Y block Roots blower

Y block Roots blower

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By dennis22 - 5 Years Ago

Hi everyone,

I’m going to put a Weiand 144 blower on a 272 y block and I’m just trying to work out correct estimated boost. I have used the formula on Weiand’s tech pages (see pic below) but I’m unsure what the 25.58 factor relates to. If anyone could help me out with a formula that would be great. Any other recommendations/experience blower related would also be welcomed.(I have had a search and found nothing specific)

The engine will be just a cruiser, nothing too serious. The project is happening, I just need to figure out how far I will dig into the engine. I am hoping to run a low PSI to keep the engine close to stock and comp down below 12:1.

Here is a list of things that I have planned/thinking about.

- new 1.54 rocker assembly + stiffer springs (still not sure how stiff)
- stock grooved cam or custom grind favoring exhaust (although I have read custom grinds have marginal gains over stock cams <5000 rpm.
- ecz-c heads or G not sure yet.
- head studs
- forged pistons

And all the usual little y block rebuild mods and all bearings changed out, also I have the manifold sorted out too.

I have probably missed some things but oh well.

By LordMrFord - 5 Years Ago

Pistons and connecting rods breaks first. Comp ratio with pump gas should be something like 9:1 not 12:1. Stock engine can handle 6psi quite long.

By Sandbird - 5 Years Ago

I believe the 25.58 is a figure they chose for an approximation of Barometric pressure. One inch of mercury equals 0.49 PSI. The last step subtracting 14.7 converts the answer back to PSI. They probably wanted to keep things simple.

By miker - 5 Years Ago

I ran a 9:1 static comp. Yblock with a Paxton centrifugal, blowing thru a 1406 Edelbrock. Only saw 5lbs at 5000rpm, and nothing until 3000. Used a 270 Mummert cam. That set up liked a 2 degree ignition retard, which came in at 2lbs on a Hobbs switch to an MSD 6 something ignition. Premium 91-92 with some ethanol. Initial and total timing was pretty much standard performance Yblock, 10-12 initial and 36 or so total.

I’ve got an OT car with a small Eaton charger, makes 7lbs right off idle. It too likes a bit of retard from 3000 rpm up, even though it’s still at 7lbs.

So you might consider that when you put the ignition system together. Hot weather and a long pull put me into light detonation, and that’s not good even with forged pistons.

By charliemccraney - 5 Years Ago

Even with G heads, a 272 will not have much more than low 8s;1 compression with flat top pistons so I probably would go with Gs of the two choices.

A custom ground cam should produce far more than marginal results, if you answer all of the grinder's questions honestly and take what they give you (unless you actually do have enough experience to specify the dimensions). The reason being that it will be custom tailored to the whole package.

By dennis22 - 5 Years Ago

Thanks everyone-

LordMrFord- Well I better get some forged rods. Thanks for the comp ratio info. I was under the impression that 12:1 was the max I could get away with but as you mentioned it seems that I’ll need to be down towards 9:1.

Sandbird- thanks, I received a reply from Holley stating that the 25.58 was a universal factor, however they didn’t elaborate.

Miker- thanks mate, yeah I will have to thing about ignition retard module. I’m not sure if there is any difference in fuel quality here in Australia?

Charlie- thanks, I did think the G heads would maybe be better with the bigger valves (even though it’s forced induction) but wasn’t sure if the C heads would be better with the lower compression.
I emailed John M with parts list and recommendation for cams.

By charliemccraney - 5 Years Ago

Stock rods are forged and considering the 40ci diference between a 272 and an F code, unless you will be running some serious boost, stock rods or stock replacements will be fine.

If you do want something beefier, hunt down some C2AE 292 rods.

By dennis22 - 5 Years Ago

Thanks Charlie! Not serious boost, just aiming for 6psi. Weiand’s stock pulleys are sized for about 5-7psi and according to the calculation, applying those sizes to the stock 8:1 272 would give me just under 12psi. so I will have to order a larger snout pulley and I’m going to have to go back to the drawing board to get compression way lower anyway. 6psi puts me up around 12:1.

Anyway thanks for the info. Appreciated!

By LordMrFord - 5 Years Ago

charliemccraney (7/10/2019)

I didnt know the originals were forged. :o
However, I dont know whats the serious boost but we dynoted a stock 292 with 0.7 bar boost with turbo and engine seemed be fine. (c2 rods)
That combination saw no highway use and were intercooled what makes a big difference. A long term heat will be hard for the pistons.
(Combination was destroyed with 1.5 bar boost when one cast piston shattered due the detonation.)

By LordMrFord - 5 Years Ago

dennis22 (7/10/2019)

I just specify, when I said 9:1 comp ratio, I mean static comp ratio.

By miker - 5 Years Ago

I wouldn’t get too carried away with the “theoretical” boost calculations. You’re going to have some losses thru the carb, and more loss thru the intake ports, etc. I know on my blow thru’s there a couple lbs loss between the top hat and the manifold. How much pressure reaches the cylinder I don’t know, but certainly there’s more loss. If you have a static ratio between 8 and 9 to 1, and 5 lbs boost, you should be ok. My OT car with the Eaton on it is a “factory stated 9.4:1 static” ratio. Now it’s a whole different motor, dual oh cams, fuel injection, etc. with a blower sized for its 96 cubic inches. But slowing the Weiand down is a lot easier than rebuilding a engine. Less boost is less horsepower, but as Gaga notes, an inter cooler is a big effect. The heat increase in more boost causes some problem, and an intercooler on your set up is a lot more complicated than his turbo units.

Should be a fun project, and a gas to drive.

By dennis22 - 5 Years Ago

Thanks LordMrFord and Miker,

Looks like I’ll just give it a run at 5-7psi boost and see what happens. I will get an ignition retard module too.

I’ll let you know how it goes. It will still be a while off yet until I get a few bits and pieces together.

By LordMrFord - 5 Years Ago

miker (7/10/2019)

The heat increase in more boost causes some problem, and an intercooler on your set up is a lot more complicated than his turbo units.

Should be a fun project, and a gas to drive.

Yes. Intercooler in roots-blower is not that effective because roots-blower only moves air not compressing it.
Turbocharger, centrifugal and screwcharger generatates pressure and therefore heat in charger it self and air is hot already when going in intercooler.
Roots just moves air forward generates heat and pressure in whole intake system from charger to piston and thats why intercooler located in the middle, cools only 50% compared to other charger types.
Water injection cooling might work better with the roots-charger because it can be located closer to piston.

By pegleg - 5 Years Ago

Keep the cam mild with not a lot of overlap (fairly wide centers) . If you tell Mummert what you are doing and want he can recommend an effective cam.

By dennis22 - 5 Years Ago

LordMrFord- yeah I did read an article the other day saying that intercoolers on roots style blowers (between blower and intake manifold) were pretty much useless. It said that because the air is already compressed once it passed through the rotors that cooling could not make it any more dense, no extra performance gains. it would just cool the air to help with detonation issues. I also read that fuel atomization cools the air by up to 50%. I thought that was pretty interesting.

Peg leg- yeah I was thinking something 112-114 with larger exhaust duration. If John gets back to me I’ll let you know.