pushrod-v8

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?

Dominic Sorresso

You should ask this on the registry. More of the motors with various configs that could answer this question.

Rkreigh

head porting definitely needs to be matched to the displacement and cam timing. putting a maxed out head on a stock displacement motor won't give you the same power obviously as a stroker

a decent head will flow around 330 cfm although I'm betting the new porting from the FBI gang is a bit better, but I honestly don't see lots of flow ratings on LT5 head

the 4 valve head has alot of things going for it, bigger valve area in comparison to 2 valve, better flow velocity (which isn't easily measured) 2 small ports is MUCH better than one big one.

great questions, and yes, post on the reg and you'll likely get more tech response.

like most engines, the lt5 really comes alive with the right porting. I think your estimates are a bit high, but not too far off. keep in mind the earlier engines benefit more as the injector housings have the bigger injector boss which can be better streamlined to give more of an improvement.

the 93-95 heads, injector housings flowed a bit more already so they don't increase quite as much when ported.

I think pete of the FBI gang might have some updated flow numbers to see how far off I am.

for comparison sake, the LS7 head STARTS at 370 cfm, so we have some catching up to do! sure wish someone would tool up some billet heads for the LT5. love to see nelson racing or someone do it and go to the much improved MY 95 LT5 design graham showed us.

that head flows quite a bit better. since we have the castings, replicating them wouldn't be such a stretch to do and the intake could be fabbed from sheet metal without too much trouble (just takes cubic $$$$)

cv67

Nelson did make some heads if I recall that went with his billet block nice stuff

Me neither. Seen a recited max # maybe 2 thats it. Doesnt really tell one much
Supposed to do a set when I can free up some time and will make a fixture/post true before/after #s. Probably sometime this fall.

There are heads out there that really dont leave much on the table. If they are well engineered a minor touch up is about all you can do short of....look at the japanese cars/bikes for example

Youll only get so much throuhg a given valve size. 2 good ports with the right air speed will trump a huge single port as Rkeigh said

Think most that port these heads guess at it or dont want to know

wdo-mkr

Well. My heads are north of 340cfm and are max port with smaller than stock valve stem titanium valves. The car has put down 590+ and is a 415 stroker. Frankly it could use a slightly larger intake cam and less on the exhaust side. Both are sgc stage 3.

Dominic Sorresso

Any chance you'd drive down to Chicagoland and join us for the wannaGoFast 1/2 mile on the 22nd June!

8388

The pent roof combustion chamber design, also allows for higher compression, which another place to find power.

lgaff

Roy Midgley handcuffed the LT-5 when he forced lotus to a 4.4 bore center vs the 4.55 Lotus wanted. This caused Lotus to go with smaller valves and gave up 25hp+ on the original motor design( more shrouding).....that's the catch 22 with DOHC motors, more valves but also the possibility of more shrouding...Ford did the same with the 5.0 Coyote Engine.

Kicker is that the 4.4 was done just to match the sbc #s not because it had to be....

Dominic Sorresso

Lee,

Lotus also wanted a wider angle between cylinder banks for a "flatter" motor but the compromise involved production line compatibility. GM wanted to use the same bore machines, therefore the 4.4 spacing and the motor had to go up through the frame rails on the assembly line.

lgaff

Bore Center is the the centerline of a cylinder bore to the centerline of the next cylinder bore; not sure if that would require a wider angle.?...from what I read it had more to do with being the same specs of the traditional SBC....

During a technical review at Lotus, however, Roy Midgley, then chief engineer of CPC's 90-degree V-type engines, found something he didn't like. According to Rudd, "Roy spotted the bore centers were 4.55 inches, and he said, 'Hey—you can't do that.' He said it had to be 4.4 inches, like the small-block V-8."

Rudd tried to tell him the bore centers for the LT5 were relatively meaningless, because the new engine wasn't going to built on the same production line, and certainly not with the same tools, as the small-block. "It can be any bore center," Rudd told Midgley. "No, no it can't," Midgley said.

