This topic came up in my post in upgrades and improvements and it's something I would like to get to the bottom of.

Like I was saying my 511 Ford stroker motor runs good, idles good,but under hevy load it develops engine Knock.

Let me tell you a little more of the specifics.

Engine Block -Lincoln 460 bored and stroked to 511 c.i.

Heads-87 ford fuel injection machined and ported to cobra jet specs., and matched to intake.

Intake- Offenhauser Dual port

Carburetor- stock Motorcraft 650 cfm that came off the original 460

Distributor- stock motorcraft that came with the new motor.

Fuel pump- Electric Holly F.P. feeding original fuel Pressure regulator.


My question? could it be possible that if it wasn't getting enough fuel could that make it Knock?

Possible, but quite unlikely. I'd first rule out timing.

My brother helped me time it we didn't have a spec. so we timed it by ear,and engine vacuum.

Wile timing it, their was spot that it liked to run really smooth, but we couldn't crank it in that position, so we adjusted it as close to that spot wile still being able to crank it( turn motor over with starter).

As mentioned before, most engines have a spec for maximum total advance, which should not be exceeded. It varies from one engine to another. It seems to be tied mostly to compression ratio and peak rpm, but not always. In general, if we run more advanced than that spec we will almost surely have knocking problems, regardless of octane.

This would be a great place to start if you can find it. Being a highly modified engine, you may have to ask the builder or shop around the internet to find what works best.

In the stock 454, for instance, they often called for about 4 degrees advanced at idle, with about 20 degrees of mechanical advance and about 10-15 degrees of vacuum advance for a total of around 35 degrees. (7.9:1 compression)

Most of the higher compression engines I've seen run even more advanced but not always. My new 454HO calls for 4 degrees base timing, 26 degrees maximum total timing. (8.75:1 compression) I suspect this may be a compromise to allow lower octane fuel without knocking. I suspect with higher octane they would allow more advance with corresponding higher output, more like the good old days.

Historically we saw higher compression and more advanced base timing, i.e. 8-10-12 degrees BTDC. Engines run better and cooler at idle that way, but hydrocarbon emissions went up. This was why they went to more retarded base settings and a "ported" vacuum source to the vacuum advance in later engines.

I would connect the vacuum advance directly to manifold vacuum to get the best off-idle performance, not the ported vacuum source.

If you can get the recommended total advance, I would start with that. Then subtract the degrees of mechanical advance in the distributor and degrees of vacuum advance to see what's left over for the base idle setting. Mechanical advance and vacuum advance can be tweaked, both in total degrees and the rate of change. Given these many variables, there is much voodoo involved in getting everything right. (BTW, be careful which degrees we're talking. Usually we mean crankshaft degrees. Distributor specs are given in degrees as well but remember the distributor (and cam) run half crankshaft speed, so double the distributor degrees to get crankshaft degrees.)

Meanwhile, most of this won't matter if the compression is too high for the octane. If we know the timing is not too advanced and we still have knocking, we need better fuel.

Bottom line: If you can find some kind gear head who knows your engine combination, he can save you a lot of time by suggesting what has worked for him. No sense in reinventing the wheel!

This is one place where the fuel injection systems can be great. If you find the right guru he can just tweak the computer chip to change any or all of these variables without turning a wrench.

Good luck, keep us posted!

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I would connect the vacuum advance directly to manifold vacuum to get the best off-idle performance, not the ported vacuum source.

Ported increases vacuum as the throttle is opened (Bernoulli Effect); manifold does the opposite - vacuum is highest with the throttle closed.

quote:

Bottom line: If you can find some kind gear head who knows your engine combination, he can save you a lot of time by suggesting what has worked for him. No sense in reinventing the wheel!

Absolutely! Before you can successfully tweak the performance, you gotta have the correct specs. It sounds like you're very close from your own knowledge and experimenting, Because every engine is different, road-testing and fine tuning is the final stage.

quote:

Originally posted by Rusty:
Ported increases vacuum as the throttle is opened (Bernoulli Effect); manifold does the opposite - vacuum is highest with the throttle closed.

