[pggto]

I just pulled apart my 71 455 HO engine, it has domed pistons in it. The TRW part number is L2394. I looking for some specs on the piston dome. How many CC's if possible. I have 2 sets of these pistons, the ones in the engine and a new set. They have the same part number but are different. According to some specs I found the new pistons have a -19.3 CC dome. I think the set in the engine are larger. I'd like to figure out what the CR is if possible with the 197 HO heads. The 197 heads are listed as 111 CC's. This engine was rebuilt in 1980, and the car was parked in the early 80's because the PO got tired of running race gas in it.

[grivera]

Have you tried the Wallace compression calculator? http://www.wallaceracing.com/cr_test2.php

[i82much]

can you have the dome removed?

[johnta1]

The old piston dome looks like the one for the SD engine. (thought they had a different part #?)
Most though end up milling those bumps done which would look about like the new piston you have.
The old SD piston had a .476" dome but don't know what the cc is. Supposedly 10.12 at standard bore and 111cc head chamber.


Since you know the new cc's use that in the calculator to see what the CR would be. The old set would be approximately .25/.50 more CR I'd say?

[steve25]

Here’s the info on that piston from my TRW catalog.

The dome is 25.8 CCs which will bring the 111 CC chamber down to 85.2 CCs.
When I run these numbers I get 10.36 to 1 compression.

Bore 4.180” for a bore size of 57.773 cid
Stock deck .023”= .3256 cid
Fel pro 1016 head gasket 9.4 CCs= .5736 cid
Chamber 85.2 CCs = 5.19 cid
Ring land volume 1.5 CCs

Any numbers in CCs must be divided by 16.386 to get to from CCs to CID for this formula to work.

[PAUL K]

At some point TRW/Speed Pro eliminated the two odd bumps on those pistons due to users complaints. Even though those bumps were "added for a reason" as part of the original design. That is why you will find two different dome volumes listed in the catalogs.

Those pistons are plenty strong and a good piston for the time.

[pggto]

Thanks for the info. What do you get for the pistons with the-19.3 dome? The new pistons? Thanks

Were did you find this info to do the calculations?

Originally Posted by steve25

Here’s the info on that piston from my TRW catalog.

The dome is 25.8 CCs which will bring the 111 CC chamber down to 85.2 CCs.
When I run these numbers I get 10.36 to 1 compression.

Bore 4.180” for a bore size of 57.773 cid
Stock deck .023”= .3256 cid
Fel pro 1016 head gasket 9.4 CCs= .5736 cid
Chamber 85.2 CCs = 5.19 cid
Ring land volume 1.5 CCs

Any numbers in CCs must be divided by 16.386 to get to from CCs to CID for this formula to work.

[Stan Weiss]

9.7845:1

Stan

[steve25]

I get 9.78 to 1 with a 19.3 CC dome.

I add the ring land volume which this formula does not include, but it’s nice to be really accurate with the compression ratio and some big bore pistons on some motors that have the top ring pretty far down the piston can have over 2.3 CCs of ring land volume detracting from the compression ratio.

Here’s what the TRW catalog shows for the compression ratio for that piston, but it’s wrong.

First this assumes that the head gasket is round and it’s not, it’s oval so it’s bigger.

2nd the stock deck clearance is not .001”

Also note in that catalog page that TRW list .015” ring groove shim stock, so whatever rings you use with those pistons make sure the back clearance is not excessive.

That bump on the peak of the dome has to go as it’s killing intake flow.

I dont know what compression your trying for, but that bump would be a deal breaker for me and send me off for a different piston that could get me to 9.5 to 1 with a 111 Cc chamber.

[pggto]

That's what I'm after, low 9's for CR.

Originally Posted by steve25

I get 9.78 to 1 with a 19.3 CC dome.

I add the ring land volume which this formula does not include, but it’s nice to be really accurate with the compression ratio and some big bore pistons on some motors that have the top ring pretty far down the piston can have over 2.3 CCs of ring land volume detracting from the compression ratio.

Here’s what the TRW catalog shows for the compression ratio for that piston, but it’s wrong.

First this assumes that the head gasket is round and it’s not, it’s oval so it’s bigger.

2nd the stock deck clearance is not .001”

Also note in that catalog page that TRW list .015” ring groove shim stock, so whatever rings you use with those pistons make sure the back clearance is not excessive.

That bump on the peak of the dome has to go as it’s killing intake flow.

I dont know what compression your trying for, but that bump would be a deal breaker for me and send me off for a different piston that could get me to 9.5 to 1 with a 111 Cc chamber.