Small Block Help

[wolfeman]

I could use some help with my 340. I rebuilt the motor two years ago. Its the first rebuild I've ever performed. I rebuilt it shooting for 10:1 static compression, used a Lunati Voodoo cam part # 10200703. The motor runs fine, vacuum is at 10" with a bit of a rough idle, which I assumed was cam related. When I put the numbers into a dynamic compression calculator I come up with an expected pressure of 193 psi. When I measure the dynamic compression I'm only getting 165psi on every cylinder +-2psi, which is way low. I performed a leak down test and get 4-6% leak-down on every cylinder, which from what I read is very good.
The numbers I used to get the static compression are 3.31" stroke, 4.07" bore, 6.123" rod length, piston are .011" in the hole, 5cc piston dish/valve recess, 63cc head chamber, head gasket thickness is .023. The cam card says the intake valve closed at 39 abdc. When I run the numbers by static compression should be around 10:1 with a dynamic around 193psi. I get thee numbers using wallaceracing.com calculators.

When I assembled the motor I measured the head to piston clearance and it measured .035, which is what I was shooting for.

Are my calculations wrong? Am I correct that my leak down number are good? If so when is my dynamic compression 30psi low? What am I missing? Even if I retarded the cam my dynamic compression shouldn't be that low. All compression test were performed with a cold motor, but I doubt that would make that big of a difference.

 

[moparleo]

What is the piston dome volume ?
10 inches vacuum should run more than a little rough. Very low. Vacuum leak ?
What are you trying to accomplish with this build ?
Throttle blades should be locked wide open, engine warm or oil into cylinders and 3 revolutions.

 

[wolfeman]

What is the piston dome volume ?
10 inches vacuum should run more than a little rough. Very low. Vacuum leak ?
What are you trying to accomplish with this build ?
Throttle blades should be locked wide open, engine warm or oil into cylinders and 3 revolutions.

5cc dish for valves. No vacuum leaks. Just trying for a slightly hot street car. I actually removed carb to make sure no restrictions. I can't believe a cold engine would drop 30psi.

 

[Steve340]

I have not seen a calculation that will tell you what reading you will get on a compression gauge.
Were did you get that from?
In any case static and dynamic compression are expressed as ratios.
The static is theoretical as it measures the swept and un-swept volume of the engine from TDC to BDC.
Dynamic or effective compression ratio takes in to account the conrod length and when the intake valve closes after bottom dead centre.

The leak down is fine.

 

[Steve340]

I used your numbers but the intake valve closing one is probably not correct. Most street cams close the valve at 50 to 70 ABDC
The dynamic is very high - that is near race engine territory.
Static 10.38
Dynamic 9.58

 

[wolfeman]

I have not seen a calculation that will tell you what reading you will get on a compression gauge.
Were did you get that from?
In any case static and dynamic compression are expressed as ratios.
The static is theoretical as it measures the swept and un-swept volume of the engine from TDC to BDC.
Dynamic or effective compression ratio takes in to account the conrod length and when the intake valve closes after bottom dead centre.

The leak down is fine.

I used a calculator on wallaceracing.com to get these numbers. The result are below. I started checking my work because I found I need to keep my throttle blades open so much to idle at 800rpm that my idle mixture screw no longer change the way it runs. They do change the idle mixture, but the throttle blade are so far open I can close them off and the car still runs the same, just leaner, the rpm doesn't go up or down. It wasn't this way prior to the rebuild with the same carb. I thought I may have chosen too large of a cam and started asking for cam recomendations to correct. Once company a asked was Hughes. They came back and said my cranking pressure is too low and won't even make a recomendasion. Also stated I must not be telling him the truth about my numbers. That's why I'm asking for opinions. I agree that my cranking pressure is lower than I expected, but if my leak down is fine, where am I losing the 30psi?

