Cam, Compression ratio and octane ratings??? Help?!

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vntned

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I know this is a potential Pandora's Box, but what's the deal with so many people saying that you cant run 10:1 compression, stock iron heads, and a "mild" cam on pump gas?!

My confusion is, stock early 273HP and 340HP came from the factory rated at 10.5:1cr, with iron heads and what was a "mild" performance cam. So were those cars unable to run without knocking on 93 octane fuel? I'm aware that much higher grades of gas were more readily available in the '60s, but was that an absolute requirement?

I'm almost ready to put my very mild 360 back together, and in the process of asking for cam advice, have developed a concern. The goal of the build was to make a warmed over copy of a stock 340HP, just with a 360 bottom end. The engine is essentially a .060" over 360, with hypereutectic pistons 0.010" below deck, stock 587 heads with 1.88/1.60 valves. Eddy LD340 intake. 750cfm carb. Stock 340HP exh manifolds. Dual 2.5" exhaust.

Having not CCd the heads, I did my math using an assumed 73cc chamber, and ended up with roughly 9.8:1cr.

The cam I was about locked in on was the Comp 268AH-10, which is just slightly hotter than a stock 340/360 4bbl cam, (with exception to the '68 4spd cam).

Specs on said cam are as follows:

Duration 268 intake 276 exhaust
@0.050" 222 intake 226 exhaust
Valve lift 0.464" int 0.464 exh
LSA 110deg
Centerline 106deg

This is the cam Id like to run, as this engine is going in my daily driver and am looking for great manners, but a little extra muscle. I'm not shooting for max power obviously! Just a reliable healthy 360.

Oh, and just so its out there, I have absolutely no qualms about having to run 93octane gasoline!
 
The key is what you dynamic CR is... the static is a starting point. I have run 10.2 SCR with a very narrow duration cam, and iron heads (351C) and it worked fine. I was not aggressive on ignition timing, installed the cam straight up (no cam advance) and stuck with premium; it would indeed knock on regular, so it was near the limit. (It did have quench.)

As an aside, the old octane ratings were 'Research' octane numbers, which were higher than 'Motor' octane. The numbers used now in the US are the average of the 2. So the older fuel was not really all that much better on octane, if at all. And, the old CR numbers were certainly quote 'optimistic'; the real numbers were lower.

I ran SCR numbers on your setup and did not come up with your level of SCR. I used the following numbers:
Bore 4.06
Stroke 3.58
Head chamber volume 72 cc ( a tiny bit more CR)
Head gasket thickness .051" (Standard Felpro's; what did you assume?)
Flat top pistons .010" below deck
Pistons eyebrow volume 5 cc (What actual pistons do you have?)

With the above numbers, I come up with SCR = 9.4:1.

With your cam installed as ground with 4 degrees cam advance, then the ICA is 60 degrees and the DCR is 7.75:1..... VERY safe IMHO.

Can you give the head gasket and pistons used?

BTW, I use the Pat Kelley CR computations.
 
Run what you want to. That's the ticket.
 
100 octane was common as late as the mid 70's.
 
Ooh, tell us about "technical editor".....
 
I have an aside question. I'm a noob to cars this old, what do you guys mean by "quench"? Code for lead additive?
 
vntned said:
I know this is a potential Pandora's Box, but what's the deal with so many people saying that you cant run 10:1 compression, stock iron heads, and a "mild" cam on pump gas?!

My confusion is, stock early 273HP and 340HP came from the factory rated at 10.5:1cr, with iron heads and what was a "mild" performance cam. So were those cars unable to run without knocking on 93 octane fuel? I'm aware that much higher grades of gas were more readily available in the '60s, but was that an absolute requirement?

I'm almost ready to put my very mild 360 back together, and in the process of asking for cam advice, have developed a concern. The goal of the build was to make a warmed over copy of a stock 340HP, just with a 360 bottom end. The engine is essentially a .060" over 360, with hypereutectic pistons 0.010" below deck, stock 587 heads with 1.88/1.60 valves. Eddy LD340 intake. 750cfm carb. Stock 340HP exh manifolds. Dual 2.5" exhaust.

Having not CCd the heads, I did my math using an assumed 73cc chamber, and ended up with roughly 9.8:1cr.

The cam I was about locked in on was the Comp 268AH-10, which is just slightly hotter than a stock 340/360 4bbl cam, (with exception to the '68 4spd cam).

Specs on said cam are as follows:

Duration 268 intake 276 exhaust
@0.050" 222 intake 226 exhaust
Valve lift 0.464" int 0.464 exh
LSA 110deg
Centerline 106deg

This is the cam Id like to run, as this engine is going in my daily driver and am looking for great manners, but a little extra muscle. I'm not shooting for max power obviously! Just a reliable healthy 360.

Oh, and just so its out there, I have absolutely no qualms about having to run 93octane gasoline!
Click to expand...
With the assumed static compression and that cam, you will have a cranking psi of about 158 which is no problem on pump gas. You could go a little higher with chamber polishing and then some with a step head piston.
 
The factory "10.5:1" motors were not actually 10.5:1.

In my 68 340 motor, I ran California oxygenated 91 octane with a MP 761 cam with stock X-head, original pistons, and Fel Pro Permatorque ~.045 compressed gaskets. That cam is a little milder than the XE268. The pistons were just a little above deck. But I adjusted my total timing so it wouldn't ping. I forgot is that number was stock or under stock for total timing.
 
Thanks guys! That answered most of my questions, and some I wasn't smart enough to ask!

As far as pistons go, I'm using SpeedPro H116CPs. And for the head gasket I think I used a way thinner compressed gasket thickness, (my error). And I used the compression calculator on Summit Racings website.
 
OK, with those pistons, and an 1121G head gasket at .028" thick, I come up with the same 9.8:1 SCR as you did, with a DCR of 8.1, using a 73 cc head chamber volume. I would run it, make sure the cam was installed at the 'straight up' timing position (which will give you just the cams 4 degrees of ground-in advance), and start out with conservative ignition timing, like with the low teen's initial timing and work from there. Keep your ear tuned for pinging/knocking under hard acceleration especially when hot.

If you are concerned or run into an issue, you can very easily drop the CR with a thicker head gasket like the Felpro 1008 at .039" or the standard Flepro at around .051". For a DD and just tooling around, I would like the low end torque of a higher CR.
 
Yep, nothing too wild about a 10:1 SCR and moderate camshaft with iron heads... its all I've ever seen built by my friends. They are monsters off the bottom and roar through a very strong midrange to a pretty strong top, no doubt only limited by camshaft choice.

Example:
Ford 408ci block (4.030"x4" stroke)
Dished pistons for quench, 11:1 compression
.500" w/230'@.050"
Fancy heads, high-rise dual plane and headers

That combo netted 500+hp from 4900rpm up and 522ftlb of torque. Off idle it had more torque than a stock 360 does peak! Very gentle giant with top street manners.

Either way you should be in good stead for the street, let us know what your ultimate combination is!

- boingk
 
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