Ford 351/427W timing...

[aarvig]

I've got a DART based Ford 351w that has been bored out to a 427w and is running AFR heads and a QuickFuel 750CFM mechanical secondary. Ignition is pertronix electronic distributor. I have had all sorts of running issues with this motor. Bottom line is that initial setup for timing was 9 degrees with no limiter straps on the distributor for total timing. It was running so rich and dieseling all the time. So, I adjusted the 4 corner idle screws, had the carb rebuilt, changed the plugs, set the timing to 14 initial with 8 degree limiter straps installed on the distributor for a total of 30 degrees timing. Helped, but still so rich that I had to adjust the 4 corner idle screws down to 1/2 turn out. This helped immensely but I still had an engine that would diesel 80% of the time.
Gave up and took it to a pro...
He hooked it up on an air/fuel ratio, got the carb dialed in by jetting down 4 sizes from where the builder had them set, and adjusting timing to 18 initial and 36 total. The pro is telling me its running like really good now but that my 750 carb is probably to big for this engine. Not ideal is what he said.
So, questions...is the 750 to big for this engine?
My builder doesn't like 36 total timing for this engine but the pro who did the set up does. Builder claims it will cause detonation. I'm lost on this...any advice?

[Hellion]

IMHO the 750 is not too big, or the 36 total timing for your setup. In an old '65 Mustang Fastback I had a while back I ran a Ford Racing 5.0 crate engine (390 HP/360 Torque which is no longer offered) mated to a T-5 that loved the 750 and tons of timing.

[OVCobra]

I would agree with Helion...the 750 is NOT too big for a 427. I used to use a 780 on a little 306 with high compression and rev limit.
In fact depending on the specific model of head (165/185/195/205/220 cfm), intake, cam and rpm limit, the 750 may be too small.
You don't mention the compression ratio but unless it is rather high (> 11:1?) I doubt 36 degrees of timing is going to cause detonation. Again depending on compression ratio and cam specs, it will dictate how much timing it wants/needs.

[Jim1855]

aarvig,
Mine is a 427w similar to yours. Heads matter, I'm running AFR 225s. Compression matters, mine is low, under 10:1.
750 mechanical DP, no choke from ProSystems. I've run a 950, that was too big but am considering a Holley 850 Ultra XP. I've also run a 670 vac Street Avenger too small but ran well at the lower RPM range, then a 780 vac Street Avenger this ran well but didn't pull like the 750 DP.
Timing is somewhere around 36/38 full in and 16/18 initial. Sorry don't remember the exact specs.
I'd stay with the 750 and your last indicated timing specs. Tune on the dyno, then drive it on the street or wherever you intend to run it and make the slight adjustments for drivability.
Hard to argue with the engine builder BUT the pro tuner is the guy seeing it run in the real world.
Jim

[bobl]

Combustion chamber design really dictates the amount of total advance needed to perform properly. I've built quite a few SBF engines with AFR heads and they have always made best power right at 36 degrees. When I dyno an engine I will keep advancing timing until it quits gaining power, then back it down a couple of degrees. Again, AFR heads work well at 36 degrees. FYI, I dyno'd a 347 with Trick Flow heads and it didn't want beyond 30 degrees total advance. If the compression is too high to run 36 degrees with AFR heads you need better fuel. IMHO. Sounds like your tuner knows what he is doing.

Bob

[aarvig]

I would agree with Helion...the 750 is NOT too big for a 427. I used to use a 780 on a little 306 with high compression and rev limit.
In fact depending on the specific model of head (165/185/195/205/220 cfm), intake, cam and rpm limit, the 750 may be too small.
You don't mention the compression ratio but unless it is rather high (> 11:1?) I doubt 36 degrees of timing is going to cause detonation. Again depending on compression ratio and cam specs, it will dictate how much timing it wants/needs.

It is AFR 205 renegade heads and the compression is 10.2:1…

[J R Jones]

The original Shelby GT350 ran a 715 CFM on 289ci and I raced it that way in SCCA. The 750 does not seem excessive.
I ran 36 to 38 degrees but always on race fuel. If you are not racing, it seems like edging your bet is not unreasonable.
I ran CI heads with 1.67 and 2.02 valves, the Shelby configuration. Gurney heads were similar.
jim

[CraigS]

I had a mild 408W and never ran that much advance. But I was never on a dyno either so I kept it on the safe side because you usually can't hear ping/detonation w/ the side pipes. All my timing adjusting was seat of the pants and I ended up at 16 initial and 32 max. With the spread between initial and max that you now have it would be easy to back out 2 deg or 4 deg and test drive to see how it feels. You can always go back to the tuner's recommendation. I ran a 650 w/ the 351 and on the 408 for the first 6 months or so. I called Quickfuel to ask about making it into a 750 and he said that all I needed was a 750 main carb body so I did that. It was an immediate and obvious bump in mid range and above torque so I agree your 750 is the right size.

[frankb]

The attached website may help (or not!) to clear the air on carb sizing. I know back in the dark ages (when I was just a tad younger) we used the formula in that article, but learned the hard way that actual carb flow is usually less than rated flow in the real world.

https://performanceimprovements.com/blogs/news/carb-size-matters-what-size-carburetor-do-i-need

[aarvig]

UPDATE:
So I picked up the car yesterday and drove it home. Tuner was telling me that he got the 4 gas numbers all dialed in properly but now I noticed a hesitation under slight acceleration. Particularly when I let the clutch out after a gear shift and put a little gas pedal into it-it will miss or hesitate. The power feels weak as well. Thoughts?

[Jim1855]

The hesitation can be cleaned up. Your tuner needs to drive it on the street or at least ride with you to experience the hesitation. Air bleeds will often correct this but its usually a slight change/test procedure. Better to tune this on the street or your normal driving conditions than the dyno as the street and dyno are not exactly the same conditions.
Jim

[J R Jones]

aarvig, You have not mentioned maximum RPM which is the challenge for carb flow capacity.
Your hesitation sounds like fuel, accelerator pump? The timing advance curve could also influence this (too little).
The tuner's comment on size may be in reference to flow velocity and signaling the carb circuits. Smaller carb size creates higher velocity and stronger circuit signals.
Based on your "slight acceleration" situation, if this is carb related, it is the primaries. If the light throttle A/F ratio is OK, the accelerator pump, or a progressive circuit are suspect.
The builder and tuner are hands-on here, not speculating long distance.
jim

[aarvig]

aarvig, You have not mentioned maximum RPM which is the challenge for carb flow capacity.
Your hesitation sounds like fuel, accelerator pump? The timing advance curve could also influence this (too little).
The tuner's comment on size may be in reference to flow velocity and signaling the carb circuits. Smaller carb size creates higher velocity and stronger circuit signals.
Based on your "slight acceleration" situation, if this is carb related, it is the primaries. If the light throttle A/F ratio is OK, the accelerator pump, or a progressive circuit are suspect.
The builder and tuner are hands-on here, not speculating long distance.
jim

Maximum RPM is 6500