i spoke with a very helpful guy for about 40 minutes about porting my heads yesterday. he kept coming back to one point that made me curious. we're talking about porting a set of stock LT1 castings to be mated with a big hyd. roller with a goal of 425 rwhp NA on the stock bottom end. he said the heads will flow a minimum of 265 cfm on the intake side and around 212 on the exhaust. he explained that this would make better power throughout the rpm band by creating better low-lift numbers through increased port velocity. it seemed to make sense to me, but....i know i have read several posts here stating that the exhaust should flow no more than 75% of the intake. is this true? if so, is what this guy is telling me incorrect? this is a big choice for me to make and i don't want to end up crying in my beer in a few months. i know other people whose heads were done at this shop and they have been very pleased with the results. one person who i've emailed with had heads done there that flowed 280ish/210ish. i'm leaning toward sending my heads to this shop, but i want some input from some of you more experienced guys before i make my final decision.

also, if i end up with heads that flow in the advertised range (265/212), does anybody have a hyd. roller cam recommendation that will help me reach my hp goal? i think i'm going to end up with a custom grind, but wouldn't mind an off the shelf grind if there is one out there that will work.

btw, car is a 97 WS6 M6 making 348/352 on the CC305, shorty headers, bolt-ons, stock heads and stock bottom end right now.

fred, ken- if this thread doesn't belong in this forum please accept my apologies and send me on my way.

[This message has been edited by rumair (edited July 03, 2002).]

 

More than 75% isn't a problem. It's a rule of thumb "minimum" for NA motors. 80%+ is better for SC or nitrous setups. If the exhaust flow is really that good you may not need quite as much of a splt between the intake and exhaust durations as is usual for an LT1. Also depends on the exhaust system you will be running.

Rich Krause

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'95 Z-28 383 with Vortech, nitrous, etc.
"1FASTZ28"

 

rich- thanks for responding so promptly to my post. he did say that the intake/exhaust duration split on the cam could be less than is usually required due to the higher exhaust flow numbers. is it to my advantage to have a smaller split on an NA motor, or is that only desired on a forced-induction/nitrous setup?

the exhaust will be hooker long-tubes and a custom exhaust, most likely 3" all the way back. i have been thinking about a 3" collector pipe (about 2' long on each side), bullet mufflers (inline) and turndowns. a friend of mine has this setup sitting in his garage ready to sell me right now. i'll have to put some more thought into the exhaust setup and solicit some more feedback after i have my heads/cam ready to go on the car.

[This message has been edited by rumair (edited July 03, 2002).]

 

Your machinist sounds like a knowlegeable fellow. Doesn't sound like he'll steer ya wrong, he may be able to point you to the best cam as well.

As far as the cam being closer to symetrical, they are right, dont' need such a large split the closer you get to 80%.
The Assymetrical cams are designed for the most part to make up for weak flowing exhaust ports.

 

Ok, I'm confused now. If you can compensate for the higher than average exhaust flow by using a cam with less split in duration, why is it better to have a 75% ratio? In theory, wouldn't you want it to be equal? Get alot in, get alot out?

 

With 265/212 you will be able to make much more than 425hp!!!
Easily.

 

Filling the cylinder on the intake stroke vs. emptying it on the exhaust side are two different processes. Filling is primarily passive. If you ignore "ram" effects the intake charge must be "pulled" into the cylinder when the piston descends and the intake valve opens. When the exhaust valve opens, the combustion gasses in the cylinder are hot and still expanding, thus they flow out much better than the unburned mixture flows in. The 75% E/I flow ratio is emprically known to work on a NA 2-valve V8.

Again, there's no disadvantage to the exhaust flowing 80% of the intake and this would be great for a nitrous or SC setup. A small advantage will be that you can shorten up the exhaust duration. By closing the exhaust a little earlier related to TDC the dynamic CR will be bit higher and there will be a small hp gain. Nothing huge, and it will require careful cam selection to take advantage of it.

Rich Krause

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'95 Z-28 383 with Vortech, nitrous, etc.
"1FASTZ28"

[This message has been edited by rskrause (edited July 04, 2002).]

