PDA

View Full Version : Carb for High-Altitude


MissileMech
08-02-2011, 06:08 PM
Many folks in here operate regularly at high altitudes so I am looking for some advice. I have read and calculated the CFM requirement for my 351W and it needs about 400CFM according to the numbers. Has anyone used a Holley 450 CFM carb either in their car or boat to match up the CFM requirement with the carb. It looks like 600 CFM is the norm for stock. Regardless of the carb I an going to rejet/plate and check vacuum to get the right power valve. I also plan to prop down to a 13.5x15.5 as recommended by OJ.

What I have read is that the lower vacuum (about 2" at 4,500ft) will cause the larger carb to have issues at the point when the secondaries open up so a smaller carb helps to compensate.

Any thoughts!!

j.mccreight@hotmail.com
08-02-2011, 10:22 PM
Not many guys here swapping out carbs for higher elevation, mostly just change out props, your might want to check out some drag racing forums for input about what you wanna do. Sorry it's probably not what you want to hear. If your want your boat to perform like at sea level your would need to invest in a supercharger to compensate for the lack of oxygen.

elevation ratio is 2% leaner per 1500 feet so you might be able to just change out your jets and tune the 600 cfm carb.

woodrowskis
08-03-2011, 12:37 PM
I am not disputing but don't immediately understand downsizing carb. You have to get more air into the engine. Downsizing fuel components of the carb for the different proportions makes since but you need all the airt you can get. Supercharging is utlimate peformance answer though I don't know I would want that in my boat.
I had an 1986. If your boat is pre GT-40, I did not see, changing to GT40 heads with larger valves and a better flowing intake manifold like an edelbrock should help a good bit. Someone else here might know how to source out help on actual amount of power increase. I made these to my 86 and it significantly increased power. Granted this was at sea level but these items mainly increase the air flow capability so it should help . Worth checking out. I was told that should change my hp from 240 to 280 at normal elevations. That was without changing the carb. Rejetting etc, is probably a good idea as air fuel proportions should be different.

MissileMech
08-03-2011, 04:01 PM
Thanks for the responses. The only reason I ask is that the race forums talk about the issues with less vacuum at altitude and the vacuum signal not being enough to let the carburetor operate properly since it relies solely on vacuum signal. They speak of down sizing the carb to increase the vacuum signal to allow the carb to operate as intended (i.e. provide a proper fuel/air mix) throughout the power band. If you are over-carbed than even re-jeting does not solve the issue because the secondaries won't open at the proper time and fuel will not flow when the do (right away) because the vacuum is too low to pull the fuel through the jets.

GT40 heads are certainly in my boats future to help the engine breathe easier. And from the responses here it appears no one is really having that much of an issue at altitude.

I have access to both carbs so I will try it out and be the guinea pig on this one.

j.mccreight@hotmail.com
08-03-2011, 04:09 PM
Good luck with your experiment, I would caution you to use marine carbs so your don't blow yourself up. I am sure other people on here that live in a high elevation would like to know the results so their boat can perform better, keep us posted..

MissileMech
08-04-2011, 01:08 AM
Both are marine carbs. I have spent too much time buffing to try to buff burn marks out of melted gelcoat.:rolleyes: I will post the results when I get them. Many other issues could use up my experimentation time.

TRBenj
08-04-2011, 11:48 AM
How much you need to rejet to compensate for a higher altitude is not an unknown thing. Do a search on the web, or call Holley directly- you should be able to find a chart that shows you how many jet sizes you want to go down to correspond with a given altitude increase.

Carbs are categorized by the rate at which they ingest air (CFM = cubic feet per minute). You do not want to ingest less air (which you would be doing by going to a smaller carb, ie 450 vs. 600). Instead, because air contains less oxygen at higher altitudes, you want to put in a corresponding lower amount of gas to maintain the proper fuel/air (oxygen) ratio.

A smaller CFM carb will likely have smaller jets in it anyways, but that is not the proper way to make the adjustment, and it may come with other driveability and performance disadvantages.

Just rejet your 600cfm properly for the higher altitude and call it a day.

MissileMech
08-11-2011, 09:46 AM
Ok - first test was the newly installed motor. There was an rpm restriction due to break-in but we spent 13 hrs on the lake so I was able operate the throttle at full range. The engine ran and pulled awesome right up to the secondaries opening up. As soon as they cracked open the engine started to miss and bog all the way through full throttle. So, I think I need to open the carb up and rebuild it. I will have the 450 cfm rebuilt before that so I will let you know how that carb works.

etduc
08-11-2011, 11:33 AM
Ok - first test was the newly installed motor. There was an rpm restriction due to break-in but we spent 13 hrs on the lake so I was able operate the throttle at full range. The engine ran and pulled awesome right up to the secondaries opening up. As soon as they cracked open the engine started to miss and bog all the way through full throttle. So, I think I need to open the carb up and rebuild it. I will have the 450 cfm rebuilt before that so I will let you know how that carb works.

Over carbing, is an over used term. It's over jetting, fuel/oxygen ratio.

What TRBenj said. :D

MissileMech
08-24-2011, 01:13 PM
Update - I couldn't determine the hole sizes for the metering plate stamped #59 so I did the conversion so I could use jets. So I set it up with .063 for the primaries and .070 for the secondaries with a purple secondary spring. The carb works great. Smooth secondary transition and seems to pull through the range. But I only got 36 mph (gauge only - taking a gps next time) at 4200 rpm. I am going to calibrate the speedo and get a new tach to make sure my numbers are right. When I get a prop that gets me to 4600 how much more speed do you think I will get?

j.mccreight@hotmail.com
08-24-2011, 03:29 PM
Prolly 40, my boat does 44 mph gps @ 4600 rpm of course im at sea level with a 4 blade 13x13 oj.