My vacumm numbers are low on my fresh 170 that has a H-P intake,390 Holley(with a 31 squirter),CompCams 264s(degreed straight up---not just dot to dot)MSD6a,DutraDuals,15 degrees initial advance with 34 total and the valves have been adjusted properly.The compression is 9.2 to one.
This engine has not been driven on the road yet,just had the cam broke in and idled in the shop and initial tuning done so I realize that the rings still need to seat but My vac numbers at idle are 13-14 inchs and that seems awfully low to me.When I rev the engine and it comes back to idle(900 rpm) the vac will hang at 15 inchs for a couple of seconds then drop to 13-14 and remain steady and it idles very smooth.The idle-air screws are off the seat one full turn each and this seems to be the "sweet spot" for this setup.
Will my vac increase after driving it or does this cam have that much of an effect on vacumm(I thought the 264 was a mild cam) or is this a effect of the Hyper-Pak intake or am I worried about nothing???
Any insight on this would be greatly appreciated.
HyperValiant
p.s.Also would the fact that this is a 170 insted of a long stroke 225 be the difference.

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1960 Hyper-Pac Valiant(rolling test-bed)
1963 Valiant V2OO(Son's summer project,- he just turned 15 )
1972 Valiant 4dr(Daughter Kelly's repair in progress)
1974 Valiant (v8) daughter Kelly's work in progress

Actually, that doesn't sound that bad, maybe a little low. At least its smooth @ 900rpm. A little known fact is fuel/air mixture can influence idle vacuum. You can also check cylinder leakage. I always vacuum check the ports on the heads I do, before I assemble the engine.

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'66 Cuda 225/4spd
'66 Dart GT convertible 225/auto
'64 Dart GT 340/4spd

I expect your fresh engine is still "tight". As it wears in over the next few hundred or even 1000 miles it will loosen up and then your idle vacuum will come up too.

Gosh, when I had mine rebuilt back in 1986 the shop must have had everything at minimum clearance. It ran hotter for a few years and the fuel mileage was down and at idle you could tell when each cylinder fired because it would make the car vibrate at engine firing frequency. When I turned off the key the engine stopped right NOW, no coast down. Now 80,000 miles later it is running fine, the operating temp is back to normal, the fuel mileage came back up, and it purs like a kitten, and has 45 PSI of oil pressure at hot idle with a stock replacement Melling oil pump.

my fresh rebuilt 225/6 was poor on vacuum first 2k miles. now I'm 3k and vac has increased a li'l. Another thing I did is recurving the dist, so now I'm 14* initial. That helped a lot. sometimes custom cam profiles can cause "poor" vac readings below 1k rpm. Don't freak out! it will improve!.

Best luck
Juan

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Juan Ignacio Caino

Please use e-mail button istead of PM'ing. I do log in sometimes but I'll be answering quicker thru e-mail.

Doc keeps saying that slants don't need much more than 30* total timing advance. Any special reason you're pushing 34*.

I've never done extensive testing, so I like to ask when someone does something different than I would expect.

Sounds like it's gonna pull pretty hard once you get it bolted into something.

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1980 Aspen 225 super six

Slant6Ram,the reason behind 34 degrees is that this is a short stroke 170 and can stand a little more timing than the 225.Also it is bolted in somthing,a 1960 Valiant V200.
BTW,thanks for the replys everyone.
HyperValiant

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1960 Hyper-Pac Valiant(rolling test-bed)
1963 Valiant V2OO(Son's summer project,- he just turned 15 )
1972 Valiant 4dr(Daughter Kelly's repair in progress)
1974 Valiant (v8) daughter Kelly's work in progress

???
Backwards.
Smaller chamber volume for same compression ratio +
smaller piston movement for each degree of crank rotation +
much higher rod ratio (1.826 vs. 1.624) =
LESS spark advance.

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Panic,that info was suggested by another board member,according to my specs,what kind of timing would you suggest?
I would appreciate your knowedge on this matter.Thanks.
HyperValiant

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1960 Hyper-Pac Valiant(rolling test-bed)
1963 Valiant V2OO(Son's summer project,- he just turned 15 )
1972 Valiant 4dr(Daughter Kelly's repair in progress)
1974 Valiant (v8) daughter Kelly's work in progress

nobody asked me but I love to carve in

I'd use 14 initial 28~30 total mech plus a 6R vacuum pod.
You have to fab a 8R dist governor
and I wouldn't use a fast-fast curve, I'd use a rather moderately faster advance curve on that engine. Say, I wouldn't use 2 light springs, I'd keep one stronger, yet a li'l lighter than stock heavy spring.

take care! juan

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Juan Ignacio Caino

Please use e-mail button istead of PM'ing. I do log in sometimes but I'll be answering quicker thru e-mail.

The main reason I run more total advance with a 170 is that they can rev. a lot higher then a 225.
30 degrees total advance is a little "short" for an engine the can rev to 7500 RPM.
As with all distributor curves, you have to do some testing to see what the engine really likes.
DD

Thanks for the info Doc,Im gonna leave mine alone until I get some real world testing done.
HyperValiant

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1960 Hyper-Pac Valiant(rolling test-bed)
1963 Valiant V2OO(Son's summer project,- he just turned 15 )
1972 Valiant 4dr(Daughter Kelly's repair in progress)
1974 Valiant (v8) daughter Kelly's work in progress

doc, so if the engine revs higer it will need more total timing? how come? ain't that would cause a leanout say, below 5k or 6k rpm? how can I test how much my 225 likes to rev? just the old way of full throttleing it and tach reading? or there's another way that I'd feel less concern in my nuts? I'm running 32 approx total, with lots of initial (15) still starts fine in the cold mornings... but I notice some "dead" or not so peppy spot when trying to regain speed from 25 to 50. Could it be the curve being too fast?

thanks for your advice in advance.
Juan

_________________
Juan Ignacio Caino

Please use e-mail button istead of PM'ing. I do log in sometimes but I'll be answering quicker thru e-mail.