 

why is it better to have a 75% ratio?

Just to clarify, that's a *minimum* -- you just don't want to have, say, a 60% ratio.

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Hear LT1 cam WAVs, DIY head porting: members.***.net/gmarengo

'95, !CAGS, CAI, ForceII, Hooker Shorties, 14.2@103

 

With a pretty big cam (he is talking RWHP, not crank), sure. I flowed more than that for peak, but you dont list what he claims you can obtain at low lift? I had good low lift numbers, with a decent sized cam (very streetable), although did not hit those numbers, may have done so with shorties and lack of ignition miss that I unfortunately had.

http://docmudge.tripod.com/dynorun030902/index.html

Good luck!

As far as your peak flow numbers are concerned though, those are pretty average for a decent head porting job on LT1 heads.

[This message has been edited by Dr.Mudge (edited July 05, 2002).]

 

good point doc. i have not talked to him about low lift numbers yet, but i have talked via email with a couple of his customers who are very happy. i realize that 265 is not an exceptional peak number, but that is a minimum flow number; i would expect the actual peak flow number to be a little north of that. the other time i've had a set of heads get a "street/strip" port job they came out flowing 17 more cfm than the advertised minimum. he also has several flow sheets from stock LT1 heads that he has done and has offered to fax a few of them to me if i want to take a look. i'll definitely take a look at a couple of these if i decide to have him port my heads. based on the power his other customers are making i don't think his low lift numbers are or will be lacking.

as for cam choice...the car is not a daily driver, so streetability is not a huge issue with me. i know that i could make the power that i'm looking for with a solid roller and have it be more streetable than any hyd. roller that will be required to make that much power. however, despite encouragement i've gotten from others, i am not interested in the valvetrain maintenance required on a solid roller setup. there are just too many variables for me to go with that kind of setup at this point. heck, i don't even have a garage!! i know going into this project that cam choice will be a critical issue. i'm also confident that if i can get the flow numbers i've talked about, 425rw is a reachable goal with a big hyd. roller on the stock bottom end. it's just a matter of choosing parts very carefully and getting things right the first time.

i'm going to make a decision on porting by the middle of this month. i'll keep everybody updated how it goes. thanks for all the helpful replies.

[This message has been edited by rumair (edited July 05, 2002).]

 

update- the machinist faxed me a couple of flow sheets yesterday. although i'm not at the office, somebody took a look at them for me and the peak numbers (.600 lift) were 260/226 and 267/227. it occurs to me that it should be nearly impossible to see that much flow on the exhaust side of a stock LT1 casting. i will have the sheets in my hands at lunch so i can get a look at the low-lift numbers. for starters, anybody care to comment on the unusally high exhaust flow numbers? his advertised I/E ratio (what he told me on the phone) of 83-85% seems to be right on. if those exhaust flow numbers are real (i.e. without a pipe), i should be able to run a cam with almost no split in duration. please let me know your opinions, i'll edit the post with the low-lift numbers early this afternoon.

got the flow sheets from the office. here's a breakdown on the 2 he sent. the exhaust is flowed with a pipe, so you can subtract 10-12 cfm on the big end. i also talked to a guy who was in the shop that had several sets of heads done there and heads done at other big name shops. he had nothing but good things to say about the quality of workmanship and the performance of his heads. he has gone as quick as 119 on a hydraulic roller setup with this port job and an automatic (TH 350) transmission. please let me know what you think.

lift intake exhaust intake exhaust

.100 67.3 65.9 65.3 65.9

.200 116.8 127.7 116.4 127.7

.300 184.9 163.5 187.7 162.6

.400 223.6 200.6 228.9 200.6

.500 253.1 216.2 263.1 221.1

.525 259 219.2 264.9 224.8

.550 262.5 222.9 266 226.6

.575 262.5 224.8 267.2 227.9

.600 260.1 226 267.2 228.5

[This message has been edited by rumair (edited July 18, 2002).]

[This message has been edited by rumair (edited July 18, 2002).]