What is correct initial timing for 1957 250hp FI?

[Peter R.]

I can't believe how many different specs there are for the initial timing on the 1957 250 hp, hydraulic lifter equipped, fuel injected engine. Here are the specs I found and didn't find; what am I missing; which one is right; how do we know?


P. 45: On all fuel injection, standard camshaft equipped engines -- 12 deg BUDC @ 550 rpm


P.44: Fuel injection (hydraulic lifter camshaft engine) -- 8 deg BUDC @ 550 rpm
P. 91: 1957-58-59 Fuel injection with hydraulic lifters -- 4 deg BTDC; rpm not specified
P.251: 250h.p. 4 deg BTDC @ 500 rpm

1956-1960 Corvette Chassis Service Operations
P. 21: 1957-58-59 fuel injection with hydraulic lifters -- 4deg BTDC; rpm not specified

Servicing the 1957 Corvette Engines and R.P.O. Equipment
P. 21: 1957 Dual four-barrel and fuel injection with hydraulic valve lifters -- 12-14 deg BTDC; rpm not specified

ST-12 Corvette Servicing Guide1957 Chevrolet Passenger Car Shop Manual

[Mike M.]

Re: What is correct initial timing for 1957 250hp FI?

my 250 hp fi engine, with dukes camshaft: i set initial at 8 degrees btdc at idle(which in my case is about 600 rpm. total 38 degrees .mike

[Peter R.]

Re: What is correct initial timing for 1957 250hp FI?

Mike, What is a "dukes" camshaft? Thanks, Pete

[Mike M.]

Re: What is correct initial timing for 1957 250hp FI?

see Restorer mag vol 37 no 2, fall 2010, pages 6 thru 18. duke williams designed the camshaft for the 250 fi engine in the article. mike

[Duke W.]

Re: What is correct initial timing for 1957 250hp FI?

The OP is too new a member to have received that issue.

There have been many threads on timing and also papers posted on Web sites. A trip to the archives should reveal...

Big port heads seem to work best with 38 deg. total WOT timing, assuming there is no detonation on the best octane pump gas available.

The smaller 283 head chambers might work best on a couple of degrees less.

Total WOT advance = initial + total centrifugal.

You should be able to find a spec for total centrifugal, but how do you know that it hasn't been modified in the last 55 years. You don't!

So test it with a dial back timing light to see what the total centrifugal is - rev the engine with the VAC disconnected/plugged until advance maxes out, then use the above formula. Also find out where the centrifugal starts and set the intial below this speed.

If it doesn't detonate, you're good to go. If it does detonate, reduce the initial in two degree increments until the detonation goes away. Chances are you will be somewhere between 8 and 12 initial.

Duke

[Tim S.]

Re: What is correct initial timing for 1957 250hp FI?

Most of my SHP small blocks have really liked 38 total. That is even where the 422" SBC in my 67 Nova worked the best. Provided there is no part throttle ping, I would start there and work you way down ( as Duke suggested) to find the "happy place". A total of 38 in my unrestored 62 340hp really woke the car up and gave it better manners.

It seems to me everyone gets real focused on intial timing. I have much better luck starting with total and working down from there.

Best of luck!

Tim

[William C.]

Re: What is correct initial timing for 1957 250hp FI?

Actually, There were three different distributors used with that engine in different engine/transmission combinations, and different times during the model year. Answering the question without knowing the distributor involved, or which one was used as the reference point for the publication spec quoted is really a futile exercise, the 889 distributor is more aggressive in it's advance curve than the '905, and the '906 has a vacuum advance from what I can see in the old Delco literature, the non-vacuum distributors carry 13-15 (distributor) degrees of advance at full advance, and the 906 carrying 10.5-12.5 degrees at full mechanical advance, all X2 for crankshaft advance. As Tim notes, total advance at "full in" state is really where to start, generally targeted at 36 degrees (initial + Distributor adv) total for a smallblock at full song thru mufflers.

[Bruce B.]

Re: What is correct initial timing for 1957 250hp FI?

On my early 250 HP FI (4360 #136) car with a L79 350 HP 327 cam (hydraulic lifters) has the initial timing set at 10 degrees btdc. It has a 889 distributor but I don't have access to a distributor machine so I am not sure of total advance.

But, the engine runs great, starts reliabably both hot and cold and has gone 4000 miles over the past 2 years with no problems.

As I mentioned in another post, we have had 14 days of 90 degree plus weather and I have never had a starting of vapor lock problem driving in this crummy weather.

[Steven B.]

Re: What is correct initial timing for 1957 250hp FI?

On my early 250 HP FI (4360 #136) car with a L79 350 HP 327 cam (hydraulic lifters) has the initial timing set at 10 degrees btdc. It has a 889 distributor but I don't have access to a distributor machine so I am not sure of total advance.

