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What has been everyones experience with best timing for a mild built 351m? I have a fresh build with .040 over. Stock exhaust manifolds with dual 2.5" and glasspacks. Weiand intake and 650 AVS2 carb and HEI dizzy. The cam is Comp flat tappet High Energy 218/218 268H. - I am right now at 14° initial 36° total. Vacuum advance is on ported vacuum all though on most other cars I have owned I run manifold vacuum advance. With the c6 auto and power brakes I think the low compression engine and this cam doesn't like the manifold vacuum advance. The idle fluxuates too much and wants to die in gear unless I turn up throttle. Ported seems to work better. Anyways I am in Denver and 5600 ft elevation. Carb is jetted perfectly. Wondering what everyone runs with a similar setup on timing. Thank you
I think you are just about right at 14 at your altitude
You run as much timing as you can get away with
I ran 18 in SLC on my old Z-28 with a 302 Chev (course it was 11 to one compression with a roudy solid lifter stock cam)
I only run about 12 degrees on my Boss 302 with factory pistons and a factory cam (roughly 11 to1 with the same cam specs as a Z-28 basically) .485 lift 290 duration
I assume you have pinging and dieseling at your present timing set of 14 degrees?
Rule of thumb is add 2 degrees for every 1000 feet of elevation
As you probably know, The 351m was just under 8 to 1 static compression stock. “IF” you used cheap rebuilder type dished pistons which have a lower compression height and stock heads then you’re down in the 7ish to 1 static. With the 268 cam your dynamic compression will probably be around 6 to 1. That coupled with your elevation will result in a bit harder fine tuning and lower than typical power/responsiveness.
As you probably know, The 351m was just under 8 to 1 static compression stock. “IF” you used cheap rebuilder type dished pistons which have a lower compression height and stock heads then you’re down in the 7ish to 1 static. With the 268 cam your dynamic compression will probably be around 6 to 1. That coupled with your elevation will result in a bit harder fine tuning and lower than typical power/responsiveness.
The builder says it is 8 to 1. What would be the better approach here moving forward? I can throw some money but I don't think I can step up to expensive pistons etc. I do have a 400 crank - its old and will need to be turned.
That sure is a perfect ratio for a supercharger or turbocharger
It would run like a raped ape and not cost a fortune IMO
Look into an annular nozzle carb for it if you go that route.
The builder says it is 8 to 1. What would be the better approach here moving forward? I can throw some money but I don't think I can step up to expensive pistons etc. I do have a 400 crank - it’s old and will need to be turned.
Manicmechanic is giving some good info there….
For naturally aspirated engines Smaller cams work better with lower compression due to the valve timing events. Something similar in specs to say the xe256h comp will build more cylinder pressure , so to speak, than the 268h. You can even install your 268 advanced to pick up a bit if you’re set on using it since it’s probably new. That is assuming you have a good quality new timing set that can be advanced, which you really do need on these or any low compression engine.
Also Don’t rule out a head swap in the future. I’d much rather spend my money on a good set of heads as convert to a 400. You can even mill your block deck and heads to a certain extent to pick up some compression and lower the (piston to block deck) deck clearance, but this can get expensive depending on your machine shop and would obviously require a tear down for the block deck.
So there are options depending on your situation. You may even be satisfied with the way it runs now once you get your ignition timing right.
Thanks @JalopyJake. its not that my timing is off - I think I got it about as good as I can. I posed the question mostly out of curiosity and if there was anything I might be missing. Would going to a roller cam help any? I was considering doing the same 268H in a roller version. I really like you idea of focusing on the heads. I have seen these Speedmaster options that are not too bad $ https://www.summitracing.com/parts/s...waAovbEALw_wcB
Thanks @JalopyJake. its not that my timing is off - I think I got it about as good as I can. I posed the question mostly out of curiosity and if there was anything I might be missing. Would going to a roller cam help any? I was considering doing the same 268H in a roller version. I really like you idea of focusing on the heads. I have seen these Speedmaster options that are not too bad $ https://www.summitracing.com/parts/s...waAovbEALw_wcB
I just quickly glanced at those heads and don’t know all the details right off but did see they have 64cc chambers. They’ll bump about a full point of compression (and add a modern high efficient design) compared to 76cc open chambers so “IF” your builder is correct in saying you’re at about 8 to 1 compression now then you’ll be at about 9 to 1 with that chamber size. That would suit much better with your cam specs.
BUT….Definitely research those heads and make sure the ports are not raised/rearranged from stock to where you couldn’t match a decent intake and maybe headers if you’re wanting them.
I think those heads might make it come alive
They look like small port C heads to me that should work with a 351-400M manifold
Do call them though and double check
Damn straight going to a roller cam will help (less than a third of rotational force required to turn one) compared to a flat tappet
Hard to get a mild roller cam for a Cleveland block though isn't it? and the expense? I thought you said budget
That should work real well.
I had no idea that decent made parts for those motors yet
Good on Comp Cams
So if you can afford that cam and those heads you will be doing it right I believe
Might even make some big HP like those motors should have all along
Like to see the specs on the cam (and the heads too) but if they say they all work and match up, go for it
I like the 10cc reduction in combustion chamber. that should help compression. I am also in colorado and your timing seems about on point, I like to get my engines going up one of these hills while up to temp and try to accelerate and listen for detonation. That is where it will have it and even with a load or trailer helps determine timing... Or atleast if its too much.
I unplug the vacuum, block the hose to the carb, rev it until the mechanical is all done, I shoot for 34* BTC. My mechanical is all in by 2500, actually just over 2,000. Shut her off and reconnect vacuum adv to ported.
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