Hi guys. I am without a boost gauge in my truck and I'm trying to figure out the exact boost my engine is under.
I'll list all the specs and my calculations and I welcome your expertise and comments. when i went from 351 to 408, i dropped compression, and my mechanic told me boost would remain the same with no pully mods.I dont understand it... as i work it out to have dropped considerably from 4psi to 1.5psi...

Blower; Eaton M112 at 1.84 Litres per revolution
Crank to blower ratio: 2:1 (6" crank, 3" pulley)
Engine C.I.D: 408
compression 8.4:1
30 thou overbore on block

Engine Cubic feet per minute; 647 Cfm (C.I.D x engine max operating rpm 5500) / 3464)

Blower Cubic feet per minute; 714 Cfm (blower Lpm(1.84) x engine max operating rpm (5500) x crank/pulley ratio (2:1) then x 0.03531 to get CFM from LPM)

Excess CFM is the differance between Blower CFM and Engine CFM / Engine CFM then x 14.7 = boost
so this is 714 - 647 = 67 , Excess cfm = 67
67 / 647 = .10355
.10355 x 14.7 = boost, or 1.5222 1.5 psi ??

.: boost = 1.5 psi by my calculations...

so how does the mechanic figure going from 351 with max rpm 5500 to a 408 max rpm 5500 will keep the boost at 4psi without snout pulley modification?
I dont understand it.
I applied this same set of formula to the 351 block and got 4.15 psi.

I look forward to any feedback you can provide, oh guru's of the engine world

- Sean

 

Checking your calculations, you look to be in the right.

The justification for having the exact same boost in a larger displacement engine doesn't sit right. You are flowing more CFM of air, so you'd drop the amount of boost output of the supercharger due to the higher volume of the engine - as you have shown through simple mathematics.

The mechanic may be thinking of the amount of volume not changing within the intake, but there's more to it than that. The volume of the engine has changed, and that's what you're dealing with.

-Kerry

 

Yup - you are right Sean...

So, how is that N/A engine treating you?
Going down to a 2" blower pulley will give you 9.6lb though

 

Thanks both of you. my engine builder is very experienced, but seems to be mistaken here.
so thanks to the math and feedback from u guys, I'm going to have a friend machine up a 2.6" pulley to put me back on 4 psi. this means also that at 5500rpm the blower will be doing 12692 rpm... 692 past its redline. then again the same blower put out 1 year later (gen 4) has a redline of 13000.

I had a look at eaton M112 pulley removers.. and they are $100+
so in the spirit of our friend frederic, I'm going to monkey one together. they were silly enough to show one in detail. easy!
my mate reckons he can turn out a 2.6 from a piece of aluminium disc no trouble at all.

- Sean

PS I'm going to put bore a hole in the burst plate and put a boost gauge on it, just to be sure.

 

The blower you have is undersized for the displacement you have. You can overdrive it but I would recommend a larger unit.

you're in at least 6-71 territory if not 8-71.

With your current compression, you should be able to run 9psi using 87 octane, and 13-14psi with 93 octane.

1-4psi is almost not worth the hassle of fabricating such a thing

 

Probably even more given he is running it on propane...

But the intake charge temps will be horrendous. Thought about water injection Sean?

 

I must have missed the propane statement.

Yeah, much more boost. And tremendous charge temps.

powerstroke intercooler wetted by water or CO2.

 

I would go ahead an whack on a 6 or 8:71 ... but I'm outta money for the time being... this last engine cost me 10500 (complete build below)
and the custom M112 build plus performance LPG conversion inclusive of dual exhaust cost me 9500.
unles someone has an old 6:71 unit lying around they wanna let go for mates rates....


Frederic, cheer me up and tell me the 4psi at LEAST beats the parisitic loss of the M112 unit.

BIGF350, I already have a water injection kit custom made, but it doesnt seem to work(when first used just killed the engine or made it run very rough). the intake manifold has a large area specifically built into it for an air-air, air-water intercooler, or water injection setup. it is like a giant NOS spray plate with a throttle-referenced incremental spray. no matter how much we buggered around with it, it just didnt seem to work.
and finally, expanding LPG = cold, then through the blower, compressed LPG = not as hot as compressed petrol. is the intercooler/injection a must?


engine build specs;
351 cleveland block (Hard Blok filled)
Scat 4340 4.00" stroke crank
Scat 6.00" H-beam Rods
ARP main Stud Kit
Probe forged Pistons (4.030")
Speed Pro moly plasma rings
Clevite bearings
Felpro Gasket set
Russell Oil Pan Modified for Bronco (reversed)
Pro Race steel balancer
Rollmaster timing chain
Brass welsh plug kit
Mellings oil pump
ARP oil pump drive shaft
AFD custom camshaft and lifter kit (mechanical solid roller)
AFD SP4V Extreme Complete Competition Ported Alloy Cylinder Heads
Custom Intake manifold
Platnium Race Shaft Roller Rocker
Crow hardened pushrods (5/16)
Ford Racing alloy valve covers

- Sean

 

Yeah, 4psi would cover the parasitic losses. All I was saying it's a lot of fabricate for little boost.

Propane and boost goes well together... 10-12psi easily with a 10:1 c/r would hurt your neck, and likely that your engine could take it no problem.