Old Man James

Hey guys. I could use a little advice on a 350 I am putting together. To make a long story short, last spring I bought an 85 T/A with the LG4 carbed 305. In short order it blew a head gasket. I decided at that point to replace the whole top end. As soon as it was back together it promptly threw a rod. So now I am on the hook for the bottom end. Problem is I put some 56cc heads on it which I am kinda married to at this point. So it looks like I need dished pistons to get the compression ratio low enough. It looks like I will be around 10.25:1, 11:1 with flat tops. I am going to lay out the direction I am leaning on parts and I would appreciate it if anyone wants to let me know if I am looking in the right direction.

Already have:
Edelbrock 600 cfm carb
Performer RPM intake
Trickflow TFS-30310003 Aluminum heads 56cc, 1.94/1.5
3' (I think) cat delete exhaust

looking at:
reman 350
Hooker 2460 headers
new torque converter 2800(?)ish stall
cam options: comp 280hr, comp XR276HR

Hows that sound? Thanks.

zraffz

10.25:1 isn't an outrageous amount of compression for pump fuel. The XR276HR should be enough cam to make at least 400hp at the crank and get you to the 12.3-12.5 1/4 mile range.


I built a 9.8-9.9:1 350 with that cam and vortec heads and the car went pretty good. I'd guess it defiantly has a 12.80 or quicker pass. You should gain quiet a bit (I'd guess between 30-50 hp) with a tad more compression and a set of heads that flow 40cfm more.

skinny z

Reman 350: 4.030" bore x 3.48" stroke.
56 cc heads.
Flat top piston with 5 cc valve relief.
Piston .025" below deck.
.039" x 4.100" head gasket.
10.75:1 static compression ratio (SCR).
It would take a cam considerably larger than the 276 to allow you to run on pump gas unless you pull all of the timing out of it. Which results in something you'd rather not drive.
I was 10.39:1 in my 355 with the 276. Dynamic compression ratio was upwards of 8.5:1 and I couldn't put a decent timing curve in it. This season I'm moving up to the XR288HR and that will drop the DCR to around 8:1 (same 10.39:1 SCR) and I don't expect any problems. Even with iron heads.
A 12 cc dish, like the L98 (I believe) will put you at 9.9:1 SCR. That'll work well with the 276 as zraffz has pointed out. I did the same thing with an earlier combination. Decent highway mileage too.

sofakingdom

iTrader: (1)

Don't guess at your compression ratio. You have WAY too many ways for it to be COMPLETELY different from whatever you "guess" at.

56cc chambers, flat-tops w 6cc valve reliefs, 0 deck clearance (pistons come up exactly even w the deck), .018" head gaskets (steel shim): 12.1:1

Same combo except .039" gaskets (FelPro composition or similar): 11.4:1

Same as the last one except .025" deck clearance (stock nominal): 10.65:1

Same as last except .035" deck clearance (stock typical): 10.4:1

Same as last except .050" deck clearance (replacement typical): 10.0:1

Same except 12cc dish w .035" deck clearance (better quality, such as Keith Black): 9.7:1

Same except w "rebuilder" pistons (.050" deck clearance) and 12cc dish: 9.4:1
This right here is what you'll get from a "reman" short block.

A little variation there by filling in the details, or what???

Notice that the ONE THING that made THE MOST difference among all those wildly variable results, was the DECK CLEARANCE: which is of course, also THE ONE THING nobody EVER measures, nobody EVER specifies in the catalog, and nobody EVER takes into account when plugging random numbers into some "calculator". People all like to argue about "well my 'calculator' says..." without realizing that ALL "calculators" are going to give the same result if you put in the same numbers, just like they'll all say that 2+2=4; meaning, the important thing ISN'T what "calculator" you use, IT'S PUTTING IN NUMBERS THAT ACTUALLY MATCH WHAT YOU'RE BUILDING.

I'd suggest using good quality D-cup ("inverse dome") pistons with an ACTUAL MEASURED FOR-REAL NOT JUST "CATALOG" compression height of 1.56" and a volume of about 10 - 12 cc, and less cam. The XR288 is ALOT of cam for a street 350 and will require gears, converter, etc. just to be bearable out of the hole; IMO too much for anything but pure strip usage. Not good for a DD.

