Piston, Head, Compression Help

[TheMattMan]

Ok guys bear with me...if more information is needed then let me know. I am looking for the correct pistons for my set up to give 11:1 compression 10:1 <--- specs from Comp Cam))) at the very least. Here is the list of what I have:

• Comp Cam Magnum Hyd Duration 286 286
• Heads I have 3 pr of heads--2 pr are cast# 3782461 which I think are 62 cc maybe 64cc
• 1 pr are cast # 3890462 which I believe are 63.3cc --all heads are steel
• Alum intake -750 holley
• Rear end 3.73 (haven't checked to make sure yet but if its not it will be )
• and all the norm stuff....ie headers, dual exhaust, etc....

The duration of the cam at .050 is 236 236 <B>
And just to note....this truck when finished will not be a daily driver...it will never tow anything... strictly a street/show truck. I was told I would need some dome pistons unless I wanted to get alum heads and shave them ...which I dont ..money wise that is... Would a forged dome piston with a .125 dome give me enough compression and if not what is the largest dome I can go to without having to buy large domes and having them shaved to fit. Also do you know any sites I can get a set of these pistons for around 400-500 dollars.
Thanks Matt

[Randy70C-10]

Why do you want 11:1 compression? Just curious, be prepair to start blending your fuel, you might need to boost your octane a little (or set your timing back). With 64cc heads, you won't need much of a dome to attain high compression, Summit racing has a tech line you can call. Tell them what heads your'e running and they will give you a part # for the pistons. Valve size and cam lift is critical, you want to make sure you have enough valve to piston clearance.

[MikeB]

I have a spreadsheet that will calculate compression ratio based on cubic inches, combustion chamber volume, deck height, head gasket thickness, and piston dish size. Unfortunately, I just discovered this site won't let me attach it to this post -- probably because of macro virus potential. Send me your e-mail and I'll send you the spreadsheet.

Use it by changing the bold numbers in column A, then read the appropriate column in the tables for your compression ratio. Notice there are two tables, so you can play with with some of the variables and look at the resulting compression ratios simultaneously. The file format is Excel 2002. If you can't read it, let me know what version of Excel you have and I will save the file in that format.

Also, there is an awesome article on compression ratios at http://www.speedomotive.com/Building%20Tips.htm
I followed their advice when I built my 383 and ended up with less than .050" piston to cylinder head clearance. I get zero detonation with 9.4:1 compression on regular gas and 32 degrees total mechanical advance.

By the way, unless you have a 3.73 or 4.10 axle, 2500 converter, high-rise manifold, and excellent heads, that cam will be overkill in a 350 in a heavy truck. I think the 268 would be a better choice, and should still have a lopey idle, if that's what you're looking for. Recommended CR for the 268 cam is probably around 9.0:1.

[bigjimzlll]

64cc witn a .015 head gasket and a .020 deck hieght....will give you 10.5 to 1 with flat top pistons....it also gives you a great quench of .035. with a cam that big...it has a lot of overlap.. you shouldnt have a detonation problem......You will need a converter of around 3000-3500...to get the best efect....that cam will idle around 1200rpm. (in gear)..and if you have power brakes...you might be in for some trouble, because of the very low manifold vacuum

[mikep]

Thats not really much of a cam. I run a similar grind with approx .500" lift at 234 duration on both sides with 108 degree centers and I have a smooth idle ( thats a relative term ) at 800-900 no problem with 10.2:1 compression. I dont dare go over 36 degrees total timing on 92 octane gas or I run into detonation. Thats with .125 domed pistons and 72 cc chamber 200 cc port world product sportsman II heads .Just from my experience having built a lot of truck motors over the years if you are never going or rarely going to haul anything and all your'e looking for is to have a hot rod truck then by all means ignore the conventional wisdom that says you need a short duration cam for low end torque. Low end torque is good for hauling around a 4000 lb camper or 2 cords of wood but too much low end will boil your tires off in short order and you will lose the race in a cloud of tire smoke if all you do is drive around with an empty bed all the time and race stoplight to stoplight. A 3000 or so stall converter and under 240 degrees @ .050" cam with 10:1 compression and good heads will give you more low end torque than you know what to do with and will come on like gangbusters on the top end. These trucks dont weigh any more than a chevelle and certainly less than a big chevy passenger car so cam away to your hearts content. Using too short of a duration cam will make you miserable if you like to get your foot into it like I do. Nothing worse than running out of steam at 5000 rpm.

