Siamesing TPI base
Thread Starter
Heel & Toe
Joined: Oct 2002
Posts: 24
Likes: 0
From: Whitby Ont.
Siamesing TPI base
After searching through some posts I have read mixed opinions on this modification. What I am concerened about is the wall between the cylinders 5 and 7. After reading some concerns about siamesing this pair of runners, I understand there is the possibility of reversion between the two cylinders due to their position in the firing order. What is the proper method for siamesing the base? Do I siamese in a couple inches for every runner but the 5-7 runner? Do I siamese the 5-7 runner at all?. With stock runners is it worth siamesing the plenum as well? Also can I grind those humps on the floor of the plenum just behind the throttle body flush with the floor? Thanks for any help, I don't want to mess up my intake, but I want to get the most possible airflow out of it to match my ported heads and cam.
Safety Car
Joined: Dec 2000
Posts: 3,767
Likes: 5
From: Jacksonville Fl
Re: Siamesing TPI base (Chevyguy1984)
Yes, grind the humps away, they make about a 25% flow restriction to the throttle body.
As for 5-7 scavengine, Cyl 7 fires at an interval of 90º crankshaft rotation after Cyl 5 does. Since the intake of No 5 will have an air mass movement, it will help charge no 7...so if anything, no 7 will run a little better than 5. Both will get the same fuel flow, since the fuel is charged directly into the valve port and not in the manifold...so there.
Honestly, even in wet manifold designs on single plenum intakes...it's not a problem, so stop worrying.
[Modified by BBA, 9:32 PM 11/13/2002]
As for 5-7 scavengine, Cyl 7 fires at an interval of 90º crankshaft rotation after Cyl 5 does. Since the intake of No 5 will have an air mass movement, it will help charge no 7...so if anything, no 7 will run a little better than 5. Both will get the same fuel flow, since the fuel is charged directly into the valve port and not in the manifold...so there.
Honestly, even in wet manifold designs on single plenum intakes...it's not a problem, so stop worrying.
[Modified by BBA, 9:32 PM 11/13/2002]
Pro
Joined: May 2002
Posts: 510
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From: Hockessin DE
Re: Siamesing TPI base (BBA)
Port the plenum, the EGR walls behind the TB, and also siamese the plenum runner holes, and the runners as well an open up these holes too. I would siamese the base 2-3 inches in. It all depends on how high you can rev, and how high your motor can rev, the more you go in the higher you can rev, obviously. A stock TPI valvetrain isn't a very high reving motor, if you want more top end you'll have to change out the springs, and maybe other parts too, im not 100% positive on that. I plan on porting and siamesing my plenum, runners, and base(2 inches). Hope this helps, good luck with whatever you do,keep us posted, I am interested to see what gains you get.
Thread Starter
Heel & Toe
Joined: Oct 2002
Posts: 24
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From: Whitby Ont.
Re: Siamesing TPI base (Red91Vette)
Well my heads and cam/valvetrain are good to 6000 rpm, so I guess I'll go 2.5 - 3" in? I realize this won't allow me to make power to 6000 but it should help considerably.
Team Owner
Joined: Dec 2000
Posts: 90,675
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From: SE NY
Cruise-In II Veteran
Re: Siamesing TPI base (Chevyguy1984)
I took the siamese cut in 1" x 1" high and gained 20hp (10%) at 5,400rpm over the stock base. My next cut (hopefully this winter) will be in another inch to make it a 2" x 1" hole, in an attempt to further extend the power band.
I too had concerns about the 5-7 reversion but cut that partition anyway after doing some research and discussing it with my mechanic.
I have some pics on my site of the siamese job along with the before and after dyno pulls. Note that on the after pull, the fuel pressure gauge was leaking fuel and the A/F ratio was much higher than in the before pull.
Since our stock runners are too small in diameter for good top end breathing I believe the siamese base overcomes the problem. I'm not sure though if it would help to use a big mouth base to aid in low rpm breathing.
I too had concerns about the 5-7 reversion but cut that partition anyway after doing some research and discussing it with my mechanic.
I have some pics on my site of the siamese job along with the before and after dyno pulls. Note that on the after pull, the fuel pressure gauge was leaking fuel and the A/F ratio was much higher than in the before pull.
Since our stock runners are too small in diameter for good top end breathing I believe the siamese base overcomes the problem. I'm not sure though if it would help to use a big mouth base to aid in low rpm breathing.
Pro
Joined: Dec 2000
Posts: 700
Likes: 20
From: Derby Kansas
Re: Siamesing TPI base (Chevyguy1984)
When you port the humps behind the TB be sure to open up the 2 plenum ports also. Open your butterflies and look into the plenum to see what I'm talking about. It looks like it chokes the stock throttle body down by at least 2-4mm.
Pro
Joined: May 2002
Posts: 510
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From: Hockessin DE
Re: Siamesing TPI base (Chevyguy1984)
Well my heads and cam/valvetrain are good to 6000 rpm, so I guess I'll go 2.5 - 3" in? I realize this won't allow me to make power to 6000 but it should help considerably.
Team Owner
Joined: Dec 2000
Posts: 90,675
Likes: 304
From: SE NY
Cruise-In II Veteran
Re: Siamesing TPI base (Red91Vette)
I guess I didn't mention that when I had the TC and tranny done I also had the shift points raised from the stock 4,800rpm.
With only a 1"x1" siamese base cut, she pulls very strongly to 5,200 on the 1-2 shift and 5,800rpm on the 2-3 shift. From other posts I firmly believe that a deeper cut will allow the engine to pull well to 6k RPM and maybe above and 2.5"-3" cut depth may just get you over 6k RPM with a good cam and head work.
BTW, I've been running this mod for most of this racing season and see no difference between the conditions of the #5 & #7 spark plugs.
If you're worried about messing up your stock TPI and/or want to experiment a little with the siamese base, pick up a used set of base and runners. For $100 I got a used set from an '87 Vette that I had cleaned up and did the siamese treatment to; my stock setup is now in a box, just in case I want to go back to a shallower cut.
[Modified by 65Z01, 9:24 PM 11/14/2002]
With only a 1"x1" siamese base cut, she pulls very strongly to 5,200 on the 1-2 shift and 5,800rpm on the 2-3 shift. From other posts I firmly believe that a deeper cut will allow the engine to pull well to 6k RPM and maybe above and 2.5"-3" cut depth may just get you over 6k RPM with a good cam and head work.
BTW, I've been running this mod for most of this racing season and see no difference between the conditions of the #5 & #7 spark plugs.
If you're worried about messing up your stock TPI and/or want to experiment a little with the siamese base, pick up a used set of base and runners. For $100 I got a used set from an '87 Vette that I had cleaned up and did the siamese treatment to; my stock setup is now in a box, just in case I want to go back to a shallower cut.
[Modified by 65Z01, 9:24 PM 11/14/2002]
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