1970 351C Dyno Numbers (you should all be interested in this)

[ratio411]

Get her dynoed and document the increase... i'm sure the old guys will appreciate #'s proving they know what they're talking about..

True

 

[70mach1]

Huge mistake.

A long time ago when I added my MSD box, I asked the question about Spark Plug Gap. Some folks on this board told me I could make it larger. Like as much as .045 or .050 with the MSD 6AL. The factory is .035

Anyway, I went to the vintage races in Seattle last weekend and many of the cars had motors similiar to mine (except fuel and compression ratio) but 3 seperate crew chiefs told me that .045-.050 is WAY WAY WAY too large and that at any RPM above 4000 will start to blow out the spark, especially with 11.0 - 1 compression (stock 1970 cleveland) MSD box or not.

So I regapped the plugs to .035.

Ummmmmmm, yeah the car revs to 6500 now ! Previously it was stalling out at 4800 (Edelbrock RPM) and 5200 (Blue Thunder).

MAN, I can't believe I made such a stupid mistake. It has been costing me about 1000RPM for 3 years now !

Lesson here, be careful with what people post, it isn't always correct. Me included ! Although I have made just about every mistake there is so hopefully I know what I am doing now......

 

[jcode68]

Wow, this is good to know. Although I think even the manufacturers of these products (MSD, Crane, Mallory etc.) document that you can run a larger plug gap as an advantage to using their product. Will have to go dig out the manual on my Crane ignition system now since I'm running at .42.

 

[ratio411]

Yeah...
MSD box won't allow an increase in gap.
You need better ignition to do that.
MSD just gives you more spark duration, not more spark power.

Why not throw a Duraspark distributor in there?
It will trigger an MSD without a brain box.
Then you can throw a high output coil on there and increase your gap .010"...

 

[D.Hearne]

I switched to the 700dp because the 670 couldn't pull enought vaccuum to open the secondaries even with the white spring. The change was night and day.
.

This doesn't add up. If your motor now is pulling harder with a larger carb, it's also pulling less vacuum than the smaller one. If you couldn't get the secondaries to open on the 670, you had some other problem. If you now stomp the pedal on the 700 DP and your motor winds up, you are pulling a vacuum but less so than a 670 with only the primaries open. There has to be a pressure differential for the venturis to pull fuel from the bowl. This is what vacuum is. A carb won't work without it.

 

[70mach1]

This doesn't add up. If your motor now is pulling harder with a larger carb, it's also pulling less vacuum than the smaller one. If you couldn't get the secondaries to open on the 670, you had some other problem. If you now stomp the pedal on the 700 DP and your motor winds up, you are pulling a vacuum but less so than a 670 with only the primaries open. There has to be a pressure differential for the venturis to pull fuel from the bowl. This is what vacuum is. A carb won't work without it.

I agree with Hearne. I didn't understand what he said either. By the way, I am now on the White Spring too (lightest). However, I started on the Purple(medium), with no secondary crash, went to long yellow, then short yellow, and now white. I still have no flat spot or hesitation. The new set up is indeed thirsty.

Perhaps my 670 Avenger Holley is too small now. I still need to dyno tune and I think the car is ready now.

 

[D.Hearne]

If anything his motor is pulling harder with the 700DP due to it's being tuned richer than a 670 would. A vacuum gage will tell you if the carb is too small for certain. The 670 is probably a better choice in day to day driving but may limit it in topend power.

 

[Gud T.B. Blown]

This doesn't add up. If your motor now is pulling harder with a larger carb, it's also pulling less vacuum than the smaller one. If you couldn't get the secondaries to open on the 670, you had some other problem. If you now stomp the pedal on the 700 DP and your motor winds up, you are pulling a vacuum but less so than a 670 with only the primaries open. There has to be a pressure differential for the venturis to pull fuel from the bowl. This is what vacuum is. A carb won't work without it.

As long as I drove the car like grandma everything was fine, but if I wanted a little more acceleration the engine would stumble and then pull. I tried changing the springs and accelerater pump cams and settings. Nothing worked.
I upgraded to an MSD ignition and had a local Mustang shop install it and also had them tune the carb as it was beyond my limited skills.
They had no better luck than I and in frustration put on a 650dp carb off one of their cars because they new it worked properly.
Bingo, no more hesitation at all. Their diagnosis was that the engine wasn't generating enough vaccuum to properly open the springs on the 670. Whether that was the case or the 670 was messed up ( I had bought it off of ebay) I don't know. All I know are the results and they are night and day.

 

[D.Hearne]

There had to be something wrong in the carb then, even a stock 302 can be made to run with a 670 SA. Bet the vacuum passage to the secondary pod was blocked.