Cranking Pressure concern

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splitdecision71
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Cranking Pressure concern

Post by splitdecision71 » Wed Dec 05, 2018 2:30 pm

Just finished putting together SBC 421. I have a concern about cranking pressure. I was thinking it should be higher than it is. Math says static compression is 14.64:1 but Cranking cylinder pressure is 150 psi across the board. I was thinking is should be in the 200s and wallace calulator says it should be. Is the 150psi cranking psi to low?
Specs on build are as follows.

4.155 bore
3.875 stroke
6.0 rods
block decked @ .0000
SCE copper .032 head gaskets
diamond 8cc dome pistons
64cc chambers
1.65 intake rocker and 1.6 exhaust rocker.
Valve lash set at .020 on intake and .019 on exhaust cold.
cam specs:
305/311 adv. duration
.645/640 lift
104 CL and 108 LSA
IO 48.5 IC 76.5
EO 87.5 EC 43.5 92 degrees overlap

splitdecision71
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Re: Cranking Pressure concern

Post by splitdecision71 » Wed Dec 05, 2018 2:35 pm

Am I overthinking this? This is a fresh build with barely any run time. Only started in the car and ran for a total of maybe 30 minutes.

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Re: Cranking Pressure concern

Post by PackardV8 » Wed Dec 05, 2018 2:58 pm

Assume you ran the cranking compression test with all spark plugs removed, throttle fully opened, battery fully charged, at least 150 RPM? All eight cylinders within 5% of that 150#?

Next test is leak-down.
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Stan Weiss
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Re: Cranking Pressure concern

Post by Stan Weiss » Wed Dec 05, 2018 3:20 pm

splitdecision71 wrote:
Wed Dec 05, 2018 2:30 pm
Just finished putting together SBC 421. I have a concern about cranking pressure. I was thinking it should be higher than it is. Math says static compression is 14.64:1 but Cranking cylinder pressure is 150 psi across the board. I was thinking is should be in the 200s and wallace calulator says it should be. Is the 150psi cranking psi to low?
Specs on build are as follows.

4.155 bore
3.875 stroke
6.0 rods
block decked @ .0000
SCE copper .032 head gaskets
diamond 8cc dome pistons
64cc chambers
1.65 intake rocker and 1.6 exhaust rocker.
Valve lash set at .020 on intake and .019 on exhaust cold.
cam specs:
305/311 adv. duration
.645/640 lift
104 CL and 108 LSA
IO 48.5 IC 76.5
EO 87.5 EC 43.5 92 degrees overlap
If the cranking compression test was done correctly.

305/311 adv. duration << Do you know @ what lifter raise this is?

Then either you have way less then 14.64:1 CR or / and your cam is not installed @ 104 ICL.

Stan
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Offering Performance Software Since 1987
Do you use engine simulation software that uses cylinder head flow files?
We have a package of more than 3000 DFW or FLW Files

splitdecision71
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Re: Cranking Pressure concern

Post by splitdecision71 » Wed Dec 05, 2018 3:21 pm

PackardV8 wrote:
Wed Dec 05, 2018 2:58 pm
Assume you ran the cranking compression test with all spark plugs removed, throttle fully opened, battery fully charged, at least 150 RPM? All eight cylinders within 5% of that 150#?

Next test is leak-down.
All plugs removed, throttle wide open, and two hot batteries. So you agree that there should be more pressure as well?

splitdecision71
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Re: Cranking Pressure concern

Post by splitdecision71 » Wed Dec 05, 2018 3:57 pm

Stan Weiss wrote:
Wed Dec 05, 2018 3:20 pm
splitdecision71 wrote:
Wed Dec 05, 2018 2:30 pm
Just finished putting together SBC 421. I have a concern about cranking pressure. I was thinking it should be higher than it is. Math says static compression is 14.64:1 but Cranking cylinder pressure is 150 psi across the board. I was thinking is should be in the 200s and wallace calulator says it should be. Is the 150psi cranking psi to low?
Specs on build are as follows.

