P90 + Dished or Flat Top Forged Pistons

[jkelly]

I'm converting my L28 to a turbo and using the P90 head with the stock 'B'  grind cam. I currently have the block torn down and I figured while I'm in there I was going to go with all ARP hardware and also buy forged pistons (I'm looking at JEs). My question is should I go with forged dished pistons for a lower CR or go with forged flat tops? I know a lot of guys run the flat top + P90 setup with success, but a lot of the reasons behind that allude me. For a reliable weekend warrior street driven car with initial horsepower goals in the mid 300's what would be the best route? I'm buying forged pistons either way. I'm also going with a larger aftermarket turbo, intercooler, and Megasquirt III.

Edited January 24, 2019 by jkelly

[rossman]

If I were to do it again with similar goals, I'd go with p90a solid lifter head modified with #4, 5 and 6 coolant bypass, stock internals with flattop pistons, stock na cam, arp hardware, modern turbo like a gtx35r with a .87 ar.

[Gollum]

1 hour ago, rossman said:

If I were to do it again with similar goals, I'd go with p90a solid lifter head modified with #4, 5 and 6 coolant bypass, stock internals with flattop pistons, stock na cam, arp hardware, modern turbo like a gtx35r with a .87 ar.

Where's the Like/ThumbsUp button for this post?!

 

But seriously, the flat tops will give you better quench (make sure they're deck-proud like stock and you're running OEM or fel-pro gasket and measure that quench!) and with programmable EFI there's no reason not to. Fuel quality in the 70's and even 80's sucked. Batches were really inconsistent, and most refineries had little programmed automation in their process to ensure consistency. When you combine that with the fact that most cars were running distributors with only a few varying curves it becomes obvious why compression ratios were kept low. The chambers were slower designs, and they couldn't pull timing around peak torque without making it difficult to add timing back in afterwards. Today, we can easily fine tune these knock prone regions and prevent knock scenarios. We also have much better fuel we can tune around and rely on the consistency being there.

 

I won't stock you from going forged pistons, but what power levels are we aiming for here? If the mid 300's is as far as you're ever going, then factory slugs are fine as long as you don't detonate under elevated boost, at which point if you SEVERELY detonate under boost at high RPM you're screwed even with forged pistons. The forged slugs will just give a bit more window to find mild knock if you're chasing every last HP.

 

And definitely spring for a decent turbo. I have nothing against the cheap ebay turbos, but if you care about response and overall experience, the new turbos are hard to beat. I'd be looking at the G-550 or EFR 7064-B with the .83 AR T3 housing. Both have show to be a capable 400+whp turbo with minimal lag and very responsive linear response instead of hard walls of boost response.

 

Also, imo I really like my MS3X, but i'm not fond of the D37 connector. I'd go with a MS3 gold box or potentially a different platform like the EMU Black if I was doing it over again.

[jkelly]

Thanks for the responses guys. This is a slow process for me, but I'm trying to figure out what direction I want to go with this engine. I'm weighing several different options from just using the pistons I have (cast flat tops) with the stock P90 head to overboring with JE flat tops and a built head (cam, race springs, etc.) from RaceTep. Again, my HP goals are mid 300s -- I think should be a pretty fun number for a weekend driver/occasional track car. I'm leaning towards Option 1 below since I already have the rebuilt head and can tune with MSIII to mitigate detonation risks and not worry too much about the cast pistons being a weak link (?).

 

I'm looking for comments suggestions on the options below. Are options 2 and 3 overkill for the goals I have?

 

'Free' means I already have it.

