308 Engine Rebuild & CIS to TWM Throttle Bodies Conversion Thread!

[Hans]

Originally Posted by mk e

45mm. There is no harm in going too large but too small costs hp.

Originally Posted by detroit

Hans, I'd suggest you be really careful about putting massive great port throttles on your car, especially if it requires opening up the intake ports. You could easily lose port flow from a steep transition, and that will most certainly cause a loss of power. Additionally, larger diameter intake runners are going to shorten the "tuned" length, pushing peak power further up the rev band, and losing you torque and idle stability in the process. I honestly do not know where the cork is on 308 in terms of flow capability, but I suspect it's in the throat area of the port. Unless you plan to make some wholesale changes to your motor in the future -cams, head work, bore diameter, etc., you may not want great big throttle bodies. Thottle response and idle stability will typically be superior with as small a volume runner as possible. Find out what 2v 308 port will flow, and then match your new throttles to it. I'm sure that Jenvey will supply you with max flow rates for their systems, and for that matter will probably recommend a diameter based on based on the 308's port flow rate.

Eric, Marc, thank you both for your insights!

They do however seem to contradict each other a bit? Or Marc, does your comment include the knowledge of the 308 flow bottleneck??

Thanks!!

Hans

[Hans]

Originally Posted by Pizzaman Chris

That's great news Hans.

Not a bad deal either for $1600. Can you use your existing linkage?

Not sure yet. I'm picking up the Dino from the paint shop next weekend. Then I'll start taking measurements to see what I can use and what I can't. I believe the guys at Jenvey are very helpful (or so I've been told), and willing to manufacture on demand if needed.

We'll see how this will go. Sounds like a fun project!! And with today's fuel prices here (over $8 per gallon), combined with my 10,000 miles-per-year in the Dino, a slight improvement in fuel mileage would be rather welcome too!! (I'm hoping to improve from an average of 14 mpg to 18 mpg or so, saving me over $1250 per year in fuel bills...)

But of course, the most welcome feature will be the no longer being dependant on the voodoo doctors of weberology for tuning the car...

[mk e]

Originally Posted by Hans

Eric, Marc, thank you both for your insights!

They do however seem to contradict each other a bit? Or Marc, does your comment include the knowledge of the 308 flow bottleneck??

Thanks!!

Hans

It's more complicated than finding A bottleneck. The throttle plate buggers up the flow and starts it tumbling into the port. You want to reduce the velocity over the throttle plate as much as practical and you will make more hp. You can cut a 30 degree angle on top of the manifold to blend it to the larger TB and that will be good.

The 45s will make more power than the 40s or 42s, no question in my mind. We aren't talking about a lot of hp, but I'd guess 5-10 hp over the 40s. Going up to 50s would probably add 1-2 over the 45s, worth it on a race engine, probably not worth it on a street engine.

The larger the TB you use the more progressive you want you throttle linkage design to let you come off idle smoother.

[Hans]

Thanks Mark! I'll give it a go.

[Newman]

I got all excited when I saw this thread up on top again, silly me I thought Chris actually did something on his car!

[AZDoug]

Originally Posted by Hans

2: I'm keeping my BSM ignition system, as it's working perfectly well. So I'll be doing fuel only. Question then is: Full sequential or batch?

And full sequential gives you a much nicer idle, even when using more radical cams and very large throttle bodies.

Batch is fine over 3K RPM, but inefficient below that. Good for boats and drag cars.

Doug

[hyenahf]

Originally Posted by Hans

Thanks Mark. I was hoping you'd chime in...

Okay, 45 mm it is, providing the casting in the intake headers is thick enough to machine a nice transition from 45 mm to port size.

I shoud probably simply google this, but what are the benefits of going with full sequential?

Would you dare to take a guess as to the gains to be had with the 45 mm's when compared to the Webers? I'm making 213 rwhp now (so 257 hp at the crank).

Thanks!!

45mm TBs for a street motor? bewary. you will lose low end responds if CR isnt picked up dramatically. but for a racecar no problem

I reckon a 40mmTB from jenvey or twm with outflow a stock 308 40 dcnf easily. just look inside your carbs, theres 34 or 36mm main venturis if memory serves and a aux venturi blocking the carb body passage. they dont call them chokes for no reason.

