I'm thinking about getting new heads, but at the same time getting new pistons to replace the dished ones that came with the L48. I'm shooting for 9.0 to 9.5 compression. I'm a fairly smart guy, but all the choices are confusing. 4 valve vs 2 valve reliefs, flat, domed, dished, hyper, forged, cast...

One of my friends says that flat tops are best because they are less prone to hot spots and therefore won't detonate as much. Me, I'm absolutely clueless, and I need a piston primer!

I'm really leaning toward the Summit/Dart heads, but also considering biting the bullet and getting one of the Trick Flow top end kits, or a set of the Edelbrock RPM heads. There are a lot of y'all that know way more than I do, so all advice is not only appreciated, but desperately needed!

Thanks in advance,

 

Your freind is correct, flat top pistons are preferred in high performance motor over dished pistons as they provide better flame travel over the top of the piston. I would get hyperutectic flat tops that would put your CR at 10:1

 

Contrail,
The piston material choice depends on $$$ and application.

The cheapest pistons are Cast Pistons. They come originaly with L-48's. No better, no worse. Stock pistons.

The next step up is the Hypereutectic Cast Piston. These pistons are stronger than plain Cast pistons but they are cheaper than a Forged piston. They work very well in many performance applications but they do not take as much abuse as a Forged Piston will. Hypereutectics can get destroyed from too much detonation and they require larger piston ring gaps on the top ring. They also will not rev as high for long periods of time as a forging without failing. Still better than stock cast pistons though. The piston is sized to fit tightly in the bore so they will last a long time and run quiet. The metalurgy of the Hypereutectics makes them have much less thermal expansion as compared to forgings.

The most expensive is the Forged Piston. They are typicaly used for high RPM and high compression. This is the most durable piston under severe service duty. The pistons are typicaly sized a little bit loose in the bores to account for the larger amount of thermal expansion that comes from the forgings. They take much more RPM and detonation as compared to the cheaper styles but the engine probably will not last as long as it would with the hypereutectics before needing to be overhauled.

Flat tops are best for flame travel and combustion. Domes are to make more compression but a smaller cylinder head chamber is more desireable to accomplish that if possible. Dishes are for reducing compression.

The 2 valve relief vs. 4 valve relief thing is just about cost. It costs less to manufacture a single design of piston that can fit in any position rather than manufacture LH and RH pistons so they have 4 valve reliefs and a notch that faces front. Piston pins are actualy bored slightly (about .060") off center so they will reduce piston slap and run quieter. Because of the off-set, you need to make sure to have the notch facing forward when assembling the rotating assembly. The more expensive pistons will have only 2 valve reliefs (or a single trough even better) but they can have a "hand" to them.

I like the Summint/Dart heads but some models to not have the exhaust heat crossover passage. I would advise you to get heads that do have the heat passage avaialble (especialy so if you are running a cast iron intake). It helps get the engine up to operating temps much faster. I have Dart heads without the passage and I can not just get in and drive from a stone cold start, I gotta wait . The "Edelbrock Performer" heads have the passage and are excellent quality pieces. The "Edelbrock Performer RPM" heads do not have the passages.

Pick heads that have either 64cc chambers to get a good amount of compression. 72cc will also work but I prefer to run the most compression you can without detonating. A lot depends on cam timing. I am running the 350/350 GM cam with the stock L-82 bottom end and 64cc Dart heads. The combination will give you about 10.4 C/R with forged flat tops and a .015" steel shim head gasket. This combo runs very well on pump gas with the thin gasket. I would not advise running this combo with a thick .038" composition head gasket or you will risk the possibility of running into detonation with pump gas. Might be OK but it is close to the limit as built.

If you run the typical .038" composition head gasket, you should probably run the 72cc heads to get 9.0 C/R. The same combo with the .015" steel shim gasket and 72cc heads will give you 9.5 C/R.

Have fun!

-Mark.

 

Stingr69,

Thanks, dude! You just answered all my questions in that one post. I'm printing that one out and putting it in my rebuild journal. I really appreciate the info.

