Compression Ratio Question

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packin3deuces

packin3deuces

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I will soon be beginning a 340 engine build using the following components: '71 cast iron block bored .60 over, Indy-Edelbrock aluminum heads, 422 stroker kit with Ross forged pistons and Eagle crank, Comp Cams hydraulic roller camshaft with a duration of 224 in. 230 ex. @ .050", Mopar Performance dual plane six pack intake, and a Promax built Holly Six Pack. It will be going into a 1973 Dodge Dart with a 3 inch TTI exhaust, a TKO 600 transmission and an 8.75 rear with either 3.23 or 3.55 gears. This car will be predominately street driven and I would like to be able to run it on 93 octane pump gas. My engine builder says go with 11:1 compression, my machinist recommends 9.5:1 and I'm confused! Which way should I go and will there be a discernable difference in performance between the two? Will an 11:1 engine even run on 93 octane pump gas or will it require a mix with 100+ octane race fuel? Help Please. :confused:
 
packin3deuces said:
I will soon be beginning a 340 engine build using the following components: '71 cast iron block bored .60 over, Indy-Edelbrock aluminum heads, 422 stroker kit with Ross forged pistons and Eagle crank, Comp Cams hydraulic roller camshaft with a duration of 224 in. 230 ex. @ .050", Mopar Performance dual plane six pack intake, and a Promax built Holly Six Pack. It will be going into a 1973 Dodge Dart with a 3 inch TTI exhaust, a TKO 600 transmission and an 8.75 rear with either 3.23 or 3.55 gears. This car will be predominately street driven and I would like to be able to run it on 93 octane pump gas. My engine builder says go with 11:1 compression, my machinist recommends 9.5:1 and I'm confused! Which way should I go and will there be a discernable difference in performance between the two? Will an 11:1 engine even run on 93 octane pump gas or will it require a mix with 100+ octane race fuel? Help Please. :confused:
Click to expand...

Firstly what are the rest of the specs for that cam, maybe paste a goto for the cam card. If the cam is what I think and you are using Edel heads I'd bump the comp. ratio up to 10-10.5 to 1 for 93 octane and think seriously about going to 3.73 gears, I definetely wouldn't go below 3.55. You're definetely going with some serious internals to make a screamer so there's no sense holding it all back with cruising gears IMHO anyway.

Terry
 
You will need more gear with the TKO 600 @ .64 OD. Even with the 3:55, your final ratio is 2:27. If you are going to go that route, build the engine so your crusing in the sweet spot for your application. I had a 70 Duster running a TKO 600 @ .64 OD with 4:30 gears. All stock except for the transmission and gear; it would run mid 13's (275-60 x 15 BFG Street Drag Radial) and cruise all day long at 75mph (2.75 final ratio)

I think you should go with the low compression build so your combination is in its power band starting around 2200 RPM. With a 3:55 gear you will be crusing at 80mph @ 2200 RPM.

If I had it to do over again, I would not have gone with the overdirve transmission. Not because it wasn't a great setup but I didn't drive it on the highway enough to make it worth the time and money I spent.
 
I know a fellow running a 422 stroker with ported Eddy. aluminum heads running 11 to 1 compression and no problem at all running 93 octane. But he is also running a much larger cam which will bleed of some cylinder pressure. That cam you've picked is really small for a 422 cubic inch engine. Should idle nearly like a stock engine. If your set on that cam I wouldn't go any higher than 10.5 to 1 like Terry said.
 
You will want a MUCH bigger cam. Strokers like more duration. The power difference between 9.5:1 and 11:1 is going to be big...probably 30-60 hp depending on the rest of the combo.
 
My 340 with eddy heads and about 10.1:1 to 10.25:1 (can't remeber the math) with a Crower solid 248/252* at 50 did fine with 91 pump gas, I had 3.91 and the car weighed about 3100. Never heard any fuss from the motor and it ran strong.

I would deff run at least 10.50:1 and a bigger cam.... 228/230?? isn't that almost a stock cam! Like others have said, I would also go with either 3.73 or 3.91s, with the OD you'll be flying.

Most of this is open to personal preference so you'll get a lot of diff answers. What kind of build are you doing?
 
demon seed said:
Firstly what are the rest of the specs for that cam, maybe paste a goto for the cam card. If the cam is what I think and you are using Edel heads I'd bump the comp. ratio up to 10-10.5 to 1 for 93 octane and think seriously about going to 3.73 gears, I definetely wouldn't go below 3.55. You're definetely going with some serious internals to make a screamer so there's no sense holding it all back with cruising gears IMHO anyway.