Static compression ratio of 10:1.
Effective stroke is 3.03 inches.
Your dynamic compression ratio is 9.24:1 .
Your dynamic cranking pressure is 195.55 PSI.
Your effective boost compression ratio, reflecting static c.r., cam timing, altitude, and boost of 0 PSI is 9.18 :1.
V/P (Volume to Pressure Index) is 185

 

[wolfeman]

I used your numbers but the intake valve closing one is probably not correct. Most street cams close the valve at 50 to 70 ABDC
The dynamic is very high - that is near race engine territory.
Static 10.38
Dynamic 9.58

Here is my cam card. I used the 39 ABDC. I calculated a static of 10:1 and expect 193 psi of cranking pressure.

 

[Mr Cuda]

I can't say for sure where the missing cranking compression went, but a couple observations from my current build.
My target compression is 10.25.
To get there with flat top custom light weight pistons, the piston is .010 above deck. There were no domed pistons availible with the spec I needed.
You mentioned yours were down .011" from the deck, but didn't mention any dome.
That would be the missing compression.
The carb primary plate needs to be closed, or have the transfer slot covered. On a holley, I try for one turn open from a shut plate. I bump the rear plate to get idle speed. Then the idle mixture and off idle enrichment works properly.
Other than that, cam advance would bump idle vaccum. 10" at idle seems low for that cam. My cam spec is just a tad more, but solid. I'm hoping for 12" of vaccum.

 

[wolfeman]

I can't say for sure where the missing cranking compression went, but a couple observations from my current build.
My target compression is 10.25.
To get there with flat top custom light weight pistons, the piston is .010 above deck. There were no domed pistons availible with the spec I needed.
You mentioned yours were down .011" from the deck, but didn't mention any dome.
That would be the missing compression.
The carb primary plate needs to be closed, or have the transfer slot covered. On a holley, I try for one turn open from a shut plate. I bump the rear plate to get idle speed. Then the idle mixture and off idle enrichment works properly.
Other than that, cam advance would bump idle vaccum. 10" at idle seems low for that cam. My cam spec is just a tad more, but solid. I'm hoping for 12" of vaccum.

What type of heads are you using? How many cc's are your heads? I'm using 63cc closed chamber heads. The pistons are flat top, with 5cc recess for the valves.

 

[Steve340]

Carburettor. You can drill small holes in the primary throttle butterfly blades to allow you to close them. Set the transfer slot to 20 thousands and check your curb idle speed. If too slow you would go up a size on the drilled hole. Keep the transfer slot at 20 thou each time.
Start at 1/16 and go bigger if you need to.
Important- Before you drill the carb do all the jetting, timing etc before you mod the carb.
Drilling the carb is the LAST thing you do. There are quite a few reasons a carb will run rich and you need to eliminate all these first.
The procedure is well documented on the internet.

You have used the wrong figures in the calculation. The diagram is the at 50 thou figures and you need to use the advertised figures.

Using your figures from your card:
7 plus 180 plus 39 = 226 at 50 - but the advertised duration that you need to use is 271
So assuming the opening and closing ramps of the camshaft are symmetrical :

271 - 226 = 45 divided by 2 = 22.5.

Add 22.5 to the 39 and you get 61.5 degrees ABDC is when the valve is actually closed.

I recalculated static is still 10.38 but the dynamic has dropped to 8.4
This is pushing it on a street motor on pump gas but if it does not detonate you are lucky.

The motors I do I pay no attention to cranking compression. I target a certain static and dynamic compression ratios at trial assembly prior to balancing the engine.
If the engine develops a dud cylinder I use a dry leak down test. Introduce no oil into the cylinder as this will seal everything and throw the test results.

My advise for what it's worth - you seem to have done a good job and if the engine runs good forget about it.
Altering the compression ratio will mean striping the engine down.

 

[Xcudame]

Lot of good info here. But one question, how did you install the camshaft? Straight up or advanced? Using a degree wheel and setting up the camshaft is very important on aftermarket camshafts and timing sets. A camshaft a few degrees off from ideal will have weird symptoms.

