How to determine Dynamic CR and intake BTDC - Chevelle Tech
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post #1 of 12 (permalink) Old Jun 3rd, 20, 5:44 PM Thread Starter
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How to determine Dynamic CR and intake BTDC

Looks like a rebuild is in the planning stage. When determining Dynamic Compression Ratio they talk about ABDC of Intake closing point and then add 15.
How do you determine that from say the information listed for a various camshaft? Thanks


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post #2 of 12 (permalink) Old Jun 3rd, 20, 8:16 PM
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Re: How to determine Dynamic CR and intake BTDC

ABDC closing point for the intake valve is listed on the cam card.

It can also be figured with simple math if you know the intake lobe duration and the intake lobe centerline.

You have to pay attention to whichever calculator you use...some want seat-to-seat duration or closing point, some want you to add 15į to the duration @ .050" lifter rise closing point. put the .050" numbers in a seat-to-seat calculator or vice versa you get wildly inaccurate results.

The ones that ask for seat-to-seat are more accurate.
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post #3 of 12 (permalink) Old Jun 4th, 20, 8:05 AM Thread Starter
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Re: How to determine Dynamic CR and intake BTDC

Thanks, I don't have a card as I am looking at different cams and trying to get the dynamic into the range of 7.8-8.0 with a static in the 10.0 to 10.5 range.

I think they list the intake lobe duration. Not sure of what you mean by intake lobe centerline. But what do you do with these numbers. ADD, Subtract, Divide?

for example: LOB 112; int duration 213 at.05 or 256 total


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Last edited by Jim_66SS; Jun 4th, 20 at 8:24 AM.
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post #4 of 12 (permalink) Old Jun 4th, 20, 8:38 AM
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Re: How to determine Dynamic CR and intake BTDC

I never worried about dynamic compression.
Not anything to miss sleep over.

I have ran in the 9.0+ dynamic many times on my own SBC 350's and no issues with anything.

Are you going to pull that 350" apart? the one with 305 heads?
What you can do is port those 305 heads a bit and get the proper retainer to guide clearances and run a beehive 313 rate valve spring and toss in a comp 268H or 270 H or 280H depending on how much power you wish to make.

Rough up the intake runners on the heads and remove all sharp edges in the combustion space and do not worry about dynamic compression.
I have ran the 268H in a 350" many times with 305 heads on a 350..Ported heads though not stock ones.

Attention to detail when assembling and all will be good.
This was to be your winter project wasn't it?

My current 350 has been 10.78 compression for decades and ran many cams and some with intake closing point earlier than a 260H. 225 psi cranking pressure.
I pulled many heavy loads for thousands of miles with this engine.

I have a 280H comp magnum cam in it now..My buddies went with LS engines and I had to step up the power to stay ahead of them.
I still run 1.84-1.50 valves in my 601 casting 305 heads.

More than you asked for.. sorry Jim
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post #5 of 12 (permalink) Old Jun 4th, 20, 9:01 AM
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Re: How to determine Dynamic CR and intake BTDC

intake lobe centerline term is probably being used as the location the cam is installed vs the crankshaft, changing the location affects the DCR even using the same camshaft, you should be able to find most cams duration at .006" which is close to the seat duration, use this number to compare DCR among different cams
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post #6 of 12 (permalink) Old Jun 4th, 20, 10:46 AM Thread Starter
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Re: How to determine Dynamic CR and intake BTDC

Quote:
Originally Posted by jeff swisher View Post
I never worried about dynamic compression.
Not anything to miss sleep over.

I have ran in the 9.0+ dynamic many times on my own SBC 350's and no issues with anything.

Are you going to pull that 350" apart? the one with 305 heads?
What you can do is port those 305 heads a bit and get the proper retainer to guide clearances and run a beehive 313 rate valve spring and toss in a comp 268H or 270 H or 280H depending on how much power you wish to make.

Rough up the intake runners on the heads and remove all sharp edges in the combustion space and do not worry about dynamic compression.
I have ran the 268H in a 350" many times with 305 heads on a 350..Ported heads though not stock ones.



Attention to detail when assembling and all will be good.
This was to be your winter project wasn't it?

My current 350 has been 10.78 compression for decades and ran many cams and some with intake closing point earlier than a 260H. 225 psi cranking pressure.
I pulled many heavy loads for thousands of miles with this engine.

I have a 280H comp magnum cam in it now..My buddies went with LS engines and I had to step up the power to stay ahead of them.
I still run 1.84-1.50 valves in my 601 casting 305 heads.

More than you asked for.. sorry Jim
Yes Jeff, its that 350, I can't seem to get enough vacuum when I shift into gear or reverse, so I have to set the idle at 1100 and when I put into gear she just jumps.

My thinking is pull the engine apart and go from there. I probably going to go with those new camel looking aluminum heads. Who knows what else after I get the engine partially disassembled. I would like to get close to 400 hp and 400 lb of torque. So looking at everything now.

My thinking is the PO put the heads, intake, and carb on with the brake and other upgrades in 97. Then who knows what he did to the block. maybe just evaluated and new gaskets. I think he didn't put i n new rings or change pistons, etc. So rings and bearing might be necessary. Maybe he threw in a different cam I am guessing due to the vacuum issues.


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post #7 of 12 (permalink) Old Jun 4th, 20, 12:02 PM
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Re: How to determine Dynamic CR and intake BTDC

Very odd situation you have. I will discuss some things.
I have worked on vehicles with only 5" idle vacuum and you did not need to idle it high.


I assume your engine dies if you idle it too low and stick it into gear.



Now if that is the case You need to look at carburetor issues.