This is why I suggested using manifold vacuum… the ported source is a hole in the carb throat above the throttle plate, sees no vacuum until the throttle is opened. (see J in diagram below) With no vacuum advance at idle, the idle is more retarded, runs hotter and has poorer off-idle throttle response.

With manifold vacuum the idle timing is more advanced, runs better, cooler, and better throttle response.

Once the throttle is fully opened, port and manifold vacuum are the same, i.e., both are low.

Nobody used ported vacuum until the EPA guys started telling us how to tune our engines, in the final days of carburetors.

It also occurs to me that a larger carburetor would be worth contemplating; IIRC, there was a Holley 750-760 for the 460. That could take advantage of the aftermarket intake and headers.

Thanks rusty and Steve, well I finally made it home, what normally takes me 20 to 25 mins. took me 1 hr 45 mins to go 15 miles.The roads are treacherous out their.

Anyway getting Back to the motor,I just went through all the papers and notes I got with it.

I found a note that says Full 32 degrees recommended.

The cam has .490 intake and .526 exhaust.

The reason I asked about it not getting enough fuel is the motor came with a bigger carb.,I just have to have it configured with the elec. choke and change the linkage around.

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The reason I asked about it not getting enough fuel is the motor came with a bigger carb.,I just have to have it configured with the elec. choke and change the linkage around.

Properly adjusted, the mixture shouldn't change regardless of fuel flow. Fuel flow is regulated by the carb's air flow; less air, less fuel.

Set your timing to the builder's spec. of 32deg total or slightly less. Do this with a timing light and with the engine revved up high enough to have maximum advance. Timing by ear on modified engines is very risky. Once that is done any spark knock at full throttle will be from lean mixture or pre-ignition caused by something internal(not real likely in this case). Now as to the distributor it's self, If i remember correctly you have a GM-type HEI built for a ford, I have one on my 408ci 5.8 in a ranchero, most of these are made in china; who only knows what your advance curve is (both vacuum and weights&springs) the good news is that standard gm parts should fit. But changing the curve will just affect the drive-ability and not spark knock. And once again-- beg borrow or steal a wide-band a/f meter, these make a dumb tuner smart. BTW we are for now going to "assume" that your timing marks are correct.

Thanks John, That Chinese distributor is no longer in my Barth, I just didn't get to that part of my engine swapping story. There is in fact a new Motorcraft distributor in. There now.

Your carb is too small-in our spec built 460 Ford we computed the cfm from the displacement and the compression ratios to get a recommended 850 Holley CFM Double Pumper with main jets set for the Lincoln 460 engine. I don't have the book nearby but if you put the coach on a dyno and use the tailpipe sniffer for CO and O2 concentrations I will bet you are very lean off idle.

Thanks Tom, That's the way I was leaning.The motor came with a Larger carburetor. I just didn't have time to have it reconfigured to fit my application. I'm sure once I get that done and dial in the timing she'll do 80 up those grades.

I have a 454 in my1987 barth motor replaced about 60,000 k back was having trouble with holly (junk)proinjection . I put on a quick fuel supper street carburetor 780 cfm electric choke . had to adjust the idle .works great .

Update

After researching all the paper work I received with this motor and deciphering all the scribbled side notes, I have come to the conclusion that this motor was designed to run at a maximum 3200 RPM.

I also found that the distributor was calibrated to 22 degrees @ 3000 RPM

I found on the internet a Offenhauser recommended carburetor sizing chart. According to the chart even if I ran the motor up to 4000 RPM's the largest carb. would be a 608 CFM.

So it looks like the builder of this motor did his homework. I would bet the carb. that came with the motor is probably Properly jetted for this motor also.

After researching Fuel Injection upgrades and the coast involved ( over $3000 )and considering The RPM limitations, I,m thinking the wiser investment would be in a GearVenders O.D.

The knocking I experienced was more then likely due to Exceeding the engine,s Designed Maximum RPM.

Any thoughts on my assessment?