Blower timing

[novaz]

Hi Guys
I think the answer to this might be try it and see but would appreciate your views
last time i had the engine on the dyno it made 757@6900 with 34* of timing and 1* per 1# of boost pulled out on the 6-BTM
my question is do you think i can run more timing
383sbc with a 6-71 @ 12# of boost now on E85.

[Ken0069]

Roy I'd sneak up on the timing if I were you. One of the best indicators is power drops off once you get past the optimum timing point. This will show up as a drop in MPH on a time slip and also as lost HP on a dyno pull. Last fall we dedicated a few runs testing to find out what my engine liked. I wound up with 30* max on gas and 26* max on methanol but in each case I went up 2* per run from a lower setting to see where MPH dropped off.

On another note, you're not likely to hurt an engine with timing set low but you could hurt one with it too high. Drag Chevette has stated that he runs the same timing whether it's gas or E85 so since he built your carbs you might want to run that question by him if he doesn't respond here.

[us7race]

My buddy with his blowers has ran 26°-30° that is with alcohol too.

[novaz]

Thanks Guys
I know Mark has said to run the same timing when going over to E85
was more thinking about the blower part of this combo
have heard that blowers like timing then heard the reverse.

[shrinker]

A Blower alters the cylinder fill percentage and it alters the scavenge on overlap. Both these factors alter the temperature achieved during compression. If the cylinder is filled with more air etc then the timing has to be treated in the same manner as if you had raised compression and were running on a very hot day, so hot you couldn't live. So the timing has to be retarded from 'normal'. If the cylinder scavenge is affected the residual exhaust is reduced and that lowers the start temperature of the compression stroke, so the timing has to be advanced compared to normal because the final temperature is affected greatly by the start temp. Some cams can cause either or both these situations to occur at points in the load range or rpm range. However its uncommon to have the over scavenge issue at high load. The safe thing to do with blowers is to retard and then look for AFR problems. Most times a blown engine running more advance than considered average is suffering from AFR issues. Cylinder distribution can be poor on blower manifolds and the engine may respond with more power by advancing the timing but it may be at the expense of wrecking a piston here or there.

[novaz]

Thanks Shrinker
I have an Erson blower cam 294/302 duration,112LC and 645/645 with 1.65 rockers
I have heard of the fuel distribution problems and to that end for my 427 build i bought an additional 1050 Dominator from Eric68 so would it be totaly out of the question to run the two Dominators on the 383 I also plan on using some 2" tapered spacers ,
apologies if this question is going off track just keen to learn
Cheers

[shrinker]

Running carburetor barrels that are spread down the length of the blower improves cylinder distribution. A single four barrel mounted in the middle generally results in the center cylinders running a richer mixture that the outer ones. The fuel droplets do not spread completely along the rotors when passing through them so starting with the 8 barrels is a good idea. Whether you need two 1050 dominators would be another question.
I assume that's .050" figures if not then disregard the following.
With that size cam you had better be revving it as it wont like the overlap much at low RPM. The cam appears to be limited in valve lift so that will create very fast air speed through the valve curtain if the heads are efficient. That cam spec is really for a poor flowing head that restricts the flow rate say at the pushrod pinch. Its not a good idea to push the fuel too fast through the valve seat because it gets turbulent across the short turn and the fuel separates and enters the cylinder in blobs. My advise to you is to install a manifold temperature gauge under the blower and monitor the temperature there, If its too cold it wont go correctly. You can find an optimum temperature when running injection because you can chose to have fuel on top of the blower or in the runner etc but with carbys on alcohol fuels its always going to be very cold in the manifold. Cold inhibits vaporization and forces more of the fuel supply to be liquid, thus greater risk of fuel separation etc. Its possible it might be faster on racing gasoline than alcohols.

[Ken0069]

Running carburetor barrels that are spread down the length of the blower improves cylinder distribution. A single four barrel mounted in the middle generally results in the center cylinders running a richer mixture that the outer ones. The fuel droplets do not spread completely along the rotors when passing through them so starting with the 8 barrels is a good idea. Whether you need two 1050 dominators would be another question.
I assume that's .050" figures if not then disregard the following.
With that size cam you had better be revving it as it wont like the overlap much at low RPM. The cam appears to be limited in valve lift so that will create very fast air speed through the valve curtain if the heads are efficient. That cam spec is really for a poor flowing head that restricts the flow rate say at the pushrod pinch. Its not a good idea to push the fuel too fast through the valve seat because it gets turbulent across the short turn and the fuel separates and enters the cylinder in blobs. My advise to you is to install a manifold temperature gauge under the blower and monitor the temperature there, If its too cold it wont go correctly. You can find an optimum temperature when running injection because you can chose to have fuel on top of the blower or in the runner etc but with carbys on alcohol fuels its always going to be very cold in the manifold. Cold inhibits vaporization and forces more of the fuel supply to be liquid, thus greater risk of fuel separation etc. Its possible it might be faster on racing gasoline than alcohols.

Now this is the kind of stuff I CAN understand.