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Old 11-18-2019, 08:58 AM
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I don't own a ram air lll I was just wanting to know what cuurent gas you run in a motor that the factory called out 10.75:1 compression.
.
91 octane should work in my 400 with 16 heads with 75 ccs. All the other questions have been answered in previous posts.

Thanks
Greg

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Old 11-18-2019, 10:26 AM
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Since it does make a difference. Are you running the stock cam?

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Yep, Mellings version of the 068

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Old 11-18-2019, 10:59 AM
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Just to add, since the YS 400 is part of this discussion.

Dad bought his 69 GTO brand new and of course was the family car for decades. Still owns the car today. He daily drove it right up through the early 1990's before the cars first semi restoration, still running the original engine that had never been opened up. Nothing but pump gas in this thing from day one, and knowing dad he didn't always put the highest octane in it either. He put 180,000 trouble free miles on that engine before the first restoration of the car itself, but the engine was left untouched.

After that it was driven as a fun weekend car for the next several years and continued to put miles on the original engine. The engine had close to 200k miles on it before it was finally pulled out for something bigger. Was still running like a top and now lives a shelved life.

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Old 11-18-2019, 11:06 AM
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A change in compression from 8.5 to 9.5 would provide some 7 more hp to a motor already making 350 hp, but if the car/ motor was set up with Cam of too much overlap or light in the CID dept or in need of more rear gear, then that 1 point change in the compression ratio would provide a bunch more drivability .

On a street motor with iron heads it just does not pay with today's fuel or what maybe tomorrow's fuel to go for that extra 1.7 hp by going for even just a compression of 10 to 1 no less the factory falicy of these motors have a comp of 10.75!

A stock 4.120" Bore RAIII motor with the 66 CC version of the casting number 48 heads along with a Fel pro 1016 gasket and a .021" deck clearance only had a true compression of 10.3 just for a reference point here.

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Old 11-18-2019, 11:44 AM
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It's why tuning is such a big part of making this stuff work. I prefer the higher compression ratios for other reasons than just power production. It also helps in the vacuum department, throttle response, drivability etc... with a cam well matched to the combo it'll run pump gas relatively easily.

Matter of fact I built our DZ 302 with a true 11:1 compression, just as the factory rated the engine. Using the original iron 186 hump heads, and the original DZ camshaft. What helps this combo work perfectly on 91 pump gas is the 30-30 camshaft as it's often referred to. 254 degrees @ .050, and a wide 114 lsa in a very small 302 CI engine. Lots of overlap. Been daily driving this thing now going on 3 years with nothing but crappy 91 pump gas here in AZ and it runs perfectly with 36 degrees of total timing. Ask dad, he'll tell ya it flat gets with the program for a little engine.

A camshaft like this needs compression anyway. If I were to build this engine and knock the compression down thinking I needed to do so to run pump gas, it would be a complete turd, sluggish throttle response and just an all around pooch to drive.

But again, to make this deal work, you need to have pretty good tuning abilities and good control over the timing and fuel curves. However once that's dialed in, you don't have to touch it. About once a year I'll check the valve lash for good measure, that's it.

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Old 11-18-2019, 11:52 AM
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A 254 @ .050" Cam is the only thing allowing that 11 to 1 302 cid motor to not spit its Rods out!

What's it's cranking compression if I might ask?

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Two dry rotted 14 x 10 Micky Thompson slicks.

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Two pairs of brown leather flip flops, never seen more then 2 mph.

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Old 11-18-2019, 01:05 PM
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You will find deep calculated gear ratio behind hi compression combinations that run well, smaller the cubic inch, deeper the gear will be.

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Old 11-18-2019, 01:31 PM
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I have a completely stock 71,000 mile 67' GP 400 with stock closed chamber 670 heads I drive daily during the season.
I curved the stock dist & tuned the carb...no detonation w/93 to date

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Old 11-18-2019, 01:36 PM
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Eh, I've run the 350 LT1's with their smaller split solid cam of 242/254 @ .050 with 11:1 compression and the stock iron hump heads on 91 octane and that engine also ran fine. The cam acts quite a bit smaller too given the extra 50 cubes and 12 degrees less intake duration.

But it's been pretty widely known in the SBC community for a lot of years that these engines run great on pump gas thanks to the generous camshafts that GM used. That's part of the equation, but the tune up is key.

As far as gears, anything was available from GM when new. Standard for the 67-69 Z was 3.73, which isn't all that steep. When coupled with the 2.20 Muncie first gear multiplication wasn't even all that great. They do love rpm but they also will loaf around town near idle without complaint and plenty of low end power to pull the car around.
I run 3.55 gears in mine, and with a TKO600 behind it now, the 2.87 first gear is perfect gear multiplication to make it snappy, yet the little 302 still has enough grunt to loaf in 5th gear at 30 mph, barely 1,000 rpm, and will gain speed easily pushing on the throttle without downshifting. Actually very docile and a pleasure to drive.

