38 mm manifolds on 34 mm SC heads

[trond]

I purchased a set of triumph throttle bodies and 38 mm manifolds now in transport. My SC heads are 34 mm. What to do with the insulators. Are they required or not ? My request to buy 38 mm insulators was turned down and I need a solution. If I get good results with the setup I can increase port size to match the manifolds but what do I do wrt spacers/insulators until then ?

[Marwil]

Try contacting Turbokraft, they may be able to do something for you. I’ve made my own from 12mm phenolic sheet to step down from a 40mm Carrera manifold to the 34mm small port head. You could also open up the port at the port face to 38mm to create a slight funnel shape transition.

[zedsn]

I would keep the insulators. They are there for heat dissipation. Shouldn't be too hard to make your own if you can't find any like Marwil said.

[T77911S]

use the 38's or
can you use 34 insulators.
you might even be able to taper the 34's to eliminate the sharp edge

you need insulators because of heat soak.

[trond]

tapering inlets in the heads mean heads off. Not what I want at all just now, but if I am happy with the move to ITBs I will next time engine is out. Still curious why insulation is required. CIS manifolds are without insulators and I have been running Accel injectors where the CIS injectors used to be with no apparent issues for many years. With throttle bodies the injectors are further away from the heads. So why should there be a heat soak issue, are the throttle bodies themselves susceptible to heat related issues ? They are not carburettors holding fuel

[KTL]

Agreed the phenolic spacers are more important on carb applications since heat soak in the carbs is to be avoided. However there are fuel injection applications that use them. The 3.2 Motronic Carrera engine has phenolic spacers between the intake and the heads. Not sure why.

['76 911S 3.0]

Originally Posted by KTL

Agreed the phenolic spacers are more important on carb applications since heat soak in the carbs is to be avoided. However there are fuel injection applications that use them. The 3.2 Motronic Carrera engine has phenolic spacers between the intake and the heads. Not sure why.

I think the main difference between the CIS and the Motronic intakes is the rubber boots on the CIS act as an isolator between the main plenum and the individual runners. This is not so much an issue at high intake velocity where the air does not have much time to absorb the heat in the plenum but rather at conditions like idle and low engine speed, low throttle input. The Motronic plenum uses the isolators to keep the intake plenum from absorbing all the heat from the engine and helps keep intake charge temps down, especially in the conditions previously described.

[trond]

no easy way then, I get the point. Hopefully I can find insulators somehow. Or make alu spacers and hope it is OK

[KTL]

I suspect the spacers also help to reduce the temperature differential between the intake and ambient air. You don't want condensation forming and icing the throttle body. I recognize that the Motronic throttle body also has a thermo valve that opens below 50F to help keep the TB warm. My guess is that the spacers complement that thermo valve?

[trond]

yes I get it. I need, and want insulators. But how to resolve the issue that I don't have them ? Anyone that could sell me a set of insulators, maybe 6 ea 34 mm? Never ever before have I experienced a vendor refusing a sale... Not complaining but not common over here so I was unprepared. One thing to sell with a warning but to refuse selling is a new one for me. Meanwhile I now have nice set of ITBs and 38 mm manifolds from Clay that I struggle to find a way to fit on my 34 mm small port heads. People who don't like modifying old cars just don't appreciate what is keeping us awake at night!

[KTL]

Sorry to hear your difficulty in getting them. Who refused to sell them to you? Have you tried this vendor?

Heat Insulators

[Steam Driver]

If I was doing this I would at least taper the bore of the insulator to ease the transition from 38mm down to 34mm. This could be readily done and doesn’t involve the head at all. The abrupt “step” the airflow would have to cross w/o a taper bothers me from a turbulence standpoint, even though it doesn’t seem like a lot. It might not be an issue, but if you take the small amount of time it would take to do it you’ve at least eliminated one variable.

Fluid dynamics can be weird that way sometimes.

[fred cook]

Originally Posted by Steam Driver

If I was doing this I would at least taper the bore of the insulator to ease the transition from 38mm down to 34mm. This could be readily done and doesn’t involve the head at all. The abrupt “step” the airflow would have to cross w/o a taper bothers me from a turbulence standpoint, even though it doesn’t seem like a lot. It might not be an issue, but if you take the small amount of time it would take to do it you’ve at least eliminated one variable.

Fluid dynamics can be weird that way sometimes.

Yep, you want to avoid any sharp corners or overhangs. In order to get the best airflow, the port needs to be smooth with no sudden changes. Ideally, you want laminar air flow in order to get the max volume of air into the engine. Non laminar flow really messes things up! When I built the 3.3SS engine for my SC, I used the large port early SC airbox and runners. They are something like 39mm inside diameter and the Carrera heads I used have 41 or 42 mm intake ports. So, I used a set of Carrera spacers and opened the runner port by 3mm diameter to make them match. With a dremel tool or some other grinder it is not hard to do and you don't have to pull the heads. Or, you could soak a piece of rag in oil and use it to plug the intake port while you do the grinding. That way, the grinding debris will stick to the rag when it is removed. Good luck!

[trond]

actually turbulence in the inlet may not be entirely bad. Thats why at least some tuners port to a rough surface. Not that anyone need a 2 mm step change (on the radius). Will think about doing a gentle ID increase at the head inlet as suggested and block inlet with rags and grease to prevent debris entering engine. Sounds wrong to me to take any risk of introducing debris into engine. But maybe it can be done safely. Then I will need 38 mm insulators which I guess were used on 70s carburetted models like the 901-108-131-00-M100 sold here by Pelican. Still there is the CIS cutout to consider which was why the deal on insulators was rejected by the vendor. Argument was the cutout does not support the insulators and it could fail. Sounded as a valid concern to me and deserves respect

[jpnovak]

The easiest way is to get the 34mm insulators. Then trace the bottom of the intake ITB manifold on the top side of the insulator. Use a dremel tool or similar to taper 2mm around the perimeter. This gives you the smoothest transition with the minimal amount of work.

[Nux]

Have your local machinist make some CNC machined adaptors that fill the CIS cutout and taper down from 38 to 34? Shouldn't be too hard or too expensive? You can even play with stack length to hit max torque.

[Nux]

or maybe these will work, if you do what Jamie described.

https://www.cbperformance.com/product-p/3171.htm

or

https://www.dellorto.co.uk/shop/vw-carburettor-kits-parts/vw-aircooled-carburettor-kits/weber-dual-34ict-carburettor-kits/ict-spacer-riser/

[trond]

Nux appreciate that. These should fit with 64.5 bolt distance center to center and 34 mm ID