512 Stroker Ignition & Fuel Curve

[73RRBob]

Hello, I'm new to this site and hope you guys don't mind but I have a 73RR. Here are a few pics.

Sorry guys but this really sucks because I spent 20 minutes writing this post and then somehow I hit a key and it took me to my profile and I lost my post. So here goes again.

I have a 73 RR, 3800 lbs w/o driver. 4 speed 23 spline, with 8 3/4 diff 489.
512 stroker, engine dynoed at over 600 torque from 3000 - 6000 rpm. its a hoot to drive, this car is mainly a street driven car and it does have good street manners and sometime I hope to take it to the track to see what she can do. But I'm a little nervous because the MT 325/50/15's I have on the rear will probably hook and my 23 spline and perhaps the rear end might not be up to task. But that's the least of my worries.

CNC'd Victor heads, victor single plane manifold, 10.5:1 CR, 950 HP Prosystems carb, Hydraulic Roller Comp Cam 247/253 at 50, 112 installed at 108 with 650 lift.
Vacuum at idle is 10 in. at 950 rpm.
It's got about 4000 miles on the build. The original builder, I found out half way through the build was shady and a crook, unfortunately, I had to wait until he finished the motor but then I took it to another reputable builder who gave it a once over i.e. checked bearings, clearances, block main lined honed, etc. He did change it from a solid roller to a hydraulic roller because I mainly drove it on the street. The engine builder said the engine liked 34 total and set it at that. Well, when I drove it, I noticed that it didn't have much get up and go out of the hole, checked the initial and it was 10. At that time, I had the firecore but I knew that I'd be taking it to a local chassis dyno shop to dial in the ignition and fuel curve. Well tuning cost money and they prefer working on msd's because they're easy to adjust on the vehicle. So I bought an MSD and took it to the dyno shop. I knew it was going to be a bad day because the first words out of the tuner's mouth were, carb first THEN ignition. It was a waste of money and time, I was there about an hour and a half and the car didn't feel any different afterwards.

About a month ago, I set the initial for what it liked and that was 24. I was able to set the timing to 24/34 with a custom 10* bushing from FBO. The car goes like stink out of the hole but I noticed it rattled just off idle, I also noticed that between shifts, as the rpm drops, the engine sounded to "growl" which it never did before, I don't know how else to describe it. So I changed the springs from heavy silver/light blue to heavy silver, heavy silver but then it was a dog off the line. I think I have heavy silver/light silver in there now, the rattle off idle is gone but not the raspy sound right after each shift.

The problem is that it's warmed up here now recently and the motor takes longer to crank and fire up. So I know I'll have to back off the initial.

If I go to the 14* custom bushing I also got from FBO, that drops my initial to 20, will I be giving up much power??

I know I can have infinite tuning from the firecore but the reason I left it off was because I knew I probably had to take it to a carb shop to dial in the fuel curve and they'd most likely balk at the firecore and want me to put in the msd so they could check the ignition curve first. We've all heard it before, msd is the way to go because it's so easy to set the curve without removing it.

But if you guys can help me with the carb tuning also, then I don't have a problem with going back to the firecore. If I can dial in the timing with the msd on it now, that would be awesome. The trouble is that with the 3 inch exhaust (sounds awesome), it's really difficult to hear detonation.

Some people say that with the configuration I have and vehicle weight, I will have detonation with anything less than 3200rpm, others say that I will be safe with it all in by 2500-2800. I guess it's a balance of playing with the springs as well as where the initial is set. I know total shouldn't exceed 34 as the engine builder determined that on his dyno, so that's 1 less piece of the puzzle I need to worry about.

Do you set initial based on highest vacuum or highest rpm? Then heat soak motor and adjust from there?

If a vacuum guage is the way to go, mine flutters a little bit, making it hard to read, it flutters about 2 inches at idle of 950rpm.

Oh one more things guys, what oil should I be running on the street?

Any and all help will be greatly appreciated. My apologies on the long post.

Thanks,
Bob

[c00nhunterjoe]

Something sounds off, that initial shouldnt be a problem on that build. Our 572 runs locked out at 42 with no issues. Could be as simple as a tad lean on the transition from idle curcuit to mains.
  The fluttering vaccum gauge could be an issue too. Can you get a video of the needle? If its just moving a little up or down at idle, that might not be an issue, but if it is actually "fluttering" pretty quick with each hit of a cylinder, there might be a valve problem.