 

[wolfeman]

Carburettor. You can drill small holes in the primary throttle butterfly blades to allow you to close them. Set the transfer slot to 20 thousands and check your curb idle speed. If too slow you would go up a size on the drilled hole. Keep the transfer slot at 20 thou each time.
Start at 1/16 and go bigger if you need to.
Important- Before you drill the carb do all the jetting, timing etc before you mod the carb.
Drilling the carb is the LAST thing you do. There are quite a few reasons a carb will run rich and you need to eliminate all these first.
The procedure is well documented on the internet.

You have used the wrong figures in the calculation. The diagram is the at 50 thou figures and you need to use the advertised figures.

Using your figures from your card:
7 plus 180 plus 39 = 226 at 50 - but the advertised duration that you need to use is 271
So assuming the opening and closing ramps of the camshaft are symmetrical :

271 - 226 = 45 divided by 2 = 22.5.

Add 22.5 to the 39 and you get 61.5 degrees ABDC is when the valve is actually closed.

I recalculated static is still 10.38 but the dynamic has dropped to 8.4
This is pushing it on a street motor on pump gas but if it does not detonate you are lucky.

The motors I do I pay no attention to cranking compression. I target a certain static and dynamic compression ratios at trial assembly prior to balancing the engine.
If the engine develops a dud cylinder I use a dry leak down test. Introduce no oil into the cylinder as this will seal everything and throw the test results.

My advise for what it's worth - you seem to have done a good job and if the engine runs good forget about it.
Altering the compression ratio will mean striping the engine down.

So I'm using the wrong intake valve closing position? If that is correct, and I plug your numbers in, I get a cranking pressure of 165, which is exactly what I measure. That means I didn't do anything wrong with the build, just the calculation to get cranking pressure. Thank you so much. Here's the number I get from the calculation using the intake valve closing position you gave me. I guess I need to learn how to read a cam card correctly.
Static compression ratio of 10:1.
Effective stroke is 2.62 inches.
Your dynamic compression ratio is 8.12:1 .
Your dynamic cranking pressure is 165.15 PSI.
Your effective boost compression ratio, reflecting static c.r., cam timing, altitude, and boost of 0 PSI is 8.06 :1.
V/P (Volume to Pressure Index) is 135

 

[Steve340]

That camshaft has 4 degrees of advance ground in to it by Lunati. This is more than enough advance.
Generally the intake centreline given by the cam grinder is pretty close for 95% of engines.
Provided you have selected the right combination of parts.
Xcudame is right degreeing the cam is important but if you don't have the equipment to properly check the cam phasing, or did not do it don't worry to much.
Just install it straight up and be done with it.
Same as static compression ratio - shifting it half a point up or down will make only a small difference to power and torque.

 

[Xcudame]

Be careful of what timing set was used. I've seen some 6 degrees off!

But really, 165 +/- 2 psi for each cylinder is pretty darn good! Like Mr Cuda said, 12" of vacuum would be best and could be just be a skight idle mixture screws adjustment away!

 

[Steve340]

I am quite sure about my calculated figures for your intake valve closing point.
I have used Wallace Racing calculators and they are good - but I have your static/dynamic numbers at a bit higher.
However this difference will not matter to most folks.
If your engine runs well and makes reasonable power don't fix what ain't broken.
Get the tuning right as this can make a hell of a difference.

 

[Steve340]

For what it's worth I do all valve lash adjustments, leak down tests etc COLD were possible.
I got tired of burning the crap out of myself.

 

[Challenger RTA]

Just in general info.
=============-NARROW LSA ----------WIDE LSA

INTAKE OPENS ----------------EARLIER-------------LATER
INTAKE CLOSES----------------EARLIER-------------LATER
EXHAUST OPENS--------------LATER-----------------EARLIER
EXHAUST CLOSES-------------LATER-----------------EARLIER
OVERLAPS----------------------MORE------------------LESS
CYLINDER PRESSURE--------GAIN-------------------LOSE
IDLE QUALITY-----------------WORSE----------------BETTER
IDLE VACUUM-----------------LESS-------------------MORE
TORQUE CURVE--------------PEAKIER---------------FLATTER
PEAK TORQUE----------------MORE------------------LESS
HIGH RPM---------------------DROPS OFF-----------HANGS ON