pitzzilla

I would like to know if rasing up the compresson in a TPI engine would affect its performance or any other way does it make a diffeance what heads you can used on a TPI or would I need speical TPI heads

F-BIRD'88

raising the compression ratio on a TPI motor or any other motor will increase power and torque. You can expect a 4% increase in output for each whole ratio increase.

There is a practical limit thou. An engine with too much compression ratio for a given octane fuel will detonate at WOT which reduces power and will damage the engine.

So it all depends on what fuel you'll be using as to how much you can raise the compression ratio to good effect and how much is too much.

Do not compare other engines with different design like a LT-1/ LT4 or LS1 engine to your TPI motor. There are specific reasons why these motor want and will tolerate more compression ratio than your motor will.
(combustion chamber design, induction system tuning, cooling system design, EFI/timing management)

For a TPI motor: for 92 octane fuel keep it just under 10:1 with a cast head
and just under 10.4:1 for a aluminum head.

You can get good effect by decking the block to decrease the quench clearance (minimum .037") and raise the cr a little. they both have to work together to work well.

What did you have in mind? what other mods are you thinking about doing? What octane fuel will you be using ( every day)? Its all about details and making the whole thing work together to get a good result.

pitzzilla

the reason I was asking I was told that I am able to place TPI 305 heads on my 350 but it will raise the comperesson to 11 and that the tpi head are diferant the regular heads ??

F-BIRD'88

305TPI heads have a 58cc combustion chamber which is a little smaller than stock 350 L-98 TPI cast heads (64cc)

So they would raise the compression ratio a little, but not to 11:1. (unless they have been shaved and the block decked a lot.)
I built a 350 with max shaved 305 heads and extreme decking and a thin head gasket and got 12.65:1 measured compression.

Generally for best results, you would want to replace the stock 1.84" intake valves in your 305 heads with larger 350sized 1.94-2.02 valves, possabily 1.60" exhaust valves which requires the chamber wall to be clearanced to fit the larger valves. This "deshrouding" will increase the chamber size closer to stock 350 64cc spec. So your 305 heads are quite usable on a 350. With larger valves and some porting to improve flow they will work well for ya.

You have to take all the variables into consideration to accuratly calculate your engines actual compression ratio.
You have to measure these variables first.

cylinder head combustion chamber volume, piston to deck clearance, head gasket volume, piston valve relief volume.
Until you know these specs your engines cr is just a guess.

Generally a 350 with flat tops and 305 heads will not have excessive cr.

rx7speed

even after you unshroud the chambers on the 305 heads though don't and get close to that 64cc chamber don't you still end up with some not so well flowing heads when compared to a 350 head being that I would assume the ports are quite a bit smaller.


and then lets say you do port the ports out that is more money.

just wondering wouldn't you just be best off taking a 350 head and using it instead?

pitzzilla

here what I would like to do use my rebuilt 350 which has flat top pistons ever thing esle is stock I what to use the heads and the cam from the 305 TPI engine on the 350 since they are new but stock the 305 is crack beyond repair

also the head on the 350 are renuilt but mis matched as to the casting numbers. I'll get them to you in the next response. will the set up work with out any problems?

Whay would you do?

pitzzilla

The 350 Casting # on the heads are the following 3973487 & 462624 what valve are in this and they R miss matched LMK anyone The 305 TPI casting # is 14014416

F-BIRD'88

Well ya know what they say about assuming stuff....

No the ports on stock 305 heads (#416) are not smaller. Infact they are bigger. When ported with the larger valves (194x 1.60) I get over 240cfm intake and over 180ex. A stock 305 heads even with the stock valves flows very close to a stock 350 head. about 190 to203 cfm.

Even if you only achieve mid 220's cfm (actually mid 230's is more typical of an honest home port effort) you'll have a very good street performance head for not too much $$'s. they will make over 400hp on a 350.
Much better than stock 350 heads. A nice budget upgrade to replace typical junk 624, 882 low compression 350 heads. There are 4 benefits to starting with a 305 head over an old 350 head. on a budget build up.
1 Compact combustion chamber. 58cc, 64cc when reworked.
2. Good unrestricted port size. good flow when ported.
3. Hardened exhaust seats
4 $$Readily available and cheap usable cores$$ I've never come across a unusable 416 casting. Most people like your self assume this and that and don't realize the potential of these heads, so they turn their nose up at them. For a budget/performance 350 in the 325 to 400hp range, they are just right.

