10 replies to this topic

[miniman92]

What are the drawbacks to having 37mm inlet and 29mm exhaust race valves in a cylinder head for a fast road engine?

Cheers

[Cooperman]

Those valves are for very high revving race or rally engines. There would be no advantage below about 6000 rpm and probably a loss of torque at lower revs, like around 2000 to 2500 rpm.
The ideal valve sizes for road engines running at beteen 2000 rpm and 6000 rpm is 36.5 mm inlet and either 29 mm or 30 mm exhausts.
However, if running a 296 or 'hotter' cam with the abiltiy to rev to over 7000 rpm then the 37 mm inlets would be good.

[miniman92]

I'm going to be running a ACD-RS+ which is similar to a 276. Probably not a good idea then!

[Cooperman]

They could just about work with a 276 above around 5700 rpm. To get the max power from a 276 you will be using around 6300 rpm at change-up point and a well-flowed big-valve head won't do any harm. Make sure the induction is OK, like twin HS4's or an HS6/HIF44 on a good inlet manifold, plus an LCB and a 1.75" exhaust system.

[Pigeonto]

Now this is interesting. I have been considering the very same.It occurred to me that there may be some worth in it for the following reason:- It is said that the difference between the 35.6 and the 37mm is 5 hp. Which is the same as going from a 270 to a 280 deg cam. I wondered if you could avoid the bad manners of a 280+ deg cam(bad idle,higher up the rev range to get the powerband) by using a 270 but going for the 37 valve.I'm no mathematician but I think the difference is about 100 sq mm. I thought,dont open out the ports stupidly, keep the air speed up and get some nice low down torque.Especially on a 1330 or more. Any one running this setup??

[ACDodd]

Pigento is on the right track intake valve size is everything if you can make it work. Which is where you will have a problem with the 37mm intake on road mileages. Basically the head will have a tendancy to crack like a 35.7 x 31mm head. Therefore its use is limited to low mileage applications. For a road engine using my cam
(ACD -RS+) use 35.7 x 29.4 for reliability.

AC

[Cooperman]

Pigento is on the right track intake valve size is everything if you can make it work. Which is where you will have a problem with the 37mm intake on road mileages. Basically the head will have a tendancy to crack like a 35.7 x 31mm head. Therefore its use is limited to low mileage applications. For a road engine using my cam
(ACD -RS+) use 35.7 x 29.4 for reliability.

AC

That's what I use on my Rally 'S' with a 286. Works fine and seems reliable.

[miniman92]

AC, do you still modify cylinder heads?

[ACDodd]

Yes I do still do heads although not so many nowadays, 1 or 2 a year. Each one is done as a one off and individually matched to the application. No tme at the moment to do another,

AC

[miniman92]

AC, when you say a 37mm will be a problem with road mileages, what mileage are we talking here? My mini will only ever be used at weekends going to and from shows and for blasts down country roads, I would be lucky to do 1000 miles a year if that? Is a 37mm valve still a no no? Only reason I am asking is I have found a nice head for sale that uses 29.4mm and 37mm inlets. Will it potentially work with your cam or not?

[ACDodd]

1k per year, is fine. Just remove the head every year to re-seat the valves to maintain it. It should last well. No doing this is the route to early failure.

The head you have found will work best as long as the intake port has not been hogged out or sleeved into the psuhrod holes (not even needed to pass the 130hp mark!). The secret to getting a 37mm road head to work is keeping a near standard MG Metro port inlet port size. Also you need a compression of 10:1 maximum.

AC

Edited by ACDodd, 06 May 2012 - 02:39 PM.