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Looking for input from folks who either have seen or recorded the manifold pressure at WOT. I recently had the car on a dyno and also hooked up a MAP sensor to me LM-1 data logger.
I noticed during a dyno run at WOT my manifold vacuum was about 3 to 4"hg of vacuum. I know when people talk about WOT it is always said one should be near 0" of vacuum. Maybe in a perfect world but I wonder what is typical with these engines. My vacuum climbed to near 5" on the 6500 rpm run. The car was originally tested with a 950 cfm carb. I swapped to a 830 body with annular boosters. The cars street manners have improved and feels more responsive down low. I am thinking about running the car with the 950 carb to see if there is a meaning difference in the numbers.
For peak power, we shoot for about .5" Hg at WOT. More vacuum than that, such as your 3-4", indicates restriction on the inlet side, such as an undersized carb or restrictions in inlet airflow. Here is an example of a pretty good dyno test on an engine I ran recently. Note the "Man Vac" column:
For peak power, we shoot for about .5" Hg at WOT. More vacuum than that, such as your 3-4", indicates restriction on the inlet side, such as an undersized carb or restrictions in inlet airflow.
Lars
Thanks Lars!! I will run it next week with the 950 and a freshly cleaned K&N filter and see what she does.
Adding a bit of relevant math to the discussion, IIRC, the "CFM" rating of a carb is at a vacuum level in the manifold of 1.5" Hg for 4 barrel carbs, and 3" Hg for 2 barrel carbs.
And somewhere in the back of my mind, theres a memory (perhaps inaccurate) that a 350CID engine draws approximately RPM/10 CFM at 100% volumetric efficiency.
From what I've read the annular boosters can be restrictive to airflow, don't know if that's true or not. Kinda makes sense. Very interested in your results.
From what I've read the annular boosters can be restrictive to airflow, don't know if that's true or not. Kinda makes sense. Very interested in your results.
That is true but I have seen dyno comparisons between annular versus non annular carbs. The annular actually made more power with less air flow because of better fuel atomization. That said I am sure that and the smaller base plate bores both contributed to the issue. Most that were involved with the build including Tony Mamo formerly from AFR agree that a 1000 cfm carb would be most appropriate for this build. The engine builder preferred the more steerable size of 950.
I just pasted in a dyno printout in my post above of a run we did recently. Note the manifold pressure column.
With an air cleaner in place you might not get into the .5" range. We run our dyno tests with a bell-mouth inlet and no air cleaner.
Lars
Thanks for the data. I am surprised to see how low. I know I am probably splitting hairs here but I have an opportunity to learn something and share with others regarding these large displacement small blocks. I am also running 34 degree advance and would like to try 36 in a separate run.
If you have good heads, you will see power drop at 36 degrees. Good aftermarket heads on a big-cube small block will produce best power at 32 - 34 degrees total. (My experience testing 420 - 427 small blocks with aftermarket aluminum and iron heads)
If you have good heads, you will see power drop at 36 degrees. Good aftermarket heads on a big-cube small block will produce best power at 32 - 34 degrees total. (My experience testing 420 - 427 small blocks with aftermarket aluminum and iron heads)
Lars
Lars, should the idle feed restrictor ports be the the same on the secondary metering blocks? I reduced the size on the primaries per you instructions and was curious about the secondaries. Should I make them match the primaries?
Thanks!
This is just an alternative thought to increasing inches of vacuum as rpm's rise. You stated up to 5 @6500.
annular boosters are a restriction to flow compared to a down leg booster. The restriction rises with air flow (rpm ) on a flow bench it can record say 18 less cfm at lower flow and higher than 60 cfm on the top end
a cheaper alternative might be down leg boosters right now I'm running down leg on my 830 demon. I drove around with a big vacuum gauge duct taped to the windshield. I'm pro annular boosters for every day driving. How about 950 or 1000 cfm with the annulars
Annular vs. Downleg has zero to do with vacuum at WOT if the carbs for both types are properly sized for the application.
Generally I have watched most annular stuff go pig fat past the power peak because vacuum will return......but this is going off track.
.5 is the number......WOT.
Annular vs. Downleg has zero to do with vacuum at WOT if the carbs for both types are properly sized for the application.
Generally I have watched most annular stuff go pig fat past the power peak because vacuum will return......but this is going off track.
.5 is the number......WOT.
Jebby
If you have one 830 carb with down leg boosters and you install annular booster the vacuum will rise
As I had stated....if properly sized for the application....apples to apples.....
Jebby
so the interesting part is there really was little difference between the WOT performance of the 830 with annular and the 950 with drop leg. My AFR was a tad richer with the 950. The 830 does better on the street. Very consistent AF ratio during all driving conditions. I am having only one problem with it and that is a lean spike right as I tip into the secondaries. My primary power valve is 6.5 but my secondary is 5.5. Thinking maybe if I can get the power valve in a tad sooner may help. My primary butterflies are closed when on the primarmaries as well. Perhaps cracking them open so I just onto the transfer slot may help some as well.
I would love love to try a 1000 cfm dominator for the heck of it. Likely much to big for street use.
I would love love to try a 1000 cfm dominator for the heck of it. Likely much to big for street use.
You have a 700 hp small block........a 1000 cfm is not too big for it. A dominator can be tuned for the street, but a 1000 cfm 4150 carb would work very well. I have a ProSystems 1000 on mine and it works very well. Correct tuning is all you need.
03-11-2018, 02:11 PM
Manifold Pressure at WOT?
...Agreed. No power valve on secondaries.
