ALFAGTV6.COM

[gday]

No, not that head, I already did that!

I have a 2.5L engine in my gtv6 racecar project. I suspect that it has at least one head gasket leak. It seems to be a small one, slightly pressurising the cooling system, but it's a 9 hour tow to the track and I don't want to take any chances, so the heads are coming off. While they are off I'll be getting a nice valve job, and perhaps some better flowing valves if I can find some.

Some extra compression is always good, there are no class restrictions on fuel, and 110 octane is readily available at all the tracks I've been to so far. So, since the heads will be at the machinists anyway, I am considering having them shaved to increase compression.

Does anyone know how much you can shave off of a stock head before you start to have clearance problems or start to affect head stiffness? Class rules allow me to shave 0.080" before I take a higher penalty.

Thanks
Graham

[gday]

I've found an old post where Jim K. says that he shaved 1.6mm off of his 3.0 heads to get an 11:1 compression ratio. Does anyone know if this is the upper limit for removing material from the 2.5 head? Has anyone done more? Does it start to have a negative impact on flow and combustion as the chamber shape and the quench area changes?

[Daniel]

I took 1mm off my old 2.5 heads and liked the result.
Seemed to give a good torque boost - seat of the pants dyno.

[gday]

Thanks for the info.

You didn't by any chance measure the change in combustion chamber volume, and calculate the new static compression ratio did you? So far the only data I have is Jim K's statement about his 3.0l. The 2.5's lower swept volume should give a lower compression number, but it has a smaller combustion chamber to start with, so there's too many variables for my small brain to figure out an estimated resulting static CR.

[zambon]

I think that some people have installed 3.0 intake valves in 2.5 heads. That would be a good upgrade since the stock intake valves are undersized relative to the exhaust valves. (If I remember correctly, I have never actually worked on heads).
Find out about that if you are going to do porting and valve work anyway.

[gday]

I'd planned to leave the intake valve sizes alone, and the heads unported for the most part. The Alfa competition code seems to place an unreasonably high penalty on bigger valves and head porting, at least at first glance. I had intended to get a high quality valve-job, some better flowing valves and whatever cleanup I'm allowed without it being considered "porting" by the rule judges. In a previous post, Jim K. seems to suggest that the stock head flows enough air to support about 216hp. I don't expect to be getting near that kind of output, so I'd prefer to spend the "points" on a hotter cam or some extra lightening. Without reworking the bottom end I'm thinking that 7200rpm is the very high-end of my RPM range, which means that big fat race cams are out of the picture. Maybe some nice autocross cams, but I need the compression bump to keep the dynamic compression in a reasonable range. Hence my interest in compression changes from shaving the head. The penalty for adding higher compression forged pistons is also unreasonably high in my oppinion, while shaving < 0.080" appears at first glance to be a bargain.

[Daniel]

It was a long time ago but I think I aimed at a static compression of 10.0:1.
I found some early calcs and I think the chamber volume is around 52cc standard and about 46cc afterwards.

[gday]

Thanks Daniel! that helps a lot.

[zambon]

Sorry, I misread "better flowing valves" to mean larger valves...
That said, I think that you can improve flow significantly just with clean up porting. As in without changing the shape of the ports, but only smoothing out rough factory casting marks.
I saw some heads that were cut to reveal a cross section of the ports. I was very surprised at how rough they were.

[gday]

No need to appologize Zambon, it's very good advice. That's exactly what "normal" people woud do. I would do that too, if the penalty were not so high.

And thanks for the heads-up on the casting marks. I'll check with the rules gurus to see whether they look for "port reshaping" or just check for grinding marks when checking for "porting".

[grant]

Graham,

I talked to my machinest about this when I had my 2.5 liter heads milled. He took a look at the amount of clearance between the head mating surface and the valve seat and he advised against taking off more material than I have to. That being said, I wouldn't get too agressive with the material removal. Who's to say your heads haven't been milled before in the first place? Early Alfa V6's blew head gasket's if you looked at them funny.

Let us know how it goes.

[gday]

But I always look at my V6 funny, and I've never had a...oh, wait, I do have a headgasket problem. I'll have to stop doing that.

Thanks Grant, I'll be sure to have a chat with my machinist about clearances before I do anything.

[Melvivio]

And what is the result? I know since fuel is better these days maybe sort of a tip for everybody

[Boazdexter]

hello what is this question ?
i really dont know about this ?
is it something funny ?


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