effects of gear and tire diameter change.

[rfoll]

I have track conditions most would die for. We occasionally see sub-sea level barometer readings with 60 degree dry air. The flow of the Brodix Spec head I quoted from Stan's website and it agrees with Brodix supplied data. i believe the Spec racing is based on smaller engines. They are definitely better than the home ported 034 Bowtie heads they replaced. I got them for $700/pair bare, and I had springs and valves on hand. They come with a cnc chamber and throat, but the casting shift was horrible. The guides lasted 30 passes. Yes you read the deck clearance correctly. In retrospect, I should have disassembled the engine and had them cut .020" off the deck. I had to port the intake to match the heads, something like 1/8" off the roof as far as I could reach. One of the things in the back of my mind is that if I come up with the money for better heads, and the a cr increase with them, the cam might be plenty. The solid roller stuff ain't cheap.

[steve cowan]

Do you have time to pull the engine??
I am buttoning up my spare 010 383 at the moment, I fitted a 1094 steel shim head gasket 0.015".
Running quench down to 28 - 30 thou
Maybe you can get a thinner head gasket and get yours around 40 thou ??
If the engine is out you can move cam timing if you have enough P/V on the exhaust side.
Also plot out lift and duration at the valve,then look at the 1.6 rocker on the intake and see how much it speeds the valve up.not a huge cost involved just time in the workshop

[rfoll]

I have the Comp cast aluminum 2 piece timing cover that will allow me to move the cam. Since I am retarding the cam I believe it will open up piston to valve clearance. 28 degrees F here this morning and my shop is unheated. High temperatures in the low 40s for the next week. Not much shop time for a while.

[rustbucket79]

Retarding the cam reduces exhaust and increases intake valve to piston clearance.

[rfoll]

I didn't invent this system but, was told it by a prominent head porter.

To check to see if a larger carb will help, run a vacuum gauge on the engine and if your pulling more than 1.5", at WOT, a larger carb, should help. A very easy test.

So, the spec I was given was you want the the vacuum reading to be between .5 to 1.5 inches at WOT.

Regarding the rocker arm issue, if your heads will flow more at a higher valve opening than your currently using, then the increased lift of the 1.6 rockers, should also increase HP. 5 cfm could equate to ~10 hp.

I have a vacuum gage in the car and it shows almost close to .5" vacuum at shift point. That doesn't mean the larger carb won't help. A 750 double pumper slows the car considerably.

[frnkeore]

I can't say that it won't help, at all but, the benefit will be marginal.

With a 400 ci+ engine, you should be able to benefit with a 850, if the heads aren't the restriction.

There is only one way to tell.

Edit:
If your heads will flow more cfm, with the the 1.6 rockers, then it should be more noticeable. If the flow is topped out at your current lift, then I go back to my first statement.

[rfoll]

I'm surprised no one made any comments about cam duration. When I change the heads down the road and kick the compression up, I can add some duration and all of this discussion should become moot. Unfortunately, the solid roller stuff is expensive.

[rustbucket79]

Are you sure the compression ratio is only 10.3:1 I find that low for that cid and flat top pistons. Even if the heads are 70cc.

What’s the deck height and head gasket thickness ?
Would it be an option to have the heads milled and installed with 0.027” thick cometic head gasket for example. You should be able to bring up the compression to around 11:1 with a little work.

What brand converter do you run and what size is it ?

Yes the compression is 10.3:1 I was running a 412 and I had gasket sealing problems. I decided to go to an aftermarket block. By the time I added up the cost of parts and machine work, it was going to cost me $800 more than buying the Dart built 415 with the Little M block and Callies rotating assembly. I called Dart beforehand and asked about deck clearance. They said .010"-.015". It was .025" in the hole. Bore volume 848.2 CCs, Deck at tdc 15 CCs, .027" 6CC thick Cometic MLS gasket, $78.00 each, and Cylinder head chamber 70CCs. The Dart catalog listing said 10.9:1 with 72 CC heads. That's why I measure everything. The 8" converter is from A-1

We found the same deck clearance on one of their 427 short blocks. Would have been nice if they came zero deck, but whatcha gonna do, pull it apart and deck it?

I suspect your engine makes a bunch of torque on the leave and runs out of steam upstairs. More contact patch on the track via taller tires and retarding your cam should be beneficial. Have you tried shifting at lower rpm’s?

[rfoll]

Short shifting has never been much benefit. With the 26" tire /3.73 gear combo in it now, everything is quite happy. I have felt for a long time the car weight could use a lower gear off the line. That is why I tried the 4.10/26" tire. I liked it better inspite of not seeing a huge drop in et. I still feel ther is something to be gained by improving the power for that last 500 rpm at the top end. I also believe the short tire didn't allow me to see much gain on the 60' time. Since I have the 4.56 axle all ready to go, it seems the tall tire should help the 60'. I might have stayed with the 4.10 but I broke it.

DSC00327.JPG

[rustbucket79]

Short shifting has never been much benefit. With the 26" tire /3.73 gear combo in it now, everything is quite happy. I have felt for a long time the car weight could use a lower gear off the line. That is why I tried the 4.10/26" tire. I liked it better inspite of not seeing a huge drop in et. I still feel ther is something to be gained by improving the power for that last 500 rpm at the top end. I also believe the short tire didn't allow me to see much gain on the 60' time. Since I have the 4.56 axle all ready to go, it seems the tall tire should help the 60'. I might have stayed with the 4.10 but I broke it.DSC00327.JPG

That picture must be of the broken axle, looks like 20 degrees of positive pinion angle.

[rfoll]

It was noisy. With a 5000+ rpm launch, turning off the key doesn't help much. The engine is still turning the rpm and the driveshaft continues to bang away until the inertia is expended. We figured the center section was welded in on a Friday after payday. The little weld slugs were laying on the pavement. I upgraded to a 3" chrome moly driveshaft with 1350 U joints after this as well.

DSC00328.JPG