'65 Barracuda FS 273 4bbl 10.5 CR on pump gas in CA

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Bob Jasinski

Bob Jasinski

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So...getting close to having my original '65 Commando 273, 10.5 CR ratio engine rebuilt. I want to keep it stock with domed pistons, iron heads and factory cam. The rebuilder I am considering asked me about reducing the compression ratio by installing flat top pistons, and I told him I really did not want to do that, and that I would ask the list about it. If you are in California running this same engine with domed pistons, are you satisfied with the performance using high grade pump gas? Should I be concerned? As a side note, I have a '61 Chrysler 300G 413 with 10.1 compression ratio that runs fine on high grade pump gas, so I was thinking it would be OK with the 273 as well. Am I in for problems with a stock rebuild?
 
I’m in NY with a 10.5 273 and had the block and heads decked so to be safe with fuel options Im set up with a MSD timing control knob in ashtray. And will keep some VP octane booster in the trunk
 
Not sure what NY has for fuel compared to CA, they have "boutique" fuels out here. I'm keeping the ignition stock with dual points or maybe a Pertronix in the distributor, but that's it.
 
Generally can get 93 no problem. We had a station that had 100 & 110 octane pumps but they closed about 2 years back.
 
I have a 67 with a 273 Commando. I had it rebuilt with domed pistons and hardened valve seats — basically stock, but with a hydraulic cam. The best gas we can get here is 91 octane with ethanol. I found I had to dial back the initial timing a few degrees, but that was it. Note that the 10.5:1 figure is an "advertised" ratio — my understanding is that in practice it was somewhat lower.
 
Just had my 383 rebuilt as a resto. Builder is very qualified so left details to him as long as engine appears original on the outside. He reduced compression down to 9.0 and went with a hydraulic roller cam. This was to better work with the poor gas and oils. Then we dyno'ed it. I picked up 100 horses over stock! She sounded sweet on the dyno, and it is running points ignition and original carb. Timing is now different- vacuum advance is present but blocked off, so initial timing is now 15*. Who says that you can't teach an old dog new tricks.
 
mvh said:
I have a 67 with a 273 Commando. I had it rebuilt with domed pistons and hardened valve seats — basically stock, but with a hydraulic cam. The best gas we can get here is 91 octane with ethanol. I found I had to dial back the initial timing a few degrees, but that was it. Note that the 10.5:1 figure is an "advertised" ratio — my understanding is that in practice it was somewhat lower.
Click to expand...
9.68 is what I got with Egge HP pistons and a .030 mill on the heads. Replacement pictons and lack of thin steel shim head gaskets won't get you 10.5/1. That's probably a good thing.
 
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Danville Ca; elevation ~350ft.
At 9.68Scr, I wonder if you can run the stock cam @ full timing @WOT.
And if you can't, then, what's the point of the pressure, or the 4bbl.

The Wallace numbers do not look encouraging, at 350 ft.
 
Hey, it's a world of difference from the flat piston 2bbl 273, let me tell you that. Check out the curve at lower left, compared to upper right.

67_back_cover.jpg
 
I agree with @toolmanmike on you being ok . I'm running a complete factory stock 273 commando in my 67 Cuda vert., factory heads, cam and pistons. With factory dual point. only change is couldn't get stock steel head gaskets, using felpro blue for 318 , no issues on pump gas in mine.
Lon
 
Thanks all, great feedback. No flat top pistons in this build! So what head gasket would be most desirable, what was stock?
 
I've been researching this very thing. So the service manual says that 0.028" compressed thickness was stock. And according to summit there is one that is available for a small block Mopar in 0.028" compressed but the few reviews it has tells a tell of leaks. Personally I'm going to run a felpro at 0.040" but I'm also running the comp 270 cam which has a tiny bit more lift and duration than stock. Hopefully it matches up well.
 
Personally, I haven't been able to run straight pump gas (91 octane) and stock ignition timing in my stock '66 273 4bbl. and not have it ping. To get it to stop pinging, I had to back off the timing to the point where performance and gas mileage suffered. It also ran hotter, so, to heck with that. I decided to just add some race fuel to each fill up to cure the pinging and keep the timing and performance up to par. Pump gas is pretty much crap around here. Luckily we have a few gas stations where you can get race fuel right at the pump. Any rebuilds I do now for a cruiser, I keep the compression ratio down to 9.0 to 1, so, I can run regular 87 octane. If I need the performance on a high horsepower rebuild, I just bite the bullet and run better (expensive) fuel, install aluminum heads, or both. On full modified engines, run a turbo.
 
I have a FelPro head gasket set here, new in the box, probably 25 years old, came with the Barracuda project. It measures .054". I'm leaning toward using it to cut down a bit on the compression and still maintain the stock engine parts. Using race fuel is out of the question for me, I just want a fun touring car that I can pump premium fuel in wherever I happen to be.
 
Just measured the bore of the Fel-Pro gasket, and it measures 4.18", whereas the bore of the stock 273 is 3.625" per the FSM. Rock Auto shows this same gasket for a 318 and 273, it appears as though 273 head gaskets are not available. If the 318 gasket were used, that would reduce compression even more I would think.
 
The bore size on those fel pro gaskets, will kill the compression, see is you can find a 4.00 bore size on the gasket.
 
I usually just added about a gallon or so of race fuel to a tank full of premium to get rid of the ping. I didn't have to run straight race fuel.
 
mvh said:
I have a 67 with a 273 Commando. I had it rebuilt with domed pistons and hardened valve seats — basically stock, but with a hydraulic cam. The best gas we can get here is 91 octane with ethanol. I found I had to dial back the initial timing a few degrees, but that was it. Note that the 10.5:1 figure is an "advertised" ratio — my understanding is that in practice it was somewhat lower.
Click to expand...

Can you tell me what cam you used?
 
Bob Jasinski said:
Just measured the bore of the Fel-Pro gasket, and it measures 4.18", whereas the bore of the stock 273 is 3.625" per the FSM. Rock Auto shows this same gasket for a 318 and 273, it appears as though 273 head gaskets are not available. If the 318 gasket were used, that would reduce compression even more I would think.
Click to expand...
The difference by math and round holes, in those two gaskets, is 5.92cc; 2.84 cc for the OEM 273 and 8.77 for the FelPro. in the 273, that's a lot.
At 8.0 true Scr, on a stocker, the total chamber volume needs to be 79.9cc. Adding 5.92cc, is 85.8 and the new SCR is 7.51, so the gasket difference alone is ! a half a point in compression.
At 10.0 the the volume needs to be 62.1cc. Adding 5.92 gets 68cc, and the new Scr is 9.2, so now 8/10s of a point DIFFERENCE

Because the stock 273 gasket does not have round holes, the difference and loss may be greater than the math shows.
 
Murray said:
Just had my 383 rebuilt as a resto. Builder is very qualified so left details to him as long as engine appears original on the outside. He reduced compression down to 9.0 and went with a hydraulic roller cam. This was to better work with the poor gas and oils. Then we dyno'ed it. I picked up 100 horses over stock! She sounded sweet on the dyno, and it is running points ignition and original carb. Timing is now different- vacuum advance is present but blocked off, so initial timing is now 15*. Who says that you can't teach an old dog new tricks.
Click to expand...
So 435hp?
 
'If you use I thicker gasket'

A 4.180 bore .046-.049 thickness #8553 permatorque felpro will take it down a substantial notch.

Unless you build a quench motor...knocking the timing out of it will ultimately reduce max output. 'One of those' doesn't want a lot of timing anyways.. right.
 
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