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Discussion Starter #1
Trying to do some compression number calcs to see if these heads would be a good fit for my 289.
Currently a stock 66 289.
Twisted Wedge 170 w/ dual springs and 61cc chambers

What's the stock comp #s with factory heads?
What would the comp be with the 61cc chambers on the TW?

Plans are for a street car. (I'm also wanting to do the roller cam conversion, or build a roller block)
 

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Discussion Starter #3
What are you starting with the C code or A code?
Honestly. Don't know the difference.
Car originally was an I6 manual car. It was converted to V8/C4 and 5 lug god knows when. It had a 2b autolite on it when I got it originally. Let me send the pictures I have from the motor tags/etc.
 

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That depends. Do you mean "static compression ratio" or compression, in psi, generated by a cranking compression test?

Regarding static compression ratio, it depends on the pistons used (stock or rebuilder, relative to compression height) and face type (flat top, valve reliefs, dish type). The rough calculation for a stock '66 C-code piston, standard bore, 1.600 compression height, standard 8.206" deck height, a .040" compressed head gasket thickness and a 61cc chamber volume would be about 8.374:1.
 

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I remember that some people back in time claimed that the TW170 heads can be milled heavily without any durability issue. TrickFlow's newer R11 head design are available with 53 cc cambers
 

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Discussion Starter #8
That depends. Do you mean "static compression ratio" or compression, in psi, generated by a cranking compression test?

Regarding static compression ratio, it depends on the pistons used (stock or rebuilder, relative to compression height) and face type (flat top, valve reliefs, dish type). The rough calculation for a stock '66 C-code piston, standard bore, 1.600 compression height, standard 8.206" deck height, a .040" compressed head gasket thickness and a 61cc chamber volume would be about 8.374:1.
Thanks Woodchuck. Seems a bit low, no?
 

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Thanks Woodchuck. Seems a bit low, no?
That's the result of increasing the combustion chamber size from the stock 53.5cc to 61cc.

Factory "Advertised" compression ratio was always inflated. Actual "C" code compression ratio was closer to 9.0:1.

FWIW, if you're going to be doing a lower-end rebuild anyway, pick a set of Twisted Wedge Pistons, #TFS-51404000, which will yield a static CR of 9.04:1 with your 289 crank and rods, or 9.4:1 with a 302/5.0 crank and rods.
 

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If you buy a new set of twisted wedge heads the springs will most likely be set up for a roller cam. So you will have to either convert to roller or buy new springs with lower seat pressure so you can use a flat tappet. I went with the 170R heads and 53cc chambers. On my 289 bored .040 over the compression is right about 10.5:1. I also choose to convert to roller cam.
 

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Discussion Starter #12
I am wanting to go roller. Decided to hold on the 170s that I had found, unless they can be had for cheap. The 61cc chambers would have put my comp numbers way too low for my liking. Now, if I can get to 10.5:1 like you noted, I'm all about it.
How happy are you with those 170Rs? What's the difference between the regular 170s and those (besides the chamber size)
 

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I am wanting to go roller. Decided to hold on the 170s that I had found, unless they can be had for cheap. The 61cc chambers would have put my comp numbers way too low for my liking. Now, if I can get to 10.5:1 like you noted, I'm all about it.
How happy are you with those 170Rs? What's the difference between the regular 170s and those (besides the chamber size)
At 10.5:1 static CR say goodbye to pump gas. Don't get hooked on a number.... for some people it's a particular horsepower...for others it's camshaft duration....yet others focus on compression ratio... Decide how you are going to use your car the majority of the time and build for THAT use. It does no good to have 10.5:1 CR if you can't enjoy using your car because you're paying $5 per gallon of gas plus octane boost, or trying to cruise around at 2,000 rpm with your motor loading up all the time or sputtering.... sure, if you're going to race it build a race car but........
 

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I am wanting to go roller. Decided to hold on the 170s that I had found, unless they can be had for cheap. The 61cc chambers would have put my comp numbers way too low for my liking. Now, if I can get to 10.5:1 like you noted, I'm all about it.
How happy are you with those 170Rs? What's the difference between the regular 170s and those (besides the chamber size)
🤷 turn it into an advantage..its a good ratio for high boost(of course you would need to step up to a 351w block if you planned on more than 450-ish HP) but a simple junkyard twin-turbo setup using say a couple of HY-35s(remote mounted if you wanted simplicity and better weight balance...though scavenge pumps are always a headache). With that route you wouldn't need to worry about any more performance parts aside from an intercooler(of course sealing a carb at more than about 15psi might be a challenge).