 

Option 1: Stock Flat Top Block and Stock Head

MSIII (Free)

Stock P90 head (already rebuilt) (Free)

Cast Flat Top Pistons in 86.5mm (Free)

ARP Hardware rods, mains, head (Free)

New rings ($100)

Rod and Main Bearings ($169)

Turbo (T3/T04E 54 Trim 0.063 A/R) ($800)

Turbo Oil Pump ($100)

MLS Gasket ($150)

Intercooler ($400)
Machine shop work ($300, hot tank, mag, bore, deck block)

Total ~$2019.00

 

Option 2: Forged Block and Built Head

MSIII (Free)

JE Forged Pistons with rings and options ($1237)

ARP Hardware (Free)

P90 Stainless Steel Valves ($250)

Race Valve Springs ($169)

Custom Cam Grind ($189)

Rod and Main Bearings ($169)

Turbo (T3/T04E 54 Trim 0.063 A/R) ($800)

Turbo Oil Pump ($100)

MLS Gasket ($150)

Intercooler ($400)

Machine shop work ($600, bore block and head assembly)

Total ~$4064.00

 

Options 3: Forged Block and Stock Head

MSIII (Free)

Stock P90 head (already rebuilt) (Free)

ARP Hardware (Free)

JE Forged Pistons with rings and options ($1237)

New rings ($100)

Rod and Main Bearings ($169)

Turbo (T3/T04E 54 Trim 0.063 A/R) ($800)

Turbo Oil Pump ($100)

MLS Gasket ($150)

Intercooler ($400)
Machine shop work ($300, hot tank, mag, bore, deck block)

Total ~$3256.00

 

 

 

 

[ZHoob2004]

I'd be inclined to go option 1, but I lean more towards the DIY/"budget" side of things.

 

I don't think cast pistons are the weak link many claim, you just have to keep detonation under control. The 5-6 head cooling mod is a well established method to reduce detonation in the rear cylinders, and is a must for this build.

 

Another thing to consider is, how willing are you to open the engine again if it doesn't meet your goals? Cylinder head work is relatively easy with the engine in the car, but it's easier still to do before the engine is in. Bottom end is doable in place, but hardly worth it.

[NewZed]

On 1/24/2019 at 12:00 PM, jkelly said:

For a reliable weekend warrior street driven car with initial horsepower goals in the mid 300's what would be the best route?

 

There's some good stuff in this thread - https://forums.hybridz.org/topic/50208-the-ultimate-l28et-guidewhat-you-need-for-350whp/

[jkelly]

3 hours ago, ZHoob2004 said:

I'd be inclined to go option 1, but I lean more towards the DIY/"budget" side of things.

 

I don't think cast pistons are the weak link many claim, you just have to keep detonation under control. The 5-6 head cooling mod is a well established method to reduce detonation in the rear cylinders, and is a must for this build.

 

Another thing to consider is, how willing are you to open the engine again if it doesn't meet your goals? Cylinder head work is relatively easy with the engine in the car, but it's easier still to do before the engine is in. Bottom end is doable in place, but hardly worth it.

 

I'm leaning that way for the same reason and because the paint shop 'broke the bank' and I know the engine will end up costing more than I estimated above when it's all said and done. I would probably not want to take the engine out of the car again since it would involve removing the hood and probably the fenders in order to minimizing scratching them.

 

I would just live with whatever power it ends up making, most likely.

 

3 hours ago, NewZed said:

 

There's some good stuff in this thread - https://forums.hybridz.org/topic/50208-the-ultimate-l28et-guidewhat-you-need-for-350whp/

 

I've read through that a few times before. It is a lot of great information, but still not really a definitive answer. From what I gather it says going with my Option 1 I should be able to get 350whp.

 

There seems to be several ways to get to 350whp(ish). Like ZHoob2004 mentioned a lot of people say you need to go with forged pistons with anything above 300hp, but is that considering that you have a MSIII tunable management computer to control detonation? Forged does buy you extra safety margin against detonation, but detonation can destroy a forged piston/ring, too, so is it really worth an extra $1200 for this kind of build? Could I create the same peace of mind and reliability with cast pistons by ensuring I tune properly?

 

EDIT: a word

Edited February 11, 2019 by jkelly

[NewZed]

Separate power from RPM.  Some of the parts you're looking at are for high RPM purposes.  If you keep the RPM down you can make power for cheap with a turbo, without destroying things.

 

Some people recommend building a throwaway engine, tuning it to the power level you want, then mimicking it with more durable parts.  You might find that 250 HP is more than enough and you don't need the expensive parts.  Your 300 HP goal is really just picked from air, isn't it?

