Compression and Gasoline

**Mike M.** · September 26, 2003, 12:21 PM

Re: Compression and Gasoline

to accuaratly determine the volume of a head, one must cc the critter. Get a burett from local high school or comm college chemistry dept( or buy one from scientific supply house). drill a hole in a piece of plexiglass and a thin film of grease between plexiglass and head's head to block interface, then add H2O thru burette to determine the vol in ccs. mike

**Duke W.** · September 26, 2003, 03:33 PM

Re: Compression and Gasoline

Also, are you including the piston-deck clearance in your calculation. Unless you know this dimension you can't compute and accurate CR.

Duke

**Joe L.** · September 26, 2003, 05:06 PM

Re: Compression and Gasoline

Mike-----

Nominally, the GM #3795896 cylinder head spec is 59.7cc WHEN AS NEW. As Mike pointed out, to know, for the purposes of the compression ratio determination, the actual CC of the combustion chambers you need to accurately measure the volume of the chambers. Head decking, if peviously performed by persons unknown, will reduce the size of the chambers. Chamber alterations or multiple valve jobs will increase the chamber size. So, you really need to check them. As Duke pointed out, you need to include the piston deck height in the calculation, too. Also, when using "flat-top" pistons you need to take into consideration the volume of the valve reliefs present in the piston tops. Most flat tops have 4 valve reliefs and some have 2. I've never seen any for small blocks that have none.

The GM #3725306 had, nominally, 55.7 cc combustion chambers WHEN AS NEW. This head was originally used only on 265 cid engines. The above-referenced GM #3795896 was originally used only on 283 cid engines. Either one is not really a great choice for a 327 cid engine, especially if you want to extract a fairly high power level from the engine. For 250 hp, or so, they might be ok.

**Mike Cobine** · September 27, 2003, 10:29 AM

Thanks, I think...

Thanks for the answers.

I know that I could check all the rest, but that means doing something I didn't want to do - take the heads off. I was hoping someone would say "I have a 250 hp that is just like that and it runs great on Amoco 93" or "mine was like that and it pinged all the time until I ran Cam II".

My temporary solution is that I can get race gas locally, although about 10 miles away. I can run that as needed until I either track down a set of 461 heads reasonable or decide on some other heads.

Thanks,
Mike

**Dennis C.** · September 27, 2003, 06:51 PM

Re: Thanks, I think...

If you plan to drive the car w/o a trailer full of $5 gas or additives, simply lower the compression to about 9.5 - 1 (lose the top of the piston). That done, you won't have to visit the issue again. Enjoy. Dennis

**Frank H.** · September 27, 2003, 06:55 PM

Re: Compression and Gasoline

I built a 327, 0.40 over ,forged 4/v reliefs, 0.20 deck, june 61 895 heads I cc
to 61cc cambers before the valve job,but were used $30.00 swap meet heads and the car screemed it guts out with out pinging for two months,is that what you want to hear? then I got tired of the plain 250hp looks and put all the 340 stuff in except the pistons,windage tray,balancer and the distributor,
for 21 years till last year added all plus 202 160 valves but no dual point,it just won't cut it like the vacuum advanced one.
But I poured octane booster in my old red 64 coupes 365hp till I pulled the heads and got those custom cut 0.93 copper reusable head gaskets ,never added octane booster again.

**Mike M.** · September 27, 2003, 07:12 PM

Re: Thanks, I think...

Dear dipstick: Mike doesn't wqant to remove his heads so i'd be interest to know how you would loose the piston domes without pulling the heads. course, i never was smart or cheat-prone enough to run with the likes of you. brother mikie.

**Mike Cobine** · September 27, 2003, 11:00 PM

Re: Thanks, I think...

ROFL. I don't know which of you guys are worse.

Of course, there is a problem in that even if I pull the heads, there aren't any domes to take off, since they are flat, with four eyebrow recesses. And I really didn't want to go in and redo the engine, or I'd drop in dished pistons or chop the top on these to set them down about .020 or so. But then you get into rebalancing, the works. And now I have to pay for machine work.

