Hello, hoping for some explination to a slight problem.

Have '68 442 convertible with an Olds 455 .060 over using Mondello's super comp aluminum heads, offey intake, .560/.565 239/249 hyd roller cam, FAST efi and a Mondello hei distributor The engine is mated to a Tremec T-56 6 speed tranny. My question is about spark knocking. The engine is right at 10.3:1cr which as I've been told is about the limit of premium pump gas. The engine has slight detonation at about 2400rpm cruising and just a hint of detonation for just a fraction of a second under hard acceleration from cruising speed. A call to Mondello and they said that I need to keep the total mechanical at 35.5 degrees and address any detonation from other aspects such as a cooler thermostat, colder plugs, and richer air/fuel ratio. I've put a 160 thermostat and 1 and then 2 numbers colder spark plugs and that has helped some but I still have the detonation. If anyone has used an aftermarket efi system then you know that you can set the idle, cruise, and wot air/fuel ratios to whatever is desired.

My question is with the information provided, what is a reasonable air/fuel ratio? Currently mine are set at: Idle 13.1, cruise 12.1, wot 12.4 My second question is would an adjustable vacuum advance help remove some of the detonation? I forgot to mention that it appears that total mechanical is all in at 2700-2800rpm. Does that sound about right? I thought that it should be a slightly higher rpm. To me the cruise air/fuel ratio seems way too rich, and its set that rich because the richer I go the less the detonation. I have yet to take a look at the springs in the distributor. Again another question, do I change the distributor springs or try to address the issue with an adjustable vacuum canister first?

Any ideas?

Thanks in advance.

 

Yes you need an adjustable vacuum advance. That'll help bigtime.

That's the problem with the EZ stuff, no spark control.
With that cam I'd try 13.5-13.8 at idle and a bit leaner at cruise(13.8-14.0). WOT should be a 12.5:1 as a starting point.

Remember all you have there is an electronic carburetor. That's a compromise however you look at it.
And by the way, Mondello doesn't have a clue, sorry.

 

To concur with Mark's statement, you need to reduce your vacuum advance by just a couple of degrees.

An adjustable can will let you dial that in very close.
You will also want a dash-mounted vacuum gauge to help discover exactly where your present timing is running to advanced, and you'll want to map the curve of your present advance can, so that you know where you're headed with the new can.

You will probably find that your pinging occurs at maximum vacuum advance, just before your vacuum begins to drop, so providing a little less maximum advance is probably all you need (you can do this with a homemade or aftermarket stop, as well, if you prefer).

- Eric

 

Thanks Cutlassefi for the response. I understand how to set the air/fuel ratios for crusing and wot, but what tells you when you got it right for idle? Temp? Ability to idle with power steering in use?

The ratios you posted are very reasonable, so should I adjust the vacuum advance down till I get I get not detonation at these ratios?

Thanks for the reply

 

What is your timing with your vacuum advance connected at 3000 rpm and does it stop there? Also have you checked your advance to make sure it's all in at 2800 or does it increase at the upper RPM ranges? Colder plugs is not going to correct your problem unless they were way to hot to start with. What is your temp running?

 

He already stated his advance was all in by 2700-2800 and that he had a 160 thermostat. We'll have to take his word for it.

As far as the best idle mixture just do it the same way you would a carb, sort of.
While it's idling in neutral, change the desired air/fuel ratios and see how it effects the idle. You may want to attach a vacuum gauge and see how it effects the idle quality and vacuum reading. The idle rpm probably won't change, the IAC will stabilize to whatever it's programmed to.

10.3:1 with aluminum heads is very doable with 93, although if you're in Ca. then you may only have 91. If so you might want to slow the advance curve a bit to be all in by 3000 or so instead.

Let us know how you make out.

 

Just double checking on the all in Mark, some people never check the upper RPM limits to make sure.

