Yes i know LS3 head does not fit on top of 3.9" bore block, minimum requirement is 4". But lets think that some magic happens, and it fits.
How would stock LS1 engine behavior change? Sure there is some power increase at some point of power curve. But does it hurt power band, due airflow velocity drop? Is LS3 heads just too flowing for 350ci engine?
Is the engine streetable anymore? Of course top of the heads is LS3 intake manifold, with throttle body, suitable camshaft with good tune, and long tube headers too, but stock LS1 bottom.

I went to local engine machine shop today. I asked if LS3 heads could be modified somehow, that they fit on top of 3.9" bore block.
Shopkeeper mentioned that it is possible to install special valve seals that support smaller valves, without need to weld new material for the seals. Maybe 2.05" or 2.08" intake valves.
With smaller valves, airflow is choked a bit. Does this little choke point wreck the whole idea, and it is same to stay in ported cathedral heads?

Why am i asking this is money, and because i think its not done before. CPP CNC ported LS3 head are so cheap. And even with LS3 intake+tb+adapters it is still cheaper package then MAST small bore LS3 heads.
Machine shop told that cost for the special valve seals with labor is 35 euros/valve, about 40 dollars. Exhaust side valves should fit to lift inside 3.9" bore, but this should be verified too.

If this idea is potential, it supports my future plans too. I will resleeve block over 4" bore with darton MID sleeves, and forced induction someday, and all i have to do is install original LS3 valves and seals back.
My current target is somewhere between 400-500rwhp natural aspirated.

I also like to invent my own ideas. Doing everything what is done before is not for me.
Maybe my rear-end choice tells the thing. Under my 2002 Firebird is modified Toyota Land Cruiser 9.5" 3rd member, 4.11 ratios inside. custom axle tubes with C5 Corvette IRS ABS wheel hubs.

 

They make small bore ls3 heads for applications like that. No matter how much you get the ls3 heads for, all the special machining, parts etc will cost as much as a set of small bore rect port heads.

That being said every gm head design has its purpose and best use. Cathedral work well for stock cid and up to 4 inch bores. The ls3 heads do not gain much on a 4 inch bore, the valves are too shrouded and you miss out on the benefits. The raised port ls7 does exactly what a raised port head is supposed to do as well.

Use the rec port head and youll be fine.

This idea has literally been beat to death since 08.

 

But Like i say, it is a budget solution. Pair of heads 1356$, intake 250$, tb 130$, rockers 250$, injectors, adapters,etc. 400$ total: 2386$ still way cheaper than MAST heads, 2913$. And there is no need for special machining. Only valve seal replacement, and new intake valves.

 

The mast heads are already designed for the small bore and come with stainless valves (lightweight intake valve iirc) and springs as well.
The reg ls3 head port isnt designed for a small bore.
Gmpp also has small bore lsx heads as well. very viable option at $1700 for the set assembled. and that port is as cast so room for improvement but designed for small bores.

 

Those LS3 cnc heads are assembled too, only rockers missing. Combustion chamber difference between small and large bore heads is only 2.6mm, that is only 1.3mm at side, i dont think that is the problem. Only problem is that valve hits cylinder wall when open. That is fixed with little bit smaller intake valves.

 

And if i buy MAST heads, i still also have to buy intake+tb etc..

 

Werth, it's good to think out of the box.

I was recently reminded it's good to think in the box first however. I wanted to have the angled snout cut off an LSXRT intake and have it reattached straight. Turns out to be very complex to do that and a new snout would have to be molded. Cost estimate was over $20,000 because a new prototype snout would have to be developed and tested. It was a cool idea in my opinion but not practical. I think putting LS3 heads on an LS1 is likely very similar in complexity and cost.

when TEA did a set of Stage 2.5 LS6 heads for me many years ago, Brian Tooley discussed ideal valve size in great detail for my new set of heads.

If I recall correctly, a 2.04 intake valve is about the max practical for a 3.9 bore. The cylinder wall shrouds more and more as the valve gets bigger. Sometimes 2.08 valves are used but to really get benefits very high lift is needed due to shrouded valves. Clearance can become critical too.

The 2.08 valve worked very well with 4 inch bore and bigger, gaining as much as 15-20 cfm at the higher lifts of .600 to .650 compared to the small valve. Big bore is where it's at.

As for just replacing the LS3 2.165 intake valve with a 2.08 intake valve, I would think the entire valve job would need to be redone for best results. Likewise, a lot of testing for the correct valve angles in this custom set up would be required for best results.

For a 5.7 LS1 I think the 260 cc stock LS3 intake runner is way too big. For a 5.7 LS1 intake runners of 245-250cc are usually too big for unless the motor is a very high rpm screamer.

The LS3 chamber volume is 70cc this would result is a big loss of compression compared to a stock LS1 head. This would be terrible for a normally aspirated NA application. The LS3 head could be decked and / or have the chambers welded up and re-machined to fix the issue but that's not a budget friendly fix. If forced induction was being used the reduce compression might be OK but a good set of 317 heads would be more budget friendly.

