Boost / Octane / Timing
 

Hey Guys, Changed the upper pulley on my blower (b&m 174 w/chiller on my 489 chevy bb with a 6 inch bottom pulley) from a 3 1/4 to 2 1/2. Found out I was in the 11500 rpm range at wot and the blower can go to 14000 ish . With the last pulley I was at 5lbs at 5200rpm with a 27p and 6lbs with a 24p double cupped at 5900rpm. With the new pulley it pulled like crazy (with the 27p) and I was at 7lbs at 5000 with a lot more throttle to go. I run mid grade 89octane fuel and never had a problem (28 total timing) but with more boost should I cut back on the timing to ease detonation or am I needing to run premium fuel? Motor runs in the 120 degree range. This is my second season with this blower so I'm still on a bit of a learning curve.
P.S. I did pull a few plugs and they look good.

Thanks for any help.

I would say 93 would be a good idea. Steel heads?

Yup, 502 marine. Keep the timing at 28?

You don't want to take to much out cause it will make more heat and the motor will be somewhat fighting itself especially with a 4" stroke. Actually turning it up a little may be beneficial but camshaft would come into play also.

You're crazy for running boost without 93 IMO, LOL.

Run 93, keep timing the same like Baja said and run it hard and check the plugs. It may want a little more fuel b/c of the boost increase.

Definatly run at least 91 octane, and preferably 93 octane. What is your static Comp ratio???

Do not reduce the timing any further. It will create more heat in the cylinders.

I'm just gonna throw this out there I had a 540 efi that was a 500 efi with a procharger on it. The initial compression was 9:1. At 8lbs of boost the CPU was pulling timing out of the motor cause it was knocking. I ended up settling for 5.5lbs of boost and it made more power cause I could put timing into it. About 35 more at 5.5 compared to 8...

I'm not looking to make 800 hp (it would be nice) but from what I had found on dyno pulls with my blower @ 6000 rpm with a 2.5 it was making 620hp. Griff, My static comp is 9:1. This past winter I pulled motor for a freshen up and had no scuffing from detonation and I ran 89 octane all last year. I have all good bottom end parts, Flat tappet 576 with a 114 and HS roller rockers but do have stock head gaskets. I didn't think 8lbs of boost was to much but like I said, having this set up is new to me. Figured it doesn't hurt to ask!

May be ok for short WOT bursts but no way I would run that sustained high with 89 octane. I think if you really want to play around without hurting your engine you should invest in a knock sensor gauge, and or have an AFR installed on the odd cyl bank side and see where you are at. The summer heat will play a factor as well, you have no chance of hearing detonation over that little blower spinning that many RPM. Chilling that little heat pump is also inexpensive insurance.

Are you running 89 because it is all you can get, or are you trying to save some money? .15 cents a gallon savings is only $15 bucks each 100 gallon fill up, not a lot of money when dropping $400 bucks at the pump

4Bus, 89 is on the water around here. I am going to keep a few cans of octane boost on board this season. Putting in 91 isn't a problem when its on the trailer. I'll call today about the knock sensor. I was thinking about that this weekend and yes I have a chiller from Teague. So far this small package has got the boat to 80.7mph on gps. I know its not flying but I'm happy so far. Thanks for all your info