Don Terrill’s Speed-Talk

How much cranking compression is too much?

Customer has 300lbs cranking compression (Snap on tester)
Thinking 240-250 best
8.20 2-valve hemi motorcycle
1500cc 15.4 CR static
Intake .470 265@.040 (as measured in engine)
Timing tag is .480 270
IVC 65 degrees ABDC

In this case, should a new camshaft with more duration be selected to aim for a certain dynamic compression?

Less LSA. Move your IVC and the EVO for more overlap to give you a little more bleed off this can be accomplished with more duration but not nessasarly If you like the profile you may just need to have a custom grind with F.E. 106 LSA instead of 108 LSA?

dhidaka wrote:How much cranking compression is too much?

Customer has 300lbs cranking compression (Snap on tester)
Thinking 240-250 best
8.20 2-valve hemi motorcycle
1500cc 15.4 CR static
Intake .470 265@.040 (as measured in engine)
Timing tag is .480 270
IVC 65 degrees ABDC

In this case, should a new camshaft with more duration be selected to aim for a certain dynamic compression?

What is you bore, stroke and rod length? Based on the above I take it you have the cam in @ 20/65 or 112 ICL (based on cam being symmetrical). I calculate using that and guessing at bore = 83.8, stroke = 68.0 and rod = 145 that your IVC @ the valve is about 76 ABDC (seat duration number not 0.04").

Is this an all out race engine?

Stan

gvx wrote:Less LSA. Move your IVC and the EVO for more overlap to give you a little more bleed off this can be accomplished with more duration but not nessasarly If you like the profile you may just need to have a custom grind with F.E. 106 LSA instead of 108 LSA?

You might want to rethink that
overlap has nothing to do with cranking compression it is on the wrong side of the equation
You do not 'bleed off" cranking compression with more overlap & if you move from 108 to 106 LSA all else equal with say 4° advance you will increase cranking not decrease it

Customer has 300lbs cranking compression (Snap on tester)

Snap On makes good stuff, but my first step would be to check the compression with another gauge, or check another engine with that gauge. (maybe you already have)

Stan Weiss wrote:

dhidaka wrote:How much cranking compression is too much?

Customer has 300lbs cranking compression (Snap on tester)
Thinking 240-250 best
8.20 2-valve hemi motorcycle
1500cc 15.4 CR static
Intake .470 265@.040 (as measured in engine)
Timing tag is .480 270
IVC 65 degrees ABDC

In this case, should a new camshaft with more duration be selected to aim for a certain dynamic compression?

What is you bore, stroke and rod length? Based on the above I take it you have the cam in @ 20/65 or 112 ICL (based on cam being symmetrical). I calculate using that and guessing at bore = 83.8, stroke = 68.0 and rod = 145 that your IVC @ the valve is about 76 ABDC (seat duration number not 0.04").

Is this an all out race engine?

Stan

Yes, Stan, racing only 8:20 class
NA
Bore 85mm
Stroke 66mm
RL 113.5mm
Cam mfg uses .040 as measurement point
I don't have .050 specs

Damn, I dont think my gage will read 300
reed

Cranking pressure is also related to cranking RPM. Most of the available CP calculations are based on a 150 RPM test from the automotive side. Are you spinning at a higher rate? My suggested CP was supposed to be 208lbs based on a SCR of 12.35:1 from my spreadsheet calc. I was shocked to find 245lbs. I traced the difference to a 450rpm starter.

Regards,
Paul

GVX has it ALL WRONG--- Dont listen to him


JOE SHERMAN RACING

How can that be right, it has to be a gauge issue

Even if we had 15 psi atmospheric pressure, you'd need 20:1 to get there. Although the exhaust can pull additional air into the cylinder, I'd be utterly amazed if it was that much when the engine was only turning on the starter and not firing

dhidaka wrote:

Stan Weiss wrote:

dhidaka wrote:How much cranking compression is too much?

