I'm getting ready to rebuild a factory roller 4-bolt main truck block to replace my aging 87 L98, and it's time to select a piston. The vehicle is a 4400lb K1500 4x4 with a 700R4, 3.42 gears 16" 285 tires (about 33" tall). The fuel injection is a stock TPI setup, in a speed density configuration with a 730 ECM and slightly modified AUJP bin with a calmer timing curve to keep the detonation in check.

I like to run 87 octane and I do OK on the stock L98 long block with R43TS plugs and 195 deg thermostat, however if I lug it at 1200-1600 RPM up steep hills in 4th gear and TCC locked it has been known to ping.

The piston/head arrangement I'm planning to use is Keith Black hypers with the D-dish design and LO5 TBI heads with a fresh 3-angle valve job and mild milling just to resurface them. The valve train will be a stock roller L31 vortec 350 cam and 1.5 rockers.

Anyway, onto the question:

KB has two pistons that would suit my needs. One with a 12cc dish (KB193), and one with a 18cc dish (KB142). Assuming a stock deck height of 9.025, KB's 1.561 compression height, and a Clevite/Victor .026 composition head gasket, I'm looking at about .050 quench. I figure I'll be around 9.4:1 with the 12cc dishes or 9:1 with the 18cc dishes.

I can't decide which pistons to go with. I know 9:1 would be the safe route to take for 87 octane especially in a 4x4 that I like to lug around in 4th gear with, but I figure with the decent quench and D shaped dish design of the KB pistons, I may be fine with the 9.4:1 arrangement since the configuration will be more of a fast-burn design.

Should I take the safe route and stick around 9:1, or look for a bit more compression and cross my fingers that 9.4:1 will be fine. I'm leaning more towards the 16cc dished pistons. Does anyone have any suggestions?

 

this question depends HEAVILY on your cam. What are you running?

I have a similar setup in a 383 and I tried to get away with .055 quench. Didnt work and I had noticable ping at highway speeds in the summer. I ported out the chambers and went to a thinner head gasket to .04 quench and no more ping. This is all with ~ 9.3 Static CR and a 4x4 cam with not much duration for a 383. And I always use 91 octane.

If I were you and didnt want to worry about using a shim gasket I would go with the 9.1 CR if you have a towing/mileage cam like me.

Now if you have a performance cam with longer duration then all bets are off and you might want higher CR.

Tell me the cam specs (timings) and Ill tell you your Dynamic compression ratio. Or you can look online for one of the many DCR calculators. The DCR will tell you what octane you can run without knock. If you keep the DCR low enough you might even be able to run the .05 quench without a problem.

 

While I did mention the camshaft is a factory L31 (Vortec 350 roller camshaft) I didn't mention the specifics.

The best I can find on it is this:

.415/.430 194/197 @ .050 111LSA

I do have a RamJet 350/HT383 roller camshaft too, but I've ruled it out because I don't like the idea of it's 109LSA and I think it will be overkill for LO5 heads. Plus I'm looking for the MOST idle-3500 RPM torque I can get.

.431/.451 196/206 E @ .050 Lift Centerline: 108/116, 109 deg sep

I do agree that I should stay of safe side and aim closer to 9:1, so the 18cc dish pistons are probably the route I will take.

 

Bad info - edited

 

LO5 heads, (I'd never consider using them) are right around 65cc, so I'd go with the 18cc pistons.

 

I'll be ordering the 18cc dished pistons next week along with the rest of the parts.

Why? They have great low lift flow figures and work perfect in low RPM heavy truck applications. My 350 sees less then 3500 RPM 90% of the time....

 

While I agree a small port cross-sectional area is great for low-rpm, the low-dynamic-compression + velocity-ramming approach isn't the most efficient. That's why GM abandoned it. While the RamJet 350 crate engine may be way past your budget, the dyno sheet showed it makes like 350 ft-lbs at just 1600 rpm. A TPI'd LO5 won't do that. I'm all for building torque without stroking, stroking hurts mileage. So, I would run Vortec heads, and a looser torque converter with the lockup feature. This is why I'd go 18cc. Later you can go to Vortecs if you ever want to, and while they are supposed to be 64cc, most usually check closer to 61cc. Just my 2 cents worth, because this is a forum, after all.

 

I certainly appropriate your input!

