Building a Chevy 302 with (only) 10:1 Compression ratio

I am in the process of building a Chevy 302, but wish to reduce the OEM compression ratio (CR) from 11:1 (which ran fine in my '69 Z-28 on 1969-82 gas) to a compression ration of 10:1 (to run on today's gas). [98 octane is the highest in my area of west-central Florida.] I am having a problem coming up with 302 pistons of less than 10.6 to 1.
It has also been suggested, if I go below 10:1 CR my 302 with roller cam, etc. will be a dog.
Also rather than building it with the flat tappet cam and BIG 4 barrel, I will be installing a full Roller CAM (cam, tappets & rockers) = NO problem here
AND
I wish to install 3 - 2 barrel systems (vice the OEM 4 bbl). Wanted to use Strombergs 97s, but am told they will not breath enough, not enough CFM. Have been suggested to use Rochester 94s. I will be using progressive linkage, so only using the center 2 barrel (same as with a 4 barrel) on the street, until I kick it in the butt. Then I get 4 more barrels vice a 4 barrel carbs 2 more. But those 2 are big and that wide open 4 barrel may flow more than my 3-2s six barrels.

I need a source for the 10:1 pistons
and
your technical know-how for the 3-2s.

This is strictly a fun street machine for a 50s-60s USN diesel submarine grease-monkey going in my 1955 Chevy Pickup 3100 and will NOT be used for racing. I have a vinatage 302 block (cleaned up at .030 over and soon to have the mains line bored) and vintage 302 heads, already rebuilt/upgraded. Trying to get the rotating assembly together and the 10:1 is the problem. Expect I'll need to find Flat-top pistons for this 302.

Thank you guys and / or gals. Murf

 

I wanted to do the same thing a few years ago...a 10 to 1 301 for my 53 ford pickup. I talked it over with my machinist and he took my TRW 11 to 1 pop-up forged pistons and milled enough off the domes to make it 10 to one with a thick head gasket and double hump 2.02/1.60 heads..64 CC I believe....charged me 7 bucks a piston to do the dirty deed and afterward I had the entire reciprocating assy balanced. Used 93 octane gas and it worked just fine..

 

How big is the cam you plan to run? If it has a decent amount of overlap and you setup the engine with good deck heights and tight quench distance(piston to head clearance) you may be able to run 11:1. a light car with steep gears could do it no problem but the weight of your truck may not help but there are more factors than just compression ratio

 

Years ago I ran flat top pistons in a punched out 283 to make 301 ci. I had 64 cc camel bump heads and never had any problems.

 

my 283 bored to 301 lasted long time in my vette....ran regular gas/premium when i could get it....its on engine stand now... could run if i had car to put in.........ran fine ......kev

 

What are the heads cc's? I was able to run flat top's w/ 305 heads on my 283 using thick blue Felpro head gaskets and a cam with .475 exhaust lift.

 

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. Re: ham "in my '69 Z-28 on 1969-82 gas)"

surfmurf
FNG

Join Date: Jan 2012
Location: Pinellas County, FL
Posts: 1



I kinda thought that this was a traditional hotrod forum

And that the 174,112 new members before you made an introduction for their first post.
just sayin


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Do you have any good pics of donks?



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Say it's going in his '55. What's issue. (Although I'd kill for that motor..) How about a trade you a 350 and you send me that high compression 302 for my '69

Either way just call ross and get some custom pistons made.

 

What year is the engine? If it is a 68 or 69, you have options. The 302 crank is worth some $$$$$$. You can purchase a 383 rotating assy very reasonably these days. Not only would you cure your compression concerns, you will have quite a bit more torque.

The main bearing size of the 68-9 is the same as 350's and you have a 4" bore.

Merely my thoughts.

Tim

 

What if you use a 6" rod you can use a readily available 350 piston and with about .040 machined of the top should be close to zero deck and have the advantage of the longer rod.

 

First of all go here and do an intro. http://www.jalopyjournal.com/forum/forumdisplay.php?f=18
Be sure you read the sticky up at the top.
Now 302's are pretty much a dog in a heavy vehicle anyway unless you build them with high comp and use a low gear to keep the R's up. they like 750 or better cfm's so the strombergs are going to kill the breathing. They are a great motor in a light car, I built one and put it in a midengine corvair and it screamed but price of racing fuel killed the fun. Build something with more cubes, less compression and you'll be happier in the long run.

