For those that read the post about the "Towing HotRod..did you note... - Chevelle Tech
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post #1 of 12 (permalink) Old Jan 20th, 20, 12:59 PM Thread Starter
 
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For those that read the post about the "Towing HotRod..did you note...

The cylinder pressure difference the "test" that go performed with the failing lobe.....No. 1 220 PSI, all other 7 170 PSI. Pump gas of 91 min was used for the entire trip. Truck has a Turbo 400 with 2 overdrive units on it. No sign of detonation on Number 1 or the other 7.

Chris Straub
Mfg Performance Parts

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Why is it we never have time or money to do it the right way in the beginning but we always have time and money to do it over again?
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post #2 of 12 (permalink) Old Jan 20th, 20, 10:51 PM
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jeff
 
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Re: For those that read the post about the "Towing HotRod..did you not

I did see that.. probably a conservative tune for the towing to be on the safe side.
I pulled 2 degrees from my 350 when i stuck my pontoon behind it . On a wheel dyno 3 degrees less was only 3 Hp difference.

Attention to detail when building really helps also. I do not care for sharp edges or stick ups in the combustion space and I like a rough intake runner with a good shape to promote good fuel atomization and you can get by with a lot of compression on pump fuel.

Another thought is the cranking pressure was much higher but due to the loss of lift that cylinder did not get filled as much as the others.

I have thought about that aspect with high flowing heads say 260-270 cfm @ .500" lift vs some heads that peak at 225 cfm at the same lift.

Would the cylinder fill more with the higher flowing head. would 9.5 compression on the high flow head want 35 total and 91 octane when the other low flowing head could tolerate 10.8 or more compression.

I have often thought about why I can get by with 11:1 with old school ported sbc heads and pump gas.
All the little massages or is it my intake runner just will not fill the cylinder like a modern head will.
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post #3 of 12 (permalink) Old Jan 21st, 20, 10:19 AM
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Scott Foxwell
 
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Re: For those that read the post about the "Towing HotRod..did you not

Quote:
Originally Posted by jeff swisher View Post
I did see that.. probably a conservative tune for the towing to be on the safe side.
I pulled 2 degrees from my 350 when i stuck my pontoon behind it . On a wheel dyno 3 degrees less was only 3 Hp difference.

Attention to detail when building really helps also. I do not care for sharp edges or stick ups in the combustion space and I like a rough intake runner with a good shape to promote good fuel atomization and you can get by with a lot of compression on pump fuel.

Another thought is the cranking pressure was much higher but due to the loss of lift that cylinder did not get filled as much as the others.

I have thought about that aspect with high flowing heads say 260-270 cfm @ .500" lift vs some heads that peak at 225 cfm at the same lift.

Would the cylinder fill more with the higher flowing head. would 9.5 compression on the high flow head want 35 total and 91 octane when the other low flowing head could tolerate 10.8 or more compression.

I have often thought about why I can get by with 11:1 with old school ported sbc heads and pump gas.
All the little massages or is it my intake runner just will not fill the cylinder like a modern head will.
The cylinder pressure on the "bad" cyl was over 220psi cranking pressure. This was from the lobe being wiped out and the intake valve closing WAY late. If someone was to do the math on the "dynamic compression" it would probably be somewhere around 11:1 or more. For all those who worry about "DCR" and how much is "too much" for pump gas, one would have to ask; why didn't that cylinder detonate itself to death with that much cylinder pressure?
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post #4 of 12 (permalink) Old Jan 21st, 20, 10:42 AM
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Re: For those that read the post about the "Towing HotRod..did you not

