this cam out of "thin air" I did a lot of research and reading and evaluating contradictory information. I didn't do a lot of math because quiet honestly it's a little over my head. I did however rely on manufacturers recommendations quite a bit, because I figure they already did the math for me and are not likely to recommend a cam that is not going to work well for me right?

Further I picked the cam for the 454 in my elcamino (on my avatar photo) using comp cams software. I picked the cam that had the highest torque at the lowest rpm and it was called a "good fit". The intake duration at .050 is 212; which I see is outside of what is recommended on techinspectors infamous chart for my stock compression 454. Is it possible this chart is a little on the conservative side? It seems like it is as compared to other information out there. It seems like there is a little bit of a "group think" thing going on here. There is more than 1 one way to do things and there isn't always 1 best answer I have learned. I watched this forum ream a guy for doubting the "all knowing", albeit it was partially deserved, I didn't think it was entirely fair. Thanks I didn't originally plan on taking a stand but it just felt right. Chuck
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Old 12-16-2011, 10:08 PM
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Changing the cam timing will not change the static (mechanical) CR. But changing the intake closing point- which happens when the cam timing is changed- WILL effect the dynamic CR.

To get you started, you need to know the exact (not estimated) SCR, the intake closing point, and the rod length to find the DRC. A couple pages for computing DCR:

http://www.empirenet.com/pkelley2/DynamicCR.html

http://www.kb-silvolite.com/calc.php?action=comp2

I don't know about 'group think' or how you perceive it being used here, or what the actual example of what you saw was. There are well documented and well understood procedures for many things. This is not to say these will be the only things that will work. But there are hundreds of combined years of experience here, and I can assure you no one will simply "follow the leader" here. If anyone puts forth patently incorrect or dangerous info, they will likely be called on it- regardless of who they are.

Last edited by cobalt327; 12-16-2011 at 10:15 PM.
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Old 12-16-2011, 10:55 PM
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streetbruiser, I couldn't find any specs on Mr. Gasket head gaskets so I used the specs for a couple popular Fel Pro gaskets for 350 based engines. You didn't say whether you used shim or composition gaskets so I figured with the #1003 composition gasket (.041" thick 4.16" bore) and #1094 steel shim gasket (.015" thick 4.1" bore). You didn't say what the dish cc's are in your pistons so I used 6.5cc (from the KB website for their flat-top 4 valve relief pistons, they show cc's of 6.5cc and 7cc for different flat top 4vr pistons). I get an intake closing point for your Lunati cam of 63* ABDC. With the composition head gasket I get a static CR of 9.61 with a dynamic CR of 7.79. With the thinner shim head gasket I get a static CR of 10.25 with a dynamic CR of 8.3.

Hope this helps,
Ed
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Old 12-17-2011, 07:48 AM
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Quote:
Originally Posted by cobalt327
Changing the cam timing will not change the static (mechanical) CR. But changing the intake closing point- which happens when the cam timing is changed- WILL effect the dynamic CR.

To get you started, you need to know the exact (not estimated) SCR, the intake closing point, and the rod length to find the DRC. A couple pages for computing DCR:

http://www.empirenet.com/pkelley2/DynamicCR.html

http://www.kb-silvolite.com/calc.php?action=comp2

I don't know about 'group think' or how you perceive it being used here, or what the actual example of what you saw was. There are well documented and well understood procedures for many things. This is not to say these will be the only things that will work. But there are hundreds of combined years of experience here, and I can assure you no one will simply "follow the leader" here. If anyone puts forth patently incorrect or dangerous info, they will likely be called on it- regardless of who they are.
Thanks these pages are helpful.
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Old 12-17-2011, 07:55 AM
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Quote:
Originally Posted by BigEd36
streetbruiser, I couldn't find any specs on Mr. Gasket head gaskets so I used the specs for a couple popular Fel Pro gaskets for 350 based engines. You didn't say whether you used shim or composition gaskets so I figured with the #1003 composition gasket (.041" thick 4.16" bore) and #1094 steel shim gasket (.015" thick 4.1" bore). You didn't say what the dish cc's are in your pistons so I used 6.5cc (from the KB website for their flat-top 4 valve relief pistons, they show cc's of 6.5cc and 7cc for different flat top 4vr pistons). I get an intake closing point for your Lunati cam of 63* ABDC. With the composition head gasket I get a static CR of 9.61 with a dynamic CR of 7.79. With the thinner shim head gasket I get a static CR of 10.25 with a dynamic CR of 8.3.

Hope this helps,
Ed
Wow it looks like you went to a lot of effort. Thank You. but my cam card says ABDC is 30 ?

I'm still wondering if there is any value in the information from what a compression test will tell me. If it is over 185 I think I'll be looking for a cam with slightly more duration. Is this reliable?
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