Hello everyone,

Yesterday while on the highway my 1988 Corvette was sitting at 1,500 rpm and it finally spun a bearing. The coolant was good, the oil level and pressure were good, and the proper weight oil, never exceeded 3,000 rpm the entire time.

So I decided to just ditch the SBC and go for an LS stroker with a T56 (wide ratio 2.97-.63), and 3.55 gear TrueTrac LSD. I'm copying the build from RoxyGTO which is an LQ4/9 built from the ground up with these exact components:
LQ9 block
PRC 227 CNC ported heads
LS9 gaskets
Diamond forged aluminum pistons -2cc Zeroed deck (11:1 compression)
4340 h beam rods
4340 stroker crank (4.00”)
ARP head studs and main caps (7/16 rod bolts)
Billet fuel rails
King bearings
LSXR 102mm intake
102 Big Mouth throttle body
1.7 stock rockers
.690 lift springs
Comp cam 235/243 .621/624 113LSA
Comp hydraulic roller Lifters
Hardened pushrods
42lb injectors

On the dyno (paired with a T56 in a 2006 GTO) this combo made a (what I consider) impressive 522rwhp and 501rwtq on the dynojet.
Here's my question. The PRC 227 heads she used are for a 3.9-4.0 bore. Now PRC makes a 247cc 4.030+ bore head for 408 strokers. Would I be able to make more power with those heads and a bigger custom grind cam? Or would I lose power from messing with this combo and lose drivability in the process? Does anyone have a build that makes more power than this? I just want to make the best drivable NA 408 stroker LS I can, with good dyno numbers. I think 550whp isn't out of the question but I believe it to be possible.

Any help is appreciated, thank you, everyone!

 

Surprised that your not going the LS3 head route? You should make some decent LS power with those parts your listed. The rear end is the weak link for T-56, so leave some budget for beefing up the dana 44.

 

Sounds like a very fun project. What rod length/piston setup are you running that puts them 0.040 in the hole!? Doing that and then decking the block will be expensive and impact intake and valvetrain geometry. Did you mean 0.004in??

Nothing wrong with the cathedral port, but personally I would run rec port heads. The cathedral port should give a little better low-mid end, but the rec port will shine up top, so whatever you care more about. I wouldn't run aftermarket rockers, the stock LS rockers are excellent and have a very low moment of inertia which makes them stable at high engine speeds with less valve spring. For peace of mind, you can swap to bushings or full captured roller bearings for not too much money. I've found a typically full roller rocker needs more valve spring and gains nothing in the way of hp. Also, a little harder to find, but if you can grab an LS2 aluminum block you'll shave ~80lbs off the front of the car.

If you have the Dana 44 I wouldn't worry about the ring and pinion, Stub shafts and u-joints could be an issue if you're drag racing, but the stock gears are beefy Dana 44 HD's. BTW, there's nothing beefier than a dana-spicer brand gear set, that's all I run in my offroad cars after breaking everything else. If you have the dana 36 then for sure I'd upgrade.

 

I’m looking at this kit here for the stroker kit and these pistons. My goal is to get them .040 in hole at the top of the stroke.
https://www.summitracing.com/parts/dpp-11519-r1-8
https://www.summitracing.com/parts/sum-cr4024125

My plan is to parallel deck it and try to get it to that .040 in the hole measurement. This car will not be seeing radials or the drag strip. Possibly the track, I do have suspension plans already set in place.

Preferably I would enjoy a 7k RPM redline, but the build I’m copying is proven (she ran it for 7 years on the street no problem) and makes decent power numbers. So if going for a different head and cam raising the rpm up to 7k vs 6.6k will impact drivability dramatically and possibly have no increase in horsepower then I might stick with this one build. She did use stock LS6 rockers and I might just do that too. Thank you for the insight on that.