42mm TB would make a lot more sense to me. first find out what peak hp you realistically looking for then find out how much flow capacity the TBs are able to support. makes no sense to put the biggest TB on first.

my 2cents

[AZDoug]

Port fuel injection doesn't suffer from too large of throttle bodies, as you are not trying to coax fuel out of a carburetor, you really can't go too large.

I was running a 1000 CFM T body on a 327 CI motor at one time with sequential injection,and a fairly hot cam, and I could drive around at 1200 RPM in top gear and accelerate gently and easily, a carb that large wouldn't have even let that motor run below 2500 RPM, as the 327 needed at most, 600 CFM.

Doug

[Newman]

Originally Posted by AZDoug

Port fuel injection doesn't suffer from too large of throttle bodies, you really can't go too large.

Doug

Put a large t-body on a stock 5.0 mustang and watch throttle resonse go down the toilet off the line or tip-in. Yes you can go too large.

[AZDoug]

No you can't go too large, within reason.

if you have a problem with a large T body, it is because some other part of the system wasn't designed for it, simply putting a large T body on a stock motor,and stock ECU is a recipe for disaster as that ECU is programmed for one set of parameters and cannot compensate for parts swapping; after market, tunable ECUs don't have that problem.

You don't have to believe me, even though I have run huge 1000 CFM T bodies on small motors with most excellent low speed response, as a matter of fact, 1000 CFM is about the smallest T body you can buy for a four barrel carb replacement. Ask Arizona Speed and Marine, or AZ TPI, or any other aftermarket FI shop.

Below is a pic on that 1000 CFM T body, on a custom edlebrock Victor Jr manifold, which was as docile as a kitten at low speed on a 327 motor. Now, that Vic Jr manifold, won't even let a motor that small run below about 3000 RPM with a 650 CFM carb sitting on top, as it is designed for larger motor circle track use in the 4000-8000 RPM range.

After I decided the 327 didn't have enough power for me, I simply pulled that manifold assembly off that motor and put it atop a 427 CI small block, and re tuned the ECU.

Tunable EFI is different animal and the usual rules don't apply

Doug

[mk e]

Originally Posted by AZDoug

No you can't go too large, within reason.

.........
Tunable EFI is different animal and the usual rules don't apply

Doug

Yup , you have to work pretty hard to have too large a TB with programable EFI. I am putting ported 54s on my engine because the flow bench says anything less is costing hp. The port 54s are what worked on 360 heads as well. $5-48 is what a stock 2v head needs.

[mk e]

Originally Posted by mk e

$5-48 is what a stock 2v head needs.

sorry, 45-48.

[Newman]

Originally Posted by AZDoug

No you can't go too large, within reason.

Doug

Thats like saying yes and no for an answer. There is a formula to calculate throttle bore size just like selecting injector size, too big is too much and has a negative effect just like too big of a cam is a mistake as well. Just clarifying that you cant blindly select the biggest throttle bore on the planet and expect it to work perfectly on every application simply with the stroke of a laptop key. Combination matters!

[mk e]

Originally Posted by Newman

Thats like saying yes and no for an answer. There is a formula to calculate throttle bore size just like selecting injector size, too big is too much and has a negative effect just like too big of a cam is a mistake as well. Just clarifying that you cant blindly select the biggest throttle bore on the planet and expect it to work perfectly on every application simply with the stroke of a laptop key. Combination matters!

I've personally never seen an engine lose hp with a TB that is oversize. I've seen a larger TB not help and make it harder to get the engien smoothly off idle, but never lose hp. This is kind of the beauty of EFI set-ups and why they make more hp than carbs, you just aren't really restricted on the TBs. I agree that there is no point in going too big, but I think "right" is a LOT bigger than the size carb I would pick for the same engine.

[Newman]

Originally Posted by mk e

I've personally never seen an engine lose hp with a TB that is oversize. I've seen a larger TB not help and make it harder to get the engien smoothly off idle, but never lose hp. This is kind of the beauty of EFI set-ups and why they make more hp than carbs, you just aren't really restricted on the TBs. I agree that there is no point in going too big, but I think "right" is a LOT bigger than the size carb I would pick for the same engine.