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Old 11-18-2019, 03:10 PM
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Quote:
Originally Posted by footjoy View Post
I don't own a ram air lll I was just wanting to know what current gas you run in a motor that the factory called out 10.75:1 compression. ...
that right there is the key;
Advertising called it 10.5 or 10.75 or whatever compression;
real world compression, as built put them just under 10.0:1.

I ran my 0.040 over 1970 YZ RAIII on regular "premium" (914 octane) pump gas just fine.

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  #31  
Old 11-18-2019, 03:25 PM
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Quote:
Originally Posted by unruhjonny View Post
that right there is the key;
Advertising called it 10.5 or 10.75 or whatever compression;
real world compression, as built put them just under 10.0:1.

I ran my 0.040 over 1970 YZ RAIII on regular "premium" (914 octane) pump gas just fine.
Thanks

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Old 11-18-2019, 03:41 PM
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When you factor in the 0.030" lash the 0.050" is not really as great as you would think. But look at the seat-to-seat duration at the valve with the lash set at 0.030". Where will find a shelve cam or custom cam like that?

Stan


VALVE_____Lift______Opens___Closes__Duration
_________________Deg_BTDC__Deg_ABDC_____________Ar ea
_________0.00000____41.41_|_105.91_|_327.31_|__38. 97
_________0.00600____32.24_|__89.66_|_301.91_|__38. 93
_________0.01000____28.63_|__83.42_|_292.05_|__38. 89
_________0.02000____22.52_|__74.85_|_277.37_|__38. 75
_________0.04000____14.37_|__64.90_|_259.27_|__38. 45
_________0.05000____11.08_|__61.12_|_252.19_|__38. 36
_________0.10000____-1.83_|__46.62_|_224.80_|__37.21
_________0.15000___-12.29_|__35.63_|_203.35_|__35.92
_________0.20000___-22.23_|__25.62_|_183.39_|__34.14
_________0.25000___-32.41_|__15.44_|_163.03_|__31.86
_________0.30000___-43.46_|___4.41_|_140.95_|__28.50
_________0.35000___-56.05_|__-8.17_|_115.78_|__24.53
_________0.40000___-71.80_|_-23.88_|__84.32_|__18.46
_________0.45000___-99.00_|_-50.82_|__30.18_|___6.83
CAM
_________0.00600___114.81_|_179.17_|_473.98_|__30. 21
_________0.01000____92.58_|_158.43_|_431.01_|__30. 03
_________0.02000____41.41_|_105.91_|_327.31_|__29. 26
_________0.04000____18.09_|__69.28_|_267.37_|__28. 44
_________0.05000____12.68_|__62.95_|_255.63_|__28. 11
_________0.10000____-6.13_|__41.99_|_215.86_|__26.66
_________0.15000___-21.24_|__26.62_|_185.38_|__24.60
_________0.20000___-36.69_|__11.17_|_154.47_|__22.10
_________0.25000___-54.69_|__-6.80_|_118.51_|__18.02
_________0.30000___-79.89_|_-32.01_|__68.10_|__10.79

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  #33  
Old 11-18-2019, 05:07 PM
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FWIW my 69cc virgin RAIV heads on a 400 picked up 8-10 HP adding race gas in the 93 octane on the dyno and had no audible detonation-and that dyno used real 3" muffler system so more things were audible.

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  #34  
Old 11-19-2019, 12:01 AM
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If you’re not a really sharp tuner then err on the conservative side, personally I wouldn’t try to run 10:1 compression with iron heads since I’m not an expert tuner like some of the posters in this thread.

If you’re not totally on top of things then you’re opening yourself up for an engine failure. If you’re conservative and keep your compression somewhere in the low 9:1 range you then have some room to compensate for a bad batch of gas or having your ignition timing a little off of the optimum setting.

I would use your 87cc 1970 #15 heads and shave about .025” to .030” off the gasket surface, enough to reduce the chamber volume by a few ccs and then tighten up the quench using the .027” thick gasket you’ve been considering. A light surface cut wouldn’t affect how your intake bolts up to the heads.

Try to end with something that you don’t have to worry about, something you can fill up anywhere and can even use 87 in an emergency if you happen to be far from your home fueling station while on an outing.

An automatic transmission vehicle will be less octane tolerant than one with a stick shift, another thing to consider.

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Old 11-19-2019, 07:06 AM
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Just to note here,the closed chamber heads like the 670's and all the 389 and 421 type early chambers are a good bit less prone to detonation then the open chamber heads.

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And he was not talking about 1/8 or 1/4 mile ETs!

1) 1940s 100% silver 4 cup tea server set.

