draw through turbo system
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Brent
draw through turbo system
Are there any modifications nessicary to either the carb or turbo when using a draw-through turbo system? Also what is a good combination for a 2276 with a fk-87 cam ported large valve 044 heads? How about for a 1904? I was thinking of using either a t03 or t04 turbo with a Street Demon 650cfm carb. Will that work?? Please help!!! I'm new to all of this
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Searoy
- Posts: 2869
- Joined: Thu Aug 23, 2001 12:01 am
draw through turbo system
Others will more than likely give opposing remarks, but I'll speak from what I understand as a traditional turbo philosophy.
The FK87 is a huge cam, meaning long duration, long overlap. Turbos don't like overlap. Turbos like very very short overlap, or none. But since you won't be under boost 100% of the time no overlap, and extremely low overlap, is unrealistic.
A stock cam (yup, stock) with 1.4 rockers is almost a perfect street turbo cam. Cheaper too. The reason lies is the way the air moves under boost versus how it moves under vacuum.
The FK87 is designed for normal aspiration, with lots of overlap between the exhaust and intake ports, with the exhaust gases leaving the chamber helping to draw in intake gasses at high velocity.
Turbos don't need that exhuast gas help under boost, since the air is being pushed in by the turbo. In fact if there is a lot of overlap under boost you will lose it out the tailpipe, since the boosted air and fuel will flow out the open exhaust valve at high speed and pressure.
Use a stock cam and 1.4:1 rockers, or a very torquey cam (low RPM torquey) with not much overlap but moderately long exhaust timing (not really necessary on a Type 1 due to it's efficient exhaust port). Run 8:1 compression with 15 psi boost and timing locked at 20 degrees (not advanced at all, but locked at 20 degrees under all RPMs).
Use 91 or better octane gas, retarding the timing 2 degrees from 20 degrees for every point under 91 you use.
Turbos in draw through applications must be able to tolerate fuel vapor flowing through them. That means different seals than blow through.
I'm a big fan of EFI with turbos, since flow requirements at all RPMs and manifold pressures can be difficult to meet with carbs.
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*** Teach a Man to Fish ***
Searoy
[This message has been edited by Searoy (edited 09-05-2001).]
The FK87 is a huge cam, meaning long duration, long overlap. Turbos don't like overlap. Turbos like very very short overlap, or none. But since you won't be under boost 100% of the time no overlap, and extremely low overlap, is unrealistic.
A stock cam (yup, stock) with 1.4 rockers is almost a perfect street turbo cam. Cheaper too. The reason lies is the way the air moves under boost versus how it moves under vacuum.
The FK87 is designed for normal aspiration, with lots of overlap between the exhaust and intake ports, with the exhaust gases leaving the chamber helping to draw in intake gasses at high velocity.
Turbos don't need that exhuast gas help under boost, since the air is being pushed in by the turbo. In fact if there is a lot of overlap under boost you will lose it out the tailpipe, since the boosted air and fuel will flow out the open exhaust valve at high speed and pressure.
Use a stock cam and 1.4:1 rockers, or a very torquey cam (low RPM torquey) with not much overlap but moderately long exhaust timing (not really necessary on a Type 1 due to it's efficient exhaust port). Run 8:1 compression with 15 psi boost and timing locked at 20 degrees (not advanced at all, but locked at 20 degrees under all RPMs).
Use 91 or better octane gas, retarding the timing 2 degrees from 20 degrees for every point under 91 you use.
Turbos in draw through applications must be able to tolerate fuel vapor flowing through them. That means different seals than blow through.
I'm a big fan of EFI with turbos, since flow requirements at all RPMs and manifold pressures can be difficult to meet with carbs.
------------------
*** Teach a Man to Fish ***
Searoy
[This message has been edited by Searoy (edited 09-05-2001).]
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Bruce M
- Posts: 89
- Joined: Thu Jul 12, 2001 12:01 am
draw through turbo system
hmmm, needs some more thought.
As above, the cam is NOT a turbo cam. Look for a genuine turbo based grind (not a relabeled existing grind). Web cam are recomended by many here.
Don't recognise the carb, therefore i'll bet it will be a b**tard to tune correctly. I turbo carb has a hell of a job to do, you will probably save a lot of time and money going with a more conventional route ie dell (modified!)
Turbo matching is very involved. it depends on capacity, flow ability and boost pressure,
very roughly the T03 will cover 1904 and the T04 will stretch to 2.2 (with the right trim)
It all comes down to application. If its a race motor, idle and smooth power delivery are not a problem. If it goes near the street start doing a lot of research.
A well set up turbo motor is a big blast. A badly set turbo motor is a very short and expensive blast!!
Good luck.
[This message has been edited by Bruce M (edited 09-05-2001).]
As above, the cam is NOT a turbo cam. Look for a genuine turbo based grind (not a relabeled existing grind). Web cam are recomended by many here.
Don't recognise the carb, therefore i'll bet it will be a b**tard to tune correctly. I turbo carb has a hell of a job to do, you will probably save a lot of time and money going with a more conventional route ie dell (modified!)
Turbo matching is very involved. it depends on capacity, flow ability and boost pressure,
very roughly the T03 will cover 1904 and the T04 will stretch to 2.2 (with the right trim)
It all comes down to application. If its a race motor, idle and smooth power delivery are not a problem. If it goes near the street start doing a lot of research.
A well set up turbo motor is a big blast. A badly set turbo motor is a very short and expensive blast!!
Good luck.
[This message has been edited by Bruce M (edited 09-05-2001).]
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Ross
- Posts: 87
- Joined: Thu Jul 12, 2001 12:01 am
draw through turbo system
I agree with the above and would add that the Turbomania book from CB performance is a very enlightening read and will help you gain some air-cooled turbo know-how. Well worth the money.