[Brad Pringle]

I hope this has not been covered too many times in the past. I did couple searchs and found nothing. Looking for advice on options for raising the compression on a .030 bore SD455. Will be putting motor basically back to stock but I would like atleast 9.2 compression maybe as high as 9.5. probably run a Ram Air IV cam. 1973 Trans Am, Auto, 3.42 Gears.
Picture just becasue people like them. LOL

[tom s]

I think with those heads you will need a piston with a small dome.Tom

[Brad Pringle]

Hi Tom,

Thank you for the input. Do yo have any experiance with Piston Manufactures who will "make" them to spec order?

I see Keith Black has a 10CC dome for the 400, but nothing similair for the 455.

Thank You

[tom s]

Racetec can make a custom piston for what ever CR you want.I think Autotec is their street piston version.I have them in 3 of my engines.Tom

[trishieldchief]

With a Ram Air IV camshaft you could move to a 10-10.5 compression in this engine and still run 93 94 Pump gas

[bird72]

Originally Posted by trishieldchief

With a Ram Air IV camshaft you could move to a 10-10.5 compression in this engine and still run 93 94 Pump gas

Comparable to what I run with HO heads, and I agree. No ethanol pump premium and it has never disliked it.

[johnta1]

I used the SD 455 'pop up' pistons with the one knobby milled flat and the big knob, milled off the extra tall protrusion.
Probably lowered the CR to around 10 or little less with SD heads.
That small increase in CR helped a bunch for HP.

['ol Pinion head]

Originally Posted by tom s

Racetec can make a custom piston for what ever CR you want.I think Autotec is their street piston version.I have them in 3 of my engines.Tom

Autotec & Diamond can both provide a small dome piston more suited as a street piston to up CR in 455HO & 455 SD engines. Autotec can mfg to accept metric ring packs. Ancient TRW dome forgings & Ross slight domes are something I've shyed away from.

[PAUL K]

If the heads are stock CC the best deal is to use a piston with a dome. If the heads have already been milled you can use a piston without valve reliefs, zero deck the block and get the compression around 9.25.

If you want your TA to rip with the RAIV cam I'd get the compression ratio closer to 10:1.


This is a build I did using some custom pistons without valve reliefs.

https://forums.maxperformanceinc.com...errerid=134156

[SD455DJ]

Brad, Dan used Diamond mini-dome .040 over pistons in the '73 SD455 engine he rebuilt back in 2015. It was already .030 over, so went to .040 with the new pistons. He was shooting for 9.8 but got 9.5 actual. Mini-domes save you from having to cut the block and heads too much.

Dennis

[PAUL K]

Originally Posted by johnta1

I used the SD 455 'pop up' pistons with the one knobby milled flat and the big knob, milled off the extra tall protrusion.
Probably lowered the CR to around 10 or little less with SD heads.
That small increase in CR helped a bunch for HP.

Removing the knobby on the spark plug side (the dome on top of the dome) is probably a good idea. Removing the knobby from the piston opposite the spark plug is a performance mistake but sadly it was highly recommended back in the day, so much that they changed the design and the piston didn't come with either knobby.. They were knobbyless

You'd loose about a quarter point of compression by removing both. Those were some tough military grade pistons that we still use occasionally when we come across some of the early forgings.

[johnta1]

True.

But this was also about 20 years ago.
(about the only pistons available cheap)

[Lee]

I used the KB's with a mini-dome last year. Just enough to negate the valve relief, so "zero" dish/dome. No problems with them, seem to work great.

[Brad Pringle]

Thank you everyone for the input. For future reference this is a complete 73 XD SD455. I Will be running all correct parts Intake, Carb, exhaust maniflods etc.

Considering I do not want to hog a bunch off heads and they will more less be 110CC and 0 deck. Now what size dome are we talking? 10 to 15CC?

Who's Compression Calculator do you recommend?

Thank you agian for your input

[starlightblack]

This is my SD with 'knobbys' (I think this name will stick) I plan to machine one off. Oddly, the guy I got this engine from still had a NOS set.
Brad I look forward to seeing what you come up with. Maybe it's time to go 'new school'.

[PAUL K]

Originally Posted by starlightblack

This is my SD with 'knobbys' (I think this name will stick) I plan to machine one off. Oddly, the guy I got this engine from still had a NOS set.
Brad I look forward to seeing what you come up with. Maybe it's time to go 'new school'.

That doesn't look like an SD block is it?

Machine the knobby off on the spark plug side but leave the one on the opposite side. It's hard to find those pistons with the knobbys intact.

johnta1 gets credit for coining the phrase.

[Formulabruce]

You guys do realize that this is all "Static" compression ratios ? Once running, you get about 1 whole point less. This is why many can say they have a 10:1 engine with pump gas. In fact its around 9:1 when running ( Dynamic compression) .

[Douglas Willinger]

Is there any factory research upon the original 1970 or 1971 high compression SD455?

Say the short block as created but with the ultra-rare #96 (I know, the same number as the 1968 Ram AIr II) high compression heads?

Was such an engine tried in say that gold 1971 TA test car seen in various publications with the custom fuel entry?

Originally Posted by Brad Pringle

I hope this has not been covered too many times in the past. I did couple searchs and found nothing. Looking for advice on options for raising the compression on a .030 bore SD455. Will be putting motor basically back to stock but I would like atleast 9.2 compression maybe as high as 9.5. probably run a Ram Air IV cam. 1973 Trans Am, Auto, 3.42 Gears.
Picture just becasue people like them. LOL

[giles]

I rebuilt my 455HO about 14 years ago. I did what you are proposing and raised the CR to 9.3 with pistons from Ross. They are very good to work with and will make exactly what you want. Tell them the bore size, the CR you want to run, valve sizes etc. Takes about 6 weeks so plan ahead. I am very happy with the result as the skirt to bore clearance is perfect and I have good ring seal. On the downside pop up domes do make the octane requirement go up so you’ll be running 92-93 octane to avoid knock.

[Matt Meaney]

if you don't have to use the stock crank and rods, i would consider a stroker crank 4.35 / 4.5 stroke should get the job done.