Okay lets sub in the word performance or torque in place of HP since peak numbers dont get me excited when im driving my car on the street. Big HP numbers dont tell all and a car losing throttle response and being a little soft on tip-in isnt a trade-off I like doing simply to see a bigger number on a sheet of paper. Now if you're making 800HP that little sag wont be noticed but on a weenie 2.9 V8 ferrari they need all the torque they can get which is what this thread is about right? Big throttle bodies (and the ones Chris bought) are a waste of money (sorry Chris they really are), you should've kept the carbs and saved your pennies.

[Pizzaman Chris]

Originally Posted by Newman

I got all excited when I saw this thread up on top again, silly me I thought Chris actually did something on his car!

Soon, my friend. Very soon.

[AZDoug]

This is pretty good low speed torque with an over size T body.

I don't have the HP chart handy, as I use torque to tell me what is really happening. HP is bragging number, wide torque band wins drag races and makes for a fun street car.

This at the rear wheels on a chassis dyno, BTW. Car is really fun, you can be rolling along at say 5-10 MPH at ~1200 RPM, punch the throttle to the floor, and you see God on the way to second gear.

Doug

[Hans]

Originally Posted by Newman

Big throttle bodies (and the ones Chris bought) are a waste of money (sorry Chris they really are), you should've kept the carbs and saved your pennies.

In my case, the main objective is not to squeeze more hp or torque from the engine, but to save pennies because:

A) One of my carbs is shot. I'm running with seven 45 idle jets and one 56, just to make it run smoothly. Someone, at one time, screwed in the idle adjustment screw at an angle and more recently, some other moron tried to resolve that by filing away at it. Replacing 4 carbs (for I am pretty sure I'll never be able to match one replacement carb - progression holes and all) costs more than going with EFI

B) I haven't found anyone yet who can REALLY tune Webers. And if I do, they cost $$$$$. This while a programmable EFI makes things soooo much easier

C) Like I wrote a few posts ago, I'm trying to achieve some better fuel economy too. I'm doing 10,000 miles per year in this car, and with our $8+ per gallon fuel costs here, going from 14 mpg to 18 mpg will save me over $1250 per year on fuel.

I DO want to keep good torque, for I agree that torque is more important than hp here.

First, let me pick the car up from the paint shop next weekend...

[mk e]

Originally Posted by Newman

Okay lets sub in the word performance or torque in place of HP since peak numbers dont get me excited when im driving my car on the street. Big HP numbers dont tell all and a car losing throttle response and being a little soft on tip-in isnt a trade-off I like doing simply to see a bigger number on a sheet of paper. Now if you're making 800HP that little sag wont be noticed but on a weenie 2.9 V8 ferrari they need all the torque they can get which is what this thread is about right? Big throttle bodies (and the ones Chris bought) are a waste of money (sorry Chris they really are), you should've kept the carbs and saved your pennies.

The torque won't go down with big TBs but some care is require to keep the tip in solid. If you just take off small TBS and bolt on big ones the result will be as you describe every time, but with some careful set-up and tuning the problems can normally be completely resolved. Adding more progression to the linkage is important so the throttle plate moves a little slower right off idle which lets the air flow rate change much closer to the stock rate even though you now have a much larger TB. The second thing is to spend time playing with the off-idle mixute, off-idle timing and the acceleration enrichment.

[Newman]

Originally Posted by mk e

. Adding more progression to the linkage is important so the throttle plate moves a little slower right off idle which lets the air flow rate change much closer to the stock rate even though you now have a much larger TB.

In other words the plate is too large to allow for a quick throttle opening without a sag or bog etc because of the large pressure drop. You would be compensating in other areas to cover up the plate issue. Still, for the street and great seat of the pants feel smaller is better. Back to the other post, I never said HP would go down with a large throttle body but it would effect tip-in and response which effects torque curve down low.

Hans, I havent needed to price 40DCNF's so I dont know but I cant see one carb costing more than a complete EFI conversion plus ones time to do the work. Chris had four new carbs in hand and decided to sell them for the EFI.

Off topic but I cant help but wonder if it wouldnt cost less to buy a TR than it is to buy a 308, rebuild the engine, convert it to EFI, rebuild the gearbox, coat the exhaust and basically restore the engine bay. The TR will eat the finsihed 308 and if its performance were looking for isnt the TR the better choice? Im just looking at the dollars and cents here of course.