 

There are stocking (on the shelf) pistons out there with some weird compression heights for different rod/stroke combos. If it were me, I'd stick a longer rod and shorter compression height combination in it to get the CR down AND help a little with torque. Side benefit is you get a lighter piston as well.

With a 9" (+.025) deck and 1.5" crank arm, you have 7.5" left. So with a typical 6" rod, you could run a 1.45 pin height with a true flat top and be .050 down the hole. Should do the trick...

 

Hard to tell from your note Moefuzz whether you are trying to be funny or a hard-ass. Reads a lot more like the latter. If so, please do not bother to respond in the future. Life has more than enough aggravation without mindless people adding to it. I've been messing with cars since the 50s and over those years a 302 cu in (Z-28) SBC in a 1955 Chevy Pickup would clearly be a traditional hot rod.

 

surfmurf, bustin balls on post number 2. nice job. but you should go do an intro. welcome aboard anyway man.

 

Surf
Some of the fellas just consider it a common courtesy for someone to introduce themselves before they start a conversation. I would probably use an engine room analogy but someone who has lived a life of honor no doubt understands courtesy. By the way if you are retired from the Dept of the Navy you have done well by me.

Someone mentioned useing a 6" rod, that is good advice, they are cheap and plentiful these days. We used to have to get them from 300" ford 6s and modify them to work.

Something that has not been mentioned is that with the longer rod you increase your dwell time and that in turn lowers your octain requirement a little. You can these days pretty much get any piston you want in any configuration you want if you just look around a bit.

I am thinking that I may shy away from the 3x2 setup myself. You are only lowering your performance for looks. if you must run the 3x2 you may consider running autolights like on a 3 duece fords. they out perform the carbs you mentioned and are easier to find alcohol compaticle components for.

Just some thoughts

 

A little testy, aren't we? You could always follow the rules.
Anyway, I built a 302 in the late '70s. At that time only TRW pistons with 12.5cr were available at a reasonable price. I massaged the domes (carefully) and smoothed out the sharp edges. I used a roller cam with similar specs to the Z28 piece and it worked. Another thing to do would be to buy some decent aluminum heads with 76cc combustion chambers. That would work out slick.

 

Actually I run right at 11.5:1 with aluminum heads on pump gas. There are a bunch of tricks to running compression on the street. But his question was concerning lowering his compression .5 or so. I think he is at least determined to build his engine and not mine that is commendable.

 

On the rod length,there's some debate on that from pro engine tuners saying a longer rod can increase detonation.More pressure builds up from the "dwell time".
Dropping a point in compression will kill some power...but...but the smaller piston dome might mean faster flame travel for less detonation and a better burn..So there may be no power loss if you can take advantage of that.
Some guys get away with 11-1 compression carbed street engines with careful tuning as P&B er says.

 

If its a true numbers 302 of 1969 vintage I would rethink it and keep that engine for a future Camaro project , then build a 383 stroker .

 

I have actually listened to both sides of the argument and both have their merits. I lean toward the better burn side on the long rod side of the argument.

Quench area is also a big part of what you can get away with fuel wise. I was reading someplace last fall where they had lowered the compression but increased the quench area and the fuel requirements went up. They had to loos a little lead in there timing to overcome the problem with the increased quench area. So I guess in theory they detunned it twice. Lower compression and less timming.

 

302 pistons made a high compression Ford 300 six so Ford 300 cast or hyper pistons should give you a low CR if you bore rods for Ford pin size.
Chevy comp ht 1.779 Ford 1.776, varies
Chevy pin 0.9273 Ford 0.9752
Flat top should put you around 9.5:1. I'd go dished and drop it further.
Ford 352 pistons also work in 300, you might still find them in TRW forgings, they have 1.816 comp ht but have 4 valve reliefs so another option for your 302

 

I wrote a LONG reply, addressing many of you and your Qs. But it disappeared. I keep getting logged out or told I am a Guest. Do not undertand.
I will reserch the longer Rod and shorter Compression Height idea. PnB you lost me on the Quench, Dwell vs Octane and "Autolight" carbs info.. You're clearly way ahead of me. Thank you to all (save the 2 stuck-in-the-muds) for your many thoughts and ideas. I have to learn what some of them mean. You brainy guys lost me in a few places. Mucho thanks gentlemen. Murf "Old age is only for the brave."

