manifold vacuum
 

ok, so i was talking to my stepdad about a 350 i wanna build for my car, but he told me about the need for manifold vacuum in order to run my brakes. we dont know how to make sure that the cam is going to supply enough of a manifold vacuum to allow the brakes to work. in fact he doesnt even really know how it powers the brakes or what creates a higher vacuum. so anyway, i wanted to find out how i could tell if my engine setup would kill my brakes or not. can anyone help me out with that? the cam i've got set up in my dyno program right now has 219.1/233.3 duration with 5.5 overlap and 110.3 lobe seperation. the more exact specs are as follows:
IVO 0.3
IVO 38.8
EVO 48.1
EVC 5.2

can anyone help me out? i'll be using a TPI 350 putting out 462 ft/lbs of torque and 396 hp so i'd hate for something like this to stop me from building such a monster 350 and spankin all the hondas around here

ok, it's pretty obvious that i actually meant to put IVC at 38.8, not IVO twice. anyway, sorry bout that

Try running the cam on a 112 LSA and it should make more than enough vacuum to operate the brakes. :)

You shouldn't have any problems. Plenty of guys are running more aggressive cams, including me, with no vacuum problems. I'm getting about 12-13" and the brakes and AC controls operate flawlessly.

I'm running a 235/235 (at .050") on 112 LSA with plenty of brake boost. The only thing that would concern me is how well TPI will react to a 110 LSA. TPI generally likes a wider seperation...it's very vacuum-sensitive.

i'll actually be running a Stealthram intake system (hopefully)
so how would I create a wider lobe seperation angle? keep the same duration and open the exhaust valve earlier and the intake valve later?

so is lobe seperation the biggest factor in manifold vacuum?

Too much overlap is what kills your vacuum. Overlap depends on both duration and LSA, so if you want to keep the same duration you'll need to widen the lobe seperation. Open the exhaust slightly earlier and the intake slightly later to widen the LSA.


Honestly I don't think you'll need it though. The shorter runners of the HSR will conduct plenty of vacuum to the plenum and MAP sensor (so the EFI won't be as picky about it,) and you only need 10-12" to run a brake booster effectively. With less than 220 duration on the intake side you shouldn't have enough overlap to seriously reduce your vac levels.

awesome-thanks for your help! do you by chance know the CFM rating of the HSR? Ive been trying to find out cause i dont wanna buy it if it's gonna be too big for my mid-range 350. my desktop dyno program claims i'll get 464ft/lbs and 396hp. you think those are actually practical numbers on 10.5:1 compression and 92 octane gas? this is my first engine build up and i have been researching each part that i've picked out for a couple of months now and Im trying to learn everything i possibly can (i read john lingenfelter's book on modifying a small block chevy. VERY good) and the only thing that doesnt really seem to piece together is the high power numbers that this program has claimed for me on a streetable engine. what do you guys think?

thanks again!
anyone know where i could get an L98 or a T-56 from an LT1 camaro/firebird/corvette?

Honestly I don't have any idea what the StealthRam will flow. It's really just your basic tunnel ram with a TPI-style throttlebody flange and some injector ports instead of a dual-carb plate, so I'd say the 400-450hp range would be a fair guess of it's factory potential.

As for the numbers you're getting.....I think they're a little bit high, but depending on which heads you're gonna be running I'd say 425ftlb/375hp is there to be had with some tuning.

those numbers were claimed using airflow research 190cc heads and 1-5/8 headers, stock 1.5" rocker arms.
im gonna see what i get if i change it to a tunnel ram intake with a 750cfm rating. lets see....

ok i got 448hp@5500 rpms and 468ft/lbs@4500rpms. with that setup it's got 407ft lbs at only 2000 rpms! ok so lets see what happens when i put in 112 LSA....

ok we've got 112.3 LSA with the same 750cfm tunnel ram and we have numbers of 445hp@5500 and 468ft/lbs@3000

both of those setups have over 400ft/lbs of torque from 2000 all the way to 5500rpms.

what do you guys think of these numbers? i mean i've entered info from crate motors that i've researched online and all of the numbers my program gives me come up to just about what the crate motor is claimed to have (most of these engines are from the jegs website or edelbrock crate motors)

any input is appreciated