Terry
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The rest of the specs for the cam are as follows: Hydraulic Roller - 3.55-4.10 gears, 9:1 compression, headers, high performance street. RPM operating range of 1800-5800. Advertized Duration 274 in. 282 ex. , Duration @ .050 224 in. 230 ex. , Valve Lift (w/1.5 rocker) 538 in. 534 ex. , Lobe Sep. Angle 110 degrees.

I also have no problem going to a 3.73 or deeper gear.
 
Dusterndart340 said:
You will need more gear with the TKO 600 @ .64 OD. Even with the 3:55, your final ratio is 2:27. If you are going to go that route, build the engine so your crusing in the sweet spot for your application. I had a 70 Duster running a TKO 600 @ .64 OD with 4:30 gears. All stock except for the transmission and gear; it would run mid 13's (275-60 x 15 BFG Street Drag Radial) and cruise all day long at 75mph (2.75 final ratio)

I think you should go with the low compression build so your combination is in its power band starting around 2200 RPM. With a 3:55 gear you will be crusing at 80mph @ 2200 RPM.

If I had it to do over again, I would not have gone with the overdirve transmission. Not because it wasn't a great setup but I didn't drive it on the highway enough to make it worth the time and money I spent.
Click to expand...

I do plan on taking 3-5 hour trips on the interstate with this car from time to time. Mostly, it will be driven locally, but I do want to be able to put it on the interstate and cruise every now and then.
 
fishy68 said:
I know a fellow running a 422 stroker with ported Eddy. aluminum heads running 11 to 1 compression and no problem at all running 93 octane. But he is also running a much larger cam which will bleed of some cylinder pressure. That cam you've picked is really small for a 422 cubic inch engine. Should idle nearly like a stock engine. If your set on that cam I wouldn't go any higher than 10.5 to 1 like Terry said.
Click to expand...

I would like to keep the cam if possible. I can increase compression to 10-10.5 to 1 like you and Terry suggest, along with a deeper gear. Or am I better off with another cam and gear set up?
 
Nitrous340Dart said:
My 340 with eddy heads and about 10.1:1 to 10.25:1 (can't remeber the math) with a Crower solid 248/252* at 50 did fine with 91 pump gas, I had 3.91 and the car weighed about 3100. Never heard any fuss from the motor and it ran strong.

I would deff run at least 10.50:1 and a bigger cam.... 228/230?? isn't that almost a stock cam! Like others have said, I would also go with either 3.73 or 3.91s, with the OD you'll be flying.

Most of this is open to personal preference so you'll get a lot of diff answers. What kind of build are you doing?
Click to expand...

The car will be driven on the street, mostly local driving, but I will put it on the interstate from time to time for 3-5 hour trips. I would like it to cruise the highway with minimal strain on the running gear, but when I'm in town and a 5.0 Mustang or ricer saddles up next to me, I want to be able to give them a good look at my TTI exhaust!
 
Packin,
I agree with the majority of the guys this will be a great build.
I am originally from south florida and you might want to rethink a few things.
93 octane is not available nation wide, however 92 is. So engineer for that, I am living in Texas and here we have 93 but I am still building for 92 because my car will see alot of highway miles.

Second the cam could be a little bigger to take advantage of the increased volume you will have.Your compression/octane requirements are governed by what is called 'Dynamic Compression' This is calculated by taking into account the 'Static compression' of your piston/stroke number ( say 10:1) and factering in the actual closing of the intake valve. This is why your DC is changed depending on the cam/piston combo you choose.

If you figure out you are on the edge of reasonable compressoin numbers you may want to concider coating the top of the pistons, chamber and the face of your valves with a thermal barrier.

Andrew
 
Did your machinist say he was going to sonic test that block? At .060 over, you will be VERY lucky (like go play the lottery that day) if you dont have to sleeve between 2-4 holes to get the wall thickness you need. If he's not going to sonic test it and gaurantee the bores don't split, get a new machinist. Theres's a great one in FL if you need a name. The truth is, you can go with either. As posted, the 11:1 is a flat top, the 9.5:1 is the dish. If it was me, I'd go flat top and 11:1 and run a cam. That one you listed is terribly small. When you build an engine that is 20% bigger you can take the catalog recommendations and toss them too. Or at least look at them using some idea that the power ranges will be much lower, and the airflow needed is much higher. Not to state the obvious, but your builder and machinist should know that. Both should be familiar with longer than factory builds, and high performance work.
 
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