[455Grandville]

What’s the Max compression you can have with 87 or 89 octane gas ?
I’m going to freshen up the never rebuilt passenger car 1971 455 in my 79 T/A (# 66 heads) but don’t want too much compression as premium 91 or 93 isn’t at most of the rural stations around here.

[steve25]

A lot of that answer hinges on the Cam you will run!
Do you have one picked?

Also note that certain blends of the same octane fuel can react better or worse to a giving compression ratio and or engine temp because they require different jetting if your really trying to stretch things!

[455Grandville]

Originally Posted by steve25

A lot of that answer hinges on the Cam you will run!
Do you have one picked?

Also note that certain blends of the same octane fuel can react better or worse to a giving compression ratio and or engine temp because they require different jetting if your really trying to stretch things!

Appreciate the responses.I might stick the old one back in as this engine has low miles, but the gaskets my friend and I put in back in high school are getting old and that’s the main reason I’m pulling it.
Looking at possibly a RA3 cam as this is just a mild engine with Air and 3.08 gears.
I went with 6x-4 heads and a Comp cams 255 DEH on my 455 GV and that combo gave me 9.4 : 1. I’m happy with that setup but I have to go out of my way to VP fuels to get 93 octane and I’d rather avoid that.

[Stan Weiss]

Originally Posted by 455Grandville

Appreciate the responses.I might stick the old one back in as this engine has low miles, but the gaskets my friend and I put in back in high school are getting old and that’s the main reason I’m pulling it.
Looking at possibly a RA3 cam as this is just a mild engine with Air and 3.08 gears.
I went with 6x-4 heads and a Comp cams 255 DEH on my 455 GV and that combo gave me 9.4 : 1. I’m happy with that setup but I have to go out of my way to VP fuels to get 93 octane and I’d rather avoid that.

What psi do you see when you do a cranking compression test on this engine?

Stan

[Jay S]

Would that 71 455 use an 067 cam?

The 66 heads are pretty the cavernous 114+cc chambers, are those heads going back in the car?. If I was picking factory heads and the cam is the 067 would use a 6x-8, that would be a 87 octane set up. I would make sure the cam is installed similar to the factory (almost straight up).

We have been as high as 10.3 SCR with stock heads in a 455 on 91 octane with a big solid cam. Ported heads with the right cam, single plane intake and headers, I don’t mind going clear up to 11 on 91. Over the years we had 455 combo’s, spc-7 (068) and the Spc-8 (041) at around 9:1 (8.8-9.3 on several engines). We were never impressed with the 041 with the compression that low, it would run on any crappy gas you put in it though. In that 9.0 compression range we have been more impressed with the Spc-7 (068) with 1.65 rockers. With the bigger rockers on the 068 I think it performs as well or better than the 2801 summit, and is has a little lower octane requirement (easier to tune).

A friend runs a 455 with ported 96 heads in a 4x4 pickup with a Crower 60919 (later version 112 LSA). and Rhoad vmaxs set at .009 lash, very ported intakes ports (260 cfm) lightly touched up exhaust (170 cfm). The compression is between 9.3-9.5, I don’t know the exact scr, he had it decked after I helped him with his heads. It tows 18,000 lbs trailers, has a mammoth aluminum radiator, q jet and rpm intake, it will start to ping a little pulling it to its knees on 87.

Octane requirements the timing curve, jetting, spark plug heat range, compression, piston design (quench and piston shape), operating temp, head flow, chamber design, head material, cam events, cam intensity, induction and exhaust all make a difference. Lol, maybe I am forgetting something. A lot of factors, any combination of factors can make a combo act different. I have seen 8:0 engines get messed up from a couple of those being wrong.

[Formulajones]

Originally Posted by Jay S

7.

Octane requirements the timing curve, jetting, compression, piston design (quench and piston shape), operating temp, head flow, chamber design, head material, cam events, cam intensity, induction and exhaust all make a difference. Lol, maybe I am forgetting something. A lot of factors, any one factor cam make a combo act different. I have seen 8:0 engines get messed up from a couple of those being wrong.

Yep, It's a hard question to answer as there are way too many variables that will affect the outcome. ALL of those things have to be paid attention to if you want success at the pumps.

We have nothing but crappy 91 here in AZ, same thing Tom deals with in Cali. It's not that great. It can be made to work on some pretty radical stuff if you cover all your bases that Jay laid out, but it's not for everyone.

For what the OP is doing I'd stay very conservative.

[455Grandville]

Originally Posted by Jay S

Would that 71 455 use an 067 cam?