Some easy things to do.
Idle it down as far as you can go in park and look down the carburetor.It should not be dribbling any fuel out of the boosters.
Boosters are the small round fuel discharge holes you can about stick a pencil through them in the middle of the primary throttle holes.

Now that it is idled down as far as you can go. And NO dribbling fuel ...Adjust the air mixture screws.
Turn them in 1/4 turn at a time or 1/8 turn at a time until you get a rough idle.

If you never get a rough idle and you have them turned in all the way you have issues with the idle passages in the carburetor or the throttle blades are just opened too far.

There is an adjustment for secondary throttle blade opening on many carbs.

What carb is on this one Again.

I assume you checked Idle vacuum with a gauge....What is idle vacuum??

I have stuck huge cams into 350" with 305 heads and still have 11+" of idle vacuum.
Not so much idle vacuum with large runner heads.. 210cc and up kills idle vacuum.
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post #8 of 12 (permalink) Old Jun 4th, 20, 12:20 PM
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Re: How to determine Dynamic CR and intake BTDC

Quote:
Originally Posted by Jim_66SS View Post
Thanks, I don't have a card as I am looking at different cams and trying to get the dynamic into the range of 7.8-8.0 with a static in the 10.0 to 10.5 range.

I think they list the intake lobe duration. Not sure of what you mean by intake lobe centerline. But what do you do with these numbers. ADD, Subtract, Divide?

for example: LOB 112; int duration 213 at.05 or 256 total
Where it can confuse people is the fact there is intake lobe centerline("ICL"), which is for symmetrical shape cam lobes the point of peak lift, .....and there is also lobe separation angle("LSA"), which is the distance in degrees between peak lift or lobe centerline of the intake and exhaust valves.

Since intake lobe centerlines are rarely ever as high as 112į, I would have to assume the spec you have is actually the Lobe Separation Angle (LSA) for that cam grind.
With that being the case, LSA and cam duration alone is not enough info to find intake lobe closing point, we need the intake lobe centerline to have enough info to do the math to find intake closing point.

If you could link the cams info you are looking at, maybe we can see enough info to figure this out.

After reading post #5 and #6 i think Jeff is right about what you are dealing with....too high idle lie,that is typically primary throttle blades open too far and engine is running of the transition slots and not the idle mixture screws. Typically happens when a bigger cam is put in, engine won't idle, so "mechanic" just cranks up the idle speed screw to get more air in so the engine will idle....now it idles but too fast and no response from mixture screws.
Fix is typically back the primary idle speed screw back down, and slightly open the secondary blades to let more air in...second choice is to drill a small bleed hole through each primary throttle blade so you can close the throttle setting down but still get enough air through.
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post #9 of 12 (permalink) Old Jun 4th, 20, 5:12 PM Thread Starter
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Re: How to determine Dynamic CR and intake BTDC

Thanks again guys. Jeff, I swapped my holley 3419 on my 66 SS On to my 350 in the conv just the other day. 350 ran the same. Idle vacuum the same 12Ē. With base timing at 14 adv and with VC hooked up 34 at 1100 rpm. When putting in gear vacuum dropped to 5 and timing down ( no sure how much) and idle dropped to 750. Put holley back on 66 SS and SS ran great. So thinking it is not carb.

Still think itís a cam issue. Doesnít make sense PO did 350TH, gear, sway bay, steering gear, disc brake and radiator upgrades. Then when it came to engine put a dirty old 350 in with new intake, new carb, and reconditioned 305 heads with roller rockers on with out touching the short block. Especially in a clean freshly painted engine bay.

I would think he at least cleaned it, inspected it, and said good to go as is except letís change the cam.

My SS has a bigger cam 284H, in a 4sp,and ran great befor; but same issues in 350; runs great again with it back home in the 396.

Thinking it needs at least rings and bearings. Then maybe a cam change and/or torque converter change.

Also adjusted idle mixture screw; but maybe that would be an issue also with the holley. I saw a holley carb rebuild talking about opening the secondaries a bit but that was to prevent them from sticking.


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post #10 of 12 (permalink) Old Jun 4th, 20, 7:25 PM
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Re: How to determine Dynamic CR and intake BTDC

350 with 305 heads will eat a huge cam and not have any idle issues.
I ran [email protected] .050 on a 106LSA it was a solid flat tappet and a [email protected] .050 on a 110 LSA hydraulic flat tappet.
Neither had the issues you speak of.

Maybe the valves need adjusted.
I have fixed a few hydraulic cams that were just set with too many turns and the things ran like poop.

Could be you have a vacuum leak somewhere also.. possibly intake gasket not sealing.

I assume you have done a compression test and had all the spark plugs out. Plugs clean?

I have seen dirty plugs cause issues.. yea simple dirty plugs.


From where you are right now i would adjust the valves and then do a compression test.
If that showed all good I would pull the intake.
If nothing shows up there you are almost to the point of pulling it anyway...so I would then pull it and go through everything.
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post #11 of 12 (permalink) Old Jun 4th, 20, 8:09 PM Thread Starter
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Re: How to determine Dynamic CR and intake BTDC

Compression test done. 7 at 165 and 1 a 150. 7 plugs good maybe a little rich, 1 plug a little dirty at the 150 cylinder. Canít find any vacuum leaks, intake, power brake, hoses, pcv,, etc checked. Some blow bye at pcv and breather.


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post #12 of 12 (permalink) Old Jun 5th, 20, 8:47 AM
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Re: How to determine Dynamic CR and intake BTDC

Could be valve adjustment then.
If not then you may just have a crappy cam in it or the cam to crank timing could be off.

If adjusting the valves does not fix it you will need to dig into that engine
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