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-   -   Old school C/H methods for boost applications? (https://forums.maxperformanceinc.com/forums/showthread.php?t=836959)

Johnny406 01-06-2020 05:28 PM

Old school C/H methods for boost applications?
 
4 Attachment(s)
Gentlemen,

I will kid you not, this is not my area of expertise but I ran into a situation today while assembling my new 428 Pontiac bracket motor. The engine is a factory 4-bolt 428 block and I am using a old 421 s/d forged crank with Eagle 6.625 floating rods. Both the pistons and rods use a .990 pin. So putting in the first couple of pistons into the motor, I noticed by eye the deck height from the piston to the block was a bit large. I confirmed by measuring it and had a WHOPPING .047. Certainly not good for a n/a motor. My good friend Jack Ferris was trying to help me figure out where the F-up was. I eventually dug out the receipt for these pistons that I bought from the late Greg Merrick (Pontiac Gregg) over a decade ago and they say 428 blower pistons. For whatever reason, I thought they had a thicker crown to go with the very heavy duty pistons.

With my digital caliper and by eye centering it, these pistons have a 1.55 C/H. A Icon piston for this application (428 4in stroke 6.625 rod) has a 1.95 C/H which covers the large gap I found.

My question is: Are/were blower builds built with a lower C/H instead of a dish piston?

Tom Vaught 01-06-2020 05:44 PM

In the old days, if you wanted to run a blower you dropped the piston down in the hole and ran a .055" compressed head gasket and you were around that .100" piston to head clearance. Basically no quench but also you were JUST BARELY on the safe side of the detonation area which is from about .070" to .100" piston crown to cylinder head surface.

Depending on the head ccs the compression ratio could be around 8.0 or even 7.5 to 1.

They errored on the safe side in the old days with blowers.

Tom V.

Johnny406 01-06-2020 05:51 PM

Quote:

Originally Posted by Tom Vaught (Post 6097903)
In the old days, if you wanted to run a blower you dropped the piston down in the hole and ran a .055" compressed head gasket and you were around that .100" piston to head clearance. Basically no quench but also you were JUST BARELY on the safe side of the detonation area which is from about .070" to .100" piston crown to cylinder head surface.

Depending on the head ccs the compression ratio could be around 8.0 or even 7.5 to 1.

They errored on the safe side in the old days with blowers.

Tom V.

Thank you Tom, so in my case I'm looking at boost or start over?

Tom Vaught 01-06-2020 06:29 PM

At the minimum a change in pistons and a slight hone job to get the correct fitting of the new pistons. I have weighed pistons once removed from the rods and pins and sometimes you can get away without a re-balance on the bob weight, SOMETIMES.

Tom V.

Johnny406 01-06-2020 06:53 PM

Tough call, in truth I have always wanted and even explored going with a 6/71 or 8/71 or even a centrifugal. I have a blower cam and a blow thru 750cfm carb. Hmm...

Tom Vaught 01-06-2020 07:10 PM

PM to you

Tom Vaught

GTOGEORGE 01-06-2020 07:11 PM

A million years ago ( I was running high12’s) i broke a piston ( I switched heads and had too much compression for cast pistons) went to the junk yard got a piston and rod assembly with rings installed the whole assembly, switched heads(to a bigger chamber and ran it for ever!.....no balancing just swapped it out! Got lucky! :)
Right now I’m changing out 3 pistons.....just plug and play! LOL!

GTO George

johnta1 01-06-2020 07:59 PM

What's the cylinder head chamber cc's?


:confused:

Johnny406 01-06-2020 08:15 PM

Quote:

Originally Posted by johnta1 (Post 6097945)
What's the cylinder head chamber cc's?


:confused:

Was planning on using my 74 cc KRE D-ports but I can also use the 87cc Edelbrocks.

GTOGEORGE 01-06-2020 08:45 PM

Quote:

Originally Posted by Johnny406 (Post 6097950)
Was planning on using my 74 cc KRE D-ports but I can also use the 87cc Edelbrocks.

Go with the KRE d-ports.


GTO George

Jay S 01-06-2020 10:27 PM

You have the parts. If it were me I would deck the block and keep with the same plan.. A lot of the aftermarket are down .030 or better anyway. Boosted or N/A, still ideal to have quench. I have an engine waiting for assembly which we cut .052 off the deck to get it to zero.

I also have one Pontiac engine going together that I used 428 flat top pistons, DSS c/h 1.585, but have a 4.21 crank and 6.625 rods, .98 pin. No one had the pistons design I was looking for on one of the engine projects so I started with flat tops and machined a step type head. It is one of the engines getting the main cap girdle system. Might depend on the piston, but if the crown is thick, that is another possibility.

And yes...putting the blow thru carb in service sounds like a winning idea to me....:)

johnta1 01-07-2020 07:40 AM

The 74 cc chamber heads would probably give you about 9.x cr.
Depends on what cr you want right now?


:confused:

taff2 01-07-2020 07:43 AM

You could get the block decks cut - I've done .019" in the past - then use a thin steel shim/Cometic head gasket,(maybe .015" thick), and the 72cc heads. You could end up with around .045" piston to head clearance and probably 10.5 to 1 ish compression.

PAUL K 01-07-2020 02:19 PM

If I'm reading correct your piston is .047 in the hole?

If So use a .022 gasket and deck the block .029

Johnny406 01-07-2020 03:53 PM

Quote:

Originally Posted by PAUL K (Post 6098159)
If I'm reading correct your piston is .047 in the hole?

If So use a .022 gasket and deck the block .029

Thank you Paul, yes the piston is .047 in the hole with these used Venolia blower pistons. Would .029 also require the intake being milled?

Tom Vaught 01-07-2020 04:07 PM

If you wanted it perfect, if you shave the block (or heads .029) , then you should shave the intake manifold flanges .029" but some have gotten away with not doing that.

Tom V.

AIR RAM 01-07-2020 08:43 PM

Get a set of longer rods to make up for the gap... of course that would require a re-balance... but it seems to be a good excuse to get a light weight set of I-beam rods...


SPEED SAFE, NICK

Tom Vaught 01-07-2020 09:19 PM

I personally would not put light weight steel rods in a Boosted engine but to each his own.
Custom rods for the length are not cheap.

Tom V.

AIR RAM 01-07-2020 09:42 PM

Quote:

Originally Posted by Tom Vaught (Post 6098274)
I personally would not put light weight steel rods in a Boosted engine but to each his own.
Custom rods for the length are not cheap.

Tom V.

Agreed... I for some reason thought it was intended to be an NA build and boost was only considered because of the situation...

SPEED SAFE, NICK

Johnny406 01-08-2020 04:20 PM

Thank you gentlemen for all your time and suggestions.

I've been and am still kicking this over. It seems my most logical options are:

Cheapest: Have the block milled .030-.035 $180.00 locally and knowing I'll likely have to have any intake I use milled and any future build on this block will require custom pistons

Not as cheap but probably the wisest: Order some new/custom pistons (Paul K) with the higher compression height and a .990 pin and hope I can get them weight matched.

Another thought: Buy a set of BBC 6.660 .990 pin rods and have my crank turned down to a 2.20 BBC journal and have it rebalanced though I'm not thrilled of cutting a rare 990 forged crank that much

Last idea: See if somebody wants to trade their Eagle .980 Pontiac rods for my Eagle .990 Pontiac rods and I'll match off the shelf the pistons by weight the best I can and likely re-balance...

What a headache!!


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