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ok. I want to understand the ins & outs of compression Vs. pumpgas

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Discussion starter · #1 ·
any ideas where to find (book,etc) to fully understand the compression vs pump gas and what alters it?If I understand this correctly there is no set CR to pump gas guide.Again, if I understand this an engine running lower full advance (say 30 deg.)and longer duration cam(to bleed of cylinder pressure) can get away with more compression than an engine running higher advance (say 36 deg.) and lower duration cam. I somewhat understand how head cc volume/pistons and even head gaskets can affect the CR.am I totally confused or on the right track.

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Michael


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Discussion starter · #2 ·
It's all here if you want to do a bunch of searching, then reading!!

It goes something like this..

Compression is good for about four percent power increase for every point going up UNTIL you run into detonation!!!

An easy way for the manufacturers to make compression is to add a dome on the piston.. Now this was fine when we had good gas, but what happens is that the dome kinda gets in the way of the fuel burning so it doesn't burn well and so will knock if the octane isn't right!!!

To keep high compression AND be able to use todays gas up to a point!!!, you can run flat-top pistons, small combustion chambers, longer rods and minimum quench dimension. Shim headgaskets help here, too...

All this fools the engine into thinking the gas is better!!!

The Fueling head design carry's this concept to an extreme as also, several past articles in POP-Hot Rodding mag. has!!!

And there have been a couple of engines built and tested using this design theory in the mags. One called "The 350 Chevy Should Have Made" is one!! They use a 400 block and a 327 crank and (I think), long rods to be able to run an 11 to 1 CR. engine on 87 octane gas!!!!!!

Fueling helped on a build up that used a 502 at 12+ CR. and their heads and shot for 700lb. ft of T on 87 octane gas!!! They only made 660 or so and had some problems that were unrelated to the concept.. They did however say, his heads b/c they were for a 454 weren't big enough to support a 502 in this state of tune!!! BUT THEY DID REAL GOOD IN MY BOOK!!!

Mr. Ray Bochaz wrote two articles on this build up and I talked to him over the phone about them!!! A great guy, although VERY TECHNICAL which to me IS GREAT!!!

Do some reading... pdq67

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#3 ·
[And there have been a couple of engines built and tested using this design theory in the mags. One called "The 350 Chevy Should Have Made" is one!! They use a 400 block and a 327 crank and (I think), long rods to be able to run an 11 to 1 CR. engine on 87 octane gas!!!!!!

pdq67,
I remember reading that article, but when I went back thru my mags I couln't find it.Do you know what issue this article is in?

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John
71 El Camino
 
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Discussion starter · #4 ·
I found this but don't temember what year and month???

Hot Rod Mag did a project buildup "The 350 GM should have built" or something like that. It was basically a 400 block with 4.155" bore and a 327 crank (3.25" stroke). Very big rod ratio (I think they used 6.125" rods), very tolerant to detonation even with 11.1 compression.

Sorry, pdq67

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#5 ·
Yeah,long rods out of a 6cyl f f f well their not chevy rods.
Image
Been through magazine collection at least 3 times over the last two years and every time I draw a blank. Some interesting theories though.
Thanks for the reply.


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John
71 El Camino
 
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#7 ·
Bruce Crower built a motor in late 70s early 80s 15-1 compression. His theory was to run so much overlap that it would remix the fuel supply. I think the artical was to sell the cams and parts but never caught on.
 
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#9 ·
Save yourself some $$$ and headache - get some 6.125" rods (off the shelf from some of the aftermarket companies like Scat) and use pistons made for a 350 stroke with 6" rod. This keeps you from having to modify the Ford rod. Don't deck the block until after you've mocked it up to check height. I helped a friend build one of these and the build-up was pretty straight forward and easy. Most will recommend using a Performer instead of Performer RPM to help bottom end with the short stroke. Theory also has it that big bore, short stroke engines like nitrous. We'll find out in a couple of months after we've broken this one in. HTH.
 
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#10 ·
The intake valve closing point is one of the biggest factors in determining the octane required. This sets the dynamic compression of the engine. This is why a long duration cam must run higher compression. The intake closes late and bleeds off the CR so a high CR to start with is needed to get the DCR in the proper range. In Smokey Yunick's book "Power Secrets" (p 74) and David Vizard's book on cams (p 25) are discussions of the hows and whys of this. If you haven't, you can follow the link in my sig for some more info.
 
#11 ·
I was not sure that I would get a response bringing back such an old post, but you guys are great. I will look into changing the rods, as that would reduce my machine shop costs. I built this engine with a friend of mine back in High School, but that was a long time ago and I was unable to remember all the details. I think that we ran a performer RPM manifold, but that car did go quite well.

Would it be advantageous to step up to larger valve in the head and maybe up the compression ratio even higher, or would I run into detonation problems.

Sorry for all the questions, but I am new to the world of engine design. Maybe you guys could recommend some reading materials?

-Geoff
 
#12 ·
David Vizard's books are great. He has several on various aspects of producing power. The Max Power on a Budget book is required reading. Smokey's "Power Secrets" is an excellent read. He advocates using the longest rods you can fit in the block. I have one of Lingenfelter's books ("On Modifying Small-Block Chevy Engines") but found it rather superficial. He may have better ones. Both Vizard and Smokey discuss dynamic compression and the effect it has on detonation or the lack thereof. Vizard talks about the effect rod length has on octane requirements and cam selection in his cam book.

Long rods do seem to favor the upper rpm band over the lower. Although, it seem to me, the better mechanical leverage long rods provide (the crank is farther around when the piston starts down, a function of the increased dwell at TDC) would help in all rpm ranges.
 
#13 ·
It is after midnight right now, and am thinking what a great read this thread is! Ok, no more drugs tonight! :D
 
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