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Opinions on 11:1 compression on 93 octane

TexasRoadRunner68

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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.

A2759FE1-CF15-4F1C-862D-0A305E156A70.jpeg
 
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
 
How good is your luck? I'd run 110 race gas mixed with 91 non ethanol 50/50 or drop your compression to 10:1. Not worth the risk to try 11:1 on pump gas unless your gas station will guarantee the quality of the gas.
 
Something to think about, you can get 93 now but look at how many places in the country that you can only get 90-91 octane. That could coming to a station near you?. I would build for 9.5-10.5 to one for a car if it does street duty.
 
Something to think about, you can get 93 now but look at how many places in the country that you can only get 90-91 octane. That could coming to a station near you?. I would build for 9.5-10.5 to one for a car if it does street duty.
Exactly. And who wants to mess around adding octane booster or mixing race gas into a tank of 91 for a street car? My last 2 motors I had built with 9.7:1 compression. No detonation on 91 and fast advance springs in the distributor. The little bit more power that 10.5:1 makes just isn't worth the hassle to me.
 
Exactly. And who wants to mess around adding octane booster or mixing race gas into a tank of 91 for a street car? My last 2 motors I had built with 9.7:1 compression. No detonation on 91 and fast advance springs in the distributor. The little bit more power that 10.5:1 makes just isn't worth the hassle to me.
True but at even 10:1 I still add race gas concentrate. Never octane booster….thats a waste of money if you are trying to up the octane. It only adds tenths of a point instead of full numbers. Why, because I do not trust the panther piss at the pump with my Hemi. It is cheap sanity and has zero do do with the extra ponies. I did the math versus mixing with race gas its almost the same versus two gas of race gas concentrate. Hell, I may even get a 55 drum of race fuel versus the 100 octane at the pump since that is a rip off these days for what you get.
 
I’m running a 11.2-1 in a 360. 93 octane is do-able but not something I’d do again with my small-ish cam. Your cam is larger than mine by a good margin, 30* @050.
 
Its a P.I.A. to mix gas etc... if I was building a new stroker motor, I would make it about 9.5:1, so I could enjoy it more often. That's the down side of running an original '68 440 HP. You will make more power than you could ever need, and be able to enjoy it more.
 
My 496 stroker has 10.2:1 w aluminum heads. I only have 91 available where I am. If it's not cool outside, below 50°, I get a bit of clatter n I'm running 32° at full advance. Now, I've been made aware by those much more knowledgeable than I that it has to do w my cam. Not enough overlap to bleed off compression. Instead of being around 160ish psi, I'm at 190.

Mixing etc... is not an option, I drive long distances and fill up frequently as only 10mpg hwy. Can't carry a trunk full of race gas etc for every gas stop. You really need to decide what "more street than track" really means. Do you just want to go to car shows or on occasion, not worry about what gas it has n just enjoy a drive?

And if it's gonna see "some" track time, what is the purpose? Just for fun, to win races? If you're there to "win", it better be set up as a drag car. If it's for fun, then lower compression won't hurt. If you still want to be a little competitive, bracket race and make it consistent and still totally street drivable.
 
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Its a P.I.A. to mix gas etc... if I was building a new stroker motor, I would make it about 9.5:1, so I could enjoy it more often. That's the down side of running an original '68 440 HP. You will make more power than you could ever need, and be able to enjoy it more.
How hard is it to dump a can or two of something into a full tank of pump gas? Not hard at all......
 
My 496 stroker has 10.2:1 w aluminum heads. I only have 91 available where I am. If it's not cool outside, below 50°, I get a bit of clatter n I'm running 32° at full advance. Now, I've been made aware by those much more knowledgeable than I that it has to do w my cam. Not enough overlap to bleed off compression. Instead of being around 160ish psi, I'm at 190.

Mixing etc... is not an option, I drive long distances and fill up frequently as only 10mpg hwy. Can't carry a trunk full of race gas etc for every gas stop. You really need to decide what "more street than track" really means. Do you just want to go to car shows or on occasion, not worry about what gas it has n just enjoy a drive?

And if it's gonna see "some" track time, what is the purpose? Just for fun, to win races? If you're there to "win", it better be set up as a drag car. If it's for fun, then lower compression won't hurt. If you still want to be a little competitive, bracket race and make it consistent and still totally street drivable.
I wish I got 10 MPG.... :lol:
 
Aluminum heads run a bit cooler than iron so that's why the motor will act like it has a bit less compression. Used to run 10.5 with a 340 in a 3560 lb Cuda and it did fine with 91 so long as I kept my foot out of it. In the winter time it was better. Summer time 93 was better until the temps hit 100 and engine temp got around 190. Keep in mind that an engine pings in the lower rpm ranges with a heavy load but at high rpm there's not enough time to ping.....and if you haven't already done it, there's plenty of info on detonation and preignition and how cams can make or break an engine.
 
How hard is it to dump a can or two of something into a full tank of pump gas? Not hard at all......
It's not hard, but its better not to have to. If OP is building a new motor, he can prevent having to do this. Its a crutch for having a high compression engine that you use to drive around and cruise. In an original motor that is already high compression, that is what you have to do to avoid rebuilding your perfectly good motor. The stroker is already not original, so might as well make it user friendly as well. You will barely notice the difference between 9.5 and 11:1 on the street in a motor that big. That is my opinion.
 
I would back it off a half point. It isn't worth the risk of having to back down the timing or screwing around mixing gas or adding additives. In the 80's I had to try everything, race gas, av gas etc. and I won't go through that again to drive a car on the street.
 
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’
 
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