None of the heads you have listed flow all that well to tell the truth and your better off selling them all off and buying yourself a set of
vortech heads from chevrolt . a set will run under $500 and the money you'll save on pistons will more than put you ahead of the game because you'll need flat tops only to give you your compression as the vortechs have something like 58 cc chambers and they flow better than anything chevrolet has ever put on a car. Or if you are dead set on using those heads .125 domed piston will give approx the compression you are looking for although remember that there isnt much of a HP difference between 10:1 and 11:1 but there is a huge difference in the type of gas you can get by with.

I use sealed power .125 domed hypereutectic pistons and am using awesome scat 4340 H beam rods with a steel crank with .003 bearing cleaance, all fully balanced. The set of 8 pistons cost less than $200 and I am fully confident running it to 7500 rpm all day long although my cam probably starts dropping power at 6500 and so will yours .

Also remember that buying 92 octane all the time sucks and that if you go to 11:1 you'll have to spike it with avgas or octane booster to keep it from rattling apart. It adds up.

A few more words of advice. Build your bottom end to handle all the power your heads and cam will give it. If the top end is going to provide 400 horsepower then build the bottom end to handle 400 horsepower ie steel crank, strong rods , 4 bolt main, ARP bolts etc. BALANCE YOUR ROTATING ASSEMBLY !!!!! It costs about $150 . Its worth it. Cut corners somewhere else .

[Stroker]

There is another way to generate the compression ratio you are targeting and it will not require pistons with domes. Get a $200 Skat 3.75 stroke crank some flat top pistons and build a 383. I personally don't care for dome pistons because of the flame travel characterisics making them more prone to detonation problems. The 286 magnum with the vortech heads or ported DBL humps make an exellent low cost combo for a hot street 383. American speed makes a crate 383 that is very similar that makes some thing like 450 HP and 500 ft lbs. BTW if you go the popup 350 way, be sure to check for piston to valve clearance carefully and allow some budget for possible piston machining. The duration and lift are getting in an area where you could possibly run into problems. Good luck!

[mikep]

.125" domed pistons with .490" lift isnt even going to get close to valve to piston interference problems.


A 383 with vortech heads and that cam with 10+:1 compression will give too much lower end torque with those 3.73 gears IMHO which looks good on paper but is a hindrance to a good launch in the real world. With a 383 you'd really need 242-245 degrees duration and thats with some pretty healthy port sizing (200+ cc) to build good midrange to top end power. Otherwise you'll run out of steam under 6K . Sure you could build a 383 with the 286 cam and be building 400+ ft lbs under 3200 rpm but what good will it do you if your tires are spinning the whole time?

All I can say is I used to race a lot of guys with 383's and short duration cams (under 250 at .050") who all had the same problem. Too much low end torque. They'd smoke it off the line and I'd beat them by a few seconds and theyd be pissed because they got beat by a Ford. OK it was a 480 horepower 351 cleveland but it was still a ford. Big ports, hi rpm, hi compression, and long duration is what its all about in small blocks. They had 12 second cars running 14 seconds because if you can't hook it up you are screwed . Add to that the lack of weight in the back of a truck and it makes even more sense to stay away from long strokes and bottom end torque and go more towards stronger mid range and higher rpms and thats where the faster revving 350 with its shorter stroke shines. Not to mention the fact that its easier ( read cheaper) to build and especially balance a 350 with good quality hi rpm capable parts. A lot of guys swear by cast cranks. Ive broken enough to not be convinced of their hi rpm longevity. Steel's the way to go.

One note on porting the chevy heads. It'd cost quite a bit more than buying decent off the shelf items to have a quality porting job done by someone who knew what they were doing. Theres just so many really good heads out there these days that it doesnt make sense to use clapped out chevrolet castings any more. A $1200 port job on a set of heads plus plates, springs and studs is putting you into the $1400+ range and thats on your heads. $700 will buy you aftermarket heads that flow as well and are ready to bolt on. I bought my sportsman II heads new in the box with valves, springs, and seals for $600 for the set out of the paper and I know Iv'e paid $1200 to port a set of angle plug bow ties that flowed about the same.

[Stroker]

mikep I agree that 95% of the time piston to valve clearance with that cam won't be a problem. But the last time I gave someone that type of a blanket response it happened to be in the other 5%. A guy at work had asked if he might have any problem with a Comp 292 Mag and flat top piston. He had recently got a good deal on a 355 SBC from a guy at the track and was going to detune it for his street machine. The seller had recommended that the 10.5 compression with flat tops and the 292 cam would be a good HIPO street engine. He never offered anymore information and I never really thought to ask. Several of us told him no problem! Well we were wrong! There was a lot of missing information: For starters the 64cc heads were not standard 23 deg heads, second it had 1.7 rocker ratio on the intake and 1.8 on the exhaust, but the killer was the pistons. They were truely flat tops and for use with heads having non standard valve angles and required custom machining for valve reliefs. He claimed everything cleared when he turned it over by hand and at low RPM. Once he rapped it up he had some serious damage. So now I always say check it "better to be safe than sorry".
I didn't get the impression from his post that he was planning to race.