"You do appreciate what this is going to do to you, Roy?" Rudd inquired. "We can't promise you 400 horsepower anymore, because the bore has to be smaller so we can't get the size of valves we want in." Midgley mulled this over and said, "4.4-inch bore centers. We'll accept 385 hp." That's how the high-revving DOHC LT5 ended up with a smaller bore and a longer stroke than the elderly pushrod V-8.

lgaff

It would be interesting to hear from Mr Behan if the stories about the 4.4 center bore being a number to please the suits is true or just hype

Dominic Sorresso

Lee,

I think the bore spacing and the V angle were two different issues and not related, but yeah it would be cool to hear what Graham has to say about that conversation.

bb62

I can fully appreciate why the decisions were made with respect to bore centers (among other parameters). While I was not on the Chevrolet team that made these decisions, I did see the decisions that were made over at Ford when the Modular engine was being developed, many of these decisions being similar to what happened with the LT5 - decisions questioned by Ford fans these days.

Consider:
- The LT5, when in early concept phase, was very likely planned to be manufactured at some point by internal GM facilities. The Mercury Marine connection was primarily to get the program off the ground.
- All OEMs hate the idea of a single engine architecture used only by one vehicle. Look at LS9 engine being placed in the Camaro and the Caddy - certainly GM doesn't mind using its high output engines in non-Corvette aplications. The Midgley decision didn't orphan the engine as the larger bore centers would have. Certainly at that early stage in the engine's development the planners would have considered a number of uses for the LT5 - but not for a $30K engine. Other decisions later by GM orphaned the LT5, but not Midgeley's.
- The production machinery that dictated the bore centers is incredibly expensive. TO give you a sense for how risk averse OEMs are about changing bore centers, Ford's domestic 4-cylinders STILL use the same bore centers from the Model T days.

Graybeard ZR1

I've seen this issue discussed for years, and that's the best assessment of it i've seen.

cv67

Id like to get my hands on a head, make a fixture flow it stock and ported also. Maybe add a larger intake valve and do it again

Ive only seen one or two max lift #s posted which dont mean much without seeing the rest. LT5 just doesnt get enough attention. Wondering if the few heads that have been flowed were done without the valve in it which skews the reading. Most shops wont want to spend the time or the owners the money to flow them right.

Im betting more power can be manipulated by playing with cam timing/opening and closing events they are just so expensive to have made.

Maybe once my move is over I can find a volunteer.

z06scentair

My assessment has been more streamed toward GM being able to use the 4v head on other V8 motors.

This was most certainly the reason why the 4.400 bore spacing was chosen, not limited to production machining capabilities.

glass slipper

GM has no other 4.4" bore center V8 engines the LT5 4V heads will fit on, it was absolutely not the reason 4.4" bore centers were chosen. I have no idea what your assessment was based on...

glass slipper

What production line machinery would the LT5 engine block have in common with the small block Chevy 4.4" bore center? The LT5 cylinder liners are forged aluminum with a Nikasil coating, they are already bored/honed before installation. LT5 cylinder liners are then installed into the cylinder block with sealant. Gen I and II small block Chevys cylinders are bored/honed while Gen III and IV small block Chevys have cast in liners or pressed in liners that are honed in place.

Like Rudd said: The new engine wasn't going to be built on the same production line, and certainly not with the same tools, as the small-block. "It can be any bore center."

So once again, what production line machinery would the LT5 engine block have in common with any other engine GM has? The answer is zero...therefore it can be any bore center. Midgley's demand for 4.4" bore centers was completely arbitrary as well as capricious.

A 4.55" bore center would have given 4.05" bores stock and allowed 4.25" bores with cast nodular iron sleeves and 4.45" bores with siamesed C350 steel sleeves. We are left to dream of what could have been...500+ cu in LT5 engines!

z06scentair

All SBC engine blocks were on a 4.400" bore center?

bb62

glass slipper,

That you could use the words capricious and arbitrary for the Midgeley decision tells me that you don't understand the OEM powertrain planning process. Some things to consider:

- Do you really think that GM was going to accept being saddled with a single use $30K engine?
- Do you really think that at the early stages of this engine development that mass production wasn't on the table?
- Are you aware of the efficiency break points for scaling up production for an engine with unique block and heads?
- Do you think that the engine assembly line is the only line in an engine plant? Do you think that installing the liners would be the only requirement of a particular block line (if boring and honing were not required)?
- Do you really think that the block bore and hone facilities were the only facility savings that would be driven by unique bore center?
- Have you considered what long term engineering support is required for each and every powertrain architecture? What would this do to the budgets of all the other powertrains?
- Have you considered the impact of union labor on the production process for a higher production engine?

That Ford produces a 4 valve per cyliner engine for it's performance oriented vehicles and has a separate assembly line for those engines (within plants that also focus on the high volume entries) does not obviate the need for many common components, processes, and facilities - often driven by bore centers.