Another thing is I’m specifically going for an LQ4/9 iron block since the block weight is almost the same as a SBC and I want to keep that weight distribution the Corvette has as I do enjoy a well balanced car. While I could get a C5 or C6 and build that instead, I really want to push the limits of the C4 and build it to its maximum potential for the street. The good thing is I have the money to do so, but unfortunately I lack when it comes to configuring an engine. Putting the engine in the car won’t be an issue. I’m mechanically inclined, but not great with making an engine from the ground up myself. Hence the use of another persons build.

I apologize I’m advance for this very long reply, just want to make sure you know what I’m looking for and to help narrow down an answer.

Thank you!

This is where I’m basing the .040 in the hole from to get the 11.01:1 compression ratio.

 

That combo should give you a ZERO piston deck clearance, which is what you want:
1.115 (compression height) + 6.125 (rod length) + 2.00 (crank radius) = 9.240 Stock LS deck height

This gives a 12.12:1 compression ratio with the parts listed. You might want a higher dish piston, or a larger combustion chamber (LS3 68cc would put you at a very nice 11.3).

If you noticed the impact of losing 80lbs up front in your handling I think it would only be for the positive. I also track a C4 and any weight loss is good ;-)

 

The vast majority of LS aftermarket crank/rod/piston combos are going to be at zero deck or even slightly out of the hole. .040 in the hole will kill the combo. You will want .040 +- ish quench and you would achieve that with a properly sized head gasket after you measure.

That cam will be nice for a street cruiser but likely a little too mild with 408 CI to achieve 550 whp. You can definitely get there with a more radical cam though.

Lots of guys are making 550+ wheel in the drag only world with just an LS3 at 376 CID but they run wild cams and a lot of RPM to do it. There are several on another forum I frequent that are at high 600s to low 700s at the crank but we are talking mostly solid rollers, big single plane intakes and 8000-8500 rpm.

 

After doing some research I found they make -10cc flat top pistons. And with an LS9 gasket at .055” compressed thickness with 4.060 gasket bore. I’m seeing about 10.99:1 with the 62cc chambers. Is that about optimal? Since this is a road cruiser I plan to keep for a while and even have some track use, I wouldn’t be one of those guys with a 8,000+ rpm redline. 7,000-7,200 max for me. I still think with the setup above 520whp isn’t too bad. I just wasn’t sure if the 247cc head and a custom grind cam would make it to the 550whp. 1 7/8 headers and 3” duals.




You have helped me quite a bit! Thank you!

 

Thank you for correcting me on that, by using what you told me I managed to find a way to use the 62cc heads with a different set of pistons for a 10.99:1 compression ratio. If you have a suggestion for a different compression ratio on 93 octane pump gas, I’d love to know. I don’t want to lose power from not taking full use of the compression for the octane we have over here! (Unfortunately E85 isn’t very common)

Edit: I found a post by HCI2000SS, he has a 6.0 with a 615 lift, 236/242 duration, 112 LSA cam, very similar to mine, and he runs 11.5:1 with no issues on 93 octane. So I think I can up my compression to 11.5:1 and I’ll be ok. (Milling the heads to a 58cc chamber will get exactly 11.5:1 according to the Diamond Pistons Compression Ratio Calculator)

 

Have you bought any of these parts yet, or is this just the plan? 11.5:1 should be fine on 93, but I wouldn't go shaving new heads. That extra 0.5 point of compression should only be worth about 1% increase in hp (here's a quick calculator you can play with to get approx. hp changes with compression: Compression Ratio HP Determination Program (bgsoflex.com)) I'd rather leave it at 11:1 or grab a different piston to get the compression you want. Diamond is excellent, but there are TONS of great pistons available for LS strokers. My personal fav shelf piston right now is Mahle Powerpak pistons in a 2618, they have a -5.8cc flattop that might put you where you want for compression.

ls-6-0-flat-top-webpage.pdf (mahle.com)

You can also gain some compression by going to a standard LS2/LQ4 head gasket. No need for a LS9 if you're not going boosted. LS2 will have thinner compressed thickness and smaller bore.

