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I have a fresh engine build I have put into my dedicated drag car, combo is a 414cid sbc single plane intake, solid roller cam 710 lift 300 duration.
I run 110 race fuel, no knock detection device and no access to a dyno at this time. Ignition is controlled through MSD Grid, the grid has a linear spark map Timing x Ignition.
The engine revs out fine without load, however once on the trans brake at around 2k it starts sputtering and spitting out the throttle body, low rpm high load. Currently the timing is 15* 0-1k 1k-4k it ramps to 25*.
it would seem to me I need more ignition timing in the lower rpm range, but without any knock detection device I am a bit leery of throwing too much timing at it.
in low load low rpm its hard to get engines to knock, make sure your timing is synced,beside that you are running race fuel you may never reach knock threshold.
add timing at idle until the rpm stops increasing and remove 5-10 degrees and use that 5-10 degree for idle correction(overspeed/underspeed)
i think sbc needs lots of timing than what you are giving it, probably 25 at 0-1k, 33 at 3k-6k
I will give this method a try and see how it responds to a little more timing.
Yup, traditionally, you're going to be looking at as much as 20@1k rpm, coming up to ~34 (could go as high as 38 at 4k, depending) around 3-4k. But it's something you're going to have play around with, yourself. If you're reasonably happy with what you have, try adding a couple of degrees at a time and see how the car reacts. Depending on your CR, with that fuel you should be able to go over the torque peak before detonation kick is, so it should show up in times first.
Regardless of timing, it shouldn't be spitting back like that - I'd suspect a valve issue (clearance or leaking) or ignition breaking down.
2k seems rather low for holding on the transmission 'brake', what's the stall, as it would normally be a little below the torque peak, so there is a little rear load transfer before it hits hard - if you have the traction, having it stalled at the torque peak 'should' give the best launch.
Timing sync was a bit off so that timing number was actually lower than 15* at 1k it was around 8*.
The converter is setup for boost, should stall around 4K with 1000hp. I was able to stall it much higher with around 650hp with my old setup. So it’s a pretty loose converter In that regard.
I have sense moved the timing to true 15* at 1k 25* at 3k increase, this helped a lot I was able to stall the converter at 3k no “spitting” or bogging is a better description. The result of this is that the headers start to glow red after a short amount of time of wot on the brake. From what I understand it needs more timing, and an eye on the AFR.
Oh, sorry, I was thinking NA, not boosted, for the timing - my suggestion 'might' be a little excessive.
Currently it is NA, however the cam and converter is setup for turbo. I am trying to setup a base timing map before I put the turbos back on.
This was all from first start up on a new engine build, set timing conservative so I could get on the trans brake and load the converter to break in the rings. I knew it would be down on power but I don’t expect it to run so poorly. I would read articles where people claimed to be running timing as low as TDC at idle.
I guess my real concern was that the duration and overlap of the cam creates a smaller timing window?
its different from engine to engine, engines that have mbt timing around 10 or 15 might idle at tdc, but your engine mbt might be around 20-28 so tdc is way too retarded for your setup,
glad you got it running