Hot Rods Hilborn injection question.

^^^^^
Typical references for alky ,
Yes a Data logger will keep You from the guessing , Tell what going on ,
Start to finish ,& or EGTs in each exhaust port.
There is a very Popular build thread here on the HAMB , That Needs a
Data Logger , His issues would have been less problematic.

The question was about Rich,
I was answering the question from Idle to WOT , witch what MFI is primary design for..
MFI ,, tec book & basic tunning
@ Idle , @ initial set up after setting spool valve, by revving & listening to for crisp revs then add 1-2 flats rich @ idle witch adds a little rich because lack of squirter shot needed off The Hit ,
Then for correct AFR @ WOT look @
Ring on bottom of spark plug for heat ring pattern/ stain on shiny plugs,
Each plug reading will tell you if up or down for jet on that particular cylinder.

 

My experience with stack injectors was with sprint car stuff.
I was building engines for one team that decided they wanted to get into the 410 class. I built a engine for them and asked about the injector and advised they would need a bigger one for this engine, they said they had a bigger injector for it. I asked about the pump they would be using and they said they were using the same pump they had been using. I told them to make sure the richen it up. I advised they make a hot lap shut it off and take a plug reading and adjust from there.
Well after the weekend I asked how it ran and they said it was a screamer for ten laps and let go. I said sounds like you didn't richen it up and they wanted to lay blame on a air lock in the cooling system because it was so rich that you couldn't stand to be around the car when at idle. All they did was adjust the barrel valve but never changed the pill in the return. I tried to explain that the car didn't race at idle.
When they brought the engine to me for tear down I found the spark plugs to show very lean, also 4 pistons torched and fire slotted both cylinder heads.
That was why I brought up the difference between idle and WOT.

 

Sorry to drudge up an old post, but it's pertinent.

Can you run too big of an injector throat on a motor? For instance I have 2 sets of Hilborns. One early that has dinky throats and then the latest set I acquired is like 2-7/16" if memory serves. It would be best suited for a 434 CID SBC, but what would happen if you run it on a 327? Can you have too much air? If you used the same nozzles and pill you always did, what would change? The CID would be the same, not like stepping up to a bigger motor.

This is a loaded question I know.

 

I LIKE this question! I hope you get a good response.

 

I was told yes there is too big! But I didn't listen. I ran my 242 inch 4 cylinder with 3 inch throttle blades on the street for two years. It has good throttle response and runs real well! Only bad thing is, it has terrible mileage, but I didn't care about that.

 

Here's another loaded question for you. Once you get past the point of no restriction in the intake tract, why would it matter? Wouldn't the port flow of the cylinder head be the determining factor and not the throat of the injector?

 

With the larger injector it will slow down the incoming air. Could have an effect on throttle response.

 

Thinking about it a little more, I am feeding 60 ci per cyl, you would be at 54.25 per cyl, with the 434 and 40.87 per cyl with the 327. Air speed has to have some play on it, plus stack length I would think.

 

It's down at the farm, but it has 4" stacks I think. The longer tubes are supposed to more for bottom end. Shorter for more rpms.

So we are talking 3" of plenum.The first set is like 1-13/16" and the second set is 2-7/16". So we have a difference a difference of 1.380" in diameter.

 

In a stack injector there really is no plenum, just runner length. Some injectors have the throttle plate up high and some are down low. I like them as high as I can get them. As you stated longer runners promote torque, short upper rpm. Mine is about 18 inches long from bell to the valve, and about 13 inches from the plates to the valve. It has lots of torque.

How did you get 3 inches of plenum, and 1.380 diameter difference? I get .625 difference between your two injectors. If you are going to try to run yours on the street, I think I would use the smaller one even though I like the bigger one. I have a 2 7/16 for my 327 but it doesn't run yet.

 

@Flatrod17
I used plenum because I couldn't think of a descriptive term for the casting.

You are absolutely correct. I mis figured. It's .625". Not sure how I came up with that.

It's not for the street. It will be for an all out drag car. I may need to motor up to be able to use it.

 

For drag racing I sure would use the bigger one!

 

What RPM will this engine see?

 

I think when it comes to the size of the injection tube or throttle blade the main consideration is how much air can your engine pump. It is like putting a bigger carb on an engine, the engine will only pull the air flow it needs. Too small will choke the flow, but too big will not starve the engine. It might reduce or kill any gains that are normally created by air flow velocity. But the A/F Ratio should still be what you valve it for. Smaller tubes with higher velocities generally give better throttle response and are considered better for drivability. There shouldn't be a horsepower advantage as long as you can provide enough air and fuel to feed the engine. Drag racing applications tend to go big tubes to ensure adequate flow at the top end.