Compression ratio with IRON heads on pump gas?

[ClassicRob]

Ive heard the rule of thumb. 9.5:1 MAX compression ratio using an iron cylinder head is the widely accepted advice. However there is so much more to this topic. For instance, would you say it depends on chamber, piston, engine temp, timing and quality of fuel?

What I am asking, is using an old style chamber (lets say a camel hump head) is there a deduction as compared to a say... vortec style head?

Same with pistons. Will a flat top allow more compression because the flame front can travel better vs a dome?

What about engine temps? How does this affect your detonation on a point per 10 degree scale?

Bottom line, I would like to know how using a 10.1 or so engine on pump gas and iron heads would fare and what good information is there to mathematically configure what allowable compression you can run?

I hope this doesn't sound too vague or simple minded. Im trying to understand when it's safe to use more compression on pump gas. Even if its terrible quality fuel.

[Joe-71]

Keep your Dynamic compression ratio at or below 8.0, and you can run 93 octane on the street with iron heads. Joe-71

[1980RS]

Ive heard the rule of thumb. 9.5:1 MAX compression ratio using an iron cylinder head is the widely accepted advice. However there is so much more to this topic. For instance, would you say it depends on chamber, piston, engine temp, timing and quality of fuel?

What I am asking, is using an old style chamber (lets say a camel hump head) is there a deduction as compared to a say... vortec style head?

Same with pistons. Will a flat top allow more compression because the flame front can travel better vs a dome?

What about engine temps? How does this affect your detonation on a point per 10 degree scale?

Bottom line, I would like to know how using a 10.1 or so engine on pump gas and iron heads would fare and what good information is there to mathematically configure what allowable compression you can run?

I hope this doesn't sound too vague or simple minded. Im trying to understand when it's safe to use more compression on pump gas. Even if its terrible quality fuel.

My SBC 358 is 9.7 to 1 and it runs on 91 pump fuel with Sportsman II heads with flat top pistons all though they are a newer style chamber.

[mt-engines]

Well you heard wrong.

[steve cowan]

In my opinion compression is your friend if you want to make any sort of power NA.
Yes pump gas is rubbish and tuning is a challenge.
30 years ago I ran a 3500 pound streeter with 12.8:1 355sbc
461 fuellie heads pumpgas on the street idling around and at the track with a 100 shot on C12 ran 10.9s @125 through mufflers and M and H treaded tyre.
Water temp 210-220 on the street it lived.
Last year 383 combo
10.6:1 cast iron dart head with 235-242 @ 0.050" 106 lsa SFT
@ 3650 pounds ran just under 114mph at the track full street trim.
This year same combo but 240deg solid roller,11.25:1 compression 32 deg timing on pump went over 116mph has run better since with a mix of 103/pump plus timing went 118.7mph.
Got piston to head clearance 31"-32".
Water temp around 180f.
It can be done and I think some people panick over nothing.

[lefty o]

how much compression is affected by cam choice. usually safe in the 10:1 and under category, but choose poorly with the cam, and it can run into trouble, choose better , and you can run higher. your tuning ability can also affect how much you can get away with.

[F-BIRD'88]

Define what "pump gas".

How much do you want to retard the ignition timing from optimum power @ WOT best spark timing?
How good do you intend to restrict engine operation to only when the water temp is low?
How much extra rich carb jetting beyond what creates best WOT power do you intend to run?
If you subscribe to the dynamic cr vs octane/compression using intended big OVERSIZED cam duration and or late less than ideal intake valve close point are you willing to try out to see if that works or not?
How many times do you want to rebuild this engine when things don't work out one day quite like the internet experts claim with various DE-TUNING schemes and operating condition restrictions qualifiers to claim suddenly don't hold in normal use?

Does your engine have electronic engine spark fuel management with detonation knock detection?

How long does this engine have to last?

[mt-engines]

1990s lt1s were 10.5:1 and up. Ran pump gas just fine.

One of the nicest running street engines i had in an 85 monte carlo was an 11:1 355 ported iron heads and a 224@.050 hft cam. Edelbrock performer rpm, and a 750dp. Ran that on pump premium. Ran 12.30s @116mph.

[steve cowan]

Ive heard the rule of thumb. 9.5:1 MAX compression ratio using an iron cylinder head is the widely accepted advice. However there is so much more to this topic. For instance, would you say it depends on chamber, piston, engine temp, timing and quality of fuel?

What I am asking, is using an old style chamber (lets say a camel hump head) is there a deduction as compared to a say... vortec style head?

Same with pistons. Will a flat top allow more compression because the flame front can travel better vs a dome?

What about engine temps? How does this affect your detonation on a point per 10 degree scale?

Bottom line, I would like to know how using a 10.1 or so engine on pump gas and iron heads would fare and what good information is there to mathematically configure what allowable compression you can run?

I hope this doesn't sound too vague or simple minded. Im trying to understand when it's safe to use more compression on pump gas. Even if its terrible quality fuel.

There are some octane booster additives that can help as well,I have used Nulon pro boost testing different ratio's of additive and found you can use way less than recommended.
I can't see 10.5:1 being a real problem in anything unless it's a heavy truck with tall tyres and highway gears.
Everything is a compromise, you have to try for yourself and record your own findings as everyone has different experiences and opinions.

[dannobee]

1990's LT1's had aluminum heads (except for the I'mpala SS/Cop Car heads). The heat dissipation is worth about 1 point of compression.

If you believe the engineers at GM do everything for a reason, the iron head LT1's had bigger chambers than the aluminum heads in the F and Y bodies. Obviously GM lowered the compression for a reason.

[mt-engines]

1990's LT1's had aluminum heads (except for the I'mpala SS/Cop Car heads). The heat dissipation is worth about 1 point of compression.

If you believe the engineers at GM do everything for a reason, the iron head LT1's had bigger chambers than the aluminum heads in the F and Y bodies. Obviously GM lowered the compression for a reason.

The B and D bodies were 10.5:1 with iron heads. The iron heads also have a 54cc chamber..

[rfoll]

My current 350 street engine has 9.63:1 with a small cam, 210/218/111 lsa. It runs like a top on regular gas. Vortec heads, iron ex manifolds, and 15 degrees initial timing. I have yet to hear it ping.

[brentry]

383sbc. Built for friend of a friend. Used alot parts he had laying around. 10.85 comp ,measured. Put 2.05 valves in his sportsman's, ported. hurricane sp intake, 232 on 106 lsp hft
Started off with torco octane boost. Now he is on pump.
But idk where his timing is now. He watching it closely and was warned about detno. Sq is around. 040 .

[1980RS]

My good 406 runs on pump gas. I can go to the pump and pump out 110 octane for it but technically it's still pump gas.

[Kipi79]

1990's LT1's had aluminum heads (except for the I'mpala SS/Cop Car heads). The heat dissipation is worth about 1 point of compression.

If you believe the engineers at GM do everything for a reason, the iron head LT1's had bigger chambers than the aluminum heads in the F and Y bodies. Obviously GM lowered the compression for a reason.

The B and D bodies were 10.5:1 with iron heads. The iron heads also have a 54cc chamber..

I’ve measured few to be closer to 64cc. And iron headed models had thinner head gaskets than others (.027), that would make it 10:1. And the water ran to heads first on all of those 90’s lt1 models.
Not really that comparable to other iron headed ones.

Intake temperatures and humidity has great impact on knock sensitivity of an engine, easily a full point in compression or more.