No, you are right....I would pass with that compression ratio as well unless you are building for forced induction.
 

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Discussion Starter #15
At 10.5:1 static CR say goodbye to pump gas. Don't get hooked on a number.... for some people it's a particular horsepower...for others it's camshaft duration....yet others focus on compression ratio... Decide how you are going to use your car the majority of the time and build for THAT use. It does no good to have 10.5:1 CR if you can't enjoy using your car because you're paying $5 per gallon of gas plus octane boost, or trying to cruise around at 2,000 rpm with your motor loading up all the time or sputtering.... sure, if you're going to race it build a race car but........
Good point. Street car, currently NA. I did eyball one of those small root type blowers once. But that's all I did.
🤷 turn it into an advantage..its a good ratio for high boost(of course you would need to step up to a 351w block if you planned on more than 450-ish HP) but a simple junkyard twin-turbo setup using say a couple of HY-35s(remote mounted if you wanted simplicity and better weight balance...though scavenge pumps are always a headache). With that route you wouldn't need to worry about any more performance parts aside from an intercooler(of course sealing a carb at more than about 15psi might be a challenge).

No, you are right....I would pass with that compression ratio as well unless you are building for forced induction.
Guy has em at $900, they look brand new. But Machining is gonna cost me $150. I'd rather just buy new ones at that point. Because those should come with free stickers. LOL
 

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Good point. Street car, currently NA. I did eyball one of those small root type blowers once. But that's all I did.

Guy has em at $900, they look brand new. But Machining is gonna cost me $150. I'd rather just buy new ones at that point. Because those should come with free stickers. LOL
Well, to be realistic, if turbocharging under 450HP you wouldn't even have to touch the heads anyway(or anything else in the longblock). But yeah...$900 is too much with as common as used aluminum heads are....you know you have to have those stickers, everyone knows its 5HP per sticker.
 

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I would suggest that if you have your old heads off you cc the valve reliefs and measure your piston distance to deck height at tdc. That and the compression height of the top ring and you can accurately figure your compression ratio with the new heads. If it is too low and you have the piston to valve distance you could look at using a thin head gasket from Cometec. I am using the .027 thickness and it has bumped my compression. You could also consider having the heads you have milled. If you look at the TW site it will tell you how much you need to mill to arrive at the chamber cc you are looking at.
I agree to what others have said though...you have to decide what your goals are...You can get away with a lot of compression if you have a big cam and/or low gears. I have almost 10.5 and it runs fine on 93.
 

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At 10.5:1 static CR say goodbye to pump gas. Don't get hooked on a number.... for some people it's a particular horsepower...for others it's camshaft duration....yet others focus on compression ratio... Decide how you are going to use your car the majority of the time and build for THAT use. It does no good to have 10.5:1 CR if you can't enjoy using your car because you're paying $5 per gallon of gas plus octane boost, or trying to cruise around at 2,000 rpm with your motor loading up all the time or sputtering.... sure, if you're going to race it build a race car but........
With aluminum heads & the timing events that typically go along with aggressive cams 10.5:1 "static" should be fine with pump gas. My 289 is 10.7:1 & runs 93-octane without issue; 17-degrees initial timing with 30 total. TBH though - haven't tried 91.
 

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I am wanting to go roller. Decided to hold on the 170s that I had found, unless they can be had for cheap. The 61cc chambers would have put my comp numbers way too low for my liking. Now, if I can get to 10.5:1 like you noted, I'm all about it.
How happy are you with those 170Rs? What's the difference between the regular 170s and those (besides the chamber size)
I like them. The valve angles have been revised and you can run stock, regular flat tops, or the twisted wedged pistons. If you don't go to radical on the lift you shouldn't have a problem. I am also running 93 octane as well without any real issues. This set up is pretty new to me and I am still getting the kinks worked out on the tune. Haven't had time to mess with jets in the carb yet. Only thing that bothers me is early morning when I first take off I get stumble off idle or when changing gears. This goes mostly goes away once the engine is good and warm. I think I need to mess with the accelerator pump cam. Not sure yet. Just been super busy with work and not much time to get to the little things.
 
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