 

Thanks for all the replies and info. I have an accel adjustable vacuum advance I'll throw in tomorrow. I'll also confirm, but the advance did seem to be all in at 2700-2800rpm, revved the engine to about 4k with no change. Total with vacuum is right around 50 degrees I believe. The temp stay right on 160 when crusing and usually when stopped in traffic, which I think is pretty good; I'm located in Kentucky with hot very humid summers. I seem to remember reading vacuum at idle at about 12 inches. I've made some minor changes since then that probably require retesting. I'll post more info/results tomorrow.

Thanks again for all the input.

God Bless

 

Let us know how it goes.

 

50 is too much. I'd say once it's all said and done you'll end up with about 44 or so under cruise and obviously less initial vacuum advance.

Thanks

 

Hi guys. Sorry for the delay. The vacuum advance I had was incompatible, the metal sleeve that attatched to the distributor was too thick, so I had to order a new one. Installed the new one, confirmed 35.5 degrees total with the vacuum disconnected. Confirmed all timing is in by 2800 rpm. Initial was 19.2 at idle. I changed the cruise a/f ratio to a lot leaner, ratio and drove around with the vacuum disconneted. I ran out of time but the last setting I was at was 14.1 at cruise, no detonation. I did notice a little popping thru the exhaust at about 13.8 or leaner when engaging the clutch or letting off the gas. I'm guessing that I'm pretty close to the right setting right? Also, I've read that once I find the right a/f ratio to enrichen the mixture by .2 to take into acoount enviromental factors such as temp and humidity. Does all this seem right? Does 14.1 for my setup seem feasible? Lastly, the temp climbed to 187, I guess I'll change the thermostat tonight to either a 180 or 195. Which one should I use?

Thanks in advance.

 

I'd go with a 180. Looks like you got your timing squared away.

 

I'd do a 160 because you have no electronic control over spark timing from the ECU.

And no you're not done with tuning, timing and/or fuel. You'll need to balance between the two to get the best results. Once again it's harder because of no spark control.
Program it for the best air/fuel, period. Who told you to richen it .2? They're an idiot, more internet crap. That's why you have a wideband and EFI. It will compensate for most changing conditions.

Hope this helps.

 

Thanks for the replies. I must confess that I'm a little surprised that Cutlassefi suggested a 160 degree thermostat, he frowned at that further up the post. Wouldn't that make the cooling system open all the time? Also, I'm confused about the spark timing/control that has been mentioned multiple times. I understand that I'm using nothing more than an electronic carburetor, but more vehicles have been produced without spark control than have and work fine. Why would I not want a 180 or 195 thermostat because the lack of spark control; I don't understand the relationship. Please explain.

Thanks in advance.

 

A good system would allow you to take advance out as the engine temp increases. Without that feature it's especially better to run it a bit cooler

 

Thanks for the response, I understand better now. I had a little time today to work with the a/f at cruise. With the vacuum advance disconnected and plugged I got up to 14.3 a/f cruising with no detonation, but I had notice that at 13.9 and leaner there seemed to be a little lack of power and a slight stumble/hesitation if I let off the gas and back on or changed gears. With that being said, should I keep the 14.3 and add a little accel fuel, or go back to 13.8 which is just before I noticed the changes?

Also, I was able to pretty closely dial in the vacuum advance at 13.8 a/f. It took 10 complete turns (pretty sure that doesn't tell you anything) to get rid of the detonation.

At idle (750 rpm), the engine pulls 9.5-9.75 inches of vacuum. The engine stayed at a rock steady temp of 165 degrees with a 14.1a/f. This actually surprised me a little because the idle a/f is 14.1 vs. 13.8 (or 14.3 depending on your feedback). Of course today probably isn't an ideal day, the temperature outside is only 70 degrees. Does any of this seem right? Any ideas or feedback?

Thanks in advance.

 

PS I forgot to mention two things in the last post. First I idle the motor at 750 rpm because I use an electric vacuum pump for my disc brakes. Second, when idling at the 14.1 a/f ratio the unit read 0-4% O2. Hope that helps. Thanks

 

Yours is a perfect example of the "self tuning" stuff not being totally self tuning. But people don't believe me when I tell them that.

You'll just need to try different things. You may want to try calling FAST, they're the ones that made the system.

Best of luck.