I think I'm just doomed to the high octane bill for awhile. It is ironic. A few years ago, we were pushing for more compression on my engines. In fact, I was having problems keeping compression up on the 302, since smaller motors require more change to get more. Everytime we got the heads done, the chambers would be too big and I'd have to cut the head some more. Now I'm trying to go the other way.

Easy enough with a set of old 882 heads with their 76 cc but then I definitely don't look stock. The 461s would look right to 90% but they are nuts on price lately. And then there are the 3795896 which are correct, but will probably only run on a 30% mix of 110 unlead added and really won't flow worth anything.

What the heck, what's $5 a gallon?

**Mike Cobine** · September 27, 2003, 11:02 PM

Re: Compression and Gasoline

Thanks, Frank.

I think I may be in for some thick gaskets.

**Eugene B.** · September 28, 2003, 08:28 AM

Re: Didn't think this pertained to me, until..

Gents,
Originally, I thought this post only pertained to the hypo guys, then I checked the octane requirements in my owner's manual.

I have a bone stock '65 250HP "mouse motor". Compression ratio is listed at 10.5 to 1. It will be going to the shop for rebuild in a couple of months. I was not considering any modifications to lower the compression because I thought that Sunoco 93 would be fine. When I looked in the owner's manual, I found the recommendation of 99-101 octane.

Are you guys telling me that I should consider some method of lowering the compression so that I can tour on today's gasoline? If 9.5 to 1 pistons were used, approx. how much horsepower would the little "mouse motor" loose?

Thanks for your comments,
Gene

**GL Anderson** · September 28, 2003, 08:54 AM

Re: Compression and Gasoline

Here is a possible solution to your problem. Go to one of the Home Depot/Loews/Menards and buy a gallon of toluene in the paint department. Mix it about 1 gal to 4 or 5 with the best unleaded you can buy. This stuff won't hurt anything and will raise the octane several full points or more. Do a google search in groups for the info. Seems to work with no down side except maybe a cancer causer but what isn't. Cost for a gallon is about $3.00. Just a thought. GL

**Clem Z.** · September 28, 2003, 09:23 AM

Re: Didn't think this pertained to me, until..

octane rating back then and now is calculated a different way. duke or hink can post a dissertation on this. if your heads have never been cut and the block decked you are below the stated CR by at least 1/2 point or more. the stated CR can only be reached by "blueprinting" the engine. you will need to "CC" the engine to get the true CR.

**Duke W.** · September 28, 2003, 11:03 AM

Re: Didn't think this pertained to me, until..

Today's Pump Octane Number - the average of the Research Octane Number and Motor Octane number is about four points lower than the RON requirememt published in the owners' manual. Todays 93 is is equivalent to about 97 RON, so it's not that far off the mark, and GM's recommendations are on the conservative side.

Prior to tearing down the engine measure the piston to deck clearance dimension for future reference, and of course, do not deck the block. If you use a thick composition gasket during the build up you will reduce CR by close to half a point versus the thin shim OE gasket. You should actually compute the CR based on piston/deck clearance, gasket thickness, cylinder head cc, and piston type and you will probably find that with a thick gasket it will be in the range of an honest 9.5:1 to 10:1, and the torque power loss will be about 2-3 percent.

SHP/FI engines with long duration, high overlap cames can run higher static CR because low speed dynamic compression is lower due to the late closing inlet valve. Detonation due to insufficienct octane is more prevalent at low engint speed than high engine speed.

Duke

**Frank H.** · September 28, 2003, 11:15 AM

Re: Compression and Gasoline

I forgot to mention all the overlap the valves had with the 288dur 488lift cam
to bleed off some of the compression on the 250s first configuration

I think you'll be fine as close to the edge without going over,unless the gas gets worse.
small blocks are forgiving,but I do remember showing my friend how I could turn the key off and hand him the key and drive my bigblock up the driveway and in the garage....Thats too much compression!

Compression and Gasoline

Compression and Gasoline

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