Midrange and low RPM grunt. Discuss please.
11-09-2014, 09:51 PM
#71
Maybe Bob M will chime in again. I tied Bobs hands on that engine. I told him I wanted a long lasting, low maintenance engine, basically I was looking for stock reliability. I further told him that to avoid drive failures not to shoot for over 700 HP or 5400 RPM and you can see he delivered perfectly. Also Bob was leaning toward the Victor / Dom combination as being potentially a better combo with this engine but the actual builder was more comfortable with the Vic Jr / 4150 combo and I ended up going that route. It's a Pro Systems carb flowing about 1000 CFM with their modified metering blocks etc. The claim was that it had Dominator flow with 4150 driveability.
That pull is with the addition of a 2 inch spacer which helped a lot across the board. We tried open and 4 hole but it made no difference. Apparently it needed more plenum volume than the Vic jr had.
I'm not happy with that Dyno session/ operator. I've been told by someone else that saw the sheet that the AFR is dangerously low for a boat and shouldn't be run like that. Any thoughts?
That pull is with the addition of a 2 inch spacer which helped a lot across the board. We tried open and 4 hole but it made no difference. Apparently it needed more plenum volume than the Vic jr had.
I'm not happy with that Dyno session/ operator. I've been told by someone else that saw the sheet that the AFR is dangerously low for a boat and shouldn't be run like that. Any thoughts?
11-09-2014, 09:53 PM
#72
Fwiw, my gas GM dually's engine, when under high load goes into power enrichment mode. The AFR drops from 14.7 to around 12.6. Gm programmed it that way, so when your pulling a trailer up a mountain with your foot to the floor, you wont be melting things. This is a port injected engine where cylinder to cylinder fuel distribution is much better than a single plane carb deal. I personally would not want that carbed 540 with no knock protection, poor fuel distribution, no timing retard capability, to be tuned to the 13's at full throttle . Just my opinion
11-09-2014, 10:05 PM
#74
Registered
I think no two engines are the same and the AFR should be treated as such. I think when tuning don't stop testing if you reach AFRs that theoretically look good, "feed the engine till it's full" in other words give it more fuel till your #s begin to fall off at that point back off just a bit. In my past experience once the engine is installed in the boat and propped right it will most likely need the extra fuel!!
11-09-2014, 10:14 PM
#75
Registered
However, you get to a point where the restriction will become more and thus more pumping losses also.
The 650Hp is my rule of thumb where a motor can take advantage of a Dom for some more power. If it doesn't, chances are the fuel metering of the carb is the issue, not the airflow.
The 650Hp is my rule of thumb where a motor can take advantage of a Dom for some more power. If it doesn't, chances are the fuel metering of the carb is the issue, not the airflow.
My favorite way to test a carb's worthiness, is on a dyno, and look at if and how the engine responds to the change, in its output. A 750 BG, 750 Holley DP, and a 750 Ultra HP, may all make a different power number. A 1050 4150 style, up against a 1050 4500 style, may again have different results.
Sometimes, just because you have an engine that may only "require" 750 CFM, doesn't mean a 750CFM carb is gonna work best, or that anything larger would be a waste. Heck, there were millions of 70's era anemic 200HP 455ci engines, and 454ci chevys, running around with 800cfm quadrajets. Did those engines NEED 800cfm worth of airflow, heck no. Did the carb's work well, provide excellent driveability, and fuel economy, they sure did. The beauty back then, was that carb would work well on Mom's station wagon, or dad's 11 second Chevelle. It was the design of the carb, not just the CFM rating.
Sometimes, just because you have an engine that may only "require" 750 CFM, doesn't mean a 750CFM carb is gonna work best, or that anything larger would be a waste. Heck, there were millions of 70's era anemic 200HP 455ci engines, and 454ci chevys, running around with 800cfm quadrajets. Did those engines NEED 800cfm worth of airflow, heck no. Did the carb's work well, provide excellent driveability, and fuel economy, they sure did. The beauty back then, was that carb would work well on Mom's station wagon, or dad's 11 second Chevelle. It was the design of the carb, not just the CFM rating.
In the end it really doesn't matter so long as they fuel requirements are met. I got tired of messing with them and that's why I focus so much on EFI. You know where your two options are a 1000CFM or 2000CFM throttle body. I can have two motors completely tuned in the time that it takes to tune a carb motor. And I can lock the ECM so people can't fool with it like anyone with a buddy, case of beer and some extra jets can a carb.
Last edited by Precision; 11-09-2014 at 10:34 PM. Reason: Fix mistakes from typing one handed holding an infant.
11-10-2014, 09:48 PM
#76
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iTrader: (2)
Damn I gave up looking for those quadrajets with the big primary ventturies .gonna have to get after it again those 850 cfm vac secondary holleys aka Volvo penta 502 and gm h.o. motors are getting hard to find for the common working man . The gen 4 454 quadrajet is the same cfm as the 260 horse 350.and 95% of the old school big block car factory set ups ,won't instantly hit 3000+rpm going wot from idle in gear (with perfect traction anyways lol ) anybody have any 850 double pumpers for sale they can't stand looking at let me know ill make room on the shelf ..
11-10-2014, 09:51 PM
#77
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iTrader: (1)
I can't believe we are talking quadrajet's here. There is reasons GM could use the 750cfm on low hp 4.3's also.
1st, not only are the primaries small, but they have a super sensitive booster also. Doesn't take much airflow to get those boosters to flow fuel.
2nd, the secondary throttle paltes are mechanical, but so what. They have an air door over them which can be manipulated to to provide the proper air flow from a lowly 4.3, to a pretty aggressive big block.
And there lies the secret to run a 'bigger' Holley than one would thing on any engine. Get a booster vs venturi/throttle plate size that will trigger the fuel (fine tuned by jet/air bleed/channel restrictions/ etc/etc) in the proper amt thru the rpm and throttle angle range.
There is a reason for different annular boosters / down leg boosters/ and etc, etc.
A carb may not be too big cfm wise - it may just have the wrong fuel metering for your application. Food for thought.
1st, not only are the primaries small, but they have a super sensitive booster also. Doesn't take much airflow to get those boosters to flow fuel.
2nd, the secondary throttle paltes are mechanical, but so what. They have an air door over them which can be manipulated to to provide the proper air flow from a lowly 4.3, to a pretty aggressive big block.
And there lies the secret to run a 'bigger' Holley than one would thing on any engine. Get a booster vs venturi/throttle plate size that will trigger the fuel (fine tuned by jet/air bleed/channel restrictions/ etc/etc) in the proper amt thru the rpm and throttle angle range.
There is a reason for different annular boosters / down leg boosters/ and etc, etc.
A carb may not be too big cfm wise - it may just have the wrong fuel metering for your application. Food for thought.
11-10-2014, 10:18 PM
#78
Registered
iTrader: (2)
The peps that have a 21' open bow filled right to rated capacity and a 3 blade prop and your average hucklberry at the controls the quadrajet (or weber ) got it on plane .merc at least gave the guys with the 525sc a 1050 dominator .yes sir I have had best results ,rpm ,wot throttle response on most 454s -502s with the 850