I would NOT, in any case, buy a reman short block. Virtually ANY such thing you buy, will have the .050" of deck clearance referenced above; your compression will ACTUALLY be lower than whatever you "calculate" it to be; and the "not enough motor for the cam" situation will just get that much worse. MEASURE IT before punching the numbers into some "calculator". MEASURE MEASURE MEASURE MEASURE MEASURE MEASURE!!!

dmccain

iTrader: (2)

Id build my own shortblock using something like a 12cc dish piston to keep it pump gas friendly at around the higher 9:?? compression and use the 276 cam. You will still have a hella strong engine with street manners. Sell the eddy 600 get a 650 Holley DP.

Old Man James

Thanks for the help guys. I am only guessing at this point because I do not yet have the motor in hand. The big decision at this point is flat top or dished. I have run a bunch of options through the compression calculator. If I go flat top there is no way I can get the compression ratio much below 11:1, and I am not really trying to push the envelope. I don't want to unnecessarily leave any power on the table but I want a platform that is daily drivable and reliable. I also don't want to push the power band too high. It looks like with a deck height gasket combo that gives me a quench in the range between 0.040-0.060 will give me a compression ratio somewhere between 9.98 and 10.5. With the XR276HR my DCR will be 8.4 or lower. I wasn't considering the XR288, skinny had mentioned that for his use.

What are youse guys thoughts on stall speed? I am not sure of my gearing yet, but will figure that out before I buy.

86LG4Bird

iTrader: (1)

I wouldn't settle for the high side of that.
It sounds like you're already committing to reverse dome (dish) pistons, so there's no reason you can't get the quench clearance down in the .035-.040" range while maintaining your desired CR. This is just as important for a good performing engine as matching cam and CR is! AND.....the tighter the quench, the more CR you can run, as best performance will demand a less aggressive timing curve.
For good street manners and still respectable performance at the strip, go with a 9.5" TC in the 2800-3200 range, assuming you don't go with an extreme (either high or low) gear ratio. Don't waste your time and money with any aftermarket 12" TC; virtually all of them are no better than the GM S-10 TC.

skinny z

I guess that comes to a definition and experience. Case in point...my engine. Or more precisely, the engine that I'm reassembling as it was in the 67 Chevelle and is now sitting between the fenders of the Camaro. Years and years upon thousands of miles of daily use. From Chicago to the Grand Canyon. Cruising main street. Driving to the drag strip, bolting on the slicks and running low 12's. We drove that car everywhere with no fear. Yes, it had a stall converter but had a lock-up. Gearing was 3.73 but the OD transmission made easy work on the highway. Oh yeah. Highway mileage, (carbed: you have to know how to tune) was always 20+ mpg. Idle vacuum was 8-10". Power brakes worked just fine too.
Then again, to each his own. One man's pure strip usage is another man's daily driver.
Back to the OP's compression ratio. The Pat Kelly calculator does take into account the piston position relative to the deck. It also allows for the valve events to be inserted so the dynamic compression can be worked out too. That's particularly helpful when you have a shortblock assembly (that's been thoroughly measured of course) and are working your way towards building the top end. Any given shortblock can be built into any number of vastly different combinations. Again, case in point. The above mentioned 350 with the 288 cam had, at one point a 276 cam. Same pistons, deck height, head gasket and combustion chamber volume yet it was totally unmanageable. I was forced to have the chambers opened up a couple of cc's and go from a .026" gasket to a .039". The result never lived up to it's potential although it too was a solid 12 second street car. Live and learn was what I took away from that. Going back to the 288 will re-visit the aforementioned daily driver and I expect the same sort of success.
For the OP, here's a good read and the calculator of my choice is attached at the end of the article.

http://cochise.uia.net/pkelley2/DynamicCR.html

sofakingdom

iTrader: (1)

Remember:

Doesn't matter WHAT "calculator" you use. If you put in THE RIGHT numbers, they'll all "calculate" the same thing. 2+2=4 no matter what "calculator" I use, and I don't have to check every calculator I own to find one that agrees. On the other hand, if you DON'T MEASURE, and the numbers you put in DON'T MATCH THE PARTS YOU HAVE, then the answer that comes out of ANY "calculator" will be GARBAGE. It will not tell you A DAMN THING about the engine you are ACTUALLY working on.

MEASURE MEASURE MEASURE MEASURE MEASURE MEASURE MEASURE