Stock throttle body MM?
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TPI,
The stock unit has 48mm bores.
If you perform a few measurements and do some easy math, you will discover that the stock throttle body has a free flow area of 4.6648 in² TOTAL. (You'll need to consider the space consumed by the throttle shaft and plate screws to get accurate numbers.)
By comparison, the stock Bosch 14094712 MAF (which many believe is a major restriction to intake air flow) has a free area of 5.965111 in². So why don't people "gut" their TBs before even thinking of the MAF? I don't understand it either. Granted, restriction is additive, so eliminating any restriction can be a good thing, but the largest single culprit certainly isn't the MAF.
By comparison, the Wells SU145 MAF = 7.036153 in² (Flow tested and rated at 750 SCFM @0.01"SP, 74-76mm I.D.). This would make a little more sense if the TB had more flow capacity, like a 52mm TB which has a free area of 5.5600 in², or a 58mm TB which has 6.8398 in².
And if I haven't already confused the question enough, consider that the GMPP RamJet 502 makes 500+HP with a stock 48mm throttle body. Obviously, the throttle body can't be undersized for a small displacement engine when it will feed over 500 cubes adequately. If you run the numbers, a 502 CID engine at 5,500 RPM uses about the equivalent amount of air as a 350 CID engine at 7,900 RPM (well, 7,888 actually). If you plan to run that kind of RPM regularly, you might need to consider a larger TB. If you're feeding a 305, forget about it entirely.
Just my 2¢
The stock unit has 48mm bores.
If you perform a few measurements and do some easy math, you will discover that the stock throttle body has a free flow area of 4.6648 in² TOTAL. (You'll need to consider the space consumed by the throttle shaft and plate screws to get accurate numbers.)
By comparison, the stock Bosch 14094712 MAF (which many believe is a major restriction to intake air flow) has a free area of 5.965111 in². So why don't people "gut" their TBs before even thinking of the MAF? I don't understand it either. Granted, restriction is additive, so eliminating any restriction can be a good thing, but the largest single culprit certainly isn't the MAF.
By comparison, the Wells SU145 MAF = 7.036153 in² (Flow tested and rated at 750 SCFM @0.01"SP, 74-76mm I.D.). This would make a little more sense if the TB had more flow capacity, like a 52mm TB which has a free area of 5.5600 in², or a 58mm TB which has 6.8398 in².
And if I haven't already confused the question enough, consider that the GMPP RamJet 502 makes 500+HP with a stock 48mm throttle body. Obviously, the throttle body can't be undersized for a small displacement engine when it will feed over 500 cubes adequately. If you run the numbers, a 502 CID engine at 5,500 RPM uses about the equivalent amount of air as a 350 CID engine at 7,900 RPM (well, 7,888 actually). If you plan to run that kind of RPM regularly, you might need to consider a larger TB. If you're feeding a 305, forget about it entirely.
Just my 2¢
Last edited by Vader; Jan 3, 2002 at 09:47 PM.
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Joined: Jan 2000
Posts: 19,651
Likes: 309
TPI,
Yeah, I think this is a very good web community, too. Keep looking around, and use the message archives like a library, and you'll find a lot of very helpful ideas.
Be careful about making too many presumptions, though. I never really said that you won't ever need to upgrade the TB size. What I did say is that while it may be a restriction, it certainly isn't the worst restriction. I also said that restriction (flow impediment/friction) is additive, so that reducing any restriction can be beneficial.
However, given the options, the TB wouldn;t be my first choice for several reasons. The flow restriction comparison is one of them. Another reason is the fact that a stock TPI setup doesn't really benfit from the increase in TB bore size. Another consideration is that for maybe a couple of HP gain, you'll spend $350-400 for a replacement TB, minimum. For that much cash, you can get a whole lot more power improvement. A roller cam kit will cost you about the same, and can net 30-40 HP depending on the profile selected. The slight increase from the TB probably wouldn't even be noticed at the track or in your "Levi's dyno", but 30-40HP will certainly be evident.
For less than $200.00 you can get a factory higher stall torque converter that will help get more power to the wheels faster. The S-10 Cyclone replacement converter stalls at about 2,400 RPM behind a V-8, and comes with a factory gaurantee. That will surely act like more than a few more HP.
For less than $100.00 you can port the plenum and produce more gains than a larger TB, and with a larger TB you'll have to port the plenum anyway to take full advantage of the larger bores.
There are a lot more modifications I would perform before considering a larger TB. Once the heads, cam, intake, and exhaust are done, you may get some gains from a larger throttles. Before that, you might just be throwing away some money that could be put to better use.
Yeah, I think this is a very good web community, too. Keep looking around, and use the message archives like a library, and you'll find a lot of very helpful ideas.
Be careful about making too many presumptions, though. I never really said that you won't ever need to upgrade the TB size. What I did say is that while it may be a restriction, it certainly isn't the worst restriction. I also said that restriction (flow impediment/friction) is additive, so that reducing any restriction can be beneficial.
However, given the options, the TB wouldn;t be my first choice for several reasons. The flow restriction comparison is one of them. Another reason is the fact that a stock TPI setup doesn't really benfit from the increase in TB bore size. Another consideration is that for maybe a couple of HP gain, you'll spend $350-400 for a replacement TB, minimum. For that much cash, you can get a whole lot more power improvement. A roller cam kit will cost you about the same, and can net 30-40 HP depending on the profile selected. The slight increase from the TB probably wouldn't even be noticed at the track or in your "Levi's dyno", but 30-40HP will certainly be evident.
For less than $200.00 you can get a factory higher stall torque converter that will help get more power to the wheels faster. The S-10 Cyclone replacement converter stalls at about 2,400 RPM behind a V-8, and comes with a factory gaurantee. That will surely act like more than a few more HP.
For less than $100.00 you can port the plenum and produce more gains than a larger TB, and with a larger TB you'll have to port the plenum anyway to take full advantage of the larger bores.
There are a lot more modifications I would perform before considering a larger TB. Once the heads, cam, intake, and exhaust are done, you may get some gains from a larger throttles. Before that, you might just be throwing away some money that could be put to better use.
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Joined: Jan 2000
Posts: 19,651
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I don't know what modifications you may have already performed, but I'm thinking you might be interested in these files to help you decide:
Air Flow.pdf
http://216.121.161.76/files/PlenumPo...um Porting.PDF
TPI-AFPR.pdf
Cooling.pdf
Air Flow.pdf
http://216.121.161.76/files/PlenumPo...um Porting.PDF
TPI-AFPR.pdf
Cooling.pdf
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