Bronco Forum - Full Size Ford Bronco Forum banner

1 - 19 of 19 Posts

·
Premium Member
Joined
·
5,396 Posts
Discussion Starter #1
So i've been thinking about forced induction lately.. and i can't seem to find a straight answer to this very simple question... what determines how much boost i can run? is it the static compression ratio, or the dynamic compression ratio?

yes i know there are other factors like head material. combustion chamber design, rod ratio etc.. but i'm thinking "main factor" sort of thing...

everybody seems to talk about compression ratio and boost, but it seems to me that if you have (for instance) a cam with the intake valve closing way way late, you're going to have pretty low dynamic compression, and therefore you could run a ton of boost (even if static was 10:1 or something)
 

·
Age. Fac ut gaudeam
Joined
·
3,881 Posts
I would keep that Bronco Naturally Aspirated... Running boost on a Bronco doesn't bode well...

Course, it could be kick ass! :D
 

·
Registered
Joined
·
9,895 Posts
If you were to go forced induction.. I would do the supercharger way.. To get a turbo big enough to make a difference it would take to long to spool to be effective..


Thought that's my opinion..
 

·
Registered
Joined
·
285 Posts
Static I believe. Most cars shoot for 8.5:1 for turbo. I know diesel are ridiculous though.

Really what determines how much boost you can run is the wastegate :p
 

·
Retired
Joined
·
16,027 Posts
I know my GN is about 8:1 compression and have ran as high as 25psi of boost. At this time I run around 19-20 psi of boost.:doh0715:

I also run pump gas now...;)
 

·
U jelly of my
Joined
·
3,204 Posts
I would keep that Bronco Naturally Aspirated... Running boost on a Bronco doesn't bode well...
I can show you a kenne bell bronco that would make you say otherwise:tinfoil

If you were to go forced induction.. I would do the supercharger way.. To get a turbo big enough to make a difference it would take to long to spool to be effective..


Thought that's my opinion..
Turbo would be bad offroad, and i know your truck isnt a total pavement pounder...


I want to put a KB 2.2 on my truck some day. I'd only run 8-10psi since it'd be non intercooled... thats the other thing, unless you have a IC you cant run alot of boost....
 

·
Premium Member
Joined
·
5,396 Posts
Discussion Starter #7
I wasn't thinking for my bronco.. i was actually thinking for my scout. the reason i ask is because when i built the engine a few years ago, i over-cammed it a bit... i should have done the math, but i just looked at the duration, thought it looked about right and slammed it in.. now it is worthless below 2700rpm, and with a redline of about 4800 (stock is 3800) it's a bit of a problem (although it does scream above 2.7k)

it has a static ratio of about 8.75:1, and i did the math for the dynamic CR last night and it was only about 8:1 (my bronco came out close to 10:1). so i thought maybe instead of re-camming it, it might be more fun to fab up some turbo manifolds, and an intake, and run a pair of small draw-through turbos.

here's the specs on the cam if anybody is interested

Performance camshaft. Great low end torque. Decent vacuum at idle. Fits ALL 266,304, 345, and 392 V-8 engines.
RPM RANGE: 1800-5000
Valve lift is calculated with zero lash and rocker ratio of 1.6
INTAKE TIMING DURATION: 256 deg.
OPEN: 18 BEFORE TDC
CLOSE: 58 AFTER BDC
CAM LIFT: .281"
VALVE LIFT: .450"
EXHAUST TIMING DURATION: 262 deg.
OPEN: 61 BEFORE BDC
CLOSE: 21 AFTER TDC
CAM LIFT: .290"
VALVE LIFT: .465"
LOBE CENTER: 110 deg.
OVERLAP: 39 deg.
CAM ADVANCE: 0 deg.
THE ABOVE TIMING IS CHECKED AT .020 (OPEN) LIFTER RISE .020 (CLOSE)
The following timing is taken at .050" lifter rise and gives a more accurate determination of camshaft position.
INTAKE DURATION: 202 deg.
OPEN: -9 BEFORE TDC
CLOSE: 31 AFTER BDC
EXHAUST DURATION: 208 deg.
OPEN: 34 BEFORE BDC
CLOSE: -6 AFTER TDC
RECOMMENDED ISKY CAM KIT PARTS AND SETUP INFORMATION
LIFTERS: 1902-HY
LIFTER TYPE: HYDRAULIC


Since i was cheap when building my bronco's engine and used the stock rods, it'd probably blow up right away if i tried to boost it.
 