Damon

Hold on- flattops in a 350 block but using 305 heads?

That's higher compression than putting 305 heads on an original L-98 (which used dished pistons). Even with no block decking and a rather thick Fel Pro .039" head gasket you're up at 10.3:1. Use a thinner gasket like the GMPP .028" gasket and you'll be at 10.6:1. With a thin steel shim head gasket you'll be at 11:1

I think you're not going to like this combo if you do it. I think you'll have a very detonation-prone engine combo on pump gas.

Sonix

iTrader: (1)

I did this, but opened up the 416 chambers to 64ccs, and decked the block to .015" deck height. Giving a finally tally of 10:1 static CR.

"shrouded" isn't really as black and white as it seems. It's not like shrouded, shrouded, shrouded, boom, unshrouded. It's a matter of *how shrouded*. I think even if you have stock valves, and open up the chambers in *right spots*, you can deshroud it more, and gain flow. I did this, and went with 1.94/1.5 valves, and polished the chambers and ended up with 64cc's. Amazed my machinist who thought there's no way I could take out 6cc's...

F-BIRD'88

Damon: here lies the false folk lore and mis-information.
Did ya get the part about fitting large valves? Increases the chamber size. You end up with a low to mid 60's cc head when done. The last set I did speced out at 61cc after unknown resurfacing cut to clean up decks.. Could have took much more out of the chamber valve deshrouding if wanted to. the motor I'll be installing them on will be 10:1

Seen 66cc finished chambers ( buddys heads)

F-BIRD'88

nice job, any track times?

rx7speed

on avg though wouldn't the typical 350 head have larger ports on both the intake and exhaust. it does generally feed a larger motor. also with the head setup if it came with 2.02/1.60 valves it would be a lot easier to work with I think. not saying all 305 heads are junk but if you get a decent set of stockers vs even a decent set of 305's I would think the 350 heads would be a better option.

which heads are you comparing your 305 heads against or would they be better then any stock 350 head?

for a very budget minded aproach with limited options I can maybe see it but I don't always see people going out to buy 305 heads all the time vs a stock set of 350 heads unless their 350 heads are nasty smoggers.

F-BIRD'88

Just so ya know. the actual as cast intake port volume of 416 heads is bigger than just about all stock sbc heads except any head with a X in the casting number like 461X 462 has smaller port volume 441 smaller 882 much smaller LT1 are larger lt4 are larger. bowtie larger.
exhaust is another matter some like 336X are bigger.
After they are all ported it doesn't matter.
Most people assume what they want and turn their nose up at these heads. But people that have messed with them a bit know better. many knoledgeable engine builders use these heads (in modded form) for "$budget buildups$" on 350's. Nothing reinvented or new here. They work very well.

Damon

No, I didn't get the part about bigger valves. I wasn't sure the original thread post involved seeking that kind of work. I was merely addressing what would happen if you bolt stock 305 heads on a typical 350 with flattop pistons. For sure, any kind of chamber work to any set of heads will only make them bigger, including installing larger valves.

Sonix

iTrader: (1)

"350" heads, as you would normally find are 90% smoggers, 76ccs. The newer ones that aren't smoggers, are far more rare and hard to find. They're usually still powering the car....

F-bird - nah, I lost the cam, so i've got some downtime as I rebuild the motor (bearings got shot). I got the O2 sensor to help tune the carb once the motor is back together though. I'm kinda rushing it, should be ready to run for next weekend, then we'll see.

F-BIRD'88

here is an example of a very typical 350 budget rebuild using about the most common federal mogul rebuild kit with flat top pistons going.
pistons are 5cc 4valve relief and 1.54" compression height. #H345p-30

Specs are 4.030" bore 3.48" stroke .045" piston to deck clearance @TDC felpro#1003 head gasket.

with stock 76cc heads the measured cr is 8.31:1
with 64cc heads the cr is 9.31:1
with stock 305 heads (58cc) the cr is 9.92:1

ok with 93 94 octane. needs slightly cooler R42T plugs.

With say, hummm; my modded 305 heads (61cc) 9.60:1

Just trying to dispell some myths,assumptions and misinformation with some facts.