I have a 383 that is currently running 10-1 compression with a set of 75cc 993 heads. I have received a set of 64cc 492 Fuelies and need to know how much this is going to up the compression? The builders said 10.25 or 10.5-1 A local engine shop have been mulling it over and have thrown numbers between 11 or 12-1. There is no way I can run that much compression on the street. This is a weekend driver, show car, and occasional strip car. I am not sure which pistons it has but I am going to have to find out.

Thanks

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69 RS Camaro (383,350t,12-bolt w/3.73s)
79 WS6 Trans Am (406SBC,factory 4-spd,3.23 posi)
87 GTA Pace Car Clone (350TPI,700r4,140k miles)
97 Bonneville SSE 40th Anniv. (3800 V-6, Power Everything, The little woman's car)

wildkatz,
without doing the math, just of the top of my head here is what I'd say. Flat top pistons with 76cc heads yeilded 9:1 on L82 small blocks. Early 70 flat tops with 64cc heads were about 10.25:1. So you're running 10:1 with 75cc on your 383. When you bolt on the 492s with 64 chambers figure to raise a little over 1 point, maybe to 11.1:1. Now if your 492s have 2.02 intake valves in them from the factory they're actually probably 66 cc chambers because GM unshrouded the chamber to allow the valve to breath near the chamber wall. This would knock .2 off so you're at 10.9:1. The figures were using from the factory were with a steel head gasket so factor out another .2 probably for a thick composite gasket and you're at 10.7:1 and if you mill the heads to clean them up you may gain a little, so my guess is you'd end up around 10.75:1. If you choose your cam profile carefully and set up your distributor curve you can run this set up and enjoy the best of both worlds. But it's tricky.

Good Luck

[This message has been edited by SY1 (edited 12-06-2001).]

My 383 with 64 cc heads and a 12cc DISH piston runs 10.4:1. If you already are at 10:1 I would suspect that you have a flat top piston. Switching to a 64-66 cc head will put you at or just over 11:1. This is a very difficult compression ratio to make run on pump gas with iron heads.

To make it run on pump gas I would suggest the following:

Have the block decked so the top of the piston is exactly even with the top of the block. Use a .039" compressed thickness head gasket. This will give a tight quech area so the engine can tolerate a little more compression. If you're going to deck you block though you might as well switch to a dished piston and not have to worry about the rest of the stuff I mention below.

Be conservative with timing. Keep total timing in the 32 - 34* range. Keep the mechanical advance curve pretty slow - heavy springs, all the mechanical advance in at 3500 RPM or so.

Use a cam with lots of overlap to bleed off cylinder pressure. Something in the 290* advertised duration range is about the max you can do in a street motor IMO.

I hope that you can see now that you will have to make a lot of compromises to run that much compression on an iron head engine; and even with the compromises it's still a crap-shoot - you could still have problems with detonation. Less timing usually means a little less power and a big cam on the street will kill your bottom end torque.

Hope this helps,

Eric

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68 Camaro, 383 small block with TH350 trans. 11.98's at 111mph and never trailered.

Here's a quick and dirty way to guess your new CR

CR=(swept volume + head volume)/(head volume)

If you are running a 383 with 10:1 compression then:

10=(SV+75cc)/75cc) so your swept volume is around 675cc. This is about 110cc off of what the sewpt volume should be (784cc) for a 383 cylinder?? So, I'm gonna add a 12cc "fudge factor" for your deck height, valve reliefs, squish area, gasket volume etc.... Now the equation is:

CR=(swept volume + head volume + fudge factor)/(head volume + fudge factor) This assumes flat or dished pistons!!!

or:

10=(swept volume + 75cc + 12cc)/(75cc + 12cc)

Now the swept volume comes out to 783cc! That's close enough! So, I would guess your new CR to be:

CR=(784cc + 12cc + 64cc)/(64cc + 12cc)

This gives me a guess of 11.3:1

Assuming that somebody actually measured and calculated your true CR at 10:1, and you dont change anything but the heads, this should be a pretty damned close guess!!

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Paul D.