And just to note....this truck when finished will not be a daily driver...it will never tow anything... strictly a street/show truck.

As far as the power band goes I have no personal experience racing these trucks. But I go out to the races every now and then and when I see one of these trucks that run well I normally try to chat with the owner and find out what he's running. I've only come across a couple or 67-72s that were really quick. All of them were running BB 454-510 cid. All using relitively small roller cams 245-255 @.050 and aftermarket or ported heads. Most had more than a 3.73 gear and came out of the hole with a few inches of air under the front tires. I'm not sure of their 60 ft times but these trucks were in the low10's to low 11's and went thru the lights at about 6500rpm. They all were using stock trailing arms that had been boxed and coilovers with 12-14 inch rear tires.Claimed they hooked up very well. But I'm sure there are plenty of trucks running as well or better with less CID. I have a buddy that shares your same veiw and he runs his 350 Camero thru the lights at 9 grand and has put plenty of the BB guys on the trailer in heads up racing.
As far as ported factory heads vs the aftermarket. Couldn't agree with you more. I really was just talking about some basic pocket porting that would only require an investment of his time. The crate engine I refered uses a set of Performer RPM with a bowl clean up and flowed about the same as the vortech heads.
Sorry about the long post.

[MikeB]

MikeP:
Great info re cams and CR. I admit I was too conservative with a 266 cam in my 383. Everyone says it's best to err on the conservative side, but I think I went 1-2 grinds too conservative. Of course, if I add lift at the cam I have to go back in a make sure my rods are relieved enough to clear the cam lobes. It was hard to measure, but I think I had around .050" clearance with my .440" lift cam, after some rod grinding. I think that's the minimum recommended clearance. What do you think of one of the "lift rule" cams with .450" lift and maybe 220 degrees duration (@ .050")? Used with my 1.6 rockers, that would effectively be maybe 222 duration and .480 lift. An option is a small base circle cam, but I wonder about their durability.

By the way, a buddy of mine has that exact same Comp 286 in his 72 LWB truck -- around 4000lbs, by the way. With a 3.08 axle, it is flat on the bottom and even lopes at 60mph! Maybe some of his problem is too little compression.

[mikep]

Thats the first time Iv'e heard of anyone remark about too little compression with a 383. Generally guys complain about too much compression even with flattops and 76 cc heads your getting into the high 9's compression wise. I'm not sure if theres commonly availabe dished pistons and if its not a d cup design then quench goes to hell in a hurry. That, the long stroke to bore ratio and the rod to cam clearance issue is why I'm not completely sold on the 383 even if its with an internally balanced steel Scat crank and 5.7 inch rods. Ideally for street use with decent heads with at least 200 cc port size I'd want .500-.520" of gross lift and 236 to 242 degrees duration on tight lobe centers for a 383 using a nominal 10:1 compression. Much less duration than that isnt going to fill the cylinders to their potential and much more than that would be sacrificing too much bottom end without real gains on the top end as hydraulic cams are going to run out of steam at 7000 or so no matter what you do. Ive never seen a hydraulic lifter that didnt pump up at around 7G and nothing gets into my stomach like the sensation of floating valves.


BTW MikeB

Thats too much cam for 3.08 gears in a truck.

[mikep]

I just described the Edelbrock performer RPM cam didnt I?

[TheMattMan]

I talked to some techs (and u guys are pretty convincing <g>) And they can set me up with a set of Vortecs with a upgrade kit with a max lift of .55 for $317 each and Edelbrock Vortec performer Intake for $158. So I should be able to get some forged flat top pistons and end up with a high 9 or low 10:1 CR. Seems to me this would be a pretty decent setup.

Thanks for the replys, U all have been really helpful...I am sure I will have many questions to come.

TheMattMan

[MikeB]

Matt - Here are some numbers from my spreadsheet assuming a 355, 64cc combustion chambers, 9.025" deck height (stock un-milled), and .038" head gasket.

Flat top pistons w/ -4cc total valve relief: 9.96:1

-12cc dished pistons: 9.16:1

So, you were right on the money guessing 10:1 w/ flat top pistons!

For different gasket thicknesses and deck heights, each .010" changes compression by prox 0.2 points. For example, a thinner .028" gasket results in 10.2:1 compression, a thicker .048" gasket = 9.75:1.

Have fun!