I'm in the progress of having my LT1 heads ported. My porter currently has them flowing the following:

Intake
.200 127
.300 188
.400 230
.500 258
.600 261
Exhaust
.600 190 (only one I know off-hand)

This seems to be about consistent with most "Stage II" LT1 porting jobs. However, my porter says he can get the intake up to about 270 cfm for $100 more.

Would this be worth it? Or would I have to rev my engine higher to take advantage of it? Does higher flow help performance at all RPMs or just allow the engine to breath at higher RPMs?

I'm going to be running either a 230/236/110 or 236/242/110 duration cam in my 383 LT1 with 11.5:1 compression if it matters. The rest of my setup is in my sig.

Thanks

 

If all your intake, valvetrain and exhaust components can support the flow level, then you can never have "too much" cfm, but it is important on how and at what combination of rpm and valve lift the air is delivered, and how your porter plans on getting extra cfm. Obviously, a larger port can deliver more air, but velocity at lower rpm and valve lift usually suffers. The goal is to flow the most cfm with the smallest runner as possible. 270 cfm @ .600 valve lift probably wouldn't help you out too much unless you had the cam and valve train to support it. Decide where you want your power and how streetable you want your motor, then talk to your head porter and tell him the intake, cam and exhaust you will use. Any reason you went to a carb over the LT1 intake? Just as an aside, a wet manifold (carb) needs to flow about 10% more than a dry manifold (PFI). Food for thought.

 

The intake numbers you have are pretty good and will support up to 525 to 550hp with open headers. 270 over 261 is only 3.5% increase in flow and not add much hp no matter what kind of cam you use.

 

The biggest thing is combination. You only need to flow as much cfm through your runners, that your carb and intake can support and also that your engine needs. High flow numbers can actually hurt overall. If your "port man" is just increasing port size to increase your flow numbers, is he doing so correctly? I've seen many port jobs from "reputable porters" that actually ruined the guys cyl. heads. They had good flow numbers on the flow bench, but on the motor with fuel introduced into the mix, horsepower and overall drivability was gone! Make sure that your "porter" is well based in wet airflow as well as runner velocity. Does he have the ability to flow your carb?, your intake? Can he flow your combination together? How much material is he removing from where? Did he increase port width or height or both? When he increased your port height, did he remove much from the floor area? What has he done with your short side and long side radius? How much increase in flow came from your bowl area? Make sure he can tell you what he has done and why! If he works alot on the floor and floor configuration or widens your port, you might want to see some of his other results!! These are two of the most critical areas of the port that can increase "flow #'s", but decrease velocity and "wet flow"! Did he ask for your complete cam specs, as well as does he have your intake manifold and carb? If not, ask him how he knows what your set up is going to require as far as port desing/configuration is concerned. I'm alwas a little "sceptical" of "porters". There are a few good "port men" out there, but most "potrers" I've ever met were guys that should be in charge of the restroom!! Be sure to ask question, I've done alot of port work for about 17 years, and you'd be suprised how many guys just drop off there parts, and never even think to ask or see anything! It's your hard earned $, make sure you get what you pay for! Anyway, to answer your question, yes you can have to much, but if the combination is right, you can use alot!! :cheers:

 

wow sinister :p: that was interesting stuff. My heads are getting ported now... it really got me thinking. I went with porting lt1 heads with lt4 intake because of revised injector location. The porter tells me my heads will flow about 270 cfm @.450 lift. He says low lift is the key and he seems to know how to do it. He also picked the cam for me, a solid roller that will go 7200 rpm. he said to stay with a 2.00 valve and not go bigger unless we build a stroker. Bigger valve = less velocity. He seems to know better than anyone in canada thats for sure. What do you think ??? he also built a 383 stroker LT1 .. the thing put out 418 rwhp!! i watched the build and the dyno runs. The motor sounds like a ferarri motor when revved... he says its the airflow that makes the sound. anyhow i just wanted some input. thanxx! :cheers:

 

