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Tried and True 304 Builds? |
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farna
Supporter of TheAMCForum Moderator Lost Dealership Project Joined: Jul/08/2007 Location: South Carolina Status: Offline Points: 19676 |
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"The calculation is woefully wrong for performance use, not bad for mild cruising around."
The calculations are based on engine pumping capacity only. There are other factors, especially in performance use. A smallish carb usually works well on the street and for low speed pulling, but becomes the choke point on a drag strip or any time the engine will be running high rpm (over 4K) often, or should I say is expected to deliver performance at 4K and above. Velocity of the charge is important, especially for low speed work. That's why it's hard to get a good street/strip combo that delivers good gas mileage AND good high speed performance with a carb. Small carb works best for economy, big for performance. Most street driving is done in the 2000-3000 rpm range, with occasional short bursts up to 4000-5000. Most engines built strictly for street (most stock engines) start to fall off power around 4000-5000 rpm, a drag engine hasn't peaked yet! |
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Frank Swygert
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Greyhounds_AMX
AMC Addicted Joined: Nov/14/2009 Location: Kansas City Status: Offline Points: 1268 |
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The 268H would give you a lower DCR by about 0.3 points, but about 1% more overall intake flow capacity than the XE256.
So with the 268H I'd expect you'd probably have the same power within a few hp, but you'd have a rougher sounding idle, and you'd be able to run 87 octane if you wanted to. Plus the valvetrain would be quieter and last longer (lower hydraulic intensity). Edited by Greyhounds_AMX - Aug/17/2017 at 1:18pm |
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1968 AMX 390 w/T5
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WhatISit
AMC Addicted Joined: Aug/07/2017 Location: Phoenix, AZ Status: Offline Points: 0 |
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Oooh, good info! Thanks! |
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Class Guy
AMC Addicted Joined: Jul/02/2007 Location: Arkansas Status: Offline Points: 969 |
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I recommend you pay attention to compression ratio when you are assembling parts and building. I have seen too much that low compression height of aftermarket pistons combined with thick head gaskets and increased combustion chamber volumes due to valve job and "performance porting" can lead to compression ratios in the low 8's. That's giving away "free" HP and will make for a very lazy feeling engine at low RPMs. It takes time and effort (and a few specialized tools) to establish compression ratio where you want it, but it is always worth the effort.
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WhatISit
AMC Addicted Joined: Aug/07/2017 Location: Phoenix, AZ Status: Offline Points: 0 |
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Well, if I could really get compression where I want it, I'd have 9.5-10.0:1, but right now I'm gonna have to to do whatever little tricks I can to squeeze out as much CR as possible. (Which will probably barely be measurable. I can't think of anything besides using a really thin gasket, honestly. |
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Class Guy
AMC Addicted Joined: Jul/02/2007 Location: Arkansas Status: Offline Points: 969 |
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Measure combustion chamber volumes, deck clearance (distance from the top of the block to the top of the pistons at TDC) and volume of any dish or dome on the pistons. There are formulas on the internet that allow you to calculate the compression from those measurements along with bore & Stroke and head gasket thickness. You can have the block machined to reduce deck clearance and you can machine the heads to reduce the combustion chamber volumes.
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Sonic Silver
Supporter of TheAMCForum Joined: Nov/23/2011 Location: East Tennessee Status: Offline Points: 7949 |
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Also, you are correct on valvetrain noise and life. I have usually used older type cams for that reason.
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Boris Badanov
AMC Addicted Joined: Dec/14/2013 Location: NH USA Status: Offline Points: 4209 |
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Flat top single valve relief pistons
and a .04 quench with a 58CC head should give you right around 10:1 compression. A mild performance cam with a Eddy 600 and a free flow exhaust will easily net you 280-300 hp at the flywheel. Premium fuel only of course. 304 heads are very good stock as long as you are not going to run it at over 5500 rpm all day. Even the stock valve size is plenty big. The only thing the AMC 304 fails to deliver as compared to the 283-307 Chebby is power to weight and a HUGE amount of aftermarket support. the Chevy oil system is way better, but that is a minor thing when doing a comparison. Edited by Boris Badanov - Aug/19/2017 at 2:07pm |
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Gremlin Dreams
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232jav3sp
AMC Addicted Joined: Jan/09/2013 Location: Texas Status: Offline Points: 2451 |
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There is a set of new flat top 290 pistons on ebay. If your 304 is stock bore, and the bores are in really good condition, you could put the 290 pistons in it (I'm sure someone will confirm this can be done after reading this). They'll bump up compression over the stock 304 dished pistons. Put moly rings on them, measure combustion chamber volume, measure deck height and choose a head gasket in the thickness and diameter you need to achieve the comp ratio you want. Cometic sells a few off the shelf head gaskets for the 304. I think the thinnest is .027". Go with a Comp High Energy cam for a quiet valvetrain that stock valve springs should be able to handle, and you'd have a stout bottom end. Wouldn't hurt to upgrade springs to something slightly stiffer, but, most seem to have no issues on stock springs with the High Energy cams under .500" lift.
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farna
Supporter of TheAMCForum Moderator Lost Dealership Project Joined: Jul/08/2007 Location: South Carolina Status: Offline Points: 19676 |
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I don't like ANY of the XE cams for a street engine (well, maybe the smallest.. least lift) because they require stiff valve springs and won't work with stock (max lift with stock springs is about 0.455"). The extra stiff springs mean you HAVE to run racing oil or an additive. Anything over about 320# per inch open needs a high pressure additive or capable oil. Anything less can use off-the-shelf 10W30, no additives except for break-in. Diesel oils have a it more high pressure additive, but are usually 15Wxx, not 10Wxx. I don't particularly like running 15Wxx, especially in winter, in a new engine. If it's got over 75K or so miles it shouldn't be an issue unless in really cold (well below freezing).
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Frank Swygert
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