im running a bbc with a s475 and I have already ran it at 10-11psi and will go to 13 I was wondering what kind of timing you are running in your setup.I start out at around 35degrees total and then after 4psi take out .7 degrees per psi until 32degrees and then leave it there.Would love to know how your doing your timing curve for your bbc.I know the ls engines are running a lot less but want something like a bbc to compare my curve to..thanks in advacne

 

Every engine combo is going to like something different, and factory bbc heads don't have the best chamber ever, but 32* on 13psi sounds like a recipe for disaster imo. The only way to know is test... start low, and add a degree or two at most until mph starts to level off, and keep an eye on plugs.

 

are you saying that 32 at 13psi is to much timing or not enough.

 

too much....the only "real" ways for determining correct timing is at the track by MPH or a dyno....anything else you are just guessing......Im guessing you are on the excess cylinder pressure side of guessing.

 

what would the difference be between a N/A and boosted engine be.Lets say you have a N/A ls and it liked 21 degrees and all you did was put a turbo on it what would be the timing differences between the 2.And lets use 13psi as the boost number...Is there a formula you can use for boosted stuff.Im going to do the MPH test next time out just trying to get a idea on the timing aspect

 

there isnt a formula.....every combo is different.....make passes at the same boost level while varying timing (make a few at the same timing to get an average)....some combos dont want any taken out until 5 lbs ....some dont want additional retard past 15 lbs....some are relatively linear........

start low and work your way up (both boost and timing)....for starters run the lowest boost you can and take 2* or more out per lb and work you way up.....then bump the boost up and back the timing back off and repeat the procedure for the high boost levels.

its easy to add timing...not easy to replace pistons and headgaskets.

 

You guys that are suggesting taking out 2° per lb., WHY? His car is working fairly well at .7° per lb. I'd suggest tweaking the curve he's already got, like maybe going 1° per lb., or .5° per lb., and/or maybe continuing the curve to less than 32°, monitoring AFR, plugs, and MPH.

 

because he may be past mbt timing....we are simply guessing at this point...hence the need for testing the verify.

Should start low and work up....not the other way around....why run more cylinder pressure than necessary to obtain peak power...additional timing past mbt just adds additional stress and heat to parts without any gain.

 

Way late timing can be almost as bad as too much, that's why I suggested tweaking the curve he has rather than starting from scratch....his max is probably about right, adding a little more or less retard per lb. and checking via MPH and reading the timing mark on the plugs would be my approach.

 

agreed on damage can be done with too little...but I dont think any damage will be done running 26* of timing (36* NA - 10 for 2*/lb= 26) for a 1/4 run on `~5 lbs (guessing he can turn it down that far)....need to figure out what side of the bell curve that he (im assuming "joe" is a he) is on.

 

ss496 thanks for you post.I would love to hear the bell curve your talking about.Im just getting my feet wet and still have a lot to learn..

 

Rickracer you have helped me a ton and really appreciate when you offer advice.Im going to try alittle more boost retard as suggested and go to .1 to see what it does.Im also going to start at 32-33 degrees and do the boost retard from there.I feel alittle uneasy going to low on the timing so will retard it down to 28 or so just to see what happens.If it runs the same or better I will go alittle more.I still new to the boost thing so don't wont to hurt anything going either way on the timing..thanks so much for your post

 

I tend to start low and work up from there turbo/iron head BBC. My 454 single 85mm I started around mid 20's in boost (15-17)
If you go too low it will run like crap - if you go too high while tuning it will detonate. So far so good

 

boosted are you running your timing in the mid 20s at 15-`17psi of boost.Not trying to be to nosey but what kind of timing curve are you currently running as far as where do you start your timing and what kind of curve do you use...We have a somewhat similar engine..thanks for coming on here and helping

 

Its just common practice to start low and go from there. To just throw a number out there seems like a bad idea to me as you have no idea what it wants, or how timing sensitive it is. Add that to the fact that he plans to try the 1/4 mile. I have seen some that live for 660, but don't make it to 1000' with a questionable tune.

 

rickracer for your 2 step do you just set your rev limiter for your transbrake less than what it would normally stall at.For example say when you hit your transbrake and your car would normally stall at 4000 you set your rev limiter for transbrake at 3500 and this rev limiting and or cutting out gives you the 2 step...haven't done this yet was wondering if this is how you do it

 

My stall is supposed to be 3800~4200, when it builds boost it will go higher though, two step is set at 4000 for now....

 

Re: bell curve

a bell curve is a statistical function that shows the distribution of something, however Im using it in this case to help you visualize engine power as a function of timing...

In this case the peak of the bell (x axis is timing, y axis is engine power at a given RPM) would be the timing at which the greatest shaft horsepower is produced. Increasing or decreasing timing results in a decrease in power. The variable slope each way is combination dependent....some combos lose power quickly as you advance or retard timing...some combos have a dwell period where changing timing a few degrees doesn't make a measurable difference. This is a function of many things. The important takeaway here is to stay at the peak or to the low timing side of the peak of the bell curve (some for safety opt to reduce timing from peak power for safety reasons) so there isnt wasted excess cylinder pressure. Do some research on in cylinder pressure traces with excess timing to further understand what I'm describing.



Well evidently my pic didnt work.... just google image search a bell curve.

 

thanks so much for the posting