     Having a full roller motor gives you more options on oil. I still prefer bradpenn, but if you want to go full synthetic then the lucas racing oil is my next choice with a roller cam and big spring pressures.

Those mt's sound like radials, if so, you will never get it to "hook" on a 4 speed. You need a good soft bias compound for the track

[73RRBob]

Thanks for your reply,

I'll get a video of the vacuum gauge at idle within a day or so. So you're thinking that the initial is fine at around 25 and the rattling or detonation just off idle is carb related?

Then should I be adjusting the carb at this point?

If so, where do I start? Should the primary and secondary idle speed screws be out about the same amount or the primary more than the secondary?

I've decided to put the firecore dizzy back in and figure out how much mechanical advance is dialed in. I'll go back to 24/34.

Any thoughts on when it should all be in by? 2500-2800 or is 3000-3200?

Thanks again for your help, I'd really like to get this motor dialed in.

Bob

[c00nhunterjoe]

I would like to see the gauge 1st before you go changing things.
What fuel are you running.
You said its warmed up and that's caused issues. How hot is it there?
I dont see 20/34 causing problems. Like i said, our 13:1 572 is locked out at 40+ and our 13.5:1 63 426 max wedge runs 30/42 with no problems on iron heads.
Its possible that it could be an idle transition, but its possible being a stick car that it may want a different ignition curve, but its also possible that you flickering vaccum gauge could be a problem. Lets not forget too altitude and octane.

[firefighter3931]

Hi Bob,

Have you done a cranking compression test ? If it has more than 180 psi cranking compression it will not run on pump gas.

The detonation is is either too much cylinder pressure or not enough fuel. The 34* total timing is about right for a closed chamber aluminum head. That's where mine made peak power.

Are you running a mini starter ? Mine cranks over fine with the distributor locked out at 35* 

A wideband O2 guage would be really helpful in diagnosing the fuel curve. Do you have access to one ?



Ron

[73RRBob]

To answer some of your other questions:

It's 20 Celsius or 68 Fahrenheit here in Vancouver.
Altitude is between 0-500 ft
Mini starter yes
I use 94 octane, no detonation at 10 initial but because I wanted to dial in the most initial I can and max of 34, it started rattling at 24 off idle.

I took the msd dizzy out, fiddled with the bushings and springs for a bit then decided to install the firecore instead,
but didn't mark any reference points so I have to find tdc and get the motor started. I'll set it at about 15-16 initial then see what the vacuum gauge shows.

Thanks again guys for your help.

I'll also get the cranking compression numbers.

What brand of O2 wide band sensor should I be looking at Ron?

[justcruisin]

I have had good success with Innovate, logging the data along with RPM and vacuum inputs is a real plus.

[73RRBob]

I have had good success with Innovate, logging the data along with RPM and vacuum inputs is a real plus.

Thanks, is there any specific model or type? I see there's a hand held meter and there's also one with a gauge. LM1 & LM2? Just want to make sure I buy the right one.

Thanks again "Just Cruisin".

Bob

[BSB67]

You might be able to slow the curve down to get rid of the rattle, but you will be giving up low end power.

Like Ron stated, check cylinder cranking pressure.  I think you can get away with a little more that 180 psi if everything is right, but not much.

I suggest trying the following:
1) Put in either R5671A-8 (stock#4554) or R5672A-8 (stock #7173) NGK spark plugs
2) Put in a 160°F thermostat
3) Make sure you have 4 to 4.5 psi fuel pressure at WOT at the carb.
4) Cold intake manifold
5) Carb insolating spacer.

You can throw more jet at it, but that could also be taking your A/F ratio in the wrong direction.

[justcruisin]

I use the MTX gauge. It is a stand alone gauge which can be mounted in the car and give you A/F ratio reading. It reacts to change instantly. It comes with the log works software so you can log things on a laptop. I am not sure on what is involved with connecting other sensors for recording purposes, it would pay to look on there site to understand what you can do. The LM1 and LM2 is an interface with a hand controller that you can plug inputs into.

Being a cheap bastard and not having a laptop at the time - I just installed the gauge along with a temporary vacuum gauge and rev counter and mounted my camera in front of them for data recording - worked for me. .

Data recording is well worth it IMO. I have had zero issues with my gauge. My advice on this is to look at all options and get an understanding of what you can do with each product. Others have had good results with other brands as well.