[jkelly]

1 minute ago, NewZed said:

Separate power from RPM.  Some of the parts you're looking at are for high RPM purposes.  If you keep the RPM down you can make power for cheap with a turbo, without destroying things.

 

Some people recommend building a throwaway engine, tuning it to the power level you want, then mimicking it with more durable parts.  You might find that 250 HP is more than enough and you don't need the expensive parts.  Your 300 HP goal is really just picked from air, isn't it? 

 

I see. I've considered building a throwaway like you mentioned, learning to tune on it, and if I destroy it I'm not out too much.

 

The 300 number was picked from owning 350hp vehicle, albeit heavier, and thinking it would be a reasonable number for this project. I haven't driven a zcar with that much power, so you may be right and 250hp may be fun enough.

[NewZed]

You could do the power-to-weight ratio calculation.  Might help.  

 

Here's some interesting comments from a bunch of the race guys, all in one thread.  TimZ made a whole bunch of power.

 

[jkelly]

Interesting post! That was a good read. I will probably stay with cast pistons, especially after doing some power to weight calculations -- that really put it into perspective. Using my 345hp vehicle (5120lb curb weight) as a power-to-weight reference the equivalent 240z power would be only 155hp.

[ZHoob2004]

19 minutes ago, jkelly said:

Interesting post! That was a good read. I will probably stay with cast pistons, especially after doing some power to weight calculations -- that really put it into perspective. Using my 345hp vehicle (5120lb curb weight) as a power-to-weight reference the equivalent 240z power would be only 155hp.

 

This is my reasoning for going hot NA with my build. I'm about at that power/weight ratio now in my daily so if I can get near/above 200 with a cam and compression that should keep me busy for a while (at least that's what I've been telling myself.)

 

For reference, my build is stock flat top bottom end with an Isky l-490, valve springs, and retainers on a shaved P79.

[Zetsaz]

Honestly I did the same calculations, and realized that to match the power to weight of my G37S (which I barely even rev up to redline or even make the most of the powerband on, I'd only need about 190-195whp. Perfectly doable NA even if it is easier with turbo. 

 

We've definitely been spoiled by the crazy power modern engines can make. 

[Gollum]

And there ARE people who have made 400hp on factory pistons, let alone higher end aftermarket cast pistons. Just can't over-rev it too much and keep it out of any detonation. Tunes kill turbo engines faster than any parts or lack thereof.

[jkelly]

15 hours ago, Zetsaz said:

Honestly I did the same calculations, and realized that to match the power to weight of my G37S (which I barely even rev up to redline or even make the most of the powerband on, I'd only need about 190-195whp. Perfectly doable NA even if it is easier with turbo. 

 

We've definitely been spoiled by the crazy power modern engines can make. 

 

True! I'm starting to think 200hp would be plenty fun, for me. But I also think of how after riding a sport bike or fast car/truck for a while I always want more. So there's that. I also have a freshly rebuilt N47 head with a Stage 1 Schneider cam and fresh springs. I could continue to go the NA route for even cheaper than going turbo if all I'm shooting for is 200hp.

 

11 hours ago, Gollum said:

And there ARE people who have made 400hp on factory pistons, let alone higher end aftermarket cast pistons. Just can't over-rev it too much and keep it out of any detonation. Tunes kill turbo engines faster than any parts or lack thereof.

 

It sounds like the safe rev range for stock internals is around 6000 - 6500rpm.

[jkelly]

On 1/24/2019 at 12:12 PM, rossman said:

If I were to do it again with similar goals, I'd go with p90a solid lifter head modified with #4, 5 and 6 coolant bypass, stock internals with flattop pistons, stock na cam, arp hardware, modern turbo like a gtx35r with a .87 ar.

 

Is this the cooling mod you're talking about?

 

Looks like he does all six chambers, though.

 

[ZHoob2004]

Yeah that's the mod. Here's the thread where most/all of that was developed 

 

 

I don't remember if it's in the same thread or not, but I recall some testing being done on an engine dyno with lower coolant temperatures and they were able to get noticeable increases by lowering coolant temperature overall, which they attributed to reducing detonation in 5 and 6, suggesting the use of the "tropical" thermostat for performance engines.