I have considered Vortecs, in fact I bought a pair of 906 (with the hardened exhaust seats) cores from a machine shop and they rebuilt them for me, but when when I got the fresh heads home I noticed one was cracking right across the middle. I returned them to the machine shop I got them from and decided to just stick with the L05 heads for now. They're the beefy 810 casting with large valve stems, rotators on intake and exhaust, and sodium filled exhaust valves. I figured they would be the ultimate smooth running stump puller heads that should last forever.

Since I also have a RamJet 350 camshaft. Which would you recommend I use, the L31 or RJ 350 cam? I really don't want to sacrifice any idle-2500 RPM torque, and I enjoy the L31 Cadillac smooth idle. Perhaps the L31 cam and a pair of 1.6 rockers would be a good compromise?

 

Vortec 350s tend to lose coolant down the back of the engine, then overheat, then crack the heads as you've seen. Very common. And if you never run low on coolant, they never crack like that. anyway, a smoother idle does mean more torque at idle, but most stock torque converters stall past 1200 rpm. That being so, the RamJet cam will give you more useable torque. 1.6:1 rockers will help, but you may have to modify the heads where the pushrods pass through them.

 

Ramjet cam has KILLER low-end torque, more than the factory L31 cam. The 109* LSA actually gives the engine MORE peak torque and makes the power curve taller and shorter.

810 swirl ports are GREAT heads for any low-rpm, heavy load engine uses.

As far as a TPI'd L05 not making the same torque as a Ramjet, it WILL make MORE torque than a Ramjet if its running the same cam, 1.6:1 roller rockers, and same compression ratio. The TBI heads will generate more torque below 3,500 rpm than the Vortecs.

FWIW- I ran a TPI'd L03 with Flattop pistons, Total Seal Gapless rings, blueprinted lower end, and a Production LT4 roller cam in it. Stock 187 swirl port heads with some exhaust porting and intake bowl work. Stock TPI intake was cleaned up and gasket matched. The base was siamese ported. With Doug Thorley Tri-Y headers it RAN despite being in a 5,600 lbs fullsize van with the stock 1,600 rpm converter and 3.08 gears.

 

Fast355: How is the idle quality of the Ram Jet cam? Smooth?

Did you get my PM question about controlling TCC lockup in a 700R4 with a 411 running the Express Van calibration?

 

Ok, change in plans!

Rather then move forward with buying new 350 pistons, I already have a 383 rotating assembly balanced and ready to go that’s been just sitting in the box that I decided to use.

It's a Scat kit with beefy I-beam rods with 7/16 cap screws, SRP -16cc forged pistons, total seal rings, and of course the Scat 9000 crank.

The pistons have a 16cc dish and a 1.425 compression height. With the 5.7 rods, stock 9.025 deck, and the Victor .026 head gaskets it looks like I’m looking at .051 quench and around 9.5:1 compression assuming 65cc LO5 heads.

I’m going to use the RamJet cam since that’s the same cam GM put in the HT383.
To get away with 87 Octane, should I run thick head gaskets to get the compression down, but also kill my quench?

Do the forged pistons decrease the chance of ping since they are denser then cast and apparently pull heat away from the surface? Or is that a myth?

 

Bad info - edited

 

BTW, I know about the 383 because the kit you have is identical to mine in my truck except for the pistons. I used flat tops and ground out my heads from 76 to 82cc's so I could run .015 shims and get .04 quench.

Its a great kit but you dont need forged pistons for the power levels you are looking at unless you are thinking of forced induction or nitrous. In which case you will NEED to get better heads with bigger chambers or you will probably make holes in any piston you get.

Also, You can get away with a little more DCR than I had because of the TPI's beter fuel distribution. I have a carb and its not that great. I will be putting the TPI from my old T/A on it next year.

Bottom line, shoot for 7.6:1 DCR for 87 octane reliablility and 7.8 if you want to run 91. Both of those assume a .04 quench distance.

 

I already have the complete kit, it's been in the box in my basement for nearly a year. Everything has been balanced already.

Would I not end up with the exact same engine as GM's HT383? It would be nearly identical except for I have L05 heads instead of Vortecs, and SRP D-dished pistons instead of GM's inverted dome. GM says you can comfortably run 87 octane on their combo, and I understood D dishes to be even better for detonation resistance then those old-school inverted dome pistons. Are LO5 chambers that inefficent that Vortecs can run 87 octane with that DCR, but LO5s barely scrape by on premium?

http://www.gilbertautoparts.com/Chev...V8s/HT_383.cfm

 

Oh I see - one of my assumptions was wrong on my analysis above. I humbly appologise for the misinformation!!!!

