CFM Calculation
Printed From: CorrectCraftFan.com
Category: Repairs and Maintenance
Forum Name: Engine Repair
Forum Discription: Engine problems and solutions
URL: http://www.CorrectCraftFan.com/forum/forum_posts.asp?TID=35120
Printed Date: November-21-2024 at 4:56pm
Topic: CFM Calculation
Posted By: Riley
Subject: CFM Calculation
Date Posted: December-07-2014 at 12:05pm
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Summit Racing's web site has a CFM calculator. The calculations are quite a bit below what marine engines generally use for carburetors. They offer 2 suggestions, street and racing. According to their calculator compared to actual: PCM 240 hp street 380 CFM, Racing 429 CFM, actual 600 Indmar 310 hp 430 CFM, Racing 557 CFM, actual 600 PCM 330 hp 536 CFM, Racing 694 CFM, actual 650 We have a Ford 312 engine, their calculations are 338 CFM and Racing 437 CFM. We have a 600 CFM on it and it runs excellent. Their calculations are 460 CFM, Racing 595 CFM for a 427 ci 300 hp engine. Would a 750 CFM be better than a 600 CFM, or is it trial and error? |
Replies:
Posted By: Gary S
Date Posted: December-07-2014 at 12:35pm
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It is better to under carb than over carb Bruce. How much time are you going to spend wide open? ------------- http://www.correctcraftfan.com/diaries/details.asp?ID=1711&sort=&pagenum=1&yrstart=1966&yrend=1970" rel="nofollow - 69 Mustang HM SS 95 Nautique Super Sport |
Posted By: Riley
Date Posted: December-07-2014 at 1:49pm
| I've always heard that Gary, but it seems marine engines appear to be over carbed, especially the 240 hp PCM. |
Posted By: Gary S
Date Posted: December-07-2014 at 2:30pm
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Could be true,maybe like HP,advertised cfm is more than actual? What did Reid's partner Matt do for PCM? You need to hear from someone who actually delt with its design ------------- http://www.correctcraftfan.com/diaries/details.asp?ID=1711&sort=&pagenum=1&yrstart=1966&yrend=1970" rel="nofollow - 69 Mustang HM SS 95 Nautique Super Sport |
Posted By: 8122pbrainard
Date Posted: December-07-2014 at 3:04pm
Bruce, That calculation fits in with the 150 CFM for each of the original YH's. My original YH's on the 312 run excellent too!   ------------- /diaries/details.asp?ID=1622" rel="nofollow - 54 Atom /diaries/details.asp?ID=2179" rel="nofollow - 77 Tique 64 X55 Dunphy Keep it original, Pete < |
Riley wrote:Posted By: Riley
Date Posted: December-07-2014 at 6:34pm
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Pete, I think the YH's are rated at 128 CFM, so 2 are more like a 2 barrel, which is fine as you can under carb and still run great. If you go by street numbers, the marine engines I've noted are over carbed by 20%-78%. The 240 hp 351 is way over carbed in theory, but without doubt runs great with the 600, so it's hard to figure any rhyme or reason for carb selection. |
Posted By: 8122pbrainard
Date Posted: December-07-2014 at 7:17pm
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Bruce, I agree that many older marine engines were over carbed. It's probably due to the mussel car era when more carbs or bigger ones were better! Exact CFM ratings are hard to find on the YH carbs and I really don't know why. I searched not long ago and did find the 128 you mention but then did find the 150 I mentioned. To me, it doesn't matter since the 312's including mine run great with the set up. There are plenty of 327 Gray's out there that do the same. ------------- /diaries/details.asp?ID=1622" rel="nofollow - 54 Atom /diaries/details.asp?ID=2179" rel="nofollow - 77 Tique 64 X55 Dunphy Keep it original, Pete < |
Posted By: Riley
Date Posted: December-07-2014 at 7:30pm
| I'm trying to figure out if a 750 may be a good selection for a 300 hp Interceptor. |
Posted By: Gary S
Date Posted: December-07-2014 at 7:40pm
Must have taken a lot of "clams" to get one of those------ ------------- http://www.correctcraftfan.com/diaries/details.asp?ID=1711&sort=&pagenum=1&yrstart=1966&yrend=1970" rel="nofollow - 69 Mustang HM SS 95 Nautique Super Sport |
Posted By: 8122pbrainard
Date Posted: December-07-2014 at 10:00pm
Opps!!  I'll blame it on the spell check!  ------------- /diaries/details.asp?ID=1622" rel="nofollow - 54 Atom /diaries/details.asp?ID=2179" rel="nofollow - 77 Tique 64 X55 Dunphy Keep it original, Pete < |
Posted By: 63 Skier
Date Posted: December-07-2014 at 11:33pm
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I wonder if part of the "over carbing" has to do with when you want the secondaries to open. A larger carb should be able to run on the primaries longer, so run more efficiently at the mid range speeds we run them most. I really don't know, just thinking aloud. ------------- '63 American Skier - '98 Sport Nautique |
Posted By: Jonny Quest
Date Posted: December-08-2014 at 2:58am
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One interesting note: the Pro-Tech throttle-body fuel injection that was on my 290 HP Pro Boss engine was a 900 CFM. I got this number from PCM when I was making the painful decision to retro-fit mine back to carb. Based on that 900 cfm number, I originally looked to a 750 cfm carb. However, after performing a cfm evaluation, I went with the standard 600 cmf Holley 4160. (If I had to do it all over again, I would probably go with the QuickFuel 600 cfm unit) JQ ------------- Current 2003 Ski Nautique 206 Limited Previous 2001 Ski Nautique Open Bow 1994 Ski Nautique Open Bow Aqua skiing, ergo sum |
