Cranking Pressure concern

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splitdecision71
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Re: Cranking Pressure concern

Post by splitdecision71 » Wed Dec 05, 2018 9:35 pm

geraldtson wrote:
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.
Thank you, very good info. I will check and double check this weekend.

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Re: Cranking Pressure concern

Post by MadBill » Thu Dec 06, 2018 12:10 am

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
2X. The most common problem is someone replacing a defective Schrader valve with a regular tire valve. Its spring force is much higher and will give false low readings.

PS: 'Dot to dot' is a crapshoot. Many years ago I checked out the Lunati cam a friend installed DTD in his 455" Buick. It was out by 17°! #-o
Last edited by MadBill on Thu Dec 06, 2018 12:14 am, edited 1 time in total.
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Re: Cranking Pressure concern

Post by tresi » Thu Dec 06, 2018 12:11 am

Bring a cyl to TDC. Get a rubber tip blow gun or make a spark plug adaptor and pressurize the cyl. Listen for air flow and not where it's coming from intake, breather or exhaust. Valve leakage could be a bad valve seal or it could be something simpler such as valve timing or lash to tight. I've seen many people adjust their hyd cam way to tight causing leakage or valve lash set when they weren't on the base circle.

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Re: Cranking Pressure concern

Post by GRTfast » Thu Dec 06, 2018 8:42 am

Sounds low. I have a 9.5:1 BBC with a street friendly hydraulic roller (.560 lift, 235 @ .050 duration) and the cranking pressure is ~175psi

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Re: Cranking Pressure concern

Post by rebelrouser » Thu Dec 06, 2018 11:45 am

If you did not take the time to degree the cam, did you do the measurements to know it has over 14 to 1 compression? Did you CC the chambers? How did you arrive at that much compression? I personally never have seen the advertised compression of pistons correct. Its always less when I CC the chambers, measure the deck height, etc. and do the calculations. It has been an issue with my builds to get that much compression with out things hitting. I usually have to grind on the piston tops and the chambers when they are that close.

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Re: Cranking Pressure concern

Post by Stan Weiss » Thu Dec 06, 2018 12:47 pm

The way my computer program sees this is.

Even if he is off 3 point in CR (11.64) and has the ICL retarded 15 degree (91.5 IVC). He still should be cranking off 160 psi. Weather 60 degrees BP 29.92.

I do have one question for the poster. At what altitude are you doing this test?

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Re: Cranking Pressure concern

Post by Caprimaniac » Thu Dec 06, 2018 1:30 pm

Stan Weiss wrote:
Thu Dec 06, 2018 12:47 pm
The way my computer program sees this is.

Even if he is off 3 point in CR (11.64) and has the ICL retarded 15 degree (91.5 IVC). He still should be cranking off 160 psi. Weather 60 degrees BP 29.92.

I do have one question for the poster. At what altitude are you doing this test?

Stan
IOW; there has to be something else that is off. Pressure gauge, air pressure, valves or whatever.

However: my current engine read around 165 on all cylinders. Using the Math it should had been 200+. It still run 10's. (SB street car.)
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Re: Cranking Pressure concern

Post by splitdecision71 » Thu Dec 06, 2018 2:23 pm

Stan Weiss wrote:
Thu Dec 06, 2018 12:47 pm
The way my computer program sees this is.

Even if he is off 3 point in CR (11.64) and has the ICL retarded 15 degree (91.5 IVC). He still should be cranking off 160 psi. Weather 60 degrees BP 29.92.

I do have one question for the poster. At what altitude are you doing this test?

Stan
Im in Bryan Tx. and it is roughly 365 ft above sea level here.

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Re: Cranking Pressure concern

Post by splitdecision71 » Thu Dec 06, 2018 2:26 pm

rebelrouser wrote:
Thu Dec 06, 2018 11:45 am
If you did not take the time to degree the cam, did you do the measurements to know it has over 14 to 1 compression? Did you CC the chambers? How did you arrive at that much compression? I personally never have seen the advertised compression of pistons correct. Its always less when I CC the chambers, measure the deck height, etc. and do the calculations. It has been an issue with my builds to get that much compression with out things hitting. I usually have to grind on the piston tops and the chambers when they are that close.
I plugged everything in to the wallace calculator and it comes up with 14.64 to 1. I did cc the heads and they are 64cc. Head gaskets are copper .032 and I measured them as well. I going to measure everything again this weekend and see what I come up with.

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Re: Cranking Pressure concern

Post by Ed Wright » Thu Dec 06, 2018 2:53 pm

I have encountered timing sets with the cam sprocket mis-drilled, off as far as 8 degrees. Never encountered a cam wrong. Retarded, of course, would certainly cause low cranking pressure.
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Re: Cranking Pressure concern

Post by MadBill » Thu Dec 06, 2018 3:39 pm

Cam King or ?? could provide chapter and verse, but I believe cam cores are supplied drilled and doweled, and occasionally with an unusual LCA or whatever, the grinder will offset the lobes from index to avoid breaking through the hardened surface, leaving it up to the buyer to correct the offset.
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Re: Cranking Pressure concern

Post by splitdecision71 » Thu Dec 06, 2018 3:43 pm

Ed Wright wrote:
Thu Dec 06, 2018 2:53 pm
I have encountered timing sets with the cam sprocket mis-drilled, off as far as 8 degrees. Never encountered a cam wrong. Retarded, of course, would certainly cause low cranking pressure.
Ok, thanks for your input, Im going to go through everything again this weekend. I borrowed another gauge to test it again to eliminate that.

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Re: Cranking Pressure concern

Post by splitdecision71 » Thu Dec 06, 2018 3:46 pm

MadBill wrote:
Thu Dec 06, 2018 3:39 pm
Cam King or ?? could provide chapter and verse, but I believe cam cores are supplied drilled and doweled, and occasionally with an unusual LCA or whatever, the grinder will offset the lobes from index to avoid breaking through the hardened surface, leaving it up to the buyer to correct the offset.
I guess that could be likely as Howard's ground this cam and sent it to me. Im going to make a call to Eric and see what he has to say. Thanks for your input.

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Re: Cranking Pressure concern

Post by plovett » Thu Dec 06, 2018 3:57 pm

You don't have to pull the motor to degree the cam.

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Re: Cranking Pressure concern

Post by splitdecision71 » Thu Dec 06, 2018 4:18 pm

plovett wrote:
Thu Dec 06, 2018 3:57 pm
You don't have to pull the motor to degree the cam.
Ok thank you. Im going to run out to my buddies tonight where I have the car and run a compression test with a different gauge. I will get back with everyone as soon as I have the results.

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