I would have to redo your 350 calculations again so dont pay attention to the piston recommendation I made in my 2nd post above - my cam assumption was wrong. Again I am very sorry.

You see, I have a really agressive ramp 4x4 cam from comp where the .05 lift and the advertised duration are very close (210@.05, 260@.006) whereas the cam you wanted to use (the L31 cam) probably has a much longer advertised duration compared to the 194@.05 given. The advertised duration is what I need for my DCR calculation and I wrongly assumed the advertised to .05 duration would be similar to my cam.

That being said...

Since I cant find the L31 cam advertised duration I will use the GMPP 14097395 you have for my calcs.
This cam has durations of 196@.05 and 288@.006 - I searched. The GMPP part is a much slower ramp and hence has a much longer advertised duration than I thought.

If I use what I found on the HT383 on the page you gave and assume a dish of 16cc and a compression distance of 1.425 (from here: http://nastyz28.com/forum/showthread.php?t=113233), I get a SCR of 9.78 and a DCR of 7.42 (very low - could run 87 all day). That engine has a horrible squish distance and they get away with it by running the super low DCR they do.

If we use the same cam with your kit combo (65cc heads w/ a .026 gasket, 16cc dish, 3.75crank, 4.0 bore) I get a 9.41 SCR and a 7.15 DCR which is way low. If you ran a smaller cam that would bring the DCR back up but only if you went way smaller. There are more agressive ramp cams out there but Ill let you decide what to use.

You could also do a couple other things. If you mill the L05 heads down to 61 cc then that brings you to 9.80 SCR and 7.43 DCR which is more like the HT383 so maybe you could run this combo nicely with just milling the heads down a little. Or you could run a .015 shim gasket (what I do) and get 9.54 SCR, 7.60 DCR with 61cc (milled) heads or 9.63 SCR, 7.31 DCR with 65cc (stock) heads. The gasket gives you the additional benefit of a .04 squish assuming the piston is .025 in the hole (block is standard deck height). You should cc your heads and measure the piston to deck height relationship to be sure of the calculations but the combos above should run on 87.

Again I appologise for the misinformation since you almost ordered parts because of it.

James.

 

James, wow my head is spinning!

To keep things simple and inexpensive, would you recommend I simply run the 14097395 RamJet cam, .026 gaskets, and when I have my LO5 heads rebuilt, ask them to bring the chambers down closer to 61cc when they resurface them? How much metal can they take off before I risk the possibility of cracking, or intake sealing problems?

 

If you are willing to go with a shim gasket I would recommend taking the heads down to 63cc give or take. If you must use the .026 then yes I would recommend at least a 61cc chamber. I am not sure what the rule of thumb is for angle milling vs. flat milling for chamber size reduction is. You may need to have your TPI base milled to adjust for any misalignments a serious milling on the heads may produce.

 

I would also recommend that you or your machine shop actually cc all the chambers. I have seen quite a wide variation on stock heads...

 

It's been a long time coming but this 383 short block is finally done. It took forever because my 638 casting block was ruined by the machine shop. He took too much off with the boring machine and didn't leave enough behind for a quality hone (.003 is ideal I think) so I could still see tool marks in the bore finish. And the icing on the cake was a nice 1/4" wide ring of chatter marks about 1/2" up from the bottom of EVERY cylinder! And it took him a month to do....

I lucked out and managed to get a perfect 2002 Vortec 350 880 casting that was originally a marine engine. The short block looked perfect and probably could have been freshened up with a ball hone and rings but I have my mind made up on the 383. I took it to my local Napa and the guy there did a decent job and put the piston to wall clearance right were SRP recommended (.003") with a very nice looking finish. He had to take .005 off the decks because of a few low spots but otherwise everything was nice and square. The 880 block cleared the 3.75" stroke with my Scat "ProComp" rods beautifully with no additional pan rail clearance required at all. I still had to take a little bit off the inner opposite cylinders as usual though, but still much less then my 638 casting required.

Anyway I attached pictures of the assembled short block for anyone interested....

Now to finish porting the L05 heads so I can get them rebuilt. I've never ported heads before, hopefully I don't screw them up. If I do I have a spare set of LO5 heads so no big deal. The intake bowls take a good bit of work to clean up. I'm leaving the ramps alone and just opening up the intake bowls, especially above the intake valve seat since they are awfully shrouded. I'm half expecting the machinist to tell me I took too much off where the intake valve seats...

Anyway, enjoy the pictures!

 

Remaining pictures...