My 340 build and pump gas questions

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oliver

oliver

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I just got around to calculating my actual static and dynamic compression ratios, looks like ill be sitting right at 10.42 static compression and 7.61 dynamic.... i thought the dynamic compression sounds a little low but i checked and rechecked things and it all computes like that.
Little info about the motor, 4.1" bore, 3.31" stroke, 68cc J heads, kb243's .026 out of the block, 7.5cc reliefs, 4.140 bore and .028" thick head gaskets, 6.123" con rod, 288 duration cam, 110 lsa.
Think this combo will run on 91 okay?
 
With 15 minutes in between posts, did ya think it was going to be a onslaught of reviews? This is not a live forum.

And no, your ratio is to high. You'll need to retard the timing alot in order to run it and that will be a big power loss.
 
I am no expert but something sticks out to me about your calculations. If those pistons are .026" above the deck and you're using a .028" gasket that usually compresses to .025" then you might want to look into that particular aspect of your measurements. I realize you have an open chamber but I would double check everything before I ran it.

Obviously you must be looking for 'quench' but you may have to get a thicker gasket, like an .039" or something. I suppose as long as everything checks out with your initial plans it could work but you would need to be 100% sure that nothing is going to make contact. I could be way off though.

To answer the question though, 10.42:1 might be right on the edge of pump 91. Guess it would have to be tuned right so as to not detonate. High c.r. 340s were what, 10.5:1 from the factory? But that was back when higher octane was readily available. Your cylinder pressure needs to be taken in to account somewhere.
 
rumblefish360 said:
With 15 minutes in between posts, did ya think it was going to be a onslaught of reviews? This is not a live forum.

And no, your ratio is to high. You'll need to retard the timing alot in order to run it and that will be a big power loss.
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12 hours and 15 minutes maybe? I thought with a dynamic ratio that low, it will be streetable on pump.
 
rmchrgr said:
I am no expert but something sticks out to me about your calculations. If those pistons are .026" above the deck and you're using a .028" gasket that usually compresses to .025" then you might want to look into that particular aspect of your measurements. I realize you have an open chamber but I would double check everything before I ran it.

Obviously you must be looking for 'quench' but you may have to get a thicker gasket, like an .039" or something. I suppose as long as everything checks out with your initial plans it could work but you would need to be 100% sure that nothing is going to make contact. I could be way off though.

To answer the question though, 10.42:1 might be right on the edge of pump 91. Guess it would have to be tuned right so as to not detonate. High c.r. 340s were what, 10.5:1 from the factory? But that was back when higher octane was readily available. Your cylinder pressure needs to be taken in to account somewhere.
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Well, its .028" compressed, the quench will be in the range of .080" and .100" so too large to be effective. Does the DCR correlate to cylinder pressure? With a dynamic ratio of 7.6 its in line with 91 octane fuel.
 
LOL, YA! I stand corrected, my bad.
 
7.6 should have about 155psi which would run on pump gas. If it's more than 175-180 then you are flirting with issues.
 
crackedback said:
7.6 should have about 155psi which would run on pump gas. If it's more than 175-180 then you are flirting with issues.
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Okay, thanks. I wasn't sure about weather with 10.5 static and 7.6 dynamic weather it would run okay on 91 octane.
 
You will be fine, There is a guy DCGr33 or something is running 8.7 dcr with 89 and 33 degrees of timing with magnum heads. I want to run 8.8 dcr on 91 and magnum heads with .038 oc quench on 91 but IM not sure if I can :/
 
The open chamber heads do you no favors either.

I've had some stockish 340's that were a pain to stop pinging.
 
crackedback said:
The open chamber heads do you no favors either.

I've had some stockish 340's that were a pain to stop pinging.
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Would you run 91 octane with 8.8 in Orange County? I read an article from Car Craft about a chevy motor that used 87 on 8.77 DCR I believe and they used longer piston rods and it helped I believe, Ill try to find the article.
 
crackedback said:
The open chamber heads do you no favors either.

