I run champion rc9yc in my high c small block .
10.2 : 1 compression 360 at sea level. Options?
- Thread starter SBM4theW!n
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MOPAROFFICIAL
Oogliboogli
When people shoot off with "I run bla bla bla plug in my small block"... immediately put them on ignore... jk but seriously, to hell with THEIR small block, this about YOUR small block, RIGHT??
Try one heat range cooler, in YOUR case IF you find you can't quite tune it out with a timing adjustment.
Good luck, I still think a little more coolant flow at low speed and 2-3 degrees timing taken out will be enough, if needed.
Right on MPoffical.
Yes, it can be to loud to hear the ping. IMO, you'll be OK on 93.
The larger cam bleeds off some compression making it easier on octane requirements and pinging issues. But not by much between the two cams.
Often that is what you will read and I say it a lot as well mostly because there is a bit of truth to it but, BUT, it is combination dependent. I say it because it is a dead safe area to do much with.
Tuning a carbs mid rpm lean spot can be a huge pain. The smaller carb is only giving up 10hp or so. It's not a huge thing.
nm9stheham
Well-Known Member
What actual AFR readings are you getting at this so-called 'lean spot' and at how much is the 'part-throttle' condition? 14-15 is fine at cruise and light throttle.
People get all hung up at keeping an AFR around 13 all the time and it is just a waste of fuel; at part throttle, the cylinder filling is only partial and the cylinder pressures are much lower so pinging and detonation is very hard to have occur unless the timing is waaaaaay wrongly advanced. FYI, the Chrysler Lean Burn system would run up into the 18 range for AFR.
The O2 sensor is a great tool; just don't let things like this fool you.
nm9stheham
Well-Known Member
The explanation from mderoy340 is right and your conception is right. His advice is to start with no vacuum advance and then work it in after the mechanical advance is all set; sounds like a good plan.
BTW, IMHO going beyond 15 degrees initial in this engine is not necessary; you have all sorts of low and mid RPM compression pressure to start with and peaking it up a bit more is not needed. And no need to go into the upper 30's on the mechanical advance. If you wanted to tweak and tune for best ET's, that is another matter, but no need to go high on timing for cruising around.
SBM4theW!n
Well-Known Member
The engine hiccups and the O2 gauge would spike. Didn't matter how hard or light I stepped on the peddle, happened off idle. Drove me crazy so I pulled the carb.
moper
Well-Known Member
Everybody's guessing until you know these figures. You might not have to do much. Or you might need to do more than tuning and spark plugs. A simple compression test will tell you where you're starting point is.
nm9stheham
Well-Known Member
OK, not just a lean spot, like the air or fuel jet was off but sounds like an accelerator pump or progression circuit issue.
SBM4theW!n
Well-Known Member
Understandable. I'll run out and borrow a compression test kit from one of the auto part stores around here this weekend and report back.
SBM4theW!n
Well-Known Member
Exactly. Rebuilt the carb, played with accelerator pump adjustment, and switched around some jets. Next step was bigger squirter diameter and possibly putting small holes in the butterflies to help with off idle transition. Strang thing was the accelerator pump was shooting plenty of fuel...
nm9stheham
Well-Known Member
Before you put holes in the butterflies, you should check the stop screw on the secondary side, and adjust that. YOur cam is not so radical that I would think you need more air. (I would 'guess' less with that size carb on this engine....but that is just a guess.) That stop being misadjusted (and exposing the incorrect amount of secondary transition slot) is a good spot to have this kind of progression/transition problem. If the idle mixture screws are not as responsive as they should be, then that is another clue to that being the issue.
And the accelerator pump cams can be changed to bring in more or less accel pump action, earlier or later.
And the accelerator pump cams can be changed to bring in more or less accel pump action, earlier or later.