 

I have not bought any parts, my mindset is measure twice cut once, I will not buy any parts until I make sure everything works together. I went of off a proven build but I think over 7 years some improvements can be made. The reason I was thinking of a 58cc chamber is because the website that sells the PRC 227 heads offer what size chamber you want from 62-57cc and so I thought it would be a decent option. However changing the piston would be a better choice as I want the intake to still line up, with zeroing the deck and milling the heads that could change that. So good call on that. I’ll check for different pistons and see what I can get.

Thank you.

 

Edited my post above with comments about changing to a standard LS2/LQ4 head gasket.

 

why use a gen 4 motor when there are good gen 5s out there? a bolt on lt1 will make 480+ hp & tq all day on ethanol. my research leads me to believe that swapping a gen v motor would be much easier than a 3 or 4. also the ceiling for power is higher na. lots of hci sbe gen v motors making over 700 crank using off the shelf parts.

 

Well, LQ4 is plentiful and finding a block is cheap, an LT1 not so much. While yes it would be a better option, by buying an LQ4 I’d be putting more money in mods than money in the block. (By using the money I saved by buying an LQ4 over an LT1 for mods) I’ve also got many many guides and parts online to make an LS swap in the C4 much easier than fabricating for an LT1. So in the end I would be more comfortable with using a LQ4 block and building it for use with the Terminator X ECU. And it’s not staying the stock 364 sbc it’ll be turned into a 408 stroker.

Edit: I swapped the 4 and 6 in 364

 

i had an lsx c4 almost 8 years ago. you will have a lot of money in building an lsx. you can find gen v truck motors for 5-6k. you'll have to put a car intake manifold on it and probably c7 exhaust manifolds. gen v fuel system is easy, you can use the stock ecu with a custom harness. most of the external (cai accessories) stuff you'll need will apply to both engines. there is no excess fabrication required for a gen v motor.

just a suggestion worth looking into. the gen v motors make power. i've beat up a few lsx strokers in my 3700 lb m6 bolt on 2018 camaro.

also an LQ4 is 364"

 

Why do you believe a gen 5 motor would be easier to install?

The Gen 5's offer impressive fuel economy and power, but I typically prefer the simplicity and ease of tuning of the gen 3/4 stuff when going for big power. My preferred combo for a c4 would probably be a gen 4 engine with a gen 3 ecm using a cable TB. No doubt, with bolt-ons a gen 5 offers better power for a given displacement, but when high flow heads and aftermarket cams are brought into the equation that gap diminishes a lot.

Another cost-conscious alternative would be a cam & bolt-on LS3, which done right should get you 500rwhp+. But, this won't offer the low to mid-range power of the bigger displacement 408 stroker, or a bolt-on gen 5 LT1.

 

Thats the thing, it’s really just the block at this point. I’d be tearing it down completely to rebuild it with a different crank, rods, pistons, bearings, and bolts. I can find an LQ4 for roughly $600 and cheaper if it’s from my local pick it yard, which wouldn’t surprise me if there’s one in there. I’d rather spend the money in modifications and sudden emergency funds for things I didn’t know I needed than to be out $4,000+ for an LT1 over an LQ.

Money wise I think LQ is my better choice and you bring that out well that it’s not really a massive difference in power, the gap diminishes.

s346k is there really a $5,000ish difference in power? If it’s only 20-30whp difference I can’t justify that over an LQ.

 

if you are only looking for 500 whp, a gen v motor is the way. start pricing parts for a head/cam 408. just the rotating assembly is 2500. lifters are $600+ now, id guess pushrods are $300+. oil pump, bearings etc. there will not be a $5k difference in engines @ 480-500 whp, if anything the gen 3 stuff will cost more. the gen v motor needs no internal mods. flex fuel sensor and remote tune. not to mention the gen v motors run better hp for hp. itll take another 20-30 whp to match performance.

i say the gen v is easier bc the fuel system only requires a low side pump. returnless and the ecu regulates everything. a stock fuel system will support 550+ na. fuel system requires 1 line and 2 wires. ricks tanks offers some bad *** very simple gen v setups using the stock tank etc.