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Old 10-17-2012, 02:14 PM
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Originally Posted by mustang 671 View Post
thanks for the info - much appreciated. i take your points and will apply them and double check the timing and also the primary throttle blades are closed .

thanks for the link to the carb spec sheets.

its hard for me to work out what jets i should have as they dont list it for a application but only for a specific carb model or part number.

so with a 351w with 30 thou over bore and this supercharger and with 2x 600 vac sec holleys. is there a close idea of what jets i should be using ?

i have seen around .62 for fronts and i have .72 but also have 2 carbs. so do i go lower as i have 2x carbs ? ie 2x lower than .62

very confusing and i want to get it somewhere there before going to dyno etc. i am sure with a bit of help and advise i should be able to get it to idle and run fairly normal. under small load before going on the road
Like TI said above, baseline the carbs and let the plugs tell you what it needs. In many cases- unless it's a competition only engine and vehicle- the jetting will not be radically different from stock. Select the power valve by the vacuum the engine has, read below the blower

also you say to lock the timing out etc. i havent got a dizzy. i have a ecu with a chart that i am able to enter actual timing numbers into. so for example what timing would you advise on the following rpm's ?

as i can put any figure against those rpm's but i can also go above the scale and add differnt figures for engine load. so ie 10% engine load could be 28 btdc but at the same rpm with engine load at 70% that wouldnt be the same figure ?

thanks shaun
Use a curve instead of locked timing if possible. You will usually find that using a relatively large amount of initial timing along w/a relatively quick curve will work very well for you. Only in cases where the amount of timing needed at idle is within a few degrees of the total timing would you consider locking the timing at one setting.

AFA giving you numbers to go w/each rpm, the numbers can be plotted on a graph. It might start out at about 18-20 degrees at idle, ramping linearly up to about 30-ish degrees at 2600-2800 rpm. This is w/the engine under a light load. Now, this is not written in stone. While these numbers should be safe (no detonation), still be on guard for detonation, and increase the amount and/or rate of advance slowly, evaluating each change as you go. I would strongly recommend you start a tuning log or journal to keep tabs on each change and the result. Don't rely on memory!

You need to compensate for engine load by pulling out advance as the load increases- just like a vacuum advance would. Hopefully your ignition is vacuum referenced so it can sense the engine load.

I looked around and found the graph below which can give you an idea of how engine load should affect timing. Remember, load is estimated by the amount of vacuum the engine makes at whatever rpm it is running.

The numbers on the graph are just a representation, your engine will likely have different amounts for how much vacuum you have at idle and at what vacuum the advance is all removed. Also the amount of advance change may need to be different than the ~2.5 degrees per in/Hg vacuum shown on the graph:

How much boost are you planning on using? What bolwer drive ratio?

What is the static compression ratio? Plan on using the very best pump gas available (or E85, but that's a whole 'nuther ball o' wax). You may need to blend race gas w/pump gas.

Last edited by cobalt327; 10-17-2012 at 02:22 PM.
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