 

One more question:

Compression height? What's best?

 

Contrail,
Best question of the day.

For optimum quench, you want to have the distance from the top of the piston to the underside of the head to be around .040" or so. The factory pistons will typicaly be about .025" down in the hole with factory block deck dimensions. The thinnest steel shim gasket will be .015" so the total is right on the money. This is important because it helps the engine be less sensitive to fuel octane. You will maximize compression without detonation if you keep the top of the piston close to the head. For safety you do not want to go much closer than .035" if you run hypereutectics. .040" is good for forged pistons as they run looser in the bore and have a tendancy to "rock" at the top more than Hypers. The cheaper rebuilder type pistons may ride lower in the block but try your best to keep the piston close to the top of the block. You can also mill the deck of the block but I would only do that as a last choice. A "zero" decked block can run one of those .039" composition style gaskets and be right in the sweet spot.

One last thought, avoid buying "claimer" type pistons. They are designed to be the cheapest possible replacements for "claimer" racing motors. If you win the race, your motor can be "claimed" or taken away from you by the other racers so the parts have to be bargain basement cheap.

Glad to be of assistance,

-Mark.

 

Okay, got it, but what about the compression height quoted for the piston? I've seen them range from 1.565 to 1.250. How does this figure into all the other variables? Is that the depth of the piston from the chamber at TDC? If I understand correctly, the smaller the compression height, the closer to the top of the bore the piston will be? Thanks for your patience, just want to verify that I've got the right idea!

 

Hello
Comp ht is dictated by the rod/ stroke of your combintaion within a engine block
A 327 Chev has a taller comp ht than a 350. The rod is 5.7, block ht is fixed, so if you have a 3.25 in crank(327) as opposed to a 350 (3.48) the comp ht has to vary with it to keep the piton at the correct ht in the block
If you are not changing the rod and stroke, your comp ht will remain the same. You can measure from the top of your piston to thge centerline of the wrist pin to know what your ht is, or just specify if you have altered the combo( if it is a 350, then just say that when ordering)
take care
Tom

 

Thanks for making that clearer. Now I've got all the info I need. Thanks to Motorhead, Stingr69 and Henny496 for all the help!

 

Your welcome and happy engine building, I love it, going to build a 434 next myself

 

SpeedPro/SealedPower hypereutectic pistons widely available at good prices, very good quality.

P/N H631CP: coated skirt, flat, 2VR, -5cc dome volume, 1.560 CD.
P/N H345NCP: coated skirt, flat, 4VR, -5cc dome volume, 1.560 CD.

If using a Felpro P/N 1094 or Victor P/N 5746 gasket ... either piston will make 9 to 9.25:1 CR w/ your L48 (about 76cc) heads. Either is correct CD for use w/ your L48 rods & L48 crank.

-edit-Unlike some other brands, SpeedPro/SealedPower hypereutectic pistons do NOT require larger ring gaps.

 

Respectfully, I disagree. Claimer pistons are less expensive and heavier than $900 Ross Ludicrous Lite w/custom gas porting and additional $400 of 3 kinds of coatings ... but they're not "cheap" as in lousy quality. The least expensive cast pistons for a 350 are about $40/set ... but they're not marketed as claimer pistons. I spend a bit of time & money in the local circle track rodeo ... I don't know of a single motor around here that failed solely because a piston failed. Consider this ... IMCA has a $500 motor claim ... but not many reasonably competitive IMCA claim rule motors are built for less than 8-10 times that. Several tracks around here permit GMPP sealed circle track motors and have a claim rule too ... those motors sell new complete for about $2500 and are sealed & have hyper pistons ... the motors rarely fail at all ... usually good for at least two full seasons before needing rings & timing chain. If you find a name brand claimer piston that meets your dimensional specs you can be pretty sure it'll hold up real well in your street car.

