2.7 cam choices
 

How much cam can you put in a stock 2.7 CIS piston? Suppose I need options for a regular and S piston with CIS dome shape.

Lets assume induction is not a limiting factor and it will have an early exhaust or headers.

Basically I am looking to wake up a CIS 2.7 by swapping cams for something more fun. Plan is to run on Webers for now.

I have built other engines with bigger cams but they always have replaced stock pistons with JE or equivalent such that there is valve clearance. I have never measured or experimented with valve clearance on a stock piston.

Hi Jamie!
I can't speak too much for the S pistons but for the Stock CIS domed Pistons, the SC cam was a good upgrade for the CIS 2.7 engine I built a while back. 964 cams require reliefs in the pistons so those were out for me as well.

What about the Webcam 464/465? Their website states it is a good upgrade for a 2.7? I don't understand cams at all.. looking at the duration specs and lobe sep angle, I would have thought the CIS would hate this cam, but it seems to be recommended.
https://i.imgur.com/ofDP87Y.png

I ran 964 cams with my stock 2.7 CIS pistons. No relief needed.

Interesting!
Where did you time your cams? I didn’t like the clearances I saw with the 964s.

I was also under the impression that 964 cams would not clear. I suppose if the timing is sufficiently advanced this would help with clearance.

Mike, those cams would work great on a 2.7 RS. This has a very different piston shape with a lot more cleerance compared with a stock CIS dome.

I am just trying to figure out what clears without taking the engine apart and replacing stuff.

Interesting that no one seems to know what cam can work in a CIS 2,7!

I wonder if John the camgrinder would know?

It might be worth checking what clearance you have now. You know, every 10* screw in your valve adjuster and record the valve to piston clearance. Then knowing the lift of the stock 2,7 cam you could compare to a 3,0 and 964? Could that work?

Yes, Gordon. This could work and what I had planned. Unfortunately I do not have my hands on the car just yet. Planning phases on a SWB build for a friend.

right now just trying to see what other experiences are out there. And yes, Most people will not swap cams but jump to a 3.0 or 3.2 as a replacement for the 2.7. One of the goals of this build is lightweight so mag case is key. Plus there is already a well running 2.7 to be used. No point in scrapping good parts.

I dont know what the cam timing was set at with my 964 cams.. But I had no clearance issues. 2.7 ran strong with 40 TIN carbs. Ported heads 40 intake 35 Exhaust

Jamie, I'm running a Cam designed by William Knight and Melissa from Web cams , It is called the Melissa , in her honor . I have an 80 sc motor ,small ports, CIS pistons , Measured comp ratio of 8.3, CIS, pulls 7k in 4th gear and 6700 in 5th
Ian

Ian,

Thanks. I know what specs I would like. I am concerned about exhaust side clearance. And of course, exhaust side duration is where all the fun happens.

Webcam 21/20 should work with stock cis.

SC cams of course

964 if you advance them a bit.

DC20 if you advance them to 1.8 (standard is 2.2-2.4).

Maybe even 993ss could clear the cis domes?

I'll second those cams from William Knight. The best out there.

Melissa Cam
 

This cam is the best version I have ever run in a cis motor. It outdates the 20/21 and 993 SS, I will not share the specs because we put way too much work in these for another cam grinder to start knocking them off and calling them their invention I have them in about 11 motors. In a 3.0 euro CIS motor they made 16 hp over a 964 cam at the wheels. They also reduce the potential for predetnation in the lower RPM's caused by the ethanol in our fuel.. The cam is called the "Melissa Cam" It is the best cam you can run in a CIS piston motor. My email is KNIGHTRACE@MAC.COM if you would like to purchase a set. Thanks, William Knight

I appreciate the suggestions William. Sounds like a great cam. But, will it clear in a 2.7? The smaller diameter 2.7 has a steeper dome angle on the CIS "bump" and generally less exhaust valve clearance.

the engine will not use CIS. It is currently running webers I am looking to maximize the 2.7 with stock pistons. Owner does not want to tear down and replace PC set at this time. The engine is currently installed in a SWB. Meaning, the shorter gears and lighter weight favor a higher rpm torque curve.

Ideally, I would build a new engine and spec everything to match but that is not going to happen right now.

I have not run them in a 2.7 but I think they would work, on the 9.8:1 euro piston we have about 80 thousandths clearance on top dead center overlap, but you can change the timing around to get more clearance. Of course these would be better than the other choices that work with CIS pistons on carbs. These cams were designed around the CIS piston so it is highly likely they will clear.

cam clearance
 

This morning I installed 3.2 cams on an 82 CIS engine.At overlap TDC there was 60 thousandth clearance.So I retarded the cam a little and got 80.This engine was 9.3 to 1.The Mellisa cam sounds good.Fred

I have had a few clients gain around 20 hp at the rear wheels. It is as good as you can do on CIS in my opinion. Thanks Fred. William

Still learning at this stage...
My understanding is that the CIS pistons have a unique dome shape to help promote a better spread of the intake charge...
If changing to an aftermarket piston with higher compression, I assume none of them have that same cis-piston dome shape.. is there any downside to this? ie, is the potential gain from the compression increase, offset by the less than ideal swirling and mixing of the intake charge?