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68RS, 331, 10.5:1, 4spd, 3.90 9", lowered, 245-45-16's all 'round

I am sorry for my arogant attitude here..but I think you need to totally get down to business...
You need to step up and do this RIGHT!! Thats if you want to know what the hell is really going on with your motor..
First thing..the only way to get 10:1 with the 383 combination with flat-tops is if the block was decked so that the pistons at TDC pop up out of the block by .005"...than when you put on a .039" gasket you wind up with a .034" quench clearance....which is a very good quench to have !!!
Here are some things to consider when calculating the compression..
You need to know the piston to deck clearance...WITHOUT THE HEAD GASKET.and figure that volume with respect to the bore of 4.03"....than you need to add to this the head gasket volume..Remember that the head gasket is 4.1"....
Also with flat tops you need to add to the UNSWEPT VOLUME the cc's of the valve reliefs...this can vary greatly and is not something to FUDGE...
So here is a realistic "assumed" example:
Piston to deck clearance: +0.005"
Gasket thickness: .039"
Effective Quench clearance: .034"
valve reliefs: -5cc
Chamber: 75cc
Bore: 4.03"
Stroke: 3.76"

This works out exactly to 9.99 or 10:1 compression....
Now if you drop the chamber cc's to 64 you now will have: 11.28 compression..
Now the whole thing is ridding on what the real numbers are:
You need to measure yourself..the valve relief cc's..also you need to know what the piston to deck clearance is..use a dial indicator...
Running iron heads in the 11:1 range is very tricky and borderline impossible with pump-gas... You need to run a colder plug by atleast 2 numbers down...you need to richen the jetting a pinch..a double pumper really helps in the hole for killing some detonation..also use a low # power valve to avoid mid-transition ping like a 4.5/5 ...cam with a 110 overlap with 230 dur or better @ .05" ....
Use a dual-pattern advance curve..watch your total advance...probably 32 to 34 if your lucky..Also..make VERY sure to run vacuum advance..and limit it to about 6 to 8 degress..since this will retard the timing durring transient loading.... make sure the kick-down cable is set right to avoid improper engine loading ( you want the lower gear to kick in sooner) this way increasing RPM and decreasing loading.. There are many more tricks to get around detonation..but the biggest thing that will help is running a tight quench around .033" is about the tightest..remove any tight edges and polish the chamber to help remove heat faster..
There are many more tricks that others here can elaborate on..
Regards
Chris



[This message has been edited by cerrem (edited 12-11-2001).]

cerrem! Yeah, you are arrogant! No need for that trash on this board IMHO! If you have a different opinion, tell us but, don't be a knob about it!

I calculated the unknown volume of the valve reliefs, gasket etc... and called it a "fudge factor". There's a lot of unknown infomation about this engine including the true CR. There are countless piston, deck height and gasket combinations that could give you 10:1 with a 383, not just one! You made up some numbers that worked out nicely!! I also see that both our methods worked the new compression ratio to within .1 difference in ratio!! http://www.camaros.net/forum/smile.gif

All he asked for is a "approximately" what his new CR would be. I think either method works!


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Paul D.

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68RS, 331, 10.5:1, 4spd, 3.90 9", lowered, 245-45-16's all 'round

cerrem - PDQUICK's and my "fudge factor guesses" of "just over 11:1" and "11.3:1" matches your 11.28:1 "down to business" calculation. Lighten the heck up, we all agree here - there is no need to word your post in such an inflamatory way.

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68 Camaro, 383 small block with TH350 trans. 11.98's at 111mph and never trailered.

Yea..Sorry..
Now that I look back at my post..
I guess I was being a dick...

Regards
Chris

Thank you Chris. pdq67

I have actually been doing a little investigating in Jegs and think that I have come up with the most accurate #'s. First, I found the KB pistons and found the 383piston with the advertised compression nearest to what I was told. I then used desktop dyno and used the compression calculator until I got the numbers that were advertised in Jegs and then the numbers I was told that I had. I am pretty sure, if calculations are right, that my compression to start with was about 9.94-1. To get the compression down a little I have bought a set of Felpro 1044 headgaskets/4.200 bore and .051 thickness. With these gaskets and the 66cc heads, I am coming up with a 10.66-1 rate. I think that I will be ok with that. I will probably have to run a can of octane booster with each tank of gas to be on the safe side, but that won't be to bad.

What do you guys think?