·
U jelly of my
Joined
·
3,204 Posts
I'm not that worried about turbo lag really.. i've driven some pretty snappy turbos, and i think turbos are cool anyway

a properly sized turbo wont have lag, or of course you can run a bi turbo setup, one small turbo to spool up one big turbo
 

·
Premium Member
Joined
·
1,713 Posts
It's still a truck motor so you won't spin huge RPM's.
A small turbo will build plenty of power.
Dynamic compression is what matters. I would look for a larger than normal compressor as you with a high duration cam you will lose some boost pressure. Low duration, med intake, and high exh lift would be best I would imagine.
 

·
Premium Member
Joined
·
1,713 Posts
I know my GN is about 8:1 compression and have ran as high as 25psi of boost. At this time I run around 19-20 psi of boost.:doh0715:

I also run pump gas now...;)
Right on...I know that back when I drag raced a LT1 Camaro (1 year before getting into autoxing) I realized fast how quick the G/N's can be. Even with very mild mods.
I saw a pretty stock GN (not tubbed-caged but full interior) run a 9.4 at 149mph at a GM EFI event in Stanton MI.
 

·
Retired
Joined
·
16,027 Posts
Right on...I know that back when I drag raced a LT1 Camaro (1 year before getting into autoxing) I realized fast how quick the G/N's can be. Even with very mild mods.
I saw a pretty stock GN (not tubbed-caged but full interior) run a 9.4 at 149mph at a GM EFI event in Stanton MI.
Yep it doesn't take much. They are fun.:toothless

Mine is has been doing 7.9 in the 1/8th mile but according to the specs it should turn around 11-11.5 second 1/4. I still have the 10 bolt rear so there is no hook up on the line. I really need to put in a 9 inch.:duh


Oh and no cage and all stock appearing.
 

·
Registered
Joined
·
453 Posts
If your engine as forged internals and quality ones like the diesels do you can run big boost numbers. Static compression is key as is timing so you don't have predetonation. Boost isn't power though, it is pushing in more air so you can dump more fuel in to make the power. If you are running out of a turbo or super chargers efficientcy range then at some point you will heat the air so much it won't help you any. It will compress to 35psi say and the air temp is close to 300, but the volume of oxygen won't be that of 20 psi at 120 degree air. The best results I have seen usually involve a cam that has no overlap of intake and exhaust for forced induction. You want both valves closed so that whatever method of forced induction you use isn't getting wasted by going out the exhaust. It really only matters in high boost areas as 5-6 psi isn't going to blow past as much as 25psi. At 25psi you are making the engine work to spool the turbo or turn the SC, heat the air as it compresses it, having an IC to cool the charge since it was heated, and then blowing your cylinder pressure out the exhaust valve. There is a sweet spot and that is what tuning is about. Depending on the map for your turbo/sc but usually it is around 14/15 psi above that and it is usually deminishing returns.
Disclaimer, most of what I know is diesel related, but most crosses over.
 

·
Retired
Joined
·
16,027 Posts
If your engine as forged internals and quality ones like the diesels do you can run big boost numbers. Static compression is key as is timing so you don't have predetonation. Boost isn't power though, it is pushing in more air so you can dump more fuel in to make the power. If you are running out of a turbo or super chargers efficientcy range then at some point you will heat the air so much it won't help you any. It will compress to 35psi say and the air temp is close to 300, but the volume of oxygen won't be that of 20 psi at 120 degree air. The best results I have seen usually involve a cam that has no overlap of intake and exhaust for forced induction. You want both valves closed so that whatever method of forced induction you use isn't getting wasted by going out the exhaust. It really only matters in high boost areas as 5-6 psi isn't going to blow past as much as 25psi. At 25psi you are making the engine work to spool the turbo or turn the SC, heat the air as it compresses it, having an IC to cool the charge since it was heated, and then blowing your cylinder pressure out the exhaust valve. There is a sweet spot and that is what tuning is about. Depending on the map for your turbo/sc but usually it is around 14/15 psi above that and it is usually deminishing returns.
Disclaimer, most of what I know is diesel related, but most crosses over.
http://www.alkycontrol.com/ ;)
 

·
Premium Member
Joined
·
5,396 Posts
Discussion Starter #19
Well i'll probably just end up swapping in a more mild cam. i'll go with something that will accept a little boost so that in the future if i decide to, i can add some (since i can't do much about the low comp ratio). if i do boost it, it wouldn't be anything crazy, probably just like 7 - 10 psi.

the crank is forged, the pistons are hypereutectic.. but those international engines are such huge over-built monstors anyhow, i think they could handle quite a bit of abuse before melting down (1 INCH between cylinders of water jacket, for instance).

oh well whatever i do, it's on my list right after building the tube body on my bronco.. which i'll do as soon as i get my supercharged explorer running (so i have something to wheel in the meantime) and i'll get that done as soon as i get my lift in... which i'll be working on this weekend (pouring the cement pad) so yeah.. i've got a pretty big list of stuff to do... it's going to take me a while!
 
1 - 19 of 19 Posts
Top