[Scubasteve95]

I'm building a 455 with 11.4 to 1 compression my engine guy is confident I'll be able to run standard pump gas which around here is 91 octane but he's a race guy and I drive mine as a daily I do get into it every now and again at the local track but I think I'm going to have issues with the 91 octane on a Tri-State Cruise.
Here's a rundown of what I'm building:

1974 455 block decked to .005
1.5 harland sharp roller rockers
3/8 trick flow pushrods
74cc KRE D-port heads as cast 260cfm
Sealed power forged .030 pistons+6.7 / 4.1815 bore
Shot peened OEM crank
Shot peened OEM connecting rods 4.210
Cometic MLS head gasket .040
Comp cams evolution hydraulic lifters
Comp cams thumper roller cam 51-602-11
Holley Sniper EFI 800cfm

[Dragncar]

Ditch the cast rods (Molnar) and the pistons.
Both will hold you back.

[Scubasteve95]

lets get out of the other guys post,
not concerned with the rods or pistons( which have a 1/16 ring) they have done well for me over the last 8 yrs I'm concerned about the detonation on 91 octane with the high compression , some say with the new KRE heads it wont be an issue with the new camber and cam i have i don't have a calc to figure all that out i want to get ever once of power i can out of her and still pull up to the pump( cake and eat it to deal) just thought i would put it out there.

[steve25]

A big factor here besides the cylinder pressure you end up with is getting the Exh heat out in terms of having your motor survie on the edge of having too much compression for fuels you may be forced to run.

Are you running headers, or Exh manifolds.?

To keep info correct here, your cast Rods C to C is not 4.210", that's your cranks stroke.
The Rods C to c is 6.625.

These days with out of the box heads that can make over 500 hp I consider aftermarket Rods in motors with 4…210" strokes or larger a very smart way to go, especially seeing that you have to rebalance the motor for those Pistons anyway.

I can't think of a worse sonarero then a new build with cast Rods in it detonating from having too much compression.

Unless your a well experienced top notch tuner Your crusin for a bruisin with that set up!

Do be one of those guys that post back here in a year and a half with a ventilated block story to tell!

[ta man]

It is certainly doable..I'm at 10.8 to 1 with 190 cranking compression..and I can use 89 octane. WIth your build I think a different cam would have been a better choice.

[72projectbird]

Had a 455 with 205psi cranking compression. Was a .060 over block with forged pistons and rods, with ported #62 heads, an old Holley 850DP and a simple HEI ignition system. I didn't know if it was zero decked or not, so knowing the exact compression was up in the air. It was between 11.5 and 12.0 depending on how much was removed from the block.

Shaker455 helped dial it in. It ran on 93 pump.

[PunchT37]

It better not fall out of tune at all.


I have a 463 with Dave`s fast burn chambers. 72cc round port Eddys. 20cc Ross dish piston. It`s at 10.3 static with a 246/253 on 112 cam degreed at 107.5. Last I checked, it`s at 175-180 cranking pressure. I don`t have detonation BUT, it will run on sometimes in the summer down here. This is with 93 octane.


That will give you a clue.

[PAUL K]

I would never build an engine like yours and expect to run safely on 91 octane. Your heads won't support a cam that would let that happen. Your rod and piston choice wouldn't support the RPM the correct cam would need to operate.

I have no idea what the specs are on that cam but my guess is someone is thinking the extra exhaust duration is going to kill off cylinder pressure. That only works on a lawn mower while your pulling the rope. It's the intake valve closing point which has the most control over cylinder pressure and the Thumper cams close that valve earlier than normal and help build cylinder pressure.

[Jay S]

I assume with that bigger thumper cam a single plane intake and headers are in the plans.

I think the thumper cam with it’s 107 LSA is going the have to much mid range power (too high VE’s thru peak tq) for what you are wanting to do. It will be packing to much power in to narrow of a RPM range, and the heads won’t flow enough to spread the power out to make it work better. They don’t spread the power out as much as a ported 300+ cfm head, the extra head flow spreads the power band out more and makes it quite a bit easier to run more compression. I would want the compression to be more in the mid 10s with a good quench and that thumper cam and OTB D port head combo.

The last production run of speed pro 455 pistons appeared to have the compression height adjust down so they generally are .025” to .030” below the deck. If that is what you have, it takes some milling to get them to zero deck. The compression height was lower almost .020” from an older piston. If you don’t deck the block with the last version of those 455 pistons the compression will end up in the high 10s. But it needs a good quench to run that much compression. Something to else keep in mind. Not sure when the change was made. Can’t assume anything anymore though. Lol

[Jay S]

Paul treed me, I didn’t see his comments before I commented. My thought’s are pretty similar to his. I did not mention the cylinder pressure that cam is going to pump if it ended up at 11.4 SCR, my guess is 215 lbs.


