hey all;
I've searched this topic and still haven't got a clear idea on which would be better for my application, 1 5/8" or 1 3/4"
combo is in sig, will the larger cubes want larger primaries? its mainly a street car, so I'm thinking the 1 5/8" should promote quicker spool, but with 393 cubes, is the 1 3/4" a better choice? looking to make 700-800hp
thanks,
Rich-

 

In a fox, if you have the room, hells yes I'd go with 1 3/4. I think the difference in spool would be negligible. Shooting for a little higher HP that what you are after, not much. But with a smaller motor also. Going 1 3/4

 

How big are the exhaust ports on the heads? If a 1 5/8" tube is going to be a restriction over the hole, then definitely go with 1 3/4". It's been proven many times before that you can make big power with 1 5/8", but if there's any chance it could hinder the power, then it wouldn't be worth it.

 

x 2
1.75 is heaps
seen sbc 1800 hp with 1.75
ex pressures ok..
the turbine , and turbine outlet is the restriction

 

not sure off hand, theyr'e stock victor jr. 70cc heads, I'll need to go measure them(they're at a friends being assembled) unless somebody already knows-the chambers are bigger, otherwise they're identical to regular victor jr's(2.05/1.60)
thanx,
Rich-

 

just got off the phone with edelbrock tech, they said the exhaust ports are 1.31" tall and 1.31" wide???
doesn't seem right.....will measure my heads

 

1 5/8" primary size will cover the exhaust port(that's 1.625" and the port is approx 1.35")
so, should I go with 1 5/8?
I guess it's not much smaller than 1 3/4" which is 1 6/8" for comparison, only 1/8 of an inch bigger
plus, shorty headers are much easier to find in 1 5/8"(will be using a set turned forward)
any issues with 1 5/8" with 393 cubes?
thanx for you input,
Rich-

 

No issues. Guys go 8's with logs. So I don't think you'll look back and wish you had.

 

boosted applications dont rely on scavenging like n/a cars do so basically the bigger the better.

 

I was wondering the same thing because I can't find any 1 3/4 shorties for sale anywhere but brand new. I run the same heads but ported on a 347 with 1 3/4 longtubes.

So you're saying you measured some headers and you heads and the 1 5/8 ports are bigger than your heads?? If so, I'm sure I'm still in there.

I'd love to find a set of 1 3/4 shorties for sale somewhere................

Dave

 

just a fyi on a 351w based motor. when using flipped shorties on the 351 use 302 headers. they clear the oil filter in the stock location. i've done 2 setups like this and it worked good. if you use 351 shorties they hit the filter

 

the exhaust ports on the vic jr's are approx 1.31" x 1.31", less than the 1.625" that 1 5/8" primaries are, so they should cover the ports. I dont have 1 5/8" headers to measure but they should be ok

 

thanx,I was wondering about that...we were gonna use the 351 shorties to fire it up n/a first, then build the turbo kit....I already have the oil filter relocate kit, so that should be alright?

 

should have height/width measurements of ports on a 1 5/8" shorty tomorrow

 

You guys are mistakenly mixing engine size to primaries diameter in a turbo motor with a naturally aspirated engine. In a turbo motor the diameter of the primaries should be the same as the port opening in the head. if you open it larger you'll lose exhaust stream flow velocity, only to have to speed it back up once it reaches the turbine wheel perifery. It's after the turbine discharge where you'll want to use a 30-degree included angle type conical diffuser before necking down to a plumbable diameter to run the exhaust. This will convert the turbulent flow to smoother a smoother laminate flow. Source; the book TURBO, by Jay K. Miller.

 

I use 1 3/4 that B&G made for my 383w with Canfields. They fit great!

 

turbojon,
I'm just concerned with the exhaust ports being larger and causing a turbulence in the exhaust with smaller header primaries. I have long tube 1 3/4 headers on my N/A setup now and don't have the measurements for anything. With his sizing of 1.31 x 1.31, I can get the measurements from a set of headers and just use 1 5/8 shorties if they will not cover the ports.

I don't have the book but are you saying that it reads to have the down pipe reduced? I have a 5" flange and planned to run a 5" DP. I also have a 5-4 inch reducer but didn't plan to go that route. I thought you wanted the least possible restriction in the exhaust after the turbo to keep any back pressure on that side down? I understand velocity but I thought it was only an issue before the turbo.

Interesting.

Dave

 

have seen good results with slightly bigger flange and 2" of primary slightly bigger than port then reduced..
Helps exhaust flow and reduces reversion..
On my 331 I have 1 5/8 for 40mm then reduces down to 1 1/2 using steam pipe , then to 2" collector 90* to turbo.. Yes sounds way too small ?? Having twin turbo help also..
I have near 650 rwhp... The hydro cam seems to be what limits rpm..

 

Dave,

Sorry I may have confused you. When it comes to turbine outlets, yes you do want to reduce restrictions because the entire turbine section sees mass-flow all the way through the system. What I was describing is called a conical diffuser. Ideally the exhaust gases should be flowing axially, but they are not. There is still inertia imparted onto the exhaust flow thus when exhaust leaves the exducer it is violently swirling. This swirl tends to impede exhaust flow capacity because it has a higher force component that is perpendicular to the desired flow path. By installing a conical diffuser the swirl flow dissipates and becomes more axial flow in nature which relieves the turbine discharge pressure backup. Now, bernoulli's law will also tell you that when you slow a gas down you increase its internal pressure, which is exactly what's going on in the compressor's diffuser section. When I referred to necking the down pipe down in size I was referencing that, in many cases, the conical diffuser bell-mouth shape may become larger in diameter than you can comfortable run and route through the available path in your vehicle. but if you've done a conical exhaust diffuser that left you with a bell mouth end of say 5-7 inches, that's ok to then gradualling reduce it back down to what ever size is largest that you can fit into your available path for exhaust tubing.

If you know someone with the book that area is covered in the chapter on desiging a turbo system. I think it's chapter 6, not sure.

Good luck with your project!
Cheers!

 

the shorties measure: height =1 5/8" width=1 1/4"
and match up to a fel-pro 1415 gasket for reference
height will be alright, width is 1.25" where the port is 1.31" wide
that 6 thou should be easy to get with a grinder, no?
that way 1 5/8" headers albeit widened slightly will work...
Am I correct in my thinking?
thanx,
Rich-