Dart 12.5* Heads

[DK1]

All Aluminum super late engine. A reputable engine builder does all of our machine work, put this motor together, and freshend this motor last year. The engine comination only made 680 on the dyno last year (found this out at the end of the season) so something must be wrong with the set up. I'm upgrading from 18* brodix heads. Going to be getting a set of Dart 12.5* heads and the matching intake, fully ported and cnc'd.

I'm trying to get some info as to who is right. These heads come standard with a 36CC chamber. Dart says that is fine and machine shop just needs to make to check piston to valve clearance. Engine builder says i need a 45cc chambers and 36cc is to small and will cause clearance issues.

I'm sort of stuck as who to believe. Is the engine builder just set in their ways and just want to use already known set ups to make life easy? Or is Dart out of touch with the circle track set ups guys are running now?

[SLM98]

Whats the cid of the engine?

[save the racers]

A 4.155X3.8 with a 46 chamber is 14.64 c/r. with a 35 chamber it is 17.27. flat top piston.Valve clearance checking should be standard stuff on any motor.

[Kromulous]

Thicker head gasket, may help, not sure how thick the cometic ones are now days, but 17.27 CR is a bit much. That or a small dished piston, maybe a 10cc dish, do those exist?

I read along time ago, may not be true today, but the article read that a small dished piston will actually make more HP, all things being equal. Meaning the CR is the same for the comparison.

[SLM98]

I would stay at the 14.5-15.00 compression. You will fight detonation with any higher ratios. i also think you need to work on getting the right cam, talk to Comp about there HXL lobes. The 18C brodix heads i have used made 740-750hp in a 400 cube engine with these lobes.

[lilsumo]

Those out of the box with titanium valves are 34cc. My SB2 heads were 36cc with 2.200 intakes. You will have to run a custom piston with a dish. Find a new engine builder

[Lizardracing]

I wouldn't ask Comp about a cam either. I'd use a real cam grinder. Some whos knows more than picking a cam lobe from a computer.

Bullet
CamCraft
Mike Jones
Chris Struab

and others

[perfconn]

Comp Cams grind more cams than all 4 of those put together.What do I know? Just because most of your Nascar engines have Comp Cams in them doesn't mean you have to use one.

[let-r-eat]

Slight dish for sure and will make things happier if it mirrors the chamber. I'm sure the major piston manufactures have specs on that chamber and could get you probably anything you wanted.

[DK1]

Thanks guys, the engine is a 421. Callies Magnum crank 3.75" stroke, 6" Scat H Beam Rods I believe. Been a few season since they were new so don't quote me on those numbers.

The motor in its pervious state was a combination of 3 different motors put into one. The cam came out of a 15* motor so I'm sure that had something to do with the low power.

Dart recommend dished pistons while engine builder wanted to use flat tops. So same question, who is right? Dished or Flat top?

[lilsumo]

Again find a new engine builder

[Kromulous]

You have to achieve the proper compression ratio with the 36cc combustion chambers. Only one way to do that, create more physical space within the combustion chamber and piston, that entire volume creates the compression ratio. You can measure it with CC tools, and fluid. Search on it, on the ole interweb you can find articles with diagrams.

Ultra thick head gasket, not a good idea really, or a dished piston is the only choices. Unless you create a bigger combustion chamber, not a good idea either.

Also with 12.5 degree heads and some SCAT rods is not a real good idea either i dont think. If its built right, your gonna be dealing with 750 plus hp, and them roads i doubt will hold up.

[let-r-eat]

Senior Member
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Thanks guys, the engine is a 421. Callies Magnum crank 3.75" stroke, 6" Scat H Beam Rods I believe. Been a few season since they were new so don't quote me on those numbers.

The motor in its pervious state was a combination of 3 different motors put into one. The cam came out of a 15* motor so I'm sure that had something to do with the low power.

Dart recommend dished pistons while engine builder wanted to use flat tops. So same question, who is right? Dished or Flat top?


A dish piston can be better than a flat top. Nascar runs dish pistons so what might that tell you?

[Krooser]

Dished pistons will work fine.

I agree with Lizard's cam grinders list... I have used Mike Jones and Camcraft with good results. Bullet grinds many of UD Harold's profiles. Harold was a genius with cam profiles. Chris Straub comes with great credentials.

Remember that every part must complement each other... cam, heads, pistons, RPM's, etc.

[50j]

Does Chris S actually grind cams, or does he pick lobes and have them done somewhere? Bullet and Camcraft are good recommendations IMO. Dished piston seems like a good way to go, especially since Dart is telling you that's what needs to be done. They should have more feedback with their heads than anyone.