 

Jackson,
You may be right. One of our fourum members here had piston issues a while back and he posted pix. It was a claimer piston. I have a hard time trusting and recomending inexpensive parts to others when you consider the total cost of an engine investment. There are some very cheap claimer type parts out there and some of the ones I have used in the past were not suitable for extended road service. One of the guys that does machine shop work for me does a lot of dirt track racing engines and races himself. I will ask him what he thinks about pistons next time I see him.

-Mark.

 

[QUOTE=stingr69]Contrail,
The piston material choice depends on $$$ and application.

I am running the 350/350 GM cam with the stock L-82 bottom end and 64cc Dart heads. The combination will give you about 10.4 C/R with forged flat tops and a .015" steel shim head gasket. This combo runs very well on pump gas with the thin gasket. I would not advise running this combo with a thick .038" composition head gasket or you will risk the possibility of running into detonation with pump gas.

Mark,
I am confused by this. I think the thicker gasket will give you a lower CR and so less chance of detonation. Can you clarify this for me?
From what I understand you want to stay about 9.5 CR with iron heads and a mild performance cam and you get that by matching gasket thickness with chamber volume (ie: thicker gasket with 64cc chambers or thinner gasket with larger chambers).
Thanks a lot.
Richard

 

[QUOTE=Rich's'78]Think quench. Ideal, I believe, is about 0.040. Stock SBC pistons are about 0.025 in the hole. If using a 0.038 gasket, quench is 0.063. Not ideal. Using a 0.015 gasket you get 0.040. You can run a higher static CR with lower quench vs stock quench without detonation. Thicker head gaskets to lower compression will raise quench, possibly resulting in more detonation.

If you don't understand this, Google "quench". Lots of good sites will explain.

 

As Glen and I are saying, quench helps to reduce the engine's octane sensitivity. If you keep the piston top close to the flat part of the cylinder head, your engine will have less tendancy to ping or knock. If your piston get too far away from the head, the quench effect goes away and you have only the compression ratio to work with to fight detonation.

The reasons you can run 10.4 c/r with that GM 350/350 cam and iron heads is that the tight quench, modern combustion chamber shape and long factory effective duration all help out to reduce the octane sensitivity. If you try to run a shorter duration cam with smog heads and thick gaskets you will be more likely to have detonation for any given compression ratio.

If you keep quench tight and pay attention to the dynamic compression ratio, it will help keep you out of detonation trouble. Do a search for "Pat Kelley's dynamic compression ratio calculator" and download it. He distributes it freely. That is a great tool to use when matching a cam to the best compression ratio. It works best on typical aftermarket cams so it does not give the same results with the factory grinds. Nobody else grinds profiles like the General these days and the reccomendations used in Pat's calculator are based on geometry and empirical evidence gained from applying the calculator to typical aftermarket race/street cam profiles. It is a must-have IMHO.

Hope this helps,

-Mark.

 

Glenn, Mark,
Thanks a lot for the info. My engine ('78 L48) was rebuilt using KB hypereutectic pistons (flat top?), stock rods and milled stock block/heads with Comp Cams HE268. Unfortunately the guy didn't record the distance the pistons are in the hole or the new chamber volume so I don't know what my CR is. It runs well on premium gas.
I am considering replacing the heads with Skoggin/Dart iron (or other) ones but I guesss I will have to pull the heads and measure the distance before I can figure out what chamber volume to buy.
Do you know from experience what my dimensions will likely be and which chamber I can go with for best performance and detonation resistance? I get different answers from different builders/mechanics.
Thanks a lot!!
Richard

 

Richard,
You will need some info and make a few assumptions. First thing is to call KB and get the compression height for that piston. Add that dimension to 7.44" and subtract that total from 9.025". (Guys, check my math )That will get you in the ball park for how far down in the bore the piston is at TDC for a typical unmilled GM block.

The HE268 will not like much over 10:1 compression ratios in general. If you have over 10:1, you might be better off with a HE270.

Your heads are probably still pretty close to 76cc's. I do not think you can mill much off those lightweight castings.

Hope that helps,

-Mark.

 

Thanks a lot Mark.