Customer has 300lbs cranking compression (Snap on tester)
Thinking 240-250 best
8.20 2-valve hemi motorcycle
1500cc 15.4 CR static
Intake .470 265@.040 (as measured in engine)
Timing tag is .480 270
IVC 65 degrees ABDC

In this case, should a new camshaft with more duration be selected to aim for a certain dynamic compression?

What is you bore, stroke and rod length? Based on the above I take it you have the cam in @ 20/65 or 112 ICL (based on cam being symmetrical). I calculate using that and guessing at bore = 83.8, stroke = 68.0 and rod = 145 that your IVC @ the valve is about 76 ABDC (seat duration number not 0.04").

Is this an all out race engine?

Stan

Yes, Stan, racing only 8:20 class
NA
Bore 85mm
Stroke 66mm
RL 113.5mm
Cam mfg uses .040 as measurement point
I don't have .050 specs

Cranking Compression is a result of other parts and decisions made. If the CR and cam events are what and where you want them then Cranking Compression Pressure is what it is.

I have no problem seeing these parts produce 300 PSI

Thanks for the engine specs.

They change the engine geometry a little and I get your IVC @ the valve is about 77.6 ABDC (seat duration number at the valve) that would be about 290.2 duration seat-to-seat lash duration at the valve. I do not need the 0.050 cam numbers. For those looking at this remember that most motorcycle cams duration is measured at 0.040" lift and not the 0.050" lift used for most other cams.

Last but not least. If everything else is correct then 300 psi is will be OK.

Stan

The wallase racing dcr calculator gives 270 psi with a 15.4 comp and 65 degrees
after bdc intake closing. So its not a guage issue. My experance is even with sky high
fuel octane engines start loosing power beyond 220 225 psi. Measured at 150 cranking rpm.

I love this forum yet other than a few lower level cars (NHRA stock class and under 700HP street cars) I own, I build single cylinder small race motorcycle motors. The one I won vintage national championship was small. 55mm bore x 59mm stroke 100.5mm rod 2-valve (27.5x21) hemi motor. Ran static at 15.4-1 and cam has intake at 101 and exhaust at 104 with approx 248/252 @.050.
With kickstart only it ran 287 psi and no way to verify kicking RPM. I spent a couple years creating new parts for this combo and found by trial and error best combo for intended purpose and rules. 31 degrees total timing, stepped diameter pipe going to true reverse cone megaphone made best power in range (6-10,7000).
Don't agree with losing or no gain on power statement above 12.5 as I ran a 11, 12, 13.5 and 15.4. Peak only gained slightly above 12-1 but higher increased torque and I ended up with a motor that had a very flat powerband exactly where I needed it.
Now these are on deep hemi dome and valve limted due to valve to valve contact etc. and started as a 1971 motor. Made 3 times stock rear wheel HP and was top of class. I employed various coatings etc to make this live as air-cooled and slow speeds so basically a 20 minute motor and shut down.
I could have tried one of my smaller cams but then I think I would have been over 300 lbs cranking. I most surely am not smartest guy but was able to optimise combo for my situation. 4- cylinder above is similar to some of the old big 750-4 hondas I used to build and they were strong back then. Modern 4-5 valve heads are worlds better and rpm's are now in the 14,000+ so it will need some other smart guy to calculate valve timing events needed.
I say try it.

Even if we had 15 psi atmospheric pressure, you'd need 20:1

because cranking pressure is obviously static CR × atmospheric pressure, right?

No, it's not.

From my limited personal experience 245-ish DCR seems to work the best in engines like I am working on. Using Wallace calculator, retarding the intake 5 degrees will lower DCR about 20 lbs. Seems a better option would be a longer duration camshaft. I will retest with another compression tester, although my Snap-On is relatively new.

Back to my original questions, rephrased:

What is too much DCR?

Should a cam ever be selected with a certain DCR in mind?