GT2 Evo 107 cams clear stock 3.6 Pistons?

[rswannabe]

Looking for some guidance. I have an internally stock ‘91 964 3.6 motor running itb’s and headers. Current cams are stock and I’d like to go bigger without changing the pistons.

I have read that the GT2 Evo cams work very well in stock NA 3.6 and are about as aggressive a cam as will clear the stock pistons without additional reliefs. So when I saw a set being sold for a good price in the classifieds, I picked them up. They arrived and look to be in good shape with no significant wear or pitting. BUT, they are on a 107 lobe center instead of the normal 112 lobe center for GT2 cams. This was not mentioned in the ad. The narrower lobe center means more overlap at TDC and less clearance. So I don’t know if these will clear the pistons.

Is there a way to tell if these will clear the pistons short of tearing down the top end and measuring? Anyone run these in a stock 964 and can confirm they clear? Other ideas?

Thanks!

[rswannabe]

This thread addresses some of the issues of narrower lobe center, but not my critical question of absolute clearance.

Timing Cams with a Custom Lobe Center

[stownsen914]

If you got enough info, you could roughly calculate clearance and at least if you have a chance. Not sure if all this info. will be available.
1. The valve timing graphs for your stock cams and the GT2 ones. Both should be available. The grinder of the GT2 cam should be able to provide it for yours with the 107 lobe centers. These graphs will show lift at different crank positions, and will tell you how much different the lifts are at TDC +- 20 degrees, which is where piston to valve clearances are critical.
2. Not sure if you'll be able to find this as easily, but the valve to piston clearances for your stock cams in at TDC +- 20 degrees.

If you're able to come up wtih #2, you can calculate your clearances to at least know if you have a chance with the new cams.

[rswannabe]

I have not been able to come up with the valve timing graphs, but I did find this test method posted by Dave at Dart Auto. What do the experts think about testing with this method?

“An easy way to check is to install your cams and get #1 set up to about 30 BTDC on the overlap stroke. Turn the valve adjust screw in 1 turn and see if you have interference as you turn it till about 30 ATDC. If not back the engine up and turn it in another turn. Each screw turn is 1mm. If you get to 3mm you are fine for sure. You can probably run it as close as 1.5mm. I have checked this method against the clay method and the solder method and found it to be at least as accurate, but it does not tell you where the interference is like clay will.”

[PeteKz]

That's how I check for valve to piston clearance. I do it from 10 degrees BTDC to 10 degrees ATDC, as I don't use cams with long duration and overlap. My current engine has slightly less than 1.5mm of clearance.

[gled49]

I do it the same way. I carefully set the cam timing, then check valve to piston clearance. Exhaust clearance is closest about 10* BTDCO and intake clearance is closest about 10* ATDCO. With “0” valve clearance i screw in adjuster and record rotation, exhaust first, intake second. I use cam timing to adjust valve to piston clearance. When in doubt I’ll check it every few degrees.

[snbush67]

Icarp made a really nice tool that can measure clearance with the engine assembled. It’s a lever assembly that rotates with the rocker to open the valve.

[rswannabe]

So, I finally got around to test fitting these GT2-107 lobe center cams in my '91 964 3.6. It is tight, but they just barely "fit". My Reserarch told be the rough timing should be about 3.45mm. I installed them timed as such, and then checked the clearances by dialing in the valve adjustment screws on the rockers. The exhaust valve clearance was looking Ok with better than 2mm of clearance (full 720 rotation), but then I was not able to even get 1mm of clearance on the intake valve. So I gradually dialed back the advance (retarded the cam timing) until I got to 3.05mm (0.120"), where I was seeing 1.25mm clearance on the intake valve and 1.75mm of clearance on the exhaust. This is less clearance than I would like, and some would consider it not enough clearance, but I'm going to give them a try. This is partially because upon pulling my stock cams I found some not insignificant pitting, and partially because it would be over a month wait on a new set of DC24's which would be my go to alternative.

So I think I'm going to run these cams and see what it feels like. I'll do some tuning and have a dyno session schedule for next month to complete any tuning and see what the impact of the cams are on my motor.

[rswannabe]

[icarp]

Brooke, those P to V clearance numbers look fine to me , I have run closer but that is with
comp valve train , light weight pistons , comp rods , you get it, know your parts.
I would run what you have , the cams will work well for you .

Ian

[rswannabe]

Originally Posted by icarp

Brooke, those P to V clearance numbers look fine to me , I have run closer but that is with
comp valve train , light weight pistons , comp rods , you get it, know your parts.
I would run what you have , the cams will work well for you .

This is basically the conclusion I have come to. I’m running a stock 964 long block, so I won’t be revving past 6,800 rpm (at least not on purpose) and should not be pushing the valve train too hard.

It will be interesting to see what these feel like as they will only clear when timed to the roughly 3.00mm (0.12”) setting, which is about 0.5mm shy of “optimal” timing. This theoretically should favor the top end a little more, but that is what I want anyways. I do not expect these to be lacking in the midrange regardless. Also, I don’t think there are any other timing options without bigger valve pockets or shims under the cylinders.

I consider this a worthy experiment to document for posterity. We’ll see how well it works.

