Looks interesting. Are 45's too big for the street?
facebook.com/marketplace/item/1926689584815414
facebook.com/marketplace/item/1926689584815414
'73 CS with m30b35, 5speed, 45DCOE in San Bruno, CA
- Thread starter skk
- Start date
ElGrecko
Member
On a 3.5 45's would be fine. I had them on a 2.0 M10 and it idled like a watch even with a big cam that came off of an 1800 TI/SA. The key is to build the carbs to the cam, the head and compression ratio. On my M10 I had 40mm chokes and it had an aggressive cam but it also had 13:1 CR. While it was pretty much a race motor it idled fine an felt like a stock 2002 until you hit 3500 rpm and then it felt like someone grabbed tach needle and yanked it over to the red line. The more displacement per cylinder the more choke diameter you need and a 3.5 has 583 cc's per cylinder (16% more airflow than a 3 liter or a 2 liter M10 and 30% more than the 1800), so the displacement is there to support the bigger carb. If you put them on a stock M30B35 then you might want to use a bit smaller chokes than 40mm. 45's can be built with choke (venturi) sized from 30 to 44mm. Choke sizes available are 30mm, 32mm, 34mm, 35mm, 36mm, 37mm, 38mm, 40mm, 42mm, and 44mm. You can probably use 37, or 38 mm chokes for a stock M30B35 and 40's if you had a cam and head work. On a stock 3 liter you would want to used 36 or 37 mm chokes. Bigger chokes give you more high end, but if you aren't turning the motor to 7k or more you aren't going to need them and you sacrifice some midrange drivability.
Nice looking car if was a slick top I'd be looking at it seriously. The bigger motor and the 5 speed are nice additions and it doesn't look like the tin worm has done any real damage.
Nice looking car if was a slick top I'd be looking at it seriously. The bigger motor and the 5 speed are nice additions and it doesn't look like the tin worm has done any real damage.
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rblongboarder
Well-Known Member
Here is the equivalent CL ad for this car:
45 dcoe's aren't too big for the street, but they are generally too big for most engines. they are really for high compression / high hp engines. my last 2002 had 11:1 and a schrick 316 cam ... not your basic street engine - it turned to 7500 rpm.
I had thought that while the 45s allow for larger venturis, that you could use a 34mm or 36mm venturi and they would behave very much like the 40s. So the 45s are not exactly too big, but instead you are buying capacity your engine can't use. Is this right?
I think 34mm is as small as recommended on 45's.
ElGrecko
Member
You may not gain as much on the high end and will certainly lose some mid range compared to using the right size carb for your setup. With the smaller chokes it will run fine, but you won't gain as much as you could. With 45's you have a bigger manifold diameter between the carbs and the head. If you don't have the airflow to keep the velocity up in the manifold between the carb and the head you don't get as much mixture into the cylinder (intake ram effect) just before the valve closes. Think of the air stream going into the cylinder as a train. When the piston is going down it gets that train moving in the intake and head. Then as the piston is nearing the end of the stroke that train of flow is still moving at high speed and it rams more mixture into the cylinder just before the valve closes. The area of the intake port and manifold determines how fast that slug of mixture is going. Smaller intake manifold diameter increases flow up to the point where it becomes restrictive and then you lose top end, but you can gain in the midrange. If you aren't going to turn the motor up to near 7,000 rpm (or more) then you are likely going to lose some mid range power with the bigger carbs, but at the high end you gain because the flow losses are lower. As i said above, carbs are to be matched to your engine displacement, cam, head airflow and compression ratio. You can get away with the bigger bodies if you have a 3.5, but even if you choke them down they are probably too big for a 2800 with a stock head and cam. You'd gain a bit of high end power but it's probably a bridge too far and in that case 40's are probably a better choice in that they're better matched and could gain more across the board compared to a 45.
Another part of the gain from side drafts are the individual throttle plates for each cylinder. ITB's make for a much more responsive engine and that's another plus for side drafts. There's also the gain from intake tuning that you don't get from downdrafts like the stock setup. The intake tuning gains are mostly at the high end (6000 to 7000 rpm) but every bit helps.