270 cfm at .450 lift is impresive! I've never done a set of lt1 or lt4 heads, but I've heard that they can be massaged and will really put out. Low lift flow is usually what a street/strip type motor will get the best results from. How many guy will run .600 or more lift in a s.b. street motor? FEW! I myself, before venturing into this "EFI/TPI" world, do tend to run more lift than most guys. Why. Lift = Flow. I'm luck to live jest a few miles from a company that grind camshafts for the auto industry! I've got a couple friends that work there and can grind me what I want if I've got time to wait. I've found that .530-.580 lift is excellant for building hp in mid(3000) to high(7500) rpm range, but it also has so much more hp at 6000 rpm where most smaller lift cams are peaking, that is is worth going this route. I tend to go with a shorter duration if possible, and also I like 106-108 lobe centerline. If you can also keep overlap as low as possible, you can get your idle and throttle response back! I'm not a big 383 fan,sorry. I know there are some out there that are impresive, but most of them are torque motors, and I like rpm's and high horsepower. I built a 383 with 11:1, performer rpm alum heads(out of the box), .525I/.530E lift 254 dur @ .050, victor jr. intake, I built a 750 dp holley(flowed 810cfm) 1 3/4 hooker super comp headers, t-350 with a 9" 4500rpm conv., 4.11 gear 79 camaro. Car weighed 3620 with driver. best et was 12.65 @ 118.2. Guy was happy, but I thought there should be more. I took him for a ride in my 81 trans am. I've got a 377(400 block+.030 and 3.48" 350 cranck) 11.5:1, 292 "turbo castings" cast iron gm heads, self ported, .565I/.560E lift, 248I/255E dur.@.050 106lc. Bowtie int. self built 750dp(flows 830cfm), 1 3/4 supercomps, 2 1/2' exh. Super T-10 close ratio, 4.88 gears. TA has gone a best of 11.02 @ 127.6. My car weighs 3755 lbs with me in it, still has functional A/C!! The 383 is very strong to about 5800/5900 rpm, and seems to fall fairly fast. My 377 runs strong all the way to 7000. I shift at 7000, and limit at 7300. This thing is alot more throttle responsive that the 383, and loves to be wound up. I leave the line at 3800 rpm, have a best 60ft. of 1.48! Brian leaves at 4300rpm with trans brake, and his best 60ft is 1.56. Ive got him covered the full length of the strip. This winter he is buying my 377, I'm selling the TA. I bought a vette, wife says to sell TA!! I'll miss it, but not for long if I can get this vette to run. I'm hoping for low 11's. If I have to, I'll put this l-98/4+3 in my wife's 55 chevy, and I'll build a carb motor for my vette! It sounds like your guy has you going in the right direction, but why such high rpm cam when he's concentrating on low lift flow? Usually you have a high lift cam for high rpm use. Just a thought. Good luck, let me know what you end up with. :seeya

 

The previous owner of the heads had a tad more than a gasket match port job done. This opened the ports a little in all directions. The bowls were unmodified.

When I first bought the heads, I had bigger valves and a valve job done. I got them flowed and they did 223i/137e CFM @ .5 in.

My current porter is only working in the bowl area. By doing so, he has increased the flow numbers 35-40 CFM in the .5-.6 range. He told me he normally doesn't even touch the ports until he gets into a "stage 3/4" job.

I don't believe he has the ability to flow intake/carb/heads. He knows my carb setup and cam specifications.

 

I've got the exhaust numbers also:

190 .600
185 .500
177 .400
141 .300
104 .200

My goal is to be in the 11s. Will these flow numbers support this much HP? Or would the 7 more CFM be worth it/needed?

 

thats an interesting point about low lift airflow numbers. Maybe he was trying to sell me with the mumbo jumbo. But.. low lift #s in the 270 cfm range would mean close if not over 300cfm at higher valve lift. anyhow, from what i know i would guess low lift #s gives a good indication of velocity. if it can flow well at low lift that means the flow speed is excellent, which is really important at high rpm. I think the cam is somewhere near .600 lift. Ive seen a similar car (shhh) that makes 400rwhp @ 6000 rpm and over 420rwhp at 6800 rpm. Thats a fat -butt powerband! Solid roller is the way to go! :D

 

In my opinion, there is no such thing as too much head flow, only not enough motor under the heads. (Stolen from Justin Massingill).

I think the more head flow you get the better power you will make, obviously this is provided your head porter knows what he is doing and won't harm low lift #'s and even improve on them if possible.

For a street motor stay under 215 cc intake runner.... I personally don't think your motor will be hurt one bit by opening the runner up above 200 cc, especially with a LT-1.

As a note, LS-1's with Stage 2-3 type porting are getting over 300 cfm @ .600 lift.... I personally believe this is a big part of the reason LS-1's make the impressive power #'s they do.

good luck
Beach Bum

 

don't ls1-6 engines have a 15deg angle to start with? a 23deg SBC head is already at a loss before any comparison gets going. the swirl in the LS heads is also just insane stable & could support high compression on pump gas if the chambers were polished 11:1+ on 92 oct

 

Flow is different than size so you really can't have too much flow but you can have too much size for your app which will result in low velocity and mediocre cylinder filling. Of course at some point you must increase port size in a given design to further increase airflow so you must keep that in mind. Most larger American 2 valve V8s need all the airflow they can get so it's hard to get to big when you're looking for power but you need to be careful on the smaller engines like 302s for instance.

 

Care to share the shop's name? Always interested in who does good work in Canada!

Thanks :D