The 66 heads are pretty the cavernous 114+cc chambers, are those heads going back in the car?. If I was picking factory heads and the cam is the 067 would use a 6x-8, that would be a 87 octane set up. I would make sure the cam is installed similar to the factory (almost straight up).

We have been as high as 10.3 SCR with stock heads in a 455 on 91 octane with a big solid cam. Ported heads with the right cam, single plane intake and headers, I don’t mind going clear up to 11 on 91. Over the years we had 455 combo’s, spc-7 (068) and the Spc-8 (041) at around 9:1 (8.8-9.3 on several engines). We were never impressed with the 041 with the compression that low, it would run on any crappy gas you put in it though. In that 9.0 compression range we have been more impressed with the Spc-7 (068) with 1.65 rockers. With the bigger rockers on the 068 I think it performs as well or better than the 2801 summit, and is has a little lower octane requirement (easier to tune).

A friend runs a 455 with ported 96 heads in a 4x4 pickup with a Crower 60919 (later version 112 LSA). and Rhoad vmaxs set at .009 lash, very ported intakes ports (260 cfm) lightly touched up exhaust (170 cfm). The compression is between 9.3-9.5, I don’t know the exact scr, he had it decked after I helped him with his heads. It tows 18,000 lbs trailers, has a mammoth aluminum radiator, q jet and rpm intake, it will start to ping a little pulling it to its knees on 87.

Octane requirements the timing curve, jetting, spark plug heat range, compression, piston design (quench and piston shape), operating temp, head flow, chamber design, head material, cam events, cam intensity, induction and exhaust all make a difference. Lol, maybe I am forgetting something. A lot of factors, any combination of factors can make a combo act different. I have seen 8:0 engines get messed up from a couple of those being wrong.

I have a stack of heads and blocks. But I’d like to keep the engine original (not original to the car) as it was when my friend and I found it in high school found it in a yard and installed it as cancer took him out about 10 years ago, so it’s sentimental as he sold it to me before he died.

[455Grandville]

Originally Posted by Stan Weiss

What psi do you see when you do a cranking compression test on this engine?

Stan

145-150 psi. Engine has maybe 70,000 miles.

[Stan Weiss]

Originally Posted by 455Grandville

Appreciate the responses.I might stick the old one back in as this engine has low miles, but the gaskets my friend and I put in back in high school are getting old and that’s the main reason I’m pulling it.
Looking at possibly a RA3 cam as this is just a mild engine with Air and 3.08 gears.
I went with 6x-4 heads and a Comp cams 255 DEH on my 455 GV and that combo gave me 9.4 : 1. I’m happy with that setup but I have to go out of my way to VP fuels to get 93 octane and I’d rather avoid that.

When talking about CR remember what cam he has and that he likes.

Stan

[tom s]

For the engines I have done with our CA 91 piss gas I found if it pumps more than 180 it has a higher chance of detonation depending on the air and engine temp and what kind of load your putting on the engine.Summer traffic can be a issue.My sweet spot with iron heads has been 9.5 and KRE alu heads 10.25Tom

[lust4speed]

I've ran a true computed 8.64:1 compression with 87 octane gas with a Crane 214/224 HR cam with a 112 LSA on a 400 with 670 heads and feel that I would have been fine with that combo at 8.8:1. As it was the engine liked 38° total timing and made 297/298 horsepower at the rear wheels on two different chassis dynos. Lousy SoCal regular gas in the heat of summer with the AC blasting and never a hint of detonation from the engine.

[Half-Inch Stud]

I shot too low: Ram Air Heads with a 38cc dish on 469 CID. No ping on 87, 1/4 mile with 89.

Was going to boost for low 10s, but met the NA performance goals of low 12s to 11s with normal inhale.

Spare Engine build will be too high a compression, unless i buy the 110 mil thick Cometics Head Gaskets

[Cliff R]

Difficult if not near impossible to answer question.

My 455 is 11.3 to 1 and manages any grade of pump fuel I've ran in it. I avoid 87 octane simply because in the hottest summer months fully heat soaked I've experience difficult hot restarts (bucking the starter a bit) or a little pre-ignition when cranking.

Never heard it ping once on any octane but I try to keep 92-93 octane in it and pretty conservative with timing just to give me a "warm and fuzzy" with the whole deal.

I really never intended to run this engine or even try 87 octane in it, but my son worked for me briefly back in 2009-2010 and on his trips to town to re-fill gas cans he just put 87 in them and didn't say anything. I never really noticed and continued to drive and race the car w/o issues. Then mid-summer I noticed it bucking the starter a couple of times, asked a few questions and low and behold he said he wasn't filling the cans with 92-93 octane. Of course as these things got with having family work for you he insisted that's what i told him to do..........

