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Old 09-18-2010, 10:10 AM
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Blower surge

Hello guys-
I have a question regarding blower surge, or detroit roll (many names for it).
I just completed my BBC 468 w/6-71 and well into my tuning phase. All seems great (so far). No leaks, no temp spikes (steady 185-190). Nothing out of the ordinary. Actually better than expected, no blow by on new rings or anything. She idles steady at 800 rpm or so, smooth a a kitten. You wouldn't even know the blower was there if I couldn't see it sticking through the hood or a slight blower whine. I am running my belt alittle loose so the whine is faint.

My question is, I have no surge what so ever. I have searched here and other sites about the surge. I know that it is caused by a lean/rich condition. Does the fact that I have no surge mean that I am real close to a good tune? Plugs look dark, but too much that concerns me at this point. I have the carbs boost referenced, but I don't have them connected to a vacuum source right now ( I can, real easy). I now have 10.5 PV after I disconnected the boost ref. Just wanted to see where she ran the best for testing purposes.

I only have about 25 miles on the motor, so I have been takin it easy.

1978 BBC, 7.5-1 (CR), 2 (Holley 770's), timing at 40* total, all in at 2500

Thanks to Fbird88 and Bentwings especially, and all others that helped me through this build.

Last edited by NOlowrider; 09-18-2010 at 10:40 AM.
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Old 09-20-2010, 08:46 PM
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I'd take a little advance out, down to 34 or so to be safe and add the vacuum advance of about 10-12 degrees for better mpg. Connect to the manifold vacuum. Also hook up the manifold reference line.

Congrats on a nice setup. I'd ease into full throttle however. It would be nice to get an Air/Fuel Ratio gage. You will be much more able to see where you are on the tune.

Work on keeping the idle steady. Your idle speed will probably creep up as the motor gets run in, but just keep after it. Also make your adjustments only after a good 1/2 half hour of running to get everything up to operating temp. You would be surprised at what the AFR does when cold or cool. It can really mess you up. "Detroit Roll" is not desired or necessary.

I'm down to 8.5 PV's now and may go one more step in the interest of mpg. Your BBC will most likely idle cleaner than my SBC which will be good.

You're going to love how it drives if you have the idle down this good. It is so much fun to just jump in and crank it over and drive off like any other car.
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Old 09-21-2010, 02:43 PM
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Thanks for the reply-
I can take some timing out, but what would I gain? Lower Idle?

This weekend I played around with the PV. I went to a 10.5, blocked off the vacuum signal and my idle quality suffered. I know that the PV is not in the idle circuit, but it did effect my idle. So, I switched back to the 6.5 and she idled fine. Hmmmmmm. With the 6.5, running no vacuum signal she runs fine, but I have not seen boost yet. I will try to take some timing out and see what happens.

The dist I have now doesn't have vacuum advance. It is a MSD magnetic pick-up, my E-curve crapped out on me. Still PO'd about that.

Still having an issue with my Boost timing master. Can't get it to start once I have the BTM hooked up. Can't figure it out. MSD is of little help. I have checked and re-checked the wiring and still no go. So for the moment I have no boost retard. Thinking I may have to send it back for a replacement.

I am looking a getting a air/fuel ratio gauge. Not sure which one to go with on my budget.

Thanks for your help. I appreciate it. Kinda disappointed that after 64 views, I only got one reply. You and Fbirb88 are a great asset to this board.
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Old 09-21-2010, 09:19 PM
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Taking some timing out will slow the idle down some but it will be more for saftey. 36 deg is about all you can use on 92-93 gas.

The boost retard will probably allow a little more advance but it sounds like you have some issues with it so I'd get it solved ASAP. Same with the dist. I use a MSD billit with vac adv.

I'd hook up a vac gage to the manifold below the blower and get some readings. Also try some readings from the base of the carb. I suspect you have just under 10 inches at idle blower manifold so your 10.5 PV may be tipping in causing a very rich idle. 6.5 cures it.
I assume you have the manifold reference at the carb hooked directly to the manifold below the blower. Also make sure the floats are adjusted properly...right at the bottom of the view port.

I use an Innovate Motorsports AFR. They are very nice. Easy to install.

It would not hurt to post more spec on your motor and vehicle.
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