MopaR&D
Nerd Member
Kind of a tangent but has anyone ever tried rigging up an OEM-style knock sensor to a simple dash light indicator of some sort? I thought about doing this with my '88 5th Ave because the Lean-Burn system actually had a knock sensor which the computer would use to tell when it could add extra timing advance or take it back out depending on conditions. After wrecking my 360 due to excessive pinging I'd definitely like a way to know for sure when knock is starting because an engine can knock lightly and still not produce an audible noise. I don't know how knock sensors work though, if this is even possible to do?
MopaR&D
Nerd Member
I found an online PDF of tuning instructions for my 750 Mighty Demon from the manufacturer, not exactly the same as a Holley but similar enough. In those instructions it pointed out that unless the transition-slot opening is set correctly (throttle plate position), the accelerator pump squirts fuel AS SOON AS the throttle is touched, AND the idle mixture is sufficiently rich, it WILL produce an off-idle bog or flat-spot. Tried it myself, leaned out the idle mixture not even that much (it still idled smooth) and all of a sudden it was hiccuping every time I touched the gas off idle. I gradually richened it back up and miraculously the stumble was completely gone and throttle response was instantaneous.
nm9stheham
Well-Known Member
Knock sensors are piezo-electric sensors that respond to vibrations in a certain narrow frequency range.
All you will get is a quick electrical pulse out of them, so you would need a small circuit to detect a pulse over a certain level, and then 'latch' it (in the electronics sense of the word) to light an indicator. It could also be set to not latch in a fault (knock) on one or a few knocks, to avoid nuisance indications. So, yes, doable for sure.
Well lookie here: Caspers Electronics Knock Gauges 102003
All you will get is a quick electrical pulse out of them, so you would need a small circuit to detect a pulse over a certain level, and then 'latch' it (in the electronics sense of the word) to light an indicator. It could also be set to not latch in a fault (knock) on one or a few knocks, to avoid nuisance indications. So, yes, doable for sure.
Well lookie here: Caspers Electronics Knock Gauges 102003
MopaR&D
Nerd Member
NICE, that's super cool! Definitely gonna have to do something like that for my next build. Much better than trying to tune optimal timing by ear (listening for knock) or paying money and a couple hours of time tuning on a dyno. With one of those and a wideband O2 AFR readout there would be no excuse not to get your car tuned perfectly.
nm9stheham
Well-Known Member
Naw, I can still come up with all sorts of excuses LOL!
But you're right: AFR gauges make life so easy..... just learn the foibles of them and what AFR to tune for in what throttle openings.
mderoy340
Well-Known Member
Agree. Most folks with AFR gauges lean the idle to 14:1 and carb motors need to be rich at low rpm high load (coming off idle). Fully warmed up with airgap manifold my motor needs 12.5 at idle in gear to transition properly. Don't use the gauge to set idle mixture, give the motor what it needs to run properly.
MopaR&D
Nerd Member
Dealt with that same situation and came to the same conclusion... The bigger plenum on Air-Gap manifolds only makes it worse too. Another note, engines with high idle vacuum (mild/stock engines) actually tend to need more accelerator pump shot volume due to the bigger drop in manifold vacuum when the throttle is cracked open compared to a "racier" engine with lower idle vacuum. I read that in David Vizard's Holley Carbs book, kind of blew my mind for a minute lol. Has to do with the fuel condensation on the inside of the intake manifold I think...
SBM4theW!n
Well-Known Member
So I did a compression test today. Sorry for the delay, the rental tool kit I got was shot so I ordered a new compression test kit online.
The drivers side came back at- 170, 175, 178, 175 and the passenger side was showing 190, 170, 197, 200.
Not sure why both sides are 15-20psi different as I CC'd the heads after having them milled and got them all pretty close.
I haven't run the car in a while, so I wonder if the lifters weren't pumped up or why this is reading so high. It was a cold crank test, and I probably ran the car a few weeks ago. With my comp cam magnum 280H intake closing shows 66 degrees at .0006 lift. Does 175 or 200psi sound right? How is this going to do on pump gas?
yellow rose
Overnight Sensation
You need to warm it up and then do a compression test.
nm9stheham
Well-Known Member
The 170 numbers sound about right. I suspect some of your lifters have leaked down and are making the intake valves close a lot sooner on those high compression number cylinders, and that is building up those high cranking compression numbers. So run first it as YR sez....