4.155 bore
3.875 stroke
6.0 rods
block decked @ .0000
SCE copper .032 head gaskets
diamond 8cc dome pistons
64cc chambers
1.65 intake rocker and 1.6 exhaust rocker.
Valve lash set at .020 on intake and .019 on exhaust cold.
cam specs:
305/311 adv. duration
.645/640 lift
104 CL and 108 LSA
IO 48.5 IC 76.5
EO 87.5 EC 43.5 92 degrees overlap
If the cranking compression test was done correctly.

305/311 adv. duration << Do you know @ what lifter raise this is?

Then either you have way less then 14.64:1 CR or / and your cam is not installed @ 104 ICL.

Stan
Are you asking gross lift at cam? .430 intake and .427 exhaust.
I installed the cam straight up dot to dot. zero advance or retard.

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Re: Cranking Pressure concern

Post by Caprimaniac » Wed Dec 05, 2018 4:04 pm

splitdecision71 wrote:
Wed Dec 05, 2018 3:57 pm
Are you asking gross lift at cam? .430 intake and .427 exhaust.
I installed the cam straight up dot to dot. zero advance or retard.
I wouldn't trust the dots- you didn't degree the cam?
How to turn GURU in an instant.....

splitdecision71
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Re: Cranking Pressure concern

Post by splitdecision71 » Wed Dec 05, 2018 4:21 pm

Caprimaniac wrote:
Wed Dec 05, 2018 4:04 pm
splitdecision71 wrote:
Wed Dec 05, 2018 3:57 pm
Are you asking gross lift at cam? .430 intake and .427 exhaust.
I installed the cam straight up dot to dot. zero advance or retard.
I wouldn't trust the dots- you didn't degree the cam?
I did not as I read an article that said it is not necessary with today"s cam companies grinds and accuracy. I guess I screwed up by not doing so?!?!?!?
I guess I fell for the old "Everything is true on the internet". Looks like I will be pulling the motor.

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Re: Cranking Pressure concern

Post by allencr267 » Wed Dec 05, 2018 5:45 pm

How was the blowby?
Turned it over by hand, listening to the sound from the crankcase/oil filler?

The dots will always make it run, not always make it run as well as it could've.

splitdecision71
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Re: Cranking Pressure concern

Post by splitdecision71 » Wed Dec 05, 2018 5:55 pm

allencr267 wrote:
Wed Dec 05, 2018 5:45 pm
How was the blowby?
Turned it over by hand, listening to the sound from the crankcase/oil filler?

The dots will always make it run, not always make it run as well as it could've.
Hard to turn over by hand with plugs in motor. Didnt try that to much.

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Re: Cranking Pressure concern

Post by Dragsinger » Wed Dec 05, 2018 6:16 pm

to give you some guideline, I would expect the pressure to be at least 230 and even as much as 260
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splitdecision71
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Re: Cranking Pressure concern

Post by splitdecision71 » Wed Dec 05, 2018 6:32 pm

Dragsinger wrote:
Wed Dec 05, 2018 6:16 pm
to give you some guideline, I would expect the pressure to be at least 230 and even as much as 260
Thank you sir, that is right in line with what wallace calculator says it should be which is where I am also coming up with 14.64 to 1 compression.

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Re: Cranking Pressure concern

Post by gnicholson » Wed Dec 05, 2018 8:49 pm

i would try another guage 1st. ive been fooled by a low reading guage before. i have a screw in snapon unit that reads low with the screw in hose but fine with the push in tapered rubber seal. maybe a hose restriction. something to check anyway

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Re: Cranking Pressure concern

Post by geraldtson » Wed Dec 05, 2018 8:58 pm

Somethings not right? Recently checked cranking compression on very similar build 414ci 14.5 sbc which i checked several times just for maintenance and to record and without looking it up i think all cylinders were very close to 270 or so on average and stayed consistent during service over time. This one was 108lsa installed at 108cl (straight up). If i were you i would recheck everything, equipment and procedure as suggested and if still that low i would be doing a leakdown test next.

splitdecision71
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Re: Cranking Pressure concern

Post by splitdecision71 » Wed Dec 05, 2018 9:34 pm

gnicholson wrote:
Wed Dec 05, 2018 8:49 pm
i would try another guage 1st. ive been fooled by a low reading guage before. i have a screw in snapon unit that reads low with the screw in hose but fine with the push in tapered rubber seal. maybe a hose restriction. something to check anyway
Good idea, Thank you!

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