If memory serves me correctly... The shape was more of a design for emissions and helped the idle, changing to an aftermarket piston like CP, it will have to idle about 70 rpm more.

Loose the CIS. pistons if your running carbs. Elgin Modified S cams are nice for this engine. You’ve got to get the compression up to at least 9 to 1

I agree if your running carbs changing pistons and running a cam designed for carbs is a better way to go, but with a Mod s cam you need different valve springs and it gets expensive to start doing pistons, springs and it is a lot of work many do not want to do. These cams were designed for CIS but they work much better on carbs than a 964 cam. To be clear we didn't design these for carbs. We needed a solution that worked with cis domes.

Just to add some info that may be of help when trying to understand if a camshaft will "fit".

Do a valve to piston check first so you know the limits of what you have to work with. Then ask the camshaft supplier to give you the lift numbers at those same crank positions. Then its a easy math exercise to see if it will fit.

Any one selling camshafts will be able to give you the lift number at any centerline position.

In the case of the question above, would a certain cam fit a stock 2.7L piston, measure the valve drop to the piston from 20° BTDC to 20° ATDC and ask the cam supplier to give you the lifts at those same positions at the centerline they suggest.

If you give the cam supplier the drop numbers and the clearances you wish to run, they can easily see if the centerlines can be moved, or the performance will be compromised any.

Simple, no guess work and you know before you buy.

I was told my stock valve Springs would be fine with my MOD S cams. It's a 2.8 with J&E pistons at 9.8:1. Good up to 7500rpm.

I would like to hear more input on using stock valve springs. I have a feeling that using the heavy duty springs is overrated and only needed for some extreme situations

That's what I was told by a few reputable mechanics. I'm not using my car for full race. Canyon carving and some AutoX

I am also running Mod S cams with stock springs (motor is a 2.7) with RS pistons. Also told this was fine. Have rev limiter set at 7500 as well, no problems.

john

cam choice depends greatly on your exhaust and usage.

I'm running M&K headers and 997 GTR RS muffler

911 Slant, The Melissa cam makes more power than 964 for sure but if you like what you have keep enjoying it. Sometimes it is better to appreciate what we have and not focus on what we do not have. If you had stock cams the Melissa cam is the way to go on CIS pistons. I do my best to live by a quote by a really good person. "you have two hands one for helping yourself and the other for helping other people" I have my reputation for doing my best to do that, I am not out to just sell a product. William Knight

Be careful here when talking about valve springs. The choice of valve spring (fitment not considered here) is about the wire and the stress put upon the wire.

Some springs are made from really good steel, some are not. Typically a spring made from German steel is a good spring and has the ability to run at high loads close to or at the stress limit without failing. Many of the domestically wound springs cannot as they are wound from a lower grade of steel. We have springs wound for our use with steel that is imported but up goes the cost of each spring.

Once you know the quality of the spring and its characteristics, you can then calculate the nose pressures required with rocker arm, retainer, locks weights etc. The seat pressure is more about the seat widths cut on the seats and valve margins. This why I can never understand the seat pressures some aftermarket springs apply to the seats.

The cam design has a huge affect on the nose pressures as well. many designs have horrible harmonics built in, so in an effort to dampen these, more nose pressure is required. You are compromising the engines performance by making up for poorly designed parts.

One last requirement is the springs ability to fully compress without going solid or coil bind. The valve lift controls this.

Do you need to run more nose pressure with higher RPM. In most cases yes, with many cams available today, as the harmonics go crazy at higher RPM. If the cam is designed properly and all of the moving parts are weighed and used in the calculations, the answer is no. We have 2V air-cooled engines that run to 8000 RPM with a single spring with an open force of 160 lbs. Yes the Exhaust valve needs more, often they weigh more due to the back geometry of the valve, and you do build in a little more safety.

If you add more force because you are concerned about missing a gear shift, you are doing this because of your driving ability and are prepared to compromise the engines performance.

This could be one of the reasons why that other car out pulled you on the straight.

Subscribed

I ran my 2.7 race motor (Elgin 315 cams, Mahle 10.3:1 pistons, Weber 46s with stock valve springs to start with. Felt strong up past 8,000 RPM. Eventually most of the spring sets broke a spring. Still ran fairly strong up to 7,600 RPM to finish a race.

I suspect no issue with stock Porsche springs at more sensible RPMs.

JP I have a set of SC grind cams for the 2.7 CIS engine. I was going to used them on my build but it was so messed up when we got into the engine we decided to go full RS+ spec.

Let me know if you're interested.

edit: OOOPS, just noticed this was an old thread bump. I'm sure you're done by now, so please post up what you decided on etc.

M1 cam is much more power over SC cam on a motor. uses stock springs.