Two dry rotted 14 x 10 Micky Thompson slicks.

1) un-mailed in gift coupon from a 1972 box of corn flakes.
Two pairs of brown leather flip flops, never seen more then 2 mph.

Education is what your left with once you forget things!
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Old 11-19-2019, 08:53 AM
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Quote:
Originally Posted by b-man View Post
If you’re not a really sharp tuner then err on the conservative side, personally I wouldn’t try to run 10:1 compression with iron heads since I’m not an expert tuner like some of the posters in this thread.

If you’re not totally on top of things then you’re opening yourself up for an engine failure. If you’re conservative and keep your compression somewhere in the low 9:1 range you then have some room to compensate for a bad batch of gas or having your ignition timing a little off of the optimum setting.

I would use your 87cc 1970 #15 heads and shave about .025” to .030” off the gasket surface, enough to reduce the chamber volume by a few ccs and then tighten up the quench using the .027” thick gasket you’ve been considering. A light surface cut wouldn’t affect how your intake bolts up to the heads.

Try to end with something that you don’t have to worry about, something you can fill up anywhere and can even use 87 in an emergency if you happen to be far from your home fueling station while on an outing.

An automatic transmission vehicle will be less octane tolerant than one with a stick shift, another thing to consider.
A lot of good input here. Good point in regards to the transmission choice. My car RA III was a four speed and it needed better than 93 octane.

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Old 11-19-2019, 08:59 AM
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Originally Posted by steve25 View Post
Just to note here,the closed chamber heads like the 670's and all the 389 and 421 type early chambers are a good bit less prone to detonation then the open chamber heads.
Interesting you say that. I had a 64 GTO with original block and heads already rebuilt when I got the car, not sure what cam but I never had any concern with detonation or pinging ran 91 octane. Put a tri power on it and got 18-19 mpg on the hiway.

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Sold it 3 years ago

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Old 11-19-2019, 09:22 AM
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Quote:
Originally Posted by b-man View Post
If you’re not a really sharp tuner then err on the conservative side, personally I wouldn’t try to run 10:1 compression with iron heads since I’m not an expert tuner like some of the posters in this thread.

If you’re not totally on top of things then you’re opening yourself up for an engine failure. If you’re conservative and keep your compression somewhere in the low 9:1 range you then have some room to compensate for a bad batch of gas or having your ignition timing a little off of the optimum setting.

I would use your 87cc 1970 #15 heads and shave about .025” to .030” off the gasket surface, enough to reduce the chamber volume by a few ccs and then tighten up the quench using the .027” thick gasket you’ve been considering. A light surface cut wouldn’t affect how your intake bolts up to the heads.

Try to end with something that you don’t have to worry about, something you can fill up anywhere and can even use 87 in an emergency if you happen to be far from your home fueling station while on an outing.

An automatic transmission vehicle will be less octane tolerant than one with a stick shift, another thing to consider.
Well that makes perfect sense I am running a 200 4r didn't realize the trans would make a difference. I do plan on some long distance trips and don't want hunt and search for gas.

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Old 11-19-2019, 09:30 AM
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Read between the lines of what I said with that paragraph!
The closed chamber heads heads have far more quench to them then a open chamber head even with the same amount of deck clearance that a open chamber motor may be running in terms of octane tolerance!

Another way of looking at this is that when you run a open chambered head you need to strive for the minimum amount of deck clearance for any given compression ratio.

In light of this fact it's best with the 68 and up iron heads when your building or modding them to make the valve notches a bit deeper or bigger towards the center of the chamber to not exceed the compression ratio your looking for.

With the later open chamber heads the factory choose to make the chambers deeper to achieve the needed compression ratio while keeping the same needed deck clearance to maintain the level of quench , but in the later motors when they went the cheaper route and started to apply a chamfer around the Pistons OD to get the needed compression they went off the rails in my opinion and lost quench with that change .

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And he was not talking about 1/8 or 1/4 mile ETs!

1) 1940s 100% silver 4 cup tea server set.

Two dry rotted 14 x 10 Micky Thompson slicks.

1) un-mailed in gift coupon from a 1972 box of corn flakes.
Two pairs of brown leather flip flops, never seen more then 2 mph.

Education is what your left with once you forget things!

Last edited by steve25; 11-19-2019 at 09:35 AM.
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Old 11-19-2019, 04:30 PM
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Originally Posted by Steve C. View Post
As I understand it the stock published compression ratio was often more than actual compression ratio. Probably vehicle and/or engine specific.


.
I once calculated that in my particular case, the 'advertised' compression ratio would be correst if the piston top was at the top of the cylinder, there were no valve reliefs and no head gasket, and their was zero space around the piston above the rings. I don't know if it just worked out that way in my case, or if that was the general practice in 'advertised CR' by GM, or just pontiac.

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