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58 cc heads and flat top pistons - too much compression?
I am currently having my engine rebuilt. I am using my high performance top end parts to bolt to the rebuilt lower end. My heads are corvette aluminum heads with 58 cc chambers with (I think) a current compression rating of 9.7 to 1. The current cam, which I plan to keep, is a comp cam magnum 270 with 224 duration at .050 which in theory will bleed off some compression. The cylinders will probably be rebored.
If I rebuild with flat top pistons will this raise the compression ratio to above pump gas friendly levels? I don't mind pushing the limits of pump gas but not if it results in retarding the timing to prevent detonation. Is there any other benifit to flat top pistons (increased torque in a lower RPM range, more efficient fuel burn in cylinder, etc) besides increased compression? In other words, I know that higher compression will bring more power, but do flat top pistons in itself improve performance beside the fact that it raises compresson? thanks P.S. - the bottom end is a stock 350 '81 engine, never rebuilt previously [Modified by SpyderD, 2:52 PM 11/25/2002] |
Re: 58 cc heads and flat top pistons - too much compression? (SpyderD)
With aluminum heads, I would not go higher than 10:1 compression. You can help with a lower thermostat (assuming your running stock). But this is not a cure all.
I'm running iron heads (Vortecs) and 9.7:1 compression. With this, I'm limited to 93-94 octane. I've run less, 91, and I think I heard some displeasure from the engine. Also, my timing is less than optimal. If I were you, I'd go conservative with the compression. Unless this is going to be a bragging numbers only car or a track car that you will need to squeek every last ounce of performance from, do yourself a favor. The car will be fast. But it will also be streetable. |
Re: 58 cc heads and flat top pistons - too much compression? (SpyderD)
Many things to consider here, first you have to know the deck height of the block, tne cc of the cylinder head and what head gasket you are using. Then you have to
find out what the manufacture says he CR is for the piston with your deck height and whatever gasket they recommend. Until you know these numbers the CR is a guess |
Re: 58 cc heads and flat top pistons - too much compression? (SpyderD)
Here is a link to a website that figures what your compression is based on information you put in.
http://www.rosspistons.com/CR%20Calculator.htm I entered in information for sort of a standard setup and came up with these numbers. Bore .in 4.030 Stroke in 3.48 Head cc's 58 Gasket Thickness .in .039 Deck Height (Put .0001 for zero deck) in .01 Dish or Valve Pocket [-] or Dome [+] cc's -6 (flat tops with 4 valve reliefs) Number of Cylinders 8 Displacement Cubic Inches 355.12 Compression Ratio 10.8 to 1 Compression Ratio Formulas GV (Head Gasket Volume) = Bore(in) X Bore(in) X 12.87 X (Head Gasket Thickness in Inches) DV (Below Deck Volume) = Bore(in) X Bore(in) X 12.87 X (Inches Below the Deck) HV (Head Volume) = CC's VV (Dish, Valve Pocket, Dome Volume) = CC's (Minus for dish or valve pockets, Plus for dome) PV (Volume displaced by Piston) = Bore X Bore X Stroke X 12.87 Compression Ratio = (GV+DV+HV-VV+PV) / (GV+DV+HV-VV) Cubic Inches = Bore x Bore X Stroke X Number of Cylinders X .7854 Convert Cubic Inches to CCs = Cubic Inches X 16.386 Convert Cubic Inches to Liters = Cubic Inches X .016386 Engine in Liters = (Bore(mm) X Bore(mm) X Stroke(mm) X Number of Cylinders X 12.87) / (16386 X 1000) Engine in CCs = (Bore(mm) X Bore(mm) X Stroke(mm) X Number of Cylinders X 12.87) / 16386 Convert CCs to Cubic Inches = CCs / 16.386 Convert Liters to Cubic Inches = Liters / .016386 |
Re: 58 cc heads and flat top pistons - too much compression? (Smokehouse69)
Thank you in particular to Smokehouse for doing the math for me. That is exactly whatl I wanted, an approximation of compression which sounds like it will be in the 10.5 to 11 range. I agree that is too high. Now for the second question - any benifit to flat top pistons besides raised compression ratio?
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Re: 58 cc heads and flat top pistons - too much compression? (SpyderD)
With alum heads you can get by with 10.5 to 1 and run 93 oct, and depending on cam overlap and duration, you might be good with 11 to 1.
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Re: 58 cc heads and flat top pistons - too much compression? (SpyderD)
I have heard that flat tops promote the best flame travel and they are less prone to have carbon buildup than dished pistons.
I don't have the program, but you can input the seat to seat valve events for any cam and static compression ratio and it will give you you dynamic compression. Which is the value that you need to be concerned about. I don't see any problems with running 10.8 and aluminum heads on 91 octane. [Modified by gkull, 10:51 AM 11/26/2002] |
Re: 58 cc heads and flat top pistons - too much compression? (gkull)
Your cam choice will be critical here. If you use a short duration, high lift (almost like an RV cam...) for ultimate low-end torque, then you have the full effect of that high CR - pinging, or worse... If you use a longer duration cam, that will drop your effective CR a bit - maybe as much as a full point. When I rebuilt a different 1973 454 years ago I went the short duration route with 9.5:1 forged pistons. That car jumped! Because of the crappy gas we get today, you may want to get a set of heads with a little more chamber volume - go with open or semi-open chambers and flat-tops. Get all that in an effective CR range of 9.5 to 10 and you'll be happiest.
