Compression Contemplation - 8 vs 9.5

[CantPutTheSmokeBack]

I'm part way through a build on my truck. I'm currently at the engine, and I'm a bit stuck on which direction to take. I have a recently rebuilt (by a local legend) and running Mark IV 454 in the truck now. It was just a cheap OE style rebuild with flat tops and peanut port heads. I was able to talk to the rebuilder a bit about the engine a while back, but by the time I got around to the point I'm at now he's passed away so I'm unsure of some of the internals. I'm unsure of the bore or the deck clearance. Using an online calculator I'm likely 8:1 to 8.5:1 compression. I have purchased a set of the smaller Speedmaster 305 heads (yeah I know, but they were incredibly cheap) and just picked them up from the machine shop.

So on to my crossroads. I'm considering a piston change and I'm not sure where to go on compression. I'm not trying to run high dollar gas, so the 8:1 would be nice. But more power is always nice too, so I've considered 9.5:1. I've never experienced a before and after on a compression change, so I'm not sure what to expect. Would going higher be worth it? Does driveability change in any significant way, be it good or bad? I know the higher compression stuff is a bit snappier. However I'm not sure if this size change would even be noticeable. Does power scale linearly with compression?

The truck is a 1969 C30 flat bed. 4L80E swapped and corporate 14 bolt with 3.73's. Cam I'll likely go with is a Howards CL123515-10 219/227 @50 .555 lift. It's mostly a driver and will be empty most of the time but will occasionally be pulling a couple round bails of hay home or taking the horses to the vet, so no crazy weight going on.


For those wanting to run numbers for compression, here's what I've got.

Bore unknown I've been using 4.280 and assuming its 30 over
Stroke is stock at 4
Cylinder head volume on my new heads is 119cc
Piston dome TBD by this post
Deck clearance unknown
Gasket thickness .039 on a fel-pro 1017-1


I will, of course, be checking and double checking all of my unknown info before I ever order pistons.

[kwmech]

My best experience running big blocks on the street for working is keeping the compression under or close to 9-1. 9.5 may give you issues in the summer with pre-detonation (pinging) especially with todays fuel. The current 454 in my flatbed came out with about 8.6 or 7, up from the anemic 7 something with a mild 262 cam. Runs extremely strong and if I power brake a little bit can get the duallys to light up from a standing stop. Doesn't take much to warm up a big block from the smog era

[Accelo]

I get 460ci and 8.4 to 1 using the information given.
However, it means little without the gasket thickness & dome-dish even if it's just for valve clearance. If it's rebuilt and has flat tops, consider smaller cc heads in lieu of domed pistons. Domed pistons are not the best as they impede flame travel. You may be better off to change the chamber volume. It takes very little difference on the heads to make a big compression ratio change.
Your mild cam may still kill some effective compression. I would target 9:1 like kwmech suggested.
Many, but not all, replacement pistons have a reduced deck height. The manufacture is fudging in-case the block was decked. Often by .020 or more. If you can measure every thing up and go with the facts. You will not have any regrets that way.
One point of compression is typically worth 2% to 3%. Or about 12hp on a 400hp motor.

A really good compression calculator;
https://dssracing.com/dss_racing_pis...calculator.htm

[cadillac_al]

I'm pretty sure those truck engines had dished pistons with the peanut port heads. If you have flat top pistons then you should have gained some compression.

[CantPutTheSmokeBack]

Originally Posted by kwmech

My best experience running big blocks on the street for working is keeping the compression under or close to 9-1. 9.5 may give you issues in the summer with pre-detonation (pinging) especially with todays fuel.

Ping is actually one of the reasons I even bothered to ask. This obviously isn't some 3000lb car, and I really don't want to do the work twice. I thought close to 10:1 might be the tipping point, but I'll lean on your experience before my guess.

[CantPutTheSmokeBack]

Originally Posted by Accelo

I get 460ci and 8.4 to 1 using the information given.
However, it means little without the gasket thickness & dome-dish even if it's just for valve clearance. If it's rebuilt and has flat tops, consider smaller cc heads in lieu of domed pistons. Domed pistons are not the best as they impede flame travel. You may be better off to change the chamber volume. It takes very little difference on the heads to make a big compression ratio change.

I was initially considering a smaller cc head, and specifically was looking at the Gen 6 factory heads. However, I live in the middle of absolutely nowhere and the closest junk yards are about an hour and a half away. Yard hunting just isn't something I have time for. I could deck the heads, but I'm worried about having to take off an enormous amount and ending up with a port mismatch on the intake. And again, middle of nowhere, closest reputable machine shop that will return a phone call is out by the junk yards. I just got back from them yesterday and I close the shop down by the time I get there after work. It's easier for me to just put a set of pistons in and be done. My other option was a set of new Edelbrock heads with smaller chambers, but I already got the deal I did on the heads I have, so I'm going to stick with them.

Also, on piston dome and flame travel. I was looking at like a 22cc dome if I were down at 8:1. Really not a very tall dome and if I measure and my engine currently comes in at 8.4:1 I would need even less dome. We're talking pretty small domes, and I don't believe they would be much of an issue. That said, I do understand OE's are now doing exactly what you said and using flat top pistons with smaller combustion chambers to get compression up, as it is the better way to go.

I have the heads I have, so it looks like I'll be looking for closer to 9:1 compression. If I pull the heads and it comes out close-ish to that number, I'll just leave it alone.

[Joyridin]

With aluminum heads, you can easily run them on the street. You might get stuck with 89 or 91 octane, but you will not have any problems. If they were cast iron, then you are probably pushing 9.5 without running 93 octane.

[THI]

Open Chamber heads and flat-top pistons were actually 7.8:1, the small dish piston was 7.3:1. The peanut port heads are advertised as 118-119cc and most will measure around 122cc - same with the 781 and 049 large oval port heads. With the flat-top pistons I would look far a set of Gen VI 7400 Vortec heads, with their 99cc chambers you will have close to 9.3:1 and no problems burning low octane fuel.

I have a Mark IV block bored .030 over with 454 HP pistons (13.8cc dome), quench distance of around .040 and 781 heads - this is a 9.0:1 set up just to give you an idea. Since you already have the flat-tops, I would go with a smaller chamber head to increase your compression.