A local guy had the next size smaller thumper in his 400 (406 cid) with round port 72 cc E heads, RPM intake and RA manifolds. His compression was in the mid 10s, I think it did run ok on 91 octane. It didn’t run that well though, I had a chance to buy it but passed since it was a 557 block. I think it made 350 HP to the wheels in his 4 speed 67 GTO. He was not happy with it, he sold it and built a stroker motor with KRE heads. I think he would have been a lot happier with it if not for the poor cam choice.

[PAUL K]

Originally Posted by Jay S

Paul treed me.

That's because I typed that response before my morning coffee.

[djustice]

It seems like we have a very similar shortblock assembly, i have 73 455 casting with a 455 "N" crank with stock cast rods and arp bolts. and sealed power/speed-pro/trw L2359 .060

When i changed heads to 85cc edelbrock heads i measuerd piston deckheigth and found it to be 0.001 in the hole. i can not 100% confirm if the block has ever been 0-decked before but deck stampings where clearly visible, if that is any indicator of decking or not.

[Scubasteve95]

Thanks for all the input I was coming up with the same info with the few calculators I was able to find online.I will definitely post back after I get this done. it's more like a rebuild I'm changing the heads over to the KRE everything else I've been using for 8 years at 10.75 to 1 with single plane intake, headers and 3in exhaust going to go with a .051 gasket bring it down to 11:1 I'd like to go 060 but I can't get them for about a month. Last time I had it on the dyno I had 355hp and 450tq to the rear wheels

[Joe-Touring]

Originally Posted by Scubasteve95

going to go with a .051 gasket bring it down to 11:1 I'd like to go 060 but I can't get them for about a month.

To add to what Jay said, consensus around here seems to be that a thicker head gasket is worst way to lower compression. Most say that you want to shoot for 0.040” piston to head clearance, for good quench/squish. Reasoning is that good quench helps stratify the air/fuel mixture for a more even burn, reducing the risk of detonation.

[Jay S]

What heads were in the engine previously that it had 10.75 compression?

Sorry, I accidentally deleted the post Joe-touring was referring too, trying to do something else at the same time.

My main point on the erased post was loosing quench over a few tenths of compression is generally pretty counter productive for dropping the compression a couple tenths for pump gas.

If it is zero deck, which I am guess I it is close to zero deck with the 455 pistons that are 8 plus year old, the .051” thick head gasket is probably ok yet, I would want not go thicker than that though.

[Steve C.]

Link to the proposed cam if desired....

https://www.compcams.com/big-mutha-t...5-455-cpg.html




.

[77 TRASHCAN]

Originally Posted by djustice

It seems like we have a very similar shortblock assembly, i have 73 455 casting with a 455 "N" crank with stock cast rods and arp bolts. and sealed power/speed-pro/trw L2359 .060

When i changed heads to 85cc edelbrock heads i measuerd piston deckheigth and found it to be 0.001 in the hole. i can not 100% confirm if the block has ever been 0-decked before but deck stampings where clearly visible, if that is any indicator of decking or not.

I have ZERO experiences building 455's, BUT
I've read here, seemingly more than once...Cliff mentioned that those pistons specifically are usually ONLY about .005 in the hole, with little room to get to zero, IF block needs much taken off. Of ourse that can be adjusted with head gaskets...
Cliff???

[Johnny406]

I run pump e85 in my 74cc KRE headed 455 and it is very streetable and runs very cool. More specs in my signature

[Scubasteve95]

Johnny406,
first i want to say thank you for your service.
second nice ride.
e85 is nice if i had the option i would use it and this would not be an issue.
hard to find by me.

last heads were OEM cast 4x ported 211/166 82cc

i talked to the 2 big Pontiac company's and both said rule of thumb is 1 point higher for aluminum heads 11.4 "should be fine" i told them i was at 34* with the cast I'm going down to 30* timing now they said "should be fine".
i know its high thats why i put it out on the interwebs to see what others thought. bottom line is im going to finish it but maybe look into a different balanced rolling assembly .

here is what i came up with for specs now.

[Jay S]

82 cc in 4X iron head, are you positive on that? That is quite a cut from the 97 or 98 cc they start out with.

That point of compression from aluminum heads isn’t exactly written in stone. It depends some on what kind of port work you had done on your 4X heads. I have gotten to do quite a few custom cams for folks that got promised Aluminum heads were good for a point or more compression, and it didn’t work out like they expected, especially true on OTB head sets.

I have a 461 6X ported combo that the static compression is 11.2 and runs all the time on 91 octane. It has 14 cc domed pistons to get the compression that high.. That would put it at 12.2 on 91 octane if it had OTB D port aluminum heads with no other changes?