How much HP in the LT5 cylinder head?

[Karl]

I have been comparing the gains from ported cylinder heads, and ported cylinder heads with cam work on the lt5 and it seems the upgrade is worth between 30- 45 HP depending on the year, and a camshaft(s) upgrade another 15 -20 HP?Are the same ported heads that make the 350c.i. LT5 so strong the same that power the larger engines? 380, 415, 402, 441, etc, or are the heads ported even further?How much air do these heads flow?

[rkreigh]

the LT5 like many big v8s is definitely "flow limited" by the heads. even with the solid design the ports are actually pretty small and designed for velocity and a broad torq curve which is what really is what its all about for other than top end power. on the stock engine, the car really noses over around 6400 and "flattens out as it approaches 7k.

on my car, which is a bit old school (LPE 390) the car will peak at 7100 at 619 on the engine stand, and is down to 616 at 7400 and this is with

bifurcation intake (sawed in half and ported)
stage II cams (still streetable with the 390)
big valves
moderately ported heads (not as aggressive as modern cnc porting)

so lets assume my heads flow around 320 cfm (about right)

even the lowly LS6 heads start around here and go up to the LS7 which is 370 (with admittedly pretty BIG ports)

so IMHO while the LT5 is a great design, it makes power more on "flow quality", velocity, tumble, and overall good mixture with the closed leaf combustion chamber and dished piston than it does with ultimate big CFM numbers

much like the dyno, a flow bench doesn't tell the whole story.

now fast forward to the moder CNC porting age. Pete, Haibeck and others have gottem much more agressive with the porting and learned much more about effective cam timing.

pete is running as quick with a stock block as most strokers. but the limit of the castings is that around 36 mm or a bit more I believe is as far as you can take the ports without getting too thin and getting into the water jacket.

alot of experimentation has been done with better intakes, and unfortunately there doens't seem to be a whole bunch of power to be gained there with what has been tried. I'd love to see an LT5 with a crab style individual runner cross tube intake like the C5R or even a better intake and head design like the MY 95 LT5 that never came to be. both I think would help top end power at a pretty big sacrifice on low end torq (of which not much is to spare on the LT5)

it would be great to reproduce the more modern LT5 MY95 head or better still a new design. we at least have those castings so they could be traced and whittled out of billet on a cnc (not cheap but hey, an LT5 never is!) and with the limited market, there is no business case for doing it.

I seem to think that around 700 hp is going to be the upper limit (flow limited) of how much hp these heads can feed. smarter folks will prove me wrong I hope.

so for me, I think turbos are the way to go to "crutch" the relatively limited cfm potential of the LT5 heads.

[XfireZ51]

I think it says a lot about the LT-5 head design when it is capable of feeding (perhaps inadequately) motors from 350 to 441CID making over 650chp NA.
One can only imagine what the motor is capable of with better airflow on the larger displacement motors. Given how quickly and "easily" you can pick up
50-55 rwhp on a stock cammed motor, it suggests how overbuilt the the LT-5 really was.

[Pete]

I would have to agree with Ron on the LT5.

There was someone wanted to make a 500ci LT5 the issue is it won't make any more power at least not per cubic inch wise, I think the heads can't support it.

Most I've heard of LT5 heads flow was 365-370 these were siamesed half way down.

A set of nicely ported heads will flow 340-345.

700-750hp is not too shabby for a 93 octane N/A 427 and very nice street manors
I don't know of too many 93 octane N/A street cars that make 750hp
I'm from Missouri show me.
Pete

[XfireZ51]

Pete,

And that's been one of the advantages of a DOHC architecture over OHV. You get top end breathing while still maintaining a decent idle. Of course, the OHV design narrows the gap on this by going to VVT. I guess I decided on the LT-5 when AL showed me his 441, which runs mid-10s, sitting idling at 850rpm.

[Kevin]

it's great to have ron posting again.

[Paul Workman]

I believe Marc Haibeck's web page lays out the performance vs. what modifications pretty well, especially on the 350 motors, to give one and idea what the LT5 does in real life.

Graphing my own 500+ chp 5.7 torque, there was significant gain across the spectrum between stock and 36+mm porting and relief and free-flowing exhaust (SW headers, X pipe, etc). (graph is fairly representative of similar "500" packages - least up here.)



Torque roll-off of the big inch LT5s, from a few limited graphs I've collected (Marc has more on his site) look something like this 441 LT5 (in development - not final form). I think I see where breathing through small 5.7 heads, even with bigger cams and valves (intake), begins to be a factor: note the difference in the torque curves for the 5.7 vs. 7.2L



What we wouldn't give for some bigger heads; especially for 4"+ bores! What would they flow, compared to 2-valve OHC configs? Imagine VVT for DOHC in a big inch V8!

Yeah... IF ONLY!

Good discussion.

P.

[XfireZ51]

Someone asked me recently what were Pete's cams worth. Of course, not as much if you don't make the complimnetary changes like head porting, exhaust etc. I went back and created a graph thru WinPep of the various stages my motor has gone thru. Use whatever you want for the stock engine numbers, but here you'll see the evolution of the power from various changes to the LT-5. It starts with Top End port to ported heads/intake cam, to Intake/Exhaust cam upgrade. As Paul says, its not just the peak numbers but more the area under the curve. These are SAE corrected. The raw(actual) numbers for the top line were 465/411rw.

[rkreigh]

Originally Posted by Pete

I would have to agree with Ron on the LT5.

There was someone wanted to make a 500ci LT5 the issue is it won't make any more power at least not per cubic inch wise, I think the heads can't support it.

Most I've heard of LT5 heads flow was 365-370 these were siamesed half way down.

A set of nicely ported heads will flow 340-345.

700-750hp is not too shabby for a 93 octane N/A 427 and very nice street manors
I don't know of too many 93 octane N/A street cars that make 750hp
I'm from Missouri show me.
Pete

so Pete, what's the "next big thing" you have learned so much from all the work on head porting and cam timing and really delivered the goods.

like to see what you would charge to freshen up the LSV with improved porting. I'd like to see what I could do to get to around 550 rwhp

I think it might be tough to "add that much" Hp as the LSV is already pretty good.

thinking of doing the aes liners, better pistons, keeping the ti rods and current stroke (just a hair more than the 385)

car is starting to "feel slow" and every time I run around in the turbo car, I year for just a bit more in the ZR-1

not sure I can afford to turbo it yet, but I'm thinking a freshen up could be done much cheaper as I already have most of the good parts in the engine

might even be cheaper to sell my current short block and just get the AES short block "done"

like to hear your thoughts, pm me your phone number if you'd like to talk.

during the "winter nap" the LSV beast gets hungry during hibernation....

[tpepmeie]

Believe me you can get well north of 700 chp with LT-5 heads. I am over that now, with smallish 39.5mm inlet valves. Heads can flow a ton but a) you lose at least 25 cfm through the convoluted injector housing/plenum tract, and b) you have to run enough cam to use all the flow anyway.