But, the engine runs great, starts reliabably both hot and cold and has gone 4000 miles over the past 2 years with no problems.

As I mentioned in another post, we have had 14 days of 90 degree plus weather and I have never had a starting of vapor lock problem driving in this crummy weather.

Bruce, being close to The River I assume the humidity and temp are tracking closely. In the 80's there I remember a month of 90 degrees and 90 percent humidity.

How does the Hilborn car like the temps and humidity?

Steve

[Bruce B.]

Re: What is correct initial timing for 1957 250hp FI?

Steve,

You are right on with the So Ohio temp and unbelieveable humidity. It looks like we might drop out of the 90s next week, about time.

My 57 (POC per JD) 4360 has been running fine in this weather, contrary to what some of the experts predicted. I love it.

The Hilborn has no problems with the high heat. And much to my surprize it runs and starts fine, just like any modern FI engine. The program controls fuel while factoring in incoming air temp and coolant temp. Due to a poor vacuum signal (my 340 HP cam possibly) I am using throttle position as the main variable for fuel instead of MAP. I have another 327 which I will build this winter and swap it out with my original engine. Then hopefully I can use MAP for fueling calculations.

The whole system is interesting but frustrating at times, but I am learning. I put an 12 volt inverter in the car so I can plug in my laptop and don't have to worry about the batteries.

Who would of thought my 62 would be computer controlled....

I have attached a few pictures for your enjoyment.

Bruce B

[Peter R.]

Re: What is correct initial timing for 1957 250hp FI?

Okay, I've followed Duke's advice to use a dial-back timing light (never had one before but got one yesterday). Thank you--maybe we're onto something. (By the way, my distributor is a 905 (no vacuum advance).) Initial advance is set to around 4 degrees. I connected the new timing light and had an assistant rev up the engine until the centrifugal advance stopped increasing (2-3000 rpm). Total advance was only about 18 degrees (4 initial + 14 centrifugal). If we're trying to get total advance up to around 36 degrees, aren't I going to need about 22 degrees initial? Isn't that awfully high?

Aside from the above, would a total advance of only 18 degrees account for an engine that starts fine, idles fine, but seems underpowered, and backfires into the intake manifold on moderate acceleration?

Thanks for all the help.

Pete

[Alexander C.]

Re: What is correct initial timing for 1957 250hp FI?

I have an 879 mechanical advance dizzy like yours and from what you are saying (I'm by no means an expert) I would say you need to dis-assemble your advance weights and springs which are inside the distributor under the points plate. Most likely they just need a good careful cleaning and lube. I have been told the springs never really wear out so you shouldn't need any parts. I would also check your dwell.

[Duke W.]

Re: What is correct initial timing for 1957 250hp FI?

Okay, I've followed Duke's advice to use a dial-back timing light (never had one before but got one yesterday). Thank you--maybe we're onto something. (By the way, my distributor is a 905 (no vacuum advance).) Initial advance is set to around 4 degrees. I connected the new timing light and had an assistant rev up the engine until the centrifugal advance stopped increasing (2-3000 rpm). Total advance was only about 18 degrees (4 initial + 14 centrifugal). If we're trying to get total advance up to around 36 degrees, aren't I going to need about 22 degrees initial? Isn't that awfully high?

Aside from the above, would a total advance of only 18 degrees account for an engine that starts fine, idles fine, but seems underpowered, and backfires into the intake manifold on moderate acceleration?

Thanks for all the help.

Pete

What are the centrifugal specs for this distributor, and how does it compare to what you measured?

Previous advice is good. Your distributor has probably never been disassembled for cleaning and inspection, which means after 50 years maybe it's time for a little TLC.

Backfiring could mean improper distributor and wire indexing. Also, how old are the plug wires and are they within the accepatable resistance range. Service manuals should provide information on proper distributor installation and wire indexing.

It sounds like you need to go completely through the ignition system, check everthing, and replace or repair everything that is out of order.

Duke

[William C.]

Re: What is correct initial timing for 1957 250hp FI?

Delco specs for '879 Distributor (crank degrees and engine rpm) are 1-5 @ 800, 12-16 @ 1500 and 19-23 degrees @ 2600 with a total of 26-30 degrees at 3700 "all in". Should work well with about 5-10 degrees of initial dialed in.

[Peter R.]

Re: What is correct initial timing for 1957 250hp FI?

I've looked at distributor specs in books I have (ST-12 and Clymer's) and they don't list the 905 distributor, even though it appears to be the standard FI distributor in '57. They do list a lot of different distributors, a few of which are without vacuum and the centrifugal advance specs are all different. Anyone have specs for the 905? The 879 specs do show that there is the potential for a lot of centrifugal advance, but don't know if the 905 is similar. I will pull the distributor to check the centrifugal advance mechanism and report back on what I find.