Engine Oppos: need clarification

"My bird IS the word" (mybirdistheword) 07/25/2018 at 14:06 • Filed to: None

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So as mentioned my engine is going to need bored out, and new pistons to go with it. I have it narrowed down to ordering some nice high er compression pistons to rectify one of the Olds’ biggest problem of a 7.9:1 CR. Might as well since I am spending the money. The question is that only one part number is listed, but two compression ratios. How do I determine what I am actually getting as well as what to order?

https://www.uempistons.com/index.php?main_page=product_piston_info&cPath=3_4_23&products_id=2875&zenid=f8345ff3befce673401df184b2b7df65

Of specific mention: listed is both 83CC as well as 96CC, with matching expected CR. I would love to go to 9.5:1 but have no idea how to achieve that. I would like to order these so I can get the ball rolling.

Trans am convertible for your time.

Steve is equipped with Electronic Fool Injection > My bird IS the word 07/25/2018 at 14:18

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7.9:1 compression ratio? Hmm... that sounds more like it’s time for some forced induction.

Sovande > My bird IS the word 07/25/2018 at 14:19

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I don’t know that you can get a 1.5 point increase in compression from pistons alone. I would call the company and ask what they recommend. 

My bird IS the word > Steve is equipped with Electronic Fool Injection 07/25/2018 at 14:19

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Can’t handle it, block is too weak.

crowmolly > My bird IS the word 07/25/2018 at 14:19

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The question is that only one part number is listed, but two compression ratios. How do I determine what I am actually getting as well as what to order?

Simple.

It’s only one piston design. There are two different compression ratios listed because your static compression ratio will change based on what cc your cylinder head combustion chamber is.

If your heads are 83cc, you get that ratio with these pistons. If it’s 96cc, you get that ratio. If you know which heads you have, look up the stock combustion chamber size.

Head gasket thickness can bump compression too. As can zero decking the engine.

LOREM IPSUM > My bird IS the word 07/25/2018 at 14:20

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Idk about the p iston thing, but if you mill the heads it’ll bump the compression ratio as well. Worth considering, while you’re in there.

crowmolly > Sovande 07/25/2018 at 14:23

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Depends on the stock piston design. If it’s a deep dished stocker and you are putting in a big dome you can do it.

RickSanchez:Fury Road Edition aka Junkrat > My bird IS the word 07/25/2018 at 14:24

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Without CC’ing your head and knowing deck height it is pretty hard to make a decision on CR. Rod length and stroke come into play as well. The machine shop that does the block/heads will be your best bet. Usually for part numbers there is a dash something at the end that tells ring size/relief volume. Take a look at this.

http://blog.jepistons.com/how-to-calculate-engine-compression-ratio-and-displacement

RickSanchez:Fury Road Edition aka Junkrat > crowmolly 07/25/2018 at 14:26

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A flat top does wonders in these low compression glorified smog pumps.

CarsofFortLangley - Oppo Forever > Steve is equipped with Electronic Fool Injection 07/25/2018 at 14:30

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All that literal powahhhhhhhhhhh

crowmolly > RickSanchez:Fury Road Edition aka Junkrat 07/25/2018 at 14:31

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It’s almost like a -12cc dish ISN’T the best for power. Who would have guessed?

My bird IS the word > RickSanchez:Fury Road Edition aka Junkrat 07/25/2018 at 14:31

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Unfortunately the factory service manual doesn’t say, and I can’t find the info. I guess that makes the purchase easy though.

My bird IS the word > crowmolly 07/25/2018 at 14:35

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Ok. That is vaugeness on their part then. U nfortunately the factory manual doesn’t say what they are, and going off the aftermarket isn’t helpful either, since the edelbrock heads they make are 77 cc. At least I know what to buy then, oh well. Thanks.

My bird IS the word > Steve is equipped with Electronic Fool Injection 07/25/2018 at 14:36

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ehh close enough.

crowmolly > My bird IS the word 07/25/2018 at 14:36

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Olds 403? What year? Got the numbers off the casting?

Maybe we can just sleuth it. Might only be one cc available in your year.

My bird IS the word > crowmolly 07/25/2018 at 14:40

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Should all be the same,’79 4a Olds heads. I believe only one casting exists.

crowmolly > My bird IS the word 07/25/2018 at 14:41

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Yeah, 83cc.

Double check my Google-fu. But it appears you have the smaller chambers.

RamblinRover Luxury-Yacht > My bird IS the word 07/25/2018 at 14:44

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Let’s see. The total volume of per-cylinder displacement is 50.330 in^3, nominal. The head volume is 6cc, = .366142 in^3 nominal, and piston bowl volume (for 83cc) is 5.06 nominal (Heron piston, technically), which gives 9.26 compression ratio, assuming there isn’t a wacky gasket complication not factored into the effective head. If they meant .83cc total combustion chamber, that would give closer to 9. 9 :1, which isn’t as close.

The only way I can think of to go higher compression would be to shave the head, as the piston with its Heron (read, valves and combustion *in* piston) setup and modern levels of compression is most likely completely flush with the top of the gasket at top of stroke. Shaving the head cuts down on the clearance for the valves, unfortunately, but it doesn’t mean piston bashing head. You’d be cutting into the head’s 6cc effective volume, which isn’t above 10:1 even when eliminated. Which it can’t be. Anyway, any clearance issues with the valves might be improved by recutting the seats that additional space.

