Hi guys, this is link to a thread over on Nastyz28.com about my recent experiences with tuning the spark curve on my Weiand 142 roots-blown 383. I thought it might also be of interest to some who frequent this board. I've learned quite a bit about what a roots-blown motr does and does not like as far as an ignition advance curve.

It's way too huge to screen-scrape and drop it into a single post so here's the link:

http://www.nastyz28.com/ubb/Forum8/HTML/005775.html

 

Damon my 9.5:1 Compresion blower motor made more power at 28* total than it did with 32* total.

with 32 it would heat up real quick, signs of closing in on detonation zone. i was running 5PSI.

backing it off to 28 gave me a load more power and it didnt heat up near as fast either. i was using a BTM to retard timing however.

My initial was always 15, with 45* total highway, 36* total without vacuum advance, and 28* total under 5 PSI. the extra initial advance is always a given on motors with big cams, they like the extra timing ive noted. even with blowers. I had a 303/313 cam in it, 234/244 @ .050, and it would eat up the advance. i wouldve run more than 15 but i got starter knock.

The motor im putting together now is a 383, 9.5:1 JE pistons, eagle crank/6"rods, afr 195, comp XE270 roller, weiand 142 starting at about 5 PSI. ill be using the commander 950 to control it, so mesing with the timing and fuel will be a snap. its somthing you should look into... im sure youve seen my post... 500 HP 18 mpg

 

I suspect you're right about a lot or all of that. My old 9.2:1 iron-headed motor with the small cam didn't like total timing over about 28* either. Making one final run with it at 34* before I tore it down there were DEFINITE signs of detonation (no possilbe way of HEARING it doeonate over the blower and exhaust noise, though)

Now that I'm down in the 8.7 range with a a bigger blower-oriented cam and aluminum heads it seems I can bump it up a little further- especially down low. I agree that it has much more of a tendency to "eat" advance down low now.

I think, in general, the more the combo tends towards lower compression and a bigger cam, the more you should "flatten out" the curve. If it's higher compression and smaller cam the more the curve should resemble a N/A setup, just retarded back a few degrees.

I would LOVE to go to an EFI setup like yours. However, funds don't permit. That' more money than I spent on the blower! It would be nice to have that tunability. It would be nicer to have the extra inch of hood clearance it gives!!!! I'd like to squeeze a 1/2" plate nitrous system on top of the blower but I'm just out of hood clearance. I don't want to go to a taller cowl hood, either- this one is nice and stealthy, just the way I like it.

 

My $.02:

8.7:1 with bigger cubes and a relatively small XE cam (the more important part is the XE style lobes and 114lsa), this thing could be building more cylinder pressure then it was at 9.5:1, depending on what the ci and cam was then. Personally, I think that you would have been happier with a 112lsa with similar lobes assuming that you have a very good exhaust on the car.

WRT your timing tinkering, I think that you’re moving in the right direction. We’ve been trying to chase down a stutter and soft launch in my brother’s M-90 blown LTD (.030” over 302, 8.6:1 compression with TFS aluminum heads and a lunati 218/226/112 cam), and went from 12* initial and 36* total (no vacuum advance) to locked out at 30* this week. For the most part, driveablilty got better (ignoring the negative effects on the tune), the car is better except that it’s a little harder to start when hot. It looked like it was going to run faster at the track but it killed the posi…

 

Agreed. I actually went bigger with the cam (216/216 on a 112*LSA to 224/230 on a 114*LSA) along with the lower compression (9.2 -> 8.7) and a change from cast iron to aluminum cylinder heads. Cylinder pressure is not what it was on the old higher cpmpression motor.

My exhaust isn't so good once it leaves the AFR exhaust ports- $50 set of Coyote headers- 1-5/8" primaries into a 2-1/2" collectors into 2-1/2" duals into 2-1/2" Flowmasters.

If I had it to do over again I would have gone with a slightly smaller cam but that's about it. This combo is WAY for forgiving than the old one on pump gas.