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Old 02-03-2002, 05:11 PM   #1 (permalink)
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351C Compression w/ Aussie Heads

Compression on a stock 351C is supposed to be 11:1 right?

What is the compression after putting on a pair off aussie 302 2V CC's?
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Old 02-03-2002, 06:07 PM   #2 (permalink)
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351C Compression w/ Aussie Heads

I think thats pretty close. Performance trends compression calculator actually gave me something like 10.79 on mine.
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Old 02-04-2002, 09:54 AM   #3 (permalink)
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351C Compression w/ Aussie Heads

Depends on the piston. KB Flattops will give around 11.1:1, while TRW Forged pieces (also flattop) will yield a number closer to 10.8:1.
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Old 02-04-2002, 03:05 PM   #4 (permalink)
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351C Compression w/ Aussie Heads

I think it is safe to assume a three to four cc smaller combustion chamber ... going from American 4V quench heads to Australian (302)2V heads.

You should be able to plug the numbers in the compression calculator and get a decent "Ball Park" idea of the difference.
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Old 02-04-2002, 06:57 PM   #5 (permalink)
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351C Compression w/ Aussie Heads

Well Larry, and everyone else that wants to chime in on me:

Ok, with the 302 Closed chamber heads, you get about 11:1 Compression (ball opark figure)3" stroke. put the same heads on a 351 Cleveland, get the extra .5" to get a total of 3.5 of stroke and the Compression remains RELATIVELY 11:1. Well, what is to stop us from going to an extra .5" of stroke? I guess I am bringing up the 400 Question again. I am just trying to learn more.

Thanks a lot!

Jeff Given
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Old 02-04-2002, 06:57 PM   #6 (permalink)
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351C Compression w/ Aussie Heads

Well Larry, and everyone else that wants to chime in on me:

Ok, with the 302 Closed chamber heads, you get about 11:1 Compression (ball opark figure)3" stroke. put the same heads on a 351 Cleveland, get the extra .5" to get a total of 3.5 of stroke and the Compression remains RELATIVELY 11:1. Well, what is to stop us from going to an extra .5" of stroke? I guess I am bringing up the 400 Question again. I am just trying to learn more.

Thanks a lot!

Jeff Given
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Old 02-04-2002, 08:51 PM   #7 (permalink)
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351C Compression w/ Aussie Heads

Jeff ... Compression on a 302 ci engine with these heads would not be 11:1 ... much smaller cylinder volume than our 351C. I reckon they deliver maybe 9.5:1 ... and I'm guessing, I'm sure someone in the forum knows exactly.

What is happening in this post is taking heads with a slightly smaller combustion chamber than the "normal" 351C quench heads and sticking them on top of our bigger cube engine. Net compression ratio difference is minimal on our engines.

When you take the quench head and stick on a 400 ... You would expect the compression to jump big time ... But, it doesn't because the deck clearance is huge on the 400 which keeps compression down ...

The big down side to that is it also destroys the "Quench Affect" ...

Then if you try to cut the deck to regain the quench affect ...

You will then end up with an outrageous compression ratio.

I hope that made sense



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[ This Message was edited by: monstermach on 2/5/02 8:53am ]
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Old 02-05-2002, 01:23 AM   #8 (permalink)
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351C Compression w/ Aussie Heads

My 351 2v build ...
Bore 4.0400 inches
stroke 3.5000 inches
swept vol 735.23 cc
Deck height above piston 0.0480 inches 4.62cc
Head gasket thickness 0.047; 10.17cc
(0.038 is an option)
Piston valve reliefs 4.00 cc
Combustion chamber 59.97 cc
Combustion volume 78.76cc
My calculated comp ratio 10.34

pistons are flat-tops with 4 valve reliefs

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Old 02-05-2002, 06:22 AM   #9 (permalink)
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351C Compression w/ Aussie Heads

The compression listed in the manual for a stock 302 cleveland is 9.8:1..
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Old 02-05-2002, 11:13 PM   #10 (permalink)
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351C Compression w/ Aussie Heads

Hmm a little confused here....

I have a US 351C. I am going to put on Aussie heads and a pretty aggressive cam that requires 10:1 - 11:1 compression. I imagine I won't be under 10 by putting on the 302 2V Aussie Cleveland heads. But I was looking to see what compression I could expect only with the addition of the smaller cc 302 Aussie Heads on my stock US 351C.

From what I read though I think somewhere around 10.5 - 11.

My other question is what is the stock compression ratio on a stock 70 351C 4V engine. The manual says 11:1 which seems a little high. So I was thinking with the addition of smaller chamber heads the compression might be higher than desired. Around 12:1 or higher, but, again from the posts, I think it won't come out that high.
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Old 02-06-2002, 01:21 AM   #11 (permalink)
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351C Compression w/ Aussie Heads

70 351C 4V was 11:1 ... Going to Australian heads will decrease chamber size by around 3cc which will increase compression somewhat ... compression ratio calculator indicates (depending on pistons ... I entered -2cc for valve releif and assumed flat tops) ... comes 11.23

You will need premium fuel for wure but the quench will obviously still be working in your favor
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Old 02-11-2002, 09:58 AM   #12 (permalink)
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351C Compression w/ Aussie Heads

So what piston do we assume here? When looking at Speed Pro (formerly TRW) pistons, I see 10.7ish:1. You calculate 11.3:1 on Keith Black pistons.

What can we assume about the pistons? Different heights? Keith Blacks give back 2cc, and the Speed Pros say they give back 1.5cc. I really would have expected that Speed Pros would have advertised a higher compression if their valve reliefs ahve less cc's.

Larry, how are you gonna account for that much compression? I know you are (were) running 11.1:1 on the Beast... are you gonna be able to squeeze in 11.3:1 on pump gas?
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Old 02-11-2002, 10:25 AM   #13 (permalink)
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351C Compression w/ Aussie Heads

speedar - something looney about your numers... your head gasket is .047" with 4.62 cc, and your deck is .048" with 10.17cc Those heights are nearly identical, the cc's should be too... unless there is something goofy about the head gasket that I am not considering.
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Old 02-11-2002, 10:36 AM   #14 (permalink)
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351C Compression w/ Aussie Heads

The "engine guru" (in the circles I run in)
has indicated to me that the extra bit of compression (11.25 vs 11.1) won't be a problem with the Aussie quench heads.

Understand though, I haven't done this, so I can't speak from personal experience.

I am building an Aussie 2V motor, but will be running an existing rotating assembly including pistons that have a slight dish so I figure (best estimate) I'll be running 10.3 or something like that in my Aussie.

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[ This Message was edited by: monstermach on 2/11/02 10:38pm ]
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Old 02-11-2002, 11:04 AM   #15 (permalink)
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351C Compression w/ Aussie Heads

Oh... I thought you were gonna use Beast's shortblock... So what is this engine going in?
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