SStrokerAce

LT4 heads are a lot bigger cylinder head compared to a LT1. The stock head is 195cc to start with.

Bret

WS Sick

StrokerSS you got it accross so much better than I did, I think your knowledge probably had something to do with it.

SStrokerAce

Nah it's the BA that sits on my wall that says I somehow had a major that was half english half art. The knowledge about this just comes from osmosis.

Bret

1BadBrd

so if i walk into the machine shop, with some knowledge of what the subject is about, what should i ask for.

i was going to go in and ask for the heads to flow around 275cfms, but with your new info what else can i tell them to do to get the most fun out of the stock shortblock, cause after the heads i want to get a new cam so the heads will also be machined for better springs.

Does valve size play a lot in the flow. Curious how hard will it be to get the extra 15cfms.

My thinking is if i got the level 1 P&P ($700) would it be easy to add to what has already been done or will i be looking at spending another $700 to get 15cfms?

Fastbird93

Someone please correct me if I'm wrong, but I believe that valve size plays more into velocity than it does actual flow. Granted you'll pick up a little flow by just moving to a larger valve, but if you have too much valve for your intake runner volume, couldn't you stall out the air velocity??

SStrokerAce

http://web.camaross.com/forums/showt...hreadid=256986

This thread will help with some of the valve questions.....


It's not just the peak flow but the whole flow curve, the LT1 head is going to be hard pressed to make flow at some lift levels no matter what. It's hard to get much over 240cfm @ .400 without killing other parts of the flow curve. A different casting al la a LT4, AFR or TFS head is going to allow you to get more flow from .200-.600 and also get you a bigger cross sectional area (basically bigger port volume). A better casting means more potential in the porting. IMHO paying money for good porting and the right cam for the car and motor is the best thing you can do. That's where the big power difference is going to be.

Bret

smokindastangs

well if a 355 makes so much better power...why in the world does anyone stroke their motor?

Mindgame

Who says they do?

All things equal, the 383 makes ~10-20 more hp and ~40-50 more lbs-ft of torque. The problems come when you try to widen that gap. You need alot more induction work to make the 383 turn the revs.

-Mindgame

TCAL95Z

How do you measure the intake runner?

Also, Bret, Mindgame what do you think about the difference of flow numbers between shops??
I had my LT4 heads flowed by a local shop. They were Lingenfelter CNC'd heads.
I then sent them to a shop that is known for producing some nice LT1 heads.
This shop came up with much lower numbers. I had this shop do some additional work to the heads and in my opinion they look much better but I don't know squat.

Don't you really have to rely on the shop for their expertise because there are several factors that make up good heads?

I was told by the second shop that they have flowed several sets of heads that are supposed to have good flow numbers and they are not able to get those numbers????

also, doesn't a 355 need to be spun alot higher to make comparable numbers to a 383?

Thanks

1racerdude

Your 272CFM heads will make appox.560FWHp with everything else being dead nuts--cam quench,compression, intake tract,ect.

Mindgame

Measure what? Volume, mid-port length, what?

Man that's a toughy. I don't think there's any standardization in that industry. Everyone calibrates their flowbenches with different calibration orifices and others just inflate whatever numbers they get by some percentage. So your guess is as good as mine.

You are better off gaging a porter on experience and reputation. Not to mention, a proven track record for performance. You can't compare flow bench numbers for the reasons mentioned. The bench is just a tool porters use to quantify gains, identify trends, etc. Not a holy grail. You howver can and should look at average numbers. I'll post a link to a thread where Chuck outlined a formula for that...

http://web.camaross.com/forums/showthread.php?t=52950

If you get a chance, google over to Reher Morrison's website and go to their Tech Talk section. Read the article there titled "Flowbench Fallacies". It'll enlighten you.

As far as the LT1 goes, it's just another small block chevy cylinder head. I can't even count the guys who've been porting these heads for 20+ years to extreme degrees of success. So I won't just name one or two people. If I had too... you probably wouldn't even know their names.

Absolutely!

Yes, the 350 needs to turn more revs but don't get too hung up on RPM numbers. You are looking for piston speeds and those speeds are the product of rod/stroke ratio.

-Mindgame

TCAL95Z

Thanks Mindgame,

I was asking about if a stock LT1 head is 195cc how do you calculate that?

Do you have a amount of quench that you think is best?
Do you think it's really needed to have a piston at zero deck height?

Thanks

meengreen 94z

Wow this thread has been extremely helpful.

I have a question regarding reliability that doesnt really pertain to the other posts, but I thought Id ask. Ive heard that 6 inch rods shorten the life of an assembly in an LT1. What is the reason? I know in a 383 you can use a 5.7 or 6 inch rod, but in a 396 a 6 inch rod is the only option correct?

Mindgame

You measure the port volume just like you do combustion chamber volume. Use the valves to seal it up, get the intake face of the head level and fill er' up (water and a tad of detergent). You'll need a flask, beaker, whatever to keep track of the amount used.

I like to keep the quench between .035 and .045 on a steel rod street engine. If you're building an engine to turn really high revs... say 7500 or better, you're better off staying to the high side of that.

Piston doesn't "need" to be at zero deck, but it makes getting the quench easier. If the piston is .020 down in the hole, you're gonna have a big quench with any gasket you use. Plus, that deck surface needs to be trued when the machine work is done. On serious race engines, guys really tend to shoot for zero or just a tad above deck. Supposedly helps cut down on residual exhaust gas trapping in the cylinder but that's just the talk. I haven't seen evidence either way.

Good luck.

Meangreen,

The piston could be 5.7, 5.85, 6.0 or even 6.125. The compression height is getting tight with 6.125 and a 9.025 deck height but it can be done. To give you an idea of what can fit in there... take the deck height (stock is 9.025) subtract 1/2 the stroke and then subtract 1" from that. That 1" is the compression height of the piston... and 1" is about as tight as I'd want to go for a street engine.

-Mindgame

TCAL95Z

Thanks Mindgame,

My pistons are .010 in the hole. I decided instead of having it square decked again to get the .026 head gaskets instead of the .039
Also, because when I had my heads re-ported the combustion chamber went from a 54cc to a 56cc and that changed my compression ratio a little.

Do you see anything wrong with doing this? I ahouldn't be spinning my engine past 7k

Thanks