Opinions on 11:1 compression on 93 octane

Geoff 2

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The benefit in higher CR is not a linear curve. There is more gain going from 8 to 9 to 1 than there is from 10 to 11 to 1. If it was my engine I would drop the CR slightly.
 

dvw

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All aluminum heads are not created equal. A angled plug and modern design chamber should easily run at 10.5 assuming its not the stock cam.
Doug
 

Gunner1

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Quench helps too. In my SBC motors I run between .040 to .032 piston to head with flat tops. 400 small block works out to a little over 11 to 1. B’ut they have a smaller bore so shorter burn path. Bigger bore and chamber leaves more time for pre ignition. Like cranky said. Cam it up a bit and 3k converter with some gears.
 

5.7 hemi

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Any of you guys watch Engine Masters?
 

65Fury440

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I run the Source 528 RB kit, (520ci @ .030 over)
And TF 240 heads ported to MW.
Originally was running a Mp distributor, 36 all in by 2000, 20 initial.
Cr came to 10.1.
Ran non eth 90 octane, would ping on really hot days if you lugged the engine, when engine temp got into the 190s.
I did a refresh, got custom pistons at 11.25 now. Also switched from 2 row radiator to a 4 row.
I also went to a Progression Ignition distributor thinking I'd need to be able to bump the timing back, being on the ragged edge of detonation.
So far, with the new rad, Temps usually don't go over 180. Have not heard any pinging at all with the same local 90 octane rec gas.
Also, with 10.1-1 I was running a sft 260/266
With 11.25 it has a roller 268/272
My take is temp plays a big part.
I had a 160 Stat but it must have been off, engine was not getting over 155 degrees. A new 180 Robertshaw Stat keeps it right at 180.
When we go to the track a put 3 gallons of 110 on 7 or 8 gallons of 90.
 

cdr

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I have spoken with Hughes, who designed the cam I am considering using. They recommended it for a 500+ CI motor with 11:1+ compression and mechanical secondaries on the carb.

My 400 block just got checked and is good for the bore needed for the stroker kits. (4.375 I believe)

I do plan on using aluminum heads, which I hear can basically “take off a point”.

Hughes sells a 400-511 kit that would put me at 11:1 without zero decking the block. 440 source offers a 400-512 kit that could be 10.5:1.

I will post a photo of the cam card. Hughes seemed to think with the low altitude here and the abundance (as of now) of 93 octane gas, that 11:1 on pump gas would be doable.

The car won’t be a daily, but will probably see more street than track. Would you say 11:1 could work safely with the cam, or is it better to try 9.5-10.5?

I have heard some people say 11:1 makes the tuning window very narrow, however there are plenty of cars from the era that cam with 10.5:1 and iron heads back then. I’m assuming it should be fine, but I just want to verify.

View attachment 1283312
I am running that same cam in my 440 source 512 street car, 9.7 compression, work great
 

WileERobby

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Go with the 10.5 setup. I'm too lazy to write a page about all the variables that will affect what you want to do.
 

Greg Warhol

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I have spoken with Hughes, who designed the cam I am considering using. They recommended it for a 500+ CI motor with 11:1+ compression and mechanical secondaries on the carb.

My 400 block just got checked and is good for the bore needed for the stroker kits. (4.375 I believe)

I do plan on using aluminum heads, which I hear can basically “take off a point”.

Hughes sells a 400-511 kit that would put me at 11:1 without zero decking the block. 440 source offers a 400-512 kit that could be 10.5:1.

I will post a photo of the cam card. Hughes seemed to think with the low altitude here and the abundance (as of now) of 93 octane gas, that 11:1 on pump gas would be doable.

The car won’t be a daily, but will probably see more street than track. Would you say 11:1 could work safely with the cam, or is it better to try 9.5-10.5?

I have heard some people say 11:1 makes the tuning window very narrow, however there are plenty of cars from the era that cam with 10.5:1 and iron heads back then. I’m assuming it should be fine, but I just want to verify.

View attachment 1283312
 

Greg Warhol

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I was running a 413 with iron heads, a single plane intake, and 750 com edelbrock carb, with a ratio of at least 11:1. I had to run 93 octane plus 10% racing fuel to keep it from knocking. No amount of tweaking the timing helped. I went to a dual plane intake, Holly 750 (Summit) carb. With a lot of tweaking, I now run 93 octane and 5% racing fuel. I can get away with no racing fuel if I feather foot it!
 

wyldbee

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You can get away with higher compression and help avoid detonation by using flat top pistons with aluminum heads providing quench and a cam big enough to bleed a little compression at slow speeds

currently running 440 with old TRW dome, 906 heads and cam 284/294 @ 0.50. So maybe 11.6 or so on 92 octane here. Can tell who has best gas as some brands can detonate on hot days.

Also noticed at elevation 3000’ is less likely to ping compared to 500’
 

super-bee_ski

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Street car on pump. Why take a chance? Build it 10.5-1 and give yourself a little cushion. The power difference between 10.5-1 and 11-1 isn't going to be more than 15-20hp. Now if it was a racecar? I'd make it 15-1
Doug

I personally agree with "dvm" and "hunt2elk"
What you are currently building is a RACE CAR...period.
Tune it "down" and make It enjoyable
After all, no mater how fast you go, someone will be faster...
So, slow down, drive it for fun and relaxation and smell the coffee
Just my opinion
 

Kern Dog

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If you are in the planning stages, now is the time to make the right decision. Err to the side of caution. Whatever slight gain you might see in 1/2 to 3/4 of a point in compression severely pales in comparison to knocking, pinging, destruction or mixing fuels. Screw all that.
I trusted a known supplier that assured me that I’d run fine at 10.8 with 91 octane. I didn’t. Mine knocked at anything over half throttle. I’m at 10.1 now and rarely get any detonation even in 110 degree weather.
Advice online is cheap. This is one instance where it makes sense to play it safe.
 
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RemCharger

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If you are in the planning stages, now is the time to make the right decision. Err to the side of caution. Whatever slight gain you might see in 1/2 to 3/4 of a point in compression severely pales in comparison to knocking, pinging, destruction or mixing fuels. Screw all that.
I trusted a known supplier that assured me that I’d run fine at 10.8 with 91 octane. I didn’t. Mine knocked at anything over half throttle. I’m st 10.1 now and rarely get any detonation even in 110 degree weather.
Advice online is cheap. This is one instance where it makes sense to play it safe.
Excellent advice.
 

cdr

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here is one of it cutting out because of fuel delivery issues
 

mikerman

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500 in. 9.5:1 will do you fine. I run 30 over [email protected]:1, tf240, hughes hyd roller less than your cam, Chevron 94 and this engine is marginal. 34* total with 6 on the hub. When she is hot, shut off in gear. 11:1 to much for fun on the street.
 
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