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Author Topic:   timing a blown motor
awkwardjeff
Dirt Forum Champ
Total posts: 562
posted August 20, 2005 08:01 PM  
I have a guy living down the block that put a blower on his Camaro. Stock short block, he has the big pully on the crank and the small one on the blower.....I told him that needs to be changed. Along with some other little things.

He knows nothing about motors, but knows that he's in over his head on this deal. He has asked me to help him TUNE this thing. I told him I had no idea on total timing but would ask on here........ he wants this to be a street driver that lives..... he has a good HEI, and a pair of Holley 650's...
what would someone that has been down this road before set the total timing at??? Also it's 9 to 1 on compression, and trying to run 91 octane pump gas.....

thanks in advance

cobb
Dirt Maniac

Total posts: 131
posted August 20, 2005 10:37 PM  
i read the topic and was thinking "why would you want to time a BLOWN motor???" lol

BrianW
Dirt Freak

Total posts: 385
posted August 21, 2005 02:42 PM  
First - he could have the pullies correct - that's an 'overdriven' setup and will make more boost than a 1:1 or an underdriven. The cast piston 9:1 compression is going to hurt him - 91 octane isn't a lot for the boost that he might be after - try getting the better 93 octane - it's out there but a little harder to find. As for tuning - figure a safe start is 38-30* total and subtract 1-1.5 degree of timing for every PSI of boost (MSD makes a BTM ignition that does this) - I'd tell the guy to go get it on the dynow with a wideband O2 sensor to even get it close to tuned in...

Edit - I assumed that this is a roots type since you mentioned a pair of carbs sitting on top...

[This message has been edited by BrianW (edited August 21, 2005).]

awkwardjeff
Dirt Forum Champ
Total posts: 562
posted August 21, 2005 07:07 PM  
Brian,

I know and understand that this blower as set-up now overdriven, and will create more boost then if it were underdriven. I was under the impression this old 100,000 mile short block will start knocking chunks out if it were overdriven. Am I wrong on that?????

This fellow is a young person and wanted a blower to look cool.....and make lots of noise....... not realy wanting power. Or he has decided he doesn't want to make too much power and end up with rods in the pan.

I didn't care to get involved, but he has spent much hard earned cash.......I would like to keep him from wasting all that cash on the first ride or two......

I appreciate your input......is this thing going to live if overdriven? How much boost would you shoot for? and they sell pop-off valves for this.....CORRECT???

thanks again, Jeff

KK17
Dirt Freak

Total posts: 229
posted August 21, 2005 08:15 PM  
Jeff, If you go to Holley.com and click on tech info.. at the very bottom they have a 5 or 6 page article on blowers and engine tuning with blowers..maybe that will help you as well. no racing tonight? see ya Erik

leapinlizard
Dirt Freak

Total posts: 402
posted August 22, 2005 08:56 AM  
Cast psiton with 9.0 compression net's a max of 6 pounds. Anymore is asking for problems. Timing as stated above works well.

BrianW
Dirt Freak

Total posts: 385
posted August 24, 2005 03:31 PM  
One thing to remember - boost is dependant on the blower 'stuffing' more air than the engine can cycle through naturally (very rough explanation, but you get the point) - that said blowers are rated by size and if it's a small blower it may have to be over-driven to produce any boost at all, if it's a huge monster blower for a 700" pro-stock engine on a 305 then it would produce a ton of boost under-driven.... So - to get some idea you would need to know the displacement of the blower - or just call the MFGR and they'll give a quick guess on the boost for that engine...

As for puking the 100k mile rods... yep with a lot of boost that will probably happen... I'd stay 5-7 PSI with stock components and keep the revs down too... As for a pop-off valve - not really needed - shut the throttle and you reduce the air going into the blower - you need it in a blow-through design as when you're running 5000 RPM and slam the carb plates shut that air has to go somewhere and it can't go through the carb so it just builds up... What you might be wanting is a backfire plate or window that will blow out if he suffers an intake backfire. Probably not a huge concern with the combo...

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