Quote:
Originally Posted by steelcomp View Post
The cylinder pressure on the "bad" cyl was over 220psi cranking pressure. This was from the lobe being wiped out and the intake valve closing WAY late. If someone was to do the math on the "dynamic compression" it would probably be somewhere around 11:1 or more. For all those who worry about "DCR" and how much is "too much" for pump gas, one would have to ask; why didn't that cylinder detonate itself to death with that much cylinder pressure?
Scott, wouldn't the intake valve have to close early to raise the cranking pressure? Also, why was just one lobe affected? If the core wasn't up to the task, wouldn't more lobes show wear? The other ones in the pic look pretty good still.
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post #5 of 12 (permalink) Old Jan 21st, 20, 12:47 PM
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Re: For those that read the post about the "Towing HotRod..did you not

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Originally Posted by nitrofox View Post
Scott, wouldn't the intake valve have to close early to raise the cranking pressure? Also, why was just one lobe affected? If the core wasn't up to the task, wouldn't more lobes show wear? The other ones in the pic look pretty good still.
Yes...the valve was closing early, not late. Wrote that in a bit of a hurry. Thanks for the correction (too late to edit). Why one lobe was effected... good question. Could have been a million things. That particular valve spring spring was most likely the culprit.

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post #6 of 12 (permalink) Old Jan 21st, 20, 1:23 PM Thread Starter
 
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Re: For those that read the post about the "Towing HotRod..did you not

No 1 chamber in excellent shape and shows no sign of detonation.

Chris Straub
Mfg Performance Parts

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post #7 of 12 (permalink) Old Jan 21st, 20, 4:55 PM
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Re: For those that read the post about the "Towing HotRod..did you not

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Originally Posted by cstraub View Post
No 1 chamber in excellent shape and shows no sign of detonation.
Doesn’t surprise me. With a wiped lobe the VE of that cylinder was probably really low at cruise rpm and above.
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post #8 of 12 (permalink) Old Jan 21st, 20, 6:55 PM
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Re: For those that read the post about the "Towing HotRod..did you not

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Originally Posted by Roadknee View Post
Doesn’t surprise me. With a wiped lobe the VE of that cylinder was probably really low at cruise rpm and above.
EXACTLY ^^^
With almost nothing to burn it wouldn't detonate. That cylinder was making no power when it fired.
The cranking pressure on that hole was higher because the intake valve was closed for most of its duration. There was no bleed off from the exhaust valve overlap.
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post #9 of 12 (permalink) Old Jan 22nd, 20, 3:36 PM
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Re: For those that read the post about the "Towing HotRod..did you not

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Originally Posted by bradley67 View Post
EXACTLY ^^^

With almost nothing to burn it wouldn't detonate. That cylinder was making no power when it fired.

The cranking pressure on that hole was higher because the intake valve was closed for most of its duration. There was no bleed off from the exhaust valve overlap.
Cranking pressure is reduced because of a late closing intake valve, and has nothing to do with exhaust overlap

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post #10 of 12 (permalink) Old Jan 22nd, 20, 4:07 PM
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Scott Foxwell
 
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Re: For those that read the post about the "Towing HotRod..did you not

Quote:
Originally Posted by bradley67 View Post
EXACTLY ^^^
With almost nothing to burn it wouldn't detonate. That cylinder was making no power when it fired.
The cranking pressure on that hole was higher because the intake valve was closed for most of its duration. There was no bleed off from the exhaust valve overlap.
It didn't suddenly get that way, to that extreme. This happened slowly over time. There was a period of time where the valve timing on that lobe was dramatically reduced but still plenty of lift to fill the cylinder.
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post #11 of 12 (permalink) Old Jan 22nd, 20, 4:09 PM
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Scott Foxwell
 
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Re: For those that read the post about the "Towing HotRod..did you not

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Originally Posted by 70GS455 View Post
Cranking pressure is reduced because of a late closing intake valve, and has nothing to do with exhaust overlap

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Cranking pressure is increased because of an early closing intake valve. From the second that lobe started wearing, cylinder pressure was increasing over what it was from the start.
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post #12 of 12 (permalink) Old Jan 23rd, 20, 10:23 AM
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Re: For those that read the post about the "Towing HotRod..did you not

Interesting, a "reverse" reading from what we're used to.

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