If we assume your head to have space loosely the same size as the bore, then cutting .020 off the head (for the sake of argument) would get you .136in^3, which is around enough for 9.7. Count the valves backing off .010 or so, probably around 9.5-9.6.

Is there any literature on shaving stock Olds heads?

RamblinRover Luxury-Yacht > My bird IS the word 07/25/2018 at 14:46

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As to what compression ratio, once you have the pistons in hand, you can probably just fill the bowl with light oil and measure the oil. Ought to at least tell you which one.

My bird IS the word > RamblinRover Luxury-Yacht 07/25/2018 at 14:50

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probably, but the 403 is the red-headed step child of this engine family. The smart money goes towards building stronger engines. M y need as a 260ish hp street engine is fine, but since the racers aren’t into them the parts and knowledge are kinda scarce. I just wanted to make sure I was ordering the right thing,  so heavy machine work to maximize CR doesn’t really make much sense right now. I figure I probably will get the heads built down the line if I keep this car, since the engine won’t have to come out again.

My bird IS the word > crowmolly 07/25/2018 at 14:52

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I couldn’t find any info at all, so this is good. I may have been searching for the wrong thing though.

RamblinRover Luxury-Yacht > My bird IS the word 07/25/2018 at 15:08

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You could always have a set of heads for a different displacement built, and just slap them on when convenient. If the valves are the same size, then odds are good that the combustion chamber on 330/350 engines is smaller.

Maybe. If you can find literature on that/cross-ref the 330 pistons and their listed head volume. The only problem that I see right off is that the chamber might be smaller around the periphery in such a way that you couldn’t use a 403 head gasket. Looking at a set of 330 heads, that may or may not be the case, and there’s more of a combustion chamber in the 330 head than I expected. If the 330 has a shallower piston, then we’re back to square one.

https://www.ebay.com/itm/1964-Oldsmobile-heads-casting-1-330-350-403-Olds-442-TG-code-rocker-arm-rails/132649092222

The situations where you can pull a head swap most easily are when there are two differently stroked versions and you just throw the short stroke heads on the long stroke engine. Unfortunately, that’s not an option here, and the “undersize bore in head” is a kind of issue I don’t know much about. Maybe if throwing 350 heads on a 400 Chevy is a thing - that would be instructive.

Worst case, the piston may be coming through the gasket and into the head slightly (!), which if that’s the case, forget it.

RamblinRover Luxury-Yacht > My bird IS the word 07/25/2018 at 15:10

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I defer to crowmolly’s input. Disregard *portions* of my input, because I only know some things.

My bird IS the word > RamblinRover Luxury-Yacht 07/25/2018 at 15:11

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Alot of people put olds 350 heads on the 403, but these pistons are the other way people do it. It should be fine, I’m not overly worried. It was the spending money on the wrong thing that worried me.

RamblinRover Luxury-Yacht > My bird IS the word 07/25/2018 at 15:14

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Well, fortunately a lot of what I was saying was based on thinking re: the piston bowl volume being what they were listing as the head volume, which was dumb of me (but also you). The pistons are not Heron pistons, so shaving the head, swapping the head, shaving the block, etc. are all in some ways more viable than if that were the case.

*major* changes in CR from head changes.

My bird IS the word > RamblinRover Luxury-Yacht 07/25/2018 at 15:48

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AMGtech - now with more recalls! > My bird IS the word 07/25/2018 at 16:04

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Best bet in my opinion, tear everything down and blueprint it. Make calculations and decide on parts from there. I loathe ordering parts without knowing exactly what I already have, and before I order a single thing I like to plan out every part  and bit of machine work. No guesswork, all numbers.

My bird IS the word > AMGtech - now with more recalls! 07/25/2018 at 16:13

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Well, these are my sole piston options outside of stock. so literally any extra compression is good. They are going to dyno it for me as well so I don’t need to worry about tuning, although now it looks like I am going to have to try to find colder spark plugs that fit again.

AMGtech - now with more recalls! > My bird IS the word 07/25/2018 at 16:18

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What about custom piston options?

My bird IS the word > AMGtech - now with more recalls! 07/25/2018 at 16:25

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Likely more expensive than I ever want to get. I think I’m good now though, crowmolly was a big help. The only thing this leaves is overbore diameter and I just went with the smallest amount they offer.

Turbineguy: Nom de Zoom > Steve is equipped with Electronic Fool Injection 07/25/2018 at 21:20

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I know I’ve seen that tattoo before..

Turbineguy: Nom de Zoom > My bird IS the word 07/25/2018 at 21:21

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Is this going to be a .030 over bore, or will the cyls clean up with less? For that matter, how available are .020 over pistons for that motor?

My bird IS the word > Turbineguy: Nom de Zoom 07/25/2018 at 22:09

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The performance pistons I am looking at are either.0 24 overbore or .0 39. I asked and the .0 24 should be fine.

Turbineguy: Nom de Zoom > My bird IS the word 07/26/2018 at 09:53

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C r a z y . I ’ v e n e v e r h e a r d o f a n y o v e r s i z e pistons t h a t w e r e n ’ t . 0 1 0 , . 0 2 0 , . 0 3 0 o r . 0 4 0

My bird IS the word > Turbineguy: Nom de Zoom 07/26/2018 at 10:09

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I have no idea why it isn’t a standard size, but everything about this engine is kinda doofy. Now it looks like I am going to have to try to find colder plugs again, and I couldn’t last time although I believe they make ones that fit, none were listed.