Posted By: TX Foilhead
Date Posted: December-08-2014 at 9:51am
| ProTec (fuel injection) is a different animal. That's the max that the throttle body could flow, the computer can make it behave like any carb up to that size since it has control of how much fuel it adds in. |
Posted By: TRBenj
Date Posted: December-08-2014 at 10:18am
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Not true. Cfm is always the measure of air ingested by the engine (or the max amount of air that can flow through the throttle plate), it has nothing to do with fuel. The fuel metering on a carburetor can be adjusted just like tbi- it just can't be done in real time. Unless you want to limit an engines performance, you will always want to have a carb capable of flowing more air than the engine is capable of processing (basically a factor of its displacement and how fast it spins). This is not the definition of "over-carbed". That would imply that the carburetor is oversized to the point where performance is harmed. That doesn't mean it runs poorly, just less than optimally (so the "my boat runs just fine with a carb that is too (big/small/whatever)" is moot). Sizing a carb can be tricky, and it depends on a lot of factors- trial and error included. It is much easier to identify an "under-carbed" engine than the inverse. |
Posted By: Riley
Date Posted: December-08-2014 at 11:02am
| So it sounds like the calculations are only good for indicating the least amount of CFM in carb selection and trial and error is probably the best method of finding the upper end or optimal size? |
Posted By: phatsat67
Date Posted: December-08-2014 at 11:04am
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What Tim said. You have to watch closely using those estimators because they are set up usually for optimal usage on a drag car (ie top rpm crossing the 1/4 mile traps). Example: Joe's boat only runs a 650 Demon (Correct Joe??). This boat makes nearly double the horse power of most of our stock boats but still lives a very healthy and productive life with only 50 more CFM than our 240-285 hp boats. Like Tim said, when upgrading engines or trying different things there are a lot of trial and error processes especially with carburetors and the different types of carbs when you get out of the realm of stock style carburetors and move more towards down leg boosters, primary and secondary air bleeds, etc. I would be completely fine with a 600 CFM carb on a 300 HP 427. Mark's footer loves the 600 CFM QF marine carb and runs hotter than it did with the brand new holley 650. Plus, nobody really makes any good carbs in the less than 600 cfm flavor for performance applications. ------------- |
Posted By: TRBenj
Date Posted: December-08-2014 at 11:40am
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Correct, Bruce... The calculator will give you the cfm the engine is likely to require. A little head room probably won't hurt, but there is such a thing as too much. It's a balancing act, and like Zach said, there are more things to consider in carb selection than cfm alone. I've read all sorts of automotive accounts of people (or magazines) bolting up comparatively huge carbs onto relatively small engines (750cfm on a 302, for example) and seen performance improvements. On the other hand, Joe's 650 works better than the 750 on his 408w- but again, the carb style was different between the 2. Lots of variables. If anyone has a 800+cfm marine carb kicking around, I wouldn't be opposed to a little experiment! :) |
Posted By: JoeinNY
Date Posted: December-08-2014 at 3:24pm
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Carb ratings are a bit wonky… First you have the marketing factor, was it actually 650 cfm or was it 630 or 670 simple rounding gets you the same place with either result and when there is marketing group involved you can expect rounding, plus a finger on the scale. Then of course there is the relation to the rating method to how it will actually be used. In this case that rating method is tenuous at best, by convention 1 and 2 barrel carbs are usually rated differently than 4 barrels, 1 and 2 barrels were rated at the amount of air that would flow through them with 3 inches of mercury pressure drop across the venturis. Later 4 barrels were typically rated at 1.5 inches of mercury pressure. The only time either of these numbers can be applied is under full throttle circumstances where all throttle plates are Wide Open. One can move more or less air through the venturis by changing the pressure drop across them, so a carb rated at 650 cfm at 1.5 inches of mercury of pressure may move produce 919 cfm at 3 inchs of mercury pressure drop. What is the difference between that 1.5 in of mercury pressure drop to the big air pump that is your engine? It will likely mean that your engine sucks in a charge approx. 