I've had some stockish 340's that were a pain to stop pinging.
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But with a DCR being pretty low, do you think its worth doing with closed chamber heads to get a decent quench? My j heads will give a quench of about .08 to .1.
 
i have been running a 340 for the last 3 years.( i am switching to a 360 for next season but thats a different story...). static compression ratio was 10.4:1 with a mp 557 solid cam. i have no idea what the dynamic cr was . ran just fine on the street on premium pump gas, which is 93-94 around here. there was a barely audible ping when ran through the mufflers at 36 degrees total. best performance at the track was with 32 total, again on pump gas. same thing with c-10. as for loosing performance with less timing, forget about it, it was my best performing 340 yet ( 12.14 @ 111). also, a little less timing such as 30-32 degrees means a cooler running engine. if you hear it ping, just remove some advance, it will run better that way plus ,i always believed alittle less timing meant a little more torque( i migth be wrong on this one).just forget about what they say that a 340 needs 35-36 total, youll find out 30-32 migth work better at this level.:burnout:
 
Open chamber heads do not give any quench affect. Which is why they can be difficult to get to run on pump fuel. Properly tuned, you can run 8.25:1 dynamic at sea level on 10% ethanol pump 91 with an open chamber head. You can get another 1/2 point over that with a closed chamber head and tight quench distance (less than .040). The key is tuning, but with 7.whatever in it, you should be able to run pump 87 with a proper tune up and timing curve.
 
moper said:
Open chamber heads do not give any quench affect. Which is why they can be difficult to get to run on pump fuel. Properly tuned, you can run 8.25:1 dynamic at sea level on 10% ethanol pump 91 with an open chamber head. You can get another 1/2 point over that with a closed chamber head and tight quench distance (less than .040). The key is tuning, but with 7.whatever in it, you should be able to run pump 87 with a proper tune up and timing curve.
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just for info, I ran 8.6DCR 10.9 compression with 91 and I think it was 34-35 degrees of timing in NorCal I think it was 85ft above sea level with closed chambered heads.
 
rumblefish360 said:
With 15 minutes in between posts, did ya think it was going to be a onslaught of reviews? This is not a live forum.

And no, your ratio is to high. You'll need to retard the timing alot in order to run it and that will be a big power loss.
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That is the longest 15 Minutes I have ever seen.....
 
7e5hangten360 said:
forget about what they say that a 340 needs 35-36 total, youll find out 30-32 m


X2

Todays fuel burns quicker, run the amount of timing the engine wants, even if its 27-28/ at the track you could try racing fuel and then bump up the timing but still it might take only 32--33, maybe 35 if the fuel burns slow--very high octane
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moper said:
Open chamber heads do not give any quench affect. Which is why they can be difficult to get to run on pump fuel. Properly tuned, you can run 8.25:1 dynamic at sea level on 10% ethanol pump 91 with an open chamber head. You can get another 1/2 point over that with a closed chamber head and tight quench distance (less than .040). The key is tuning, but with 7.whatever in it, you should be able to run pump 87 with a proper tune up and timing curve.
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wow, low octane huh? I'm beginning to wonder if my cam choice is going to be a hamper on things. if the dynamic ratio matters so much more, then why does everyone base things on static ratio?
 
It's what were used to. Static ratio + cam size = useable octane ability, plus or minus variables .....etc.... The dynamic ratio changes with the cam used as well.

There's a few places that give both ratio's and there recommendations to what cam to use. Though there good choices, there is also wiggle room to there formulas or cam duration's recommended.

Look at Hugesengines.com
They give dynamic ratio with certain cams suggested and used to target for.
 
oliver said:
wow, low octane huh? I'm beginning to wonder if my cam choice is going to be a hamper on things. if the dynamic ratio matters so much more, then why does everyone base things on static ratio?
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It's like anything else. Time and technology march on. Honestly, if it's an enthusiast that's been active in hot rodding over the past 10 years and reads magazines at all, they should not be using static ratio. And any pro shop that does is relegating themselves to the non-performance genre. At least by my opinion. The truth is, it's easier for both buyer and builder to not worry about the details of physical reality. Nevermind the loss of quality fuel, and the dying art of timing and carburetor tuning.
I'm not sure what the problem might be by running cheaper fuel. How will it hamper anything?
 
moper said:
I'm not sure what the problem might be by running cheaper fuel. How will it hamper anything?
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Well, im thinking that the cam im running is bleeding off alot of cylinder pressure in the low end making it have quite a bit less torque.
 
You're definately giving up some snap that way, yes. Take the cam's intake closing event and advance it 4 or 6 degrees and see how that changes the dynamic ratio. You can probably just advance the cam and get a bit back.
 
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