Personally, I would spend my funds on a set of well proven quality CNC'D 243/799 LS6 style heads from TEA or a set of TEA CNC'D 5.3's or PRC 5.3 before spending money trying to convert a stock LS3 head to work on an NA LS1.

 

Again you would be buying cyl heads with ports designed for a large bore engine. Putting a smaller valve in does not change that. Cyl head cc has little to do with combustion chamber design also.
The gmpp small bore heads use a port that already flows very well. its as cast. their lsx-ls7 head has a cast port as well that nearly mirrors the design of the stock cnc ls7 head and the lsx-ls7 head has room to grow.

Using a HUGE cnc rec intake runner on such a small bore would not be streetable unless you have 4.56 gears+ Youd see little bottom end and likely have to run a wider split and quite a bit on the duration side as well.

You are trying to overengineer what doesnt need to be done. You'll do just as good with a set of ported cathedrals for that small bore under 3k.

 

There was a sponsor that sold small bore LS3 castings. I'm pretty sure they just cut a valvejob and used a smaller valve, but they did have some results posted and they didn't seem to be all that stellar. They're not around anymore.

 

Chevrolet Performance 19201807
Part Number: 809-19201807
Chevrolet Performance LSX-L92 Small Bore Cylinder Head, Assembled

LSX-L92 Small Bore Cylinder Head
250cc Intake Port / 80cc Exhaust Port
70cc Combustion Chamber
2.00'' Intake Valve / 1.55'' Exhaust Valve
Assembled, Sold Individually

 

That is what i had mentioned.

 

Wouldn't the cost outweigh a 4" bore short block?

It would seem that a 4" bore short block would be cheaper, easier, and more cubes.

 

Do you mean that is it cheaper to buy big bore block than resleeve small block? I said resleeve over 4" bore. maybe 4.125", with 4" stroke, it is 427ci. Then resleeving is much cheaper, and more reliable.

 

I hate to tell you that if your on a Budget and talking this way, Your Dead wrong thinking it'll be cheaper than buying a aftermarket set of heads. The only reason to do something like this is:

1st) You wanna prove a Point, by not buying a aftermarket name set of heads. Also to talk $h!7! The later is the main reason. As all or 99 % use or have the same heads.
2nd) Have the Only set of one off heads in the World but they must be able to Compete Vs the after-maket for the time and $ wasted.
3rd) You have cash to burn.

Your cost in heads will be around $2500 with out parts to be Done Correct. The normal CNC program will not work as the valve & bowls have shrunk also the runner size Would LIKE to change too, due to a smaller valve size. BTW you'd need a top head porter to do this to not feel like you got Screwed with facts of velocity,cfm along with alot of other things. BTW taking the larger runner of a ls3 and placing a smaller valve. LMAO! After all this, would you still like to do this? I don't believe your about that life. Stay among the normal way off doing things. So these valve seats your talking about ask your guy what's to be done about the Bowls being larger than the valve itself. Find a new guy!

 

Here is the heads i was talking about: https://www.summitracing.com/int/parts/nal-88958758 It is 1356$ for pair, and allready CNC ported. Why head ports should be ported again? I know LS3 head ports are not optimal for small bore heads, so what?. First question was does the engine even run, or how crappy the power curve is. Sure there is choke point at intake valve, but let it be there. If i had lots of money, i would buy those heads and test it out. If it outruns ported 243, or equals with aftermarket heads, then test is success. Machine shop guy said it is possible, but he did not mention how profitable it is. He is doing what the customer payed for.

 

Ok so there ported. But I don't think you get the point. The way your going about things the HEADS are wasted. First off so there ported so things are bigger Correct? So these New seats you speak of go in right, They can Only OBSTRUCT flow which is HP as the Bowls have not been re done to compensate for the smaller seats. Which Requires Welding. If your gonna do something Crazy @ Least do it Correct.

 

You are right, smaller valves, needs smaller valve seats. So the valve seat inner diameter is tightest point of head port. What this restriction does for the engine behavior?

 

This Crazy guy took you at face value and did it Correctly, just with Ls7 heads.

http://www.yellowbullet.com/forum/sh...+07+bore+a+6+4

 

Really don't know or care too know brother (not being rude, just facts) it just isn't right. Let someone restrict your air flow to you as a person. Well how do you feel about your engine? No tap out in me, I'd like to Breathe so let there not be any restriction on me as a person or my engine. Thank you! Also the heads are I'd say around 99% of the induction to me. I have common sense to know what intake to choose for the other 1% Welding is required to correct everything. Matching bowls/throats/valves & seats with a runner size or *CSA* for the intended Bore size. We want all these things to match which cost $ to re-engineer. BTW your CSA would & should be tiny. You could try a small intake duration lobe with some lift and fire *compression. Still a waste of $. Don't jump in that life if you are not ready. Just buy the correct heads.

 

The smallest point in the head is usually a little further into the port between the short turn radius and the valve job. It's usually called the Minimum Cross Sectional Area (MCSA), the Venturi, the throat, etc. Generally speaking, a larger MCSA will move peak torque and usually the entire powerband to a higher RPM. Smaller MCSA will do the opposite.

The MCSA of a stock unported LS3 head is just about right for a 2.080" intake valve in a performance application, but it's big for a 347ci street engine.