Pistons down hole

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Nick payne
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Pistons down hole

Post by Nick payne » Tue Sep 12, 2017 11:45 pm

Has anyone ever ran pistons .175 down in the hole to lower compression for boosted applications? Trying to figure out which way to go on my build. If so pros/cons? Thanks in advance.

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Re: Pistons down hole

Post by bigjoe1 » Tue Sep 12, 2017 11:47 pm

It is a very common practice with boosted engines- NO PROBLEM




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Re: Pistons down hole

Post by Mark O'Neal » Tue Sep 12, 2017 11:49 pm

Yes. It was normal back in the day for SB Chevy's with roots blowers.

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Re: Pistons down hole

Post by Nick payne » Tue Sep 12, 2017 11:51 pm

Guy at diamond pistons told me it would be so inofficient that it might not even idle so it kinda had me worried. Thanks for the advice.

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Re: Pistons down hole

Post by MadBill » Wed Sep 13, 2017 12:02 am

The Lenoir I.C.E. (circa 1860) ran with a 1.0:1 "compression ratio"...
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Re: Pistons down hole

Post by pdq67 » Wed Sep 13, 2017 12:33 am

MadBill wrote:The Lenoir I.C.E. (circa 1860) ran with a 1.0:1 "compression ratio"...
How about the 1st diesels running on coal dust.

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Re: Pistons down hole

Post by mag2555 » Wed Sep 13, 2017 7:53 am

I don't see .175" as being a issue, but anything over .200" I sure would!
You will however need real darn good low lift flow on the Exh side though!

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Re: Pistons down hole

Post by pamotorman » Wed Sep 13, 2017 7:54 am

I believe ford BB 360 truck engines had the piston down in the hole .250

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Post by dwilliams » Wed Sep 13, 2017 11:31 am

I did a 4.1 turbo Buick with the pistons about .300" down.

When the combustion chamber is small and you need to get the CR *way* down, you run into wristpin height and ring pack issues, as well as finding someone with a piston blank with a top thick enough to cut a deep enough bowl.

Sure, you lose quench by dropping the piston down, but at 20PSI there's plenty of turbulence anyway, and you can go nuts porting the chamber to venturi or unshroud the valve heads.

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Re: Pistons down hole

Post by Nick payne » Wed Sep 13, 2017 12:15 pm

Do you think this will also work on a low boost street application?

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Re: Pistons down hole

Post by PackardV8 » Wed Sep 13, 2017 6:19 pm

Can't answer without knowing your static and dynamic compression ratios.
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Re: Pistons down hole

Post by Geoff2 » Thu Sep 14, 2017 6:07 am

I wouldn't be buying pistons from Diamond pistons then......
When the BB Chrysler went to an open chamber head in 1968, the chamber surface to the piston top would have been close to....0.170", possibly more. And they idled just fine. There was NO quench pad at all. The chamber recess was about 0.110 - 0.120" deep, 0.02X" head gasket, & the pistons were down in the bore.

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Re: Pistons down hole

Post by psychomotors » Thu Sep 14, 2017 7:40 am

Geoff2 wrote:I wouldn't be buying pistons from Diamond pistons then......
Doesn't mean that they don't have a quality product , it just means the tech (as usual) doesn't know what he is on about. :roll:
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Re: Pistons down hole

Post by F-BIRD'88 » Thu Sep 14, 2017 8:42 am

Another way is to just stack up head gaskets. (re torque head bolts)
AND/OR use a thick copper head gasket (Custom?)

V8 intake now sits higher (port match) distributor hole sits higher pushrods effectively get shorter...But it gets it done.
The stacked gaskets can act somewhat like a safety fuse reacting to over boost detonation.
The head gasket gives out, by detonation, before the piston explodes.

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Re: Pistons down hole

Post by piston guy » Thu Sep 14, 2017 11:29 am

On BOOSTED applications ONLY , quench is not a factor like it is on naturally aspirated combinations. A round dish is more powerful but when the volume needed gets too great , a flat top "down in the hole" is the only alternative. The boosted charge has so much "mixture motion" is my opinion as to why it works. Been doing it for at least ten years. It was tough at first to convince "old schoolers" it would work , but it does.

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