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

We tried the short yellow secondary springs and it created a rich spot when the secondaries hit. Meaning they came in too early. This causes the car to stumble slightly and then catch up. My tuner says that it may feel faster (like the car just had the turbos engage) but in actuality it is slower.

So we backed down one level to the purple spring and the rich spot went away.

This fact indicates that the Carb is plenty big. If the engine needed more carb, then the vacuum would open the secondaries earlier and take the extra air/fuel that it needed while requiring a lighter spring.

I am glad there is so much interest in this topic. You will all find out May 1st when I go back to the dyno, what the numbers will be.

Any guesses?

My engine builder says that the HUGE ports of the Cleveland 4V are too big and in fact the 2V heads actually flow better and can make more power everywhere.

I installed the INTAKE MPG port plates. My engine builder HIGHLY recommended it. He said he has dyno tuned about 12 cleveland and Boss 302 cars and everyone of them benefitted from the plates. It doesn't lower or raise the PEEK Hp, but he said it brings the torque curve up and flatens it out. He also said the mid range power is much higher.

Basically he says you want the most area under the dyno curve and the plates increase the area under the curve.

I am not sure if they plates work or not, but Craig Blood (Blood Enterprises in Aurburn WA) is very sure. His web site is www.bloodenterprises.com. He is argualby the best tuner is Western Washington.

Bottom line is if you have 2V heads, stick with them. They will do just fine and could do better in certain situations. And I am a 4V guy!!!

The new dyno numbers are in ! Closure at last.

Here are the results of the dyno second dyno.

I compared the dyno info from 3 weeks ago to this info. The X pipe made a huge difference. Huge. So huge, that we couldn't fully re-dyno the car. That is because the air/fuel ratio became so dangerously high that the tech stop revving the engine. The air/fuel ratio should be at 12.5 the whole time.

As a baseline, when I first had the car baseline tested 3 weeks ago, the air fuel was at 13.0 to 14.0 with a slight umbrella shape to it. Blood enterprises brought the umbrella down to 12.2 to 12.9 by changing the primary jets from 65 to 69. So 4 sizes larger brought the air/fuel down by one step. The car was in good shape air/fuel wise.

Then I found out the exhaust had a huge restriction. It wasn't the h pipe (although it would have been if not for what I am about to tell you). The tail pipe that leaves the muffler and goes over the drive shaft was bent in an elliptical fashion. It was 2.25 originally but when the bastards put it in, they smashed it. So it was seriously oval on both sides. On top of that, my exhaust tips are only 2" diameter. And the h pipe didn't help either. The whole exhaust was f--up.

The x-pipe is beautiful. I had him install flanges so I can take it out when I do the trans. And he used 2.5". It looks huge compared to what was there. And the car is a little louder now.

Anyway, you will notice on "combined" tab of the EXCEL sheet (see JPG attached), that the air fuel starts out fine but then gradually gets leaner from 2100 RPM to 2600 RPM. This should be the primary jets being too small AGAIN ! I probably will make them 2 to 4 sizes larger (71 to 73). Now that the engine can move a proper amount of air, it is! And the fuel cannot keep up.

Next, the secondaries kick in and initially richen up the air/fuel from 2700 to 3000 RPM but then the air starts flowing so fast that the jets cannot keep up and the engine goes lean. By 4300 the car is dangerously lean. By 4600 engine damage could occur and the car is struggling to make power due to lack of fuel. By 4900 the car is simply giving up and trying to prevent piston oblivion. The tuner said the #68 jets in the secondary should be 75 (75, that is a huge jump), but if it were his car he would make them 80 (not sure why). He said richer is better than leaner due to piston damage vs fuel economy. I have to agree, but I don't want the car so rich that it looses power.

So I need to change primary jets up 2 to 4 or secondaries up 6 to 8 sizes. I thought that if I make the primaries up 3, then I would only have to raise the secondaries up by 5.

What do all you think? Alot of info, but that is what is fun about it.

Ryno...

Oh, the car made 235 hp. vs 202 previously and there should be another 25hp according to the tech when the jets are proper.

Reply to paintballtommy about power valves. Good technical info

Your power valve selection should be computed as follows. Take you initial vacuum reading at idle. Let's say it is 12 like mine. Now divide by 2 and that should be your power valve reading. So you power valve should open at 6 in of vacuum.

This is straight out of the holley tuning book. Mine is factory at 6.5. A little high, but still perfectly fine. Cars with large cams and aggresive tuning with little vacuum may use 4.5 or 3.5 power valves.


No it is definetely jets.

I am going to stick in the factory jets for the holley 770 Avenger (as compared to the 670 jets). The 770 jets are 72 primary, 75 seconday. The 670 came with 65 prim 68 secondary. As I indicated earlier, my carb was changed to 69 prim 68 secondary BEFORE THE FIRST DYNO RUN. Now after the exhaust change and second dyno run, I am making an educated guess at the jet size. I will track tune it today at 72 primary with a super heavy secondary spring (to isolate the primaries) and keep adding jet until I get a little bit of black smoke, then I will back it off 1 or 2 sizes.

I think you are confused. The stock PV in the Holley 670 Avenger is 6.5. That means is takes 6.5 in of vacuum to open. Not that it opens at 6.5in Hg. If you have an 8.5, then that is really stiff. Feel the spring difference between a 3.5 and an 8.5 The 8.5 will have a much higher spring rate, thus requiring more vacuum to open. Hence it will open later.

Large PV # = later opening.
Smaller PV # = earlier opening.