Evaluating dyno numbers


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Animal Lawyer

Well-known member
Sep 3, 2020
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A few months ago I bought a supposedly rebuilt 1970 351c and a T5 as a package, that had been sitting for about 8 years after the owner/rebuilder died. My shop finally got around to opening it up and after inspecting the engine dyno'd it. Here are the results.

351c dyno 6-29-21.pdf

It is making about 340 hp at 5800 rpm. I now need to make decisions. I have bought and am adding the Blue Thunder intake (I hope it will fir with the ram air, without too much hassle) I need to decide what carb to use (I have a 680 cfm Street Avenger on the original engine now) I am not sure whether to use the SA or look for something different. I was considering the Street Demon (not sure whether to look at the 625 or the 750).

The car is a 73 Q code vert and it is strictly street and driven regularly. So, while I am happy to add the extra HP (75-100 extra depending on what the blue thunder adds) I understand it is only useful at higher revs and I really do need to worry about low and mid range torque. The Torque values  are virtually a straight line. How do I optimize the engine for the best all around results. I am going to get a recurved duraspark dizzie (unfortunately we know nothing about the cam (other than the dyno results). the engine has no accessories or brackets, so I need to decide whether to move everything over from the original engine or purchase all the brackets, pulleys and accessories (anyone have a set of brackets etc they want to sell at a reasonable price?))

I presently have a stock 4 speed toploader and a 325 9" posi rear. Do I change the rear gearing? I know if I swap to the 5 speed I will need to change the rear ratio, do I swap to the t5? (Overdrive 5th is nice on the parkways, what are the downsides (other than the toploader is virtually bulletproof)?)  I have ps, pb (front discs) power steering & air (all of which I am keeping).

The new engine still has the pedestal rockers, is there any real advantage to be gained by drilling them for studs (other than adjustability) and am I correct there is a tool to allow it to be done on the engine (assuming there is a real advantage to doing so)?

So many questions (and I am sure there are more I haven't considered. I am open to suggestions (other than I should have gone to the Aussie heads for street torque) The accumulated wisdom of the board has been a lifesaver.



  • 351c dyno 6-29-21.pdf
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The 680 SA will be fine with that engine. Assuming it was dynoed with the stock iron intake, the BT will definitely add some power. I personally would just move my front dress to the new engine. 

T5s have a bunch of different ratios, so you'll need to know that to determine which way to go. As far as changing the rear ratio, I'd get it in and see how the engine does in OD at typical highway speeds. 

No advantage to machining the heads at this point, not on such a mild cam. You can get a set of Scorpion 3224 Endurance rockers which are adjustable if needed. Otherwise, there are many pedestal style rockers available. 

Put the table into a chart for you, a bit easier to get a read on in my opinion. First pull looks pretty good, but you could probably stand to go a hair leaner on the jets - could make more power, but if it negatively impacts drivability, maybe don't go that route. Most carb'd engines do well with around a 12.5 air/fuel ratio. Other than that, the dyno curve looks pretty typical for a mild cam - not much torque drop-off and HP levels off about 5,000. I'm not familiar enough with the blue thunder intake to tell you how that would affect your curve, but I'm sure someone who is can chime in.

As far as carb sizing goes, I like to use this calculation. If you call up Holley asking what carb you should get, they'll pretty much use the same formula: 

(Cubic Inches x RPM / 3456) = CFM (cubic feet per minute)

So, in your instance, it would be ((351 * 6,000) / 3456) = 609 CFM. Probably good to stick under 700CFM for your carb. Most people oversize their carb thinking bigger is better - don't fall into that trap lol. 


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