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Old 04-07-2018, 09:08 AM   #1
Husein-p
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inline 6 250 std rocker arms question

hi
i have a problem with some info about comp cams specs for l6 cam

factory std rocker arms have 1.5 to 1 ratio ok ? but for example if i choose 61-233-4 part# with this cam we have 0.28 for lobe lift but for valve lift we must to have 0.42 but in comp cams specs said 0.489 !!!

why ?

i checked cam specs for v8 sbc like 12-105-3 muscle 929H cam
comp cams said 0.26 for cam lift but for valve lift said 0.39
this is true because 1.5x0.26=0.39

but for l6 0.28x1.5=0.42 if 1.75x0.28=0.490

chevy l6 250 std have rocker arms with 1.5 ratio or 1.75 ?!!!!
or comp cams specs calculate with 1.75 ratio ?

tnx
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Old 04-07-2018, 10:11 AM   #2
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Re: inline 6 250 std rocker arms question

Inline six rockers are 1.75 ratio.

P. S.
Be sure to get a set of springs with your new cam. Not just because your new cam is about 0.100"
higher lift than stock. Valve float from old weak springs will kill performance.
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Last edited by GRX; 04-07-2018 at 10:21 AM.
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Old 04-07-2018, 10:29 AM   #3
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Re: inline 6 250 std rocker arms question

Quote:
Originally Posted by GRX View Post
Inline six rockers are 1.75 ratio.

P. S.
Be sure to get a set of springs with your new cam. Not just because your new cam is about 0.100"
higher lift than stock. Valve float from old weak springs will kill performance.
hah
so how much lift of factory cam ?

Last edited by Husein-p; 04-07-2018 at 12:02 PM.
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Old 04-07-2018, 03:00 PM   #4
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Here is an image for you.
Note the Clevite stock replacement specs. Factory cams were not exactly performance oriented. We put marine cams in them a lot back in the day.
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Old 04-08-2018, 02:48 AM   #5
Husein-p
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Re: inline 6 250 std rocker arms question

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Originally Posted by GRX View Post
Here is an image for you.
Note the Clevite stock replacement specs. Factory cams were not exactly performance oriented. We put marine cams in them a lot back in the day.
ok i got it

but i have a plan !
can i use v8 350 rocker arms on 6 250 ?

12 v8 sbc roker arms with 1.5 ratio can drop valve lift from 0.490 to 0.420

this work good for cam lobe (no need add zddp to oil) and no need to change valve springs so i can use std springs without zddp additive but valve lift lower than 1.75 ratio
this can effect on power and engine vaccum yeah ?
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Old 04-08-2018, 12:25 PM   #6
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350 rockers on a 250 ... no can do. Distance from fulcrum/stud to valve tip is greater. That is why the ratio is different.
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Old 04-08-2018, 01:25 PM   #7
Husein-p
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Re: inline 6 250 std rocker arms question

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350 rockers on a 250 ... no can do. Distance from fulcrum/stud to valve tip is greater. That is why the ratio is different.
alright
1.75 ratio is compulsory ratio not selectable from chevy engineer ?
but i see after market rockers for l6 250 in summitracing with 1.5 so what they are ??!!
in overall is my theory right about lower valve lift with lower ratio of rockers(my mean lower than 0.425 valve lift) and no need zddp or not ?

Last edited by Husein-p; 04-08-2018 at 01:33 PM.
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Old 04-09-2018, 06:59 AM   #8
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Re: inline 6 250 std rocker arms question

Summit's catalog was typed in in error. Small block chevy rockers are not compatible with the 250. The ratio is off and the distance from the stud to the valve is off. The ratio was driven by the block geometry and there are no factory alternatives.

If you replace the rocker studs with screw in big block studs you can run big block rockers, and get access to some different ratios. Almost all of these will be LARGER as this technique is usually used to increase lift at the valve. Also note that these will not not fit under the valve cover so you would need a custom cover.

Your theory of lower ratio means lower lift is correct, but that will not negate the need for proper break in lube. The cam to lifter is the critical problem area and that will be under high pressure no matter what. You have to have some valve spring to make the motor run. A better bet would be to use stock springs for break in then upgrade the springs if desired.
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Old 04-09-2018, 08:25 AM   #9
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Re: inline 6 250 std rocker arms question

Quote:

Your theory of lower ratio means lower lift is correct, but that will not negate the need for proper break in lube. The cam to lifter is the critical problem area and that will be under high pressure no matter what. You have to have some valve spring to make the motor run. A better bet would be to use stock springs for break in then upgrade the springs if desired
break in for 20 minutes of first runing need but my means is
if we have lower valve lift ratio than 0.425 then we need lower pressure springs so with low pressure springs and low lift no need to add zddp additive for every change of engine oil

anyway if i use recommended springs for 0.500 lift then if i use sj oil again need to add zddp or with sj grade oil no need ? in iran all grade availble
sc sf sg sj sl sm sn
someone tell me if you use sj oil in engine with high lift cam and high pressure springs no need to use zddp but if you go on sl sm sn oil grade then zddp is important to add to your engine oil is this true ?

in iran zddp additive unavailble for this reason i talked about this subject

overall we cant change 250 rocker arms for lower ratio no problem i got it
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