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Re: 58 cc heads and flat top pistons - too much compression? (Smokehouse69)
Suppose you could have a good machine shop perform a little work on the heads and 'un-shroud' the area around the intake valve, which may open up the combustion camber area in the heads from 58 ccs to 60 to 62 ccs, which will give you a little more 'comfrort' room in your compression ratio.........
I understand that you want the tightest/smallest 'quinch' area which promotes fast flame travel, so you don't want to use a thickner than necessary head gasket, and you do want to use flat top pistons if possible. Perhaps the 'new generation piston, which is flat on the quench side of the piston is something you should look into. (I believe I read something about these new pistons in Chevy High Performance magazine a while back.) |
Re: 58 cc heads and flat top pistons - too much compression? (Wardog)
Hmmm, more things to think about. I don't really want to replace or machine the heads just to fit flat top pistons. Here are the cam specs copy and pasted from a web site:
Gross valve lift: .470" intake/ exhaust with 1.5 to 1 rockers Duration at .006" Tappet Lift: 270 degrees intake/ exhaust Duration @ .050": 224 degrees intake/ exhaust Lobe lift: .313" intake/ exhaust open close intake 29 BTDC 61 ABDC exhaust 69 BBDC 21 ATDC Lobe Separation Angle is 110 degrees with 106 degree intake centerline STRAIGHT UP = 106 degrees; cam is ground 4 degrees advanced ...so I think that duration is relatively high, not sure about the overlap. Is it high enough to bleed down the 10.8 to 1 compression? |
Re: 58 cc heads and flat top pistons - too much compression? (SpyderD)
I am letting the pro decide - My engine builder is measuring the head combustion chamber (he thinks it may be larger than 58cc), entering it into a computer with all the specs including my cam and seeing what pistons I can use to get the absolute highest compression ratio I can get away with on pump gas. He says 10.5 up to 11 should be OK, and the car will fly!
Well that's why I pay him the big bucks (hopefully not to big), to figure all this out. The engine will also have a 3 year warranty so if my car detonates to destruction they will replace it. |
Re: 58 cc heads and flat top pistons - too much compression? (SpyderD)
I'm running 52cc with factory pistons and it calculated to about 9.5 to 1. I memory serves me correct, flat top would have brought it up to 10.5 to 1.
You should be safe with that duration. |
Re: 58 cc heads and flat top pistons - too much compression? (Budman78)
Dave, with the flat top pistons you get a better flame travel and this cuts down on detonation to some degree.
With the aluminum heads you can run 10.5-1 compression with no problems on 91-93 octane gas. You will not have a zero deck height on the piston unless your engine builder decks the block for a zero deck height. Ask him if he plans to do this. The engine should be fine with the 10.5 compression but I think you will be a little below that unless he does deck the block for zero deck height. |
Re: 58 cc heads and flat top pistons - too much compression? (candyapplevette)
Rob - I just emailed you. After he told me he was checking the heads with a straight edge I was imagining Bubba with a ruler eyeballing the heads (this is North East Tennessee afterall), turns out he plans to magnaflux test the heads and a few other things and running all my specs in a computer program to determine parts and pistons and so forth. I think he did say he was decking the block. I will keep you up to date...
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Re: 58 cc heads and flat top pistons - too much compression? (SpyderD)
Rob - I just emailed you. After he told me he was checking the heads with a straight edge I was imagining Bubba with a ruler eyeballing the heads (this is North East Tennessee afterall), turns out he plans to magnaflux test the heads and a few other things and running all my specs in a computer program to determine parts and pistons and so forth. I think he did say he was decking the block. I will keep you up to date... |
Re: 58 cc heads and flat top pistons - too much compression? (jackson)
In days of old they had to run the cr high because nobody made cams with tighter lobe centerlines than 114 degrees. With todays grinds 108-110 degerres is more the norm. This effectivly increases the dynamic cr,making the engine think it't got more compression. In fact it does. Back in the 60's it took a big block running domed pistons to break 450 lbs. of torque. I pull almost 500 lbs out of my 406,running flattops.
Save the money you'll spend on pistons,take the wife out to dinner,you'll both be happier. |
Re: 58 cc heads and flat top pistons - too much compression? (jackson)
Bubba from Tennessee or Hans from Stuttgart ... there's plenty BS everywhere. Now y'all correct me if you disagree, but my understanding is that aluminum (heads, blocks etc) cannot be magnafluxed (aka eddy current testing). Use of a fluorescent penetrant dye (aka Zyglo) can expose surface cracks in aluminum ... but the ability of magnaflux to indicate sub-surface abnormalities cannot be used on aluminum. https://www.corvetteforum.com/zeroimg/smilies/skep.bmp |
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