[rswannabe]

Final cam timing was 0.109” (2.77mm). That was dictated by valve clearance rather than optimal timing. Motor is buttoned up, cams were run in at 2,000rpm for 20 minutes, oil was changed, itb’s were rebalanced, and I am now retuning for the new cams. All is going well so far. The car is running great and pulling hard to redline. I hope to have it on the dyno next week to document the change compared to the 964 cams.

[dannobee]

What was the suggested cam timing on the cam card? "Stock" GT2 Evo's call for 2.3mm, but that's on 111-112 deg LSA.

[rswannabe]

While these were virtually new Webcam sticks (previously test fit with blackening not even worn off the backside of the cam lobes) I did not receive a timing card with them and Webb would not give me any info when I called. My research showed they should likely be in the 3.45mm area optimally (https://forums.pelicanparts.com/911-engine-rebuilding-forum/904629-timing-cams-custom-lobe-center.html), but these will not come close to clearing the pistons at that setting. Testing got me down to the 2.77mm range for best valve clearance (1.25mm intake and 1.75mm exhaust). So really no wiggle room for adjusting the timing outside that range while running stock pistons.

This more retarded cam timing (versus optimal) was a little concerning to me, but it is 0.7mm more retarded than “optimal”, and people commonly retime stock 964 cams by almost that much (1.8mm vs 1.26). The retarded setting is also biased towards top end, which is why I’m swapping to these cams anyways. So I said let’s try it and see what it feels like.

[rswannabe]

OK, so I hit the dyno again back in July, but then headed out on vacation with the family, so I've been slow posting the update. Here we go:

The new cams only added 8 WHP, peaking at 242 wheen hp (285 at the crank) and peak torque was the same at 230 Wheel lb ft (270 crank). Here's the graph.



For reference, here is the previous dyno run with same exhaust and configurations, but with stock 964 cams.



Something that seems weird to me is that peak torque is now occurring at a lower RPM than with the 964 cams (4,300 rpm vs 5,000 rpm). That should not be the case. The more aggressive cam should be peaking later in the rpm range. I'd love to get people's thoughts on why this would be. My current theory is that the GT2-107 cam is a bigger bump (more duration and a lot more lift at .470" vs .430") on the exhaust side than on the intake side compared to the 964 grind. While I have 1.75" B&B headers, I was back to the Dansk sport 2 in 1 out muffler for this dyno test and I'm wondering if it is really starting to restrict on the exhaust side? The test will be to go back to the freer flowing Monty muffler and see what it does then.



I would be lying if I said I wasn't a bit disappointed with the gross numbers, but I am absolutely loving the character of the motor now and how it drives. The motor feels like it is smoother, smaller displacement (in a good way), and higher strung that with the stock 964 cams. Regardless of the numbers, the tune is pretty well dialed in and running the best it ever has and I am very happy with the cam upgrade.

[dannobee]

Those results are what I would have expected with a cam that has a tighter LSA. Smaller LSA's will give more "peaky" torque at a lower rpm, all else equal, at the expense of lower peak hp (compared to the "stock" 112 LCA Evo cams) No surprise there.

As long as you're happy with the results, that's all that really matters. And you did have the valve to piston clearance as a limitation.

[stownsen914]

Looks like a nice result. You picked up power at the top end where the 964 cams ran out of oomph. And power and torque curves were substantially similar otherwise.

[mikedsilva]

I'm far from an expert with these, but my personal experience has shown that our engines are VERY sensitive to exhaust and muffler changes.
I've also found that 'off the shelf' mufflers for modified engines, simply don't work well enough.

Pretty sure you'll need to have something custom fabbed up. What I would try first, is open pipes on the dyno, to see what the max potential of the motor is, then add your Monty back and retest.

[PeteKz]

That much torque in your '74 will really make it scoot. I'm a fan of torque for street motors.

[targa72e]

I second what Mikedsilva says, test it open and see if muffler is a issue.
My previous dyno testing (3.0SC small port) has shown that mufflers can kill HP. Dyno below is a car with two glass packs straight off the headers compared to no glasspacks. The Glass packs were straight through but cheap with lots of metal pushed into the exhaust flow.




My research shows that the B&B headers are not bad on stockish 3.6 (i have a set to test). I have a similar 3.6 with DC 993SS cams but stock intake. It's in my 91 964. I have dynoed it in many configurations. I compared my most current dyno results with yours (wondering if I should try your cams). Power and torque were similar to yours until 5K, then mine was higher. Latest dyno below was as above with Steve Wong chip (not optimized for cams and a little lean above 5K) FVD headers and Muffler. The FVD system has 1-5/8 headers (short), 200 cell cats and a 2 in 2 out muffler that should be free flowing (x design). The other test (blue)was stock heat exchangers with cat bypass and secondary muffler bypass.
If you still had stock intake I would question if your resonance flap was working. With ITB's i would expect your 5K plus to be better.



I recently built a new exhaust with 1-3/4 headers that are a little longer. They have merge style collectors and expansion, then into a custom X flow muffler. I need to dyno this configuration and get a custom chip to maximize my A/F ratio and see how it compares.
My previous testing has shown that similar cars tested on different Dynojets show very close results when tested back to back on the same Dynojet

john