5% less dense. So let us say that you are really requiring 700 cfm out of that 650 cfm carb, the additional pressure drop as a result of the carb is going to be considerably smaller maybe enough to decrease the density of the charge by 1%. That may translate directly into engine hp if both charges are equal in quality and there are not exhaust limitations… Generally pressure drop is not your friend, but neither are poor quality fuel mixtures due to low flow velocities in the venturis. The carb is of course not the only point of pressure drop between the cylinder and the atmosphere, valves, intake, spark arrester, bilge ventilation all play a role. Realistically a properly designed 4 barrel carb does a very good job at keeping the fuel flowing and reasonably well mixed at a large range of flows/velocities so they don’t need to make a bunch of different sizes to insure minimal pressure drop and good fuel mixing. IMHO, 450 is generally sufficient for a 302 with reasonable head room, it is just a tad too small to never be a significant restriction on a 351 (although you would likely still have a very useful 351). So they jump to 600 which was the next relatively available size, depending on rpm that 600 will take you up to most 400cu inch engines before you start looking for larger bores, you might squeak by a 427 with a 600, but a 650 is probably as low as I would go. And if it was me that would even depend on the intake manifold/cam/exhaust, with a dual plane intake manifold but good exhaust I would probably go bigger on the carb. But again all 650’s aren’t equal, even within a brand a 650 with downleg boosters will have less pressure drop at a particular rpm than a 650 with larger annular boosters. The quality and design of the carb also comes into play, a smooth venturi without a choke horn on it can be expected to cover a larger range more effectively, not simply work at its rating point. ------------- http://www.correctcraftfan.com/diaries/details.asp?ID=1477 - 1983 Ski Nautique 2001 1967 Mustang 302 "Decoy" http://www.youtube.com/watch?v=cO5MkcBXBBs - Holeshot Video |
Posted By: Riley
Date Posted: December-08-2014 at 7:30pm
| Joe, sounds like a 750 Edelbrock is worth trying out. We have a dual plane Wiend intake, Interceptor logs not sure how the flow is on those and a mild cam. If the right deal on a carb comes up I'll buy it, but wanted to make sure it wasn't a for sure wrong choice. |
Posted By: JoeinNY
Date Posted: December-08-2014 at 11:25pm
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If you are just looking to try out a 750 cfm edelbrock I can lend you one, its automotive in nature but I could swap in the jets and rods to make it the same as the marine tune and it would give you an idea as to whether you wanted to spend money on a marine version for day to day use. ------------- http://www.correctcraftfan.com/diaries/details.asp?ID=1477 - 1983 Ski Nautique 2001 1967 Mustang 302 "Decoy" http://www.youtube.com/watch?v=cO5MkcBXBBs - Holeshot Video |
Posted By: Riley
Date Posted: December-09-2014 at 10:09am
| That sounds like a great idea, I'll send you a PM. |
Posted By: 63 Skier
Date Posted: December-09-2014 at 10:54am
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I have a Quadrajet in my jet boat right now, stock 454. Before I had it rebuilt I had thought I'd switch to a Holley with mechanical secondaries, seems like I'm into the secondaries so often in that application that the efficiency of vacuum was negated. The guy I talked to about this suggested I stay with the Rochester, that they performed very well as long as the engine was mostly stock, providing a lot of vacuum. In reading about these carbs, which have noticeably small primary bores and oversized secondaries, the claim is that though they can flow 750-800 cfm, they can work very well on smaller engines as well because the small primaries will flow less, and the large secondaries will only flow what the engine "asks" for. I'm wondering if any vacuum secondary carb will work this way, only open up enough to flow what the engine needs, so that there isn't a reason to limit carb size to 600 cfm if there's any thought that it needs more. Makes sense as I think about it. ------------- '63 American Skier - '98 Sport Nautique |
Posted By: 8122pbrainard
Date Posted: December-09-2014 at 11:16am
David, I'm sure the carb pros will speak but I have a feeling you're into the secondaries so much because of the higher RPM's the jets run in. Yes, they do go through the fuel and you almost need to own a gas station to run them! ------------- /diaries/details.asp?ID=1622" rel="nofollow - 54 Atom /diaries/details.asp?ID=2179" rel="nofollow - 77 Tique 64 X55 Dunphy Keep it original, Pete < |
Posted By: GottaSki
Date Posted: December-09-2014 at 11:41am
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Perhaps noteworthy is a true dual plane setup will require a larger cfm carb than an open plenum, for the same reason IDF setups require larger CFM throttles. As David suggested, no way would these boats would be as precise at 30-36 mph if they were into the vac secondaries due to cutting the cfm too close. This 800 cfm list 9022 is what PCM put on the LH HP454 engines. It is also sometimes listed as an 850cfm depending which holley document one finds. My friend has a spare he keeps in rebuild-rotation for his boat. Tim if it ever becomes available, i'll let you know! https://www.holley.com/products/fuel_systems/carburetors/specialty_carburetors/marine_carburetors/parts/0-9022 ------------- "There is nothing, absolutely nothing, half so much worthwhile as messing around with boats...simply messing." River Rat to Mole |
Posted By: TRBenj
Date Posted: December-09-2014 at 12:02pm
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Posted By: 63 Skier
Date Posted: December-09-2014 at 12:50pm
Yes, that's exactly why. They build rpms instantly and at low planing speed, in my boat around 24-26 mph, I can maybe stay on primaries but anything more on the throttle and I'm sucking down fuel. Not a problem, part of the fun! But, it does bring into play the discussion of mechanical vs. vacuum, I lean toward mechanical for jets. ------------- '63 American Skier - '98 Sport Nautique |