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69 RS Camaro (383,350t,12-bolt w/3.73s)
79 WS6 Trans Am (406SBC,factory 4-spd,3.23 posi)
87 GTA Pace Car Clone (350TPI,700r4,140k miles)
97 Bonneville SSE 40th Anniv. (3800 V-6, Power Everything, The little woman's car)

Chris,
Sometimes we don't realize when typing a post how it will look to others. You come back and read it later and it can look nastier than intended...

Personally, I think a lot of someone who comes back later, re reads his post and explains he didn't mean it that way.
Posts can be edited by clicking on the icon above with the pencil.
Thanks, David

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Check my web page for First Gen Camaro suspension info:
David's Motorsports page (http://ourworld.compuserve.com/homepages/David_Pozzi/)
First Gen Suspension Page (http://ourworld.compuserve.com/homepages/David_Pozzi/first_gen_suspension.htm)
67 RS 327
69 Camaro Vintage Racer
65 Lola T-70 Chev 350 Can-Am Vintage Racer

Here are some things to consider...
I am a firm believer in running a tight quench ( .030" to .035")....total clearance, including gasket thickness..
It's been my experience that this not only has HP benefits but it also reduces the tendency to ping and detonate...better flame travel propagation and increased swirl velocity leads to a mucho better burn rate..
What I am trying to say is that I have had 9:1 motors with large quenches rattle and ping while I have built motors with 10:1 with .030" quench and not heard a peep out of that motor...
If your going to pull the heads anyway..why not measure the piston to deck clearance..than put some clay in the valve reliefs to figure those cc's....this way getting a closer figure..
Another thing to consider is that these calculations are STATIC compression ratios...and typically the cam design and heads will dictate if and when the DYNAMIC compression ratio will approach this "IDEAL" number...since the compression ratio formula does not acount for over-lap, intake charge-back fuel dilution, effeciency..ect..
Since efficiency varies dynamically as well ..compression is not even close to being a constant...
You may be able to tolerate the higher compression with a proper cam choice..actually a cam that is IN-EFFICIENT in the RPM band that you would use in he street with pump gas..this would be "effectively" lowering the dynamic compresion ratio at a given lower band.. and if you decide to go to the track with this motor..you can simply rev it higher in the band it was ment for and produce some better power with gas sold at the track..
Regards
Chris

Thanks for the reply Chris!

I agree with your thoughts on quench and larger cams. I've run quite a few "street" engines with very tight quench area and aggressive cam profiles without any detonation trouble. If you run a cam with long duration and a lot of overlap, it will make the engine very "volumetrically inefficient" at low speeds. The intake and exhaust valves are open together for such a large amount of crank rotation it effectively lowers the compression by pumping some of the incoming charge out the exhaust.

The only problem I see with this approach on an SBC as large as 383CID is that the true power band might get very limited. Most 383's make their best power under 6000 RPM,
If the engine doesn't wake up until 3-4K with a large cam then it might make for a hard to drive and "peaky" combination???

I would think about it long and hard before running that much CR on a street 383.

I agree with Chris - a quench in the .030-.040" range would help some as would a cam with some overlap.

There are two ways to create more overlap - 1. more duration 2. tighter Lobe Separation Angle. Rather than just running a cam with lots of duration and killing your lower RPM power, I'd recommend a cam with tighter LSA. Comp Cams uses 110* and Isky uses 108*. I'd stay away from grinds with 112* or more. Something in the 230-240* duration at .050" lift with a 108-110* LSA would be where I'd want to be with your compression ratio.

Even with the cam and quench area changes, you will still have trouble making it run on ordinary pump gas with iron heads and a 10.66:1 comp ratio. Keep in mind that you cannot always hear detonation, so you'll want to check your plugs for the tell-tale tiny spots that indicate detonation. I have run 10.0:1 with iron heads before on 93 octane though so you just may be able to do it, it will be a challenge though.

[This message has been edited by Eric68 (edited 12-10-2001).]

Here's a calculator for you.
http://cochise.uia.net/pkelley/crc.htm

I like .040 to .045 quench. Any less, I think you are asking for trouble as your engine wears, but that's just me.


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67 Camaro 410sb 11.63 @117.6
67 Camaro 388 12.10 @110.6, best MPH 112.96
website (http://www.geocities.com/lonnie67_1966)

[This message has been edited by Lonnie67 (edited 12-10-2001).]