The good news is that is none of them are low.
The good news is that is none of them are low.
AJ/FormS
68 Formua-S fastback clone 367/A833/GVod/3.55s
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This is what Mr Wallace Says
Static compression ratio of 10.2:1.
Effective stroke is 2.74 inches.
Your dynamic compression ratio is 8.04:1 .
Your dynamic cranking pressure is 160.92 PSI.
V/P (Volume to Pressure Index) is 135................................................135
This is what your compression tester is saying
Static compression ratio of 12:1.
Effective stroke is 2.74 inches.
Your dynamic compression ratio is 9.42:1 .
Your dynamic cranking pressure is 197.35 PSI.
V/P (Volume to Pressure Index) is 166.............................................166
If after repeating the test with a warmed up,pumped up engine,and the numbers haven't changed much, you will need a second opinion
160 should be fine on pumpgas with that 650, and a solid cooling system.
200 is guaranteed trouble.......with iron heads.
To get a 10.2 with a 365 engine would require a swept of 748, and a total chamber volume of 81.3cc.
With your parts, say 65cc heads, an 8.9 gasket and 5 in the eyebrows, will require 2.4cc in the decks or; pistons down .011, so that sounds about right. I wouldn't call your gauge a liar,....yet
Some guys have reported success with 200psi,aluminum heads,tight Q, and pumpgas. You got the right cam, to make that a funtastic street engine. A VP of 166 is pretty awesome. With a manual trans VP becomes very important.Your 3.23 may be able to carry you all the way to 60mph
for comparison, here's a 69 440 with a 268/284/113/.450 cam, ICA of 67*
Static compression ratio of 10:1.
Effective stroke is 2.83 inches.
Your dynamic compression ratio is 7.79:1 .
Your dynamic cranking pressure is 154.44 PSI.
V/P (Volume to Pressure Index) is 154................................................ 154
As you can see 166 is way more than you need, but it sure is fun
Read about V/P here .
V/P Index Calculation
Static compression ratio of 10.2:1.
Effective stroke is 2.74 inches.
Your dynamic compression ratio is 8.04:1 .
Your dynamic cranking pressure is 160.92 PSI.
V/P (Volume to Pressure Index) is 135................................................135
This is what your compression tester is saying
Static compression ratio of 12:1.
Effective stroke is 2.74 inches.
Your dynamic compression ratio is 9.42:1 .
Your dynamic cranking pressure is 197.35 PSI.
V/P (Volume to Pressure Index) is 166.............................................166
If after repeating the test with a warmed up,pumped up engine,and the numbers haven't changed much, you will need a second opinion
160 should be fine on pumpgas with that 650, and a solid cooling system.
200 is guaranteed trouble.......with iron heads.
To get a 10.2 with a 365 engine would require a swept of 748, and a total chamber volume of 81.3cc.
With your parts, say 65cc heads, an 8.9 gasket and 5 in the eyebrows, will require 2.4cc in the decks or; pistons down .011, so that sounds about right. I wouldn't call your gauge a liar,....yet
Some guys have reported success with 200psi,aluminum heads,tight Q, and pumpgas. You got the right cam, to make that a funtastic street engine. A VP of 166 is pretty awesome. With a manual trans VP becomes very important.Your 3.23 may be able to carry you all the way to 60mph
for comparison, here's a 69 440 with a 268/284/113/.450 cam, ICA of 67*
Static compression ratio of 10:1.
Effective stroke is 2.83 inches.
Your dynamic compression ratio is 7.79:1 .
Your dynamic cranking pressure is 154.44 PSI.
V/P (Volume to Pressure Index) is 154................................................ 154
As you can see 166 is way more than you need, but it sure is fun
Read about V/P here .
V/P Index Calculation
Last edited:
SBM4theW!n
Well-Known Member
So I was able to get all the plugs back in and warmed it up before doing the cranking compression test again. This time everything came back pretty close to 155 PSI.
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