[ta man]

Originally Posted by Cliff R

Difficult if not near impossible to answer question.

My 455 is 11.3 to 1 and manages any grade of pump fuel I've ran in it. I avoid 87 octane simply because in the hottest summer months fully heat soaked I've experience difficult hot restarts (bucking the starter a bit) or a little pre-ignition when cranking.

Never heard it ping once on any octane but I try to keep 92-93 octane in it and pretty conservative with timing just to give me a "warm and fuzzy" with the whole deal.

I really never intended to run this engine or even try 87 octane in it, but my son worked for me briefly back in 2009-2010 and on his trips to town to re-fill gas cans he just put 87 in them and didn't say anything. I never really noticed and continued to drive and race the car w/o issues. Then mid-summer I noticed it bucking the starter a couple of times, asked a few questions and low and behold he said he wasn't filling the cans with 92-93 octane. Of course as these things got with having family work for you he insisted that's what i told him to do..........

Same results here..quite often 87 octane goes into my tank since my pump at the shop is 87 octane..then filled with 91 octane when in town. I'm at 10.8 to 1 also with KRE Aluminum heads.
On a side note..this car is actually fuel efficient 20mpg on long stretches of highway..91 octane..almost 600hp with a Holley 950hp and a Torker 2!

[PontiacJim1959]

ta man "On a side note..this car is actually fuel efficient 20mpg on long stretches of highway..91 octane..almost 600hp with a Holley 950hp and a Torker 2!
Today 07:36 AM"

You may want to match your speedometer gears to the rear axle - I am not a believer of those number at all, no offense. Was the car on a trailer being pulled?

[77 TRASHCAN]

Originally Posted by PontiacJim1959

ta man "On a side note..this car is actually fuel efficient 20mpg on long stretches of highway..91 octane..almost 600hp with a Holley 950hp and a Torker 2!
Today 07:36 AM"

You may want to match your speedometer gears to the rear axle - I am not a believer of those number at all, no offense. Was the car on a trailer being pulled?

Jim, remember that Ernie has a 3.08 gear in his car. A well thought out and efficient engine can do crazy things such as is! His combo isn’t a “share tree” thrown together deal. If I remember... Dave at SD Performance had influence on this build, AND Ernie has been tune testing his combo for quite a while!
My 77 TA got 22, MPG a few times, back in the day, stock 7.6:1 400 engine, that would seem far fetched also, although the 2.41 gear made that happen!

[pastry_chef]

Originally Posted by PontiacJim1959

You may want to match your speedometer gears to the rear axle - I am not a believer of those number at all, no offense. Was the car on a trailer being pulled?

A friend and I have seen 20.8 MPG.

Heavy 80 Firebird with two men.
455 engine - SD 310 CFM heads,
288HR - 110 LSA hyd roller
2004R with PTC 3200 stall - locked up.
Self built engine and trans.

56 Miles/ 10.2 Liters of gas consumed. RPM was right about 1700 RPM for the drive. Owner spent many hours tweaking the fuel and timing curves. Autometer speedo was calibrated less than 1% error.

[ta man]

Originally Posted by PontiacJim1959

ta man "On a side note..this car is actually fuel efficient 20mpg on long stretches of highway..91 octane..almost 600hp with a Holley 950hp and a Torker 2!
Today 07:36 AM"

You may want to match your speedometer gears to the rear axle - I am not a believer of those number at all, no offense. Was the car on a trailer being pulled?

It's accurate...I actually went by the map route that I took to take the mileage..
It took maybe 2 years of a mixture of carb work and timing to get the mileage good.

[Chris.H]

I am going to build 400 with #16 heads and I am concerned about the 10.75:1 CR . over the yrs I have talked and seen quite a few that run pump gas as high as 12:1 . (any scraps of paper I wrote on have been misplaced and disappeared) but I do remember that it was down to setting your timing differently ( retard) than the old days , and some mentioned cams as well. My questions are if I am using 94 octane
#1--could I go stock rebuild for 400HO and just retard the timing a couple of degrees or try re-calibrating the distributor advance curve,
#2--should I use a colder spark plug than the stock
#3-- tune carb to a richer mix. or get one of those modern fuel injected self tuning carb set up
#4--would i have to use use a different cam than the stock 068

and if the Pontiacs CR 10.75:1 actually is over rated by at least 0.5 then theoretically I should be able to run on 94. on the stock rebuild..

One more question when they changed the octane back in the day did all the old motors start detonating around the world that year

[tom s]

If your going to build it build it right the first time.Lower the CR to be safe.Retarding the timing will hurt performance and will cause the engine to over heat.Most people dont seem to have time to do it right but always find time to do it over.JMHO,tom