 

Id shoot for .035" - .040" quench (with a steel rod) and 240+ duration@ .050" and you will get away with hi 10s/11:1 cr on premium pump.

Id try to zero deck it if possible and dial in you actual quench with your actual "crushed" head gasket thickness if possible

Rat

 

IF it's a large journal 3" stroke crank, use the L-99 5.94" long rods and any 350 piston you want (anything from dished to dome). You wind up with a 9.0" stack height against a 9.025 deck height. And, it uses stock parts. There used to be spacer bearings made to put large journal rods on a small journal crank; they were King bearing # CR867HP. You can also do the same thing, but with a 327 piston and 5.85" long rods (aftermarket). That gives a stack height of 9.020. I "think" the 5.85" rods are available in small journal also. No matter which way you go, balancing/rebalancing should be done. Butch/56sedandelivery.

 

Surfmurf, I will offer you this from SRP, the entry level division of JE Pistons one of thee best piston manufacturers in the world. Part # 202890, this is a SOLID domed piston, so you can pretty much mill away on these. This is after checking and within reason for the hardheads reading. 4.030" X 3.000' stroke will net you 306.134 cubes. For what you are doing, forgo the long rod and go with the 5.700" long rod and these pistons. 1.800" C.H. or C.D. if you prefer. With a 64c.c. chamber volume will net approx. 10.4 and with 70 c.c. chamber vol., willl net approx. 9.7 C.R. static of course. You should be able to touch up mill the deck and still end up just below deck with these. Your not a experienced engine builder, save the zero deck to the experts OK. Keep your pistons slightly in the hole. .005" - .010" in the hole, the right head gasket and you will arrive at your 10:1 C.R. Again, these are SOLID domes and you can mill to give you the compression { static ] you are after if needed. These slugs take the common ring stack of 1/16, 1/16, and 3/16. These are very good pistons BTW, and good for well up to 450-500 H.P., assembled and clearanced correctly. Best of luck, TR

 

x2! Set it up with 0 deck,.042 headgasket(.042 quench) flattop pistons lotsa duration to bleed off comp at low speeds my choked down 383 is set up like that and I dynoed 544 hp on pumpgas

 

Race engineering sells the spacer bearings too

 

I have not yet bought / had made. the 302 crank. I only have the 350 rotating assembly and the 302 block and markedly reworked 302 (2.02 int / 1.60 exh) heads. Yours is yet another interesting idea to check out. Thank you

 

May be the best answer yet. You guys sure come up with some interesting stuff. You are so right, not currently a pro. Have done head jobs, but no bottom-up build in decades and never built a performance engine (like I'm trying to design now), unless rebuilding my '69 Z, after it dropped a keeper, valve and punched the block - thus died my DZ block - so I rebuilt it, but with a 350 block. WHAAAAAAA!!

I love the site, except for the "traditional" crap I've been getting. I've suggested - someone should post a sign outside the door, not after one comes in.

 

Not ahead of you at all, while you were keeping me safe I was busy trying to screw up the environment as much as I could. Not to worry I wasn't able to get the job completely done, there is still a little left for you to play with.

Let me get hold of one of the fellas to get you the numbers on the autolight carbs. They are basically just Holley 2 bbls. I think about 300-350 cfm. Your 302 even at the lower compression rate will like the air flow a lot.

Don't get hung up over quench or dwell. Dwell time is just the amount of time that your piston spends going over the top. Overly simplified with the longer rod it take a little longer for the piston to transition from the up stroke to the down stroke.

Quench area is the area that you have for the explosion when your fuel is lit off. Bigger combustion chamber more area. I am probably going to get this part wrong I usually do but the measurement I gave you of .040 is the distance from the top of the piston to the bottom of the combustion chamber. If I am wrong someone will correct me. Basically all I was trying to say is that just lowering your compression can have adverse effects.

Case in point, I have a 307 in an old military vehicle. It is a very low compression engine, like about 8:1 I think. You would think that it would run on panther piss and it probably will before all is said and done. If I run more than about 12 degrees initial advance on my distributer I get compression rattle under load. My 11:1 engine runs on pump gas but I can advance my spark to 14-16 initial with no problems. The hot rod mill has a smaller quench area than the 307.

There are actually a lot of tricks to running a hot mill on the street aside from just compression but they are no doubt discussions for another day.

By the way I failed to offer you a courtesy yesterday and I damned well should have. Welcome to the HAMB friend.