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Bigger intake smaller exhaust valves

General engine tech -- Drag Racing to Circle Track

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Little Mouse
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Bigger intake smaller exhaust valves

Post by Little Mouse » Fri Mar 27, 2020 1:11 am

How big of an intake valve could you put in the common 23 degree head without it being 60/40. Would putting in a smaller then 1.60 exhaust valve help do it to maximise the intake valve size.

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Re: Bigger intake smaller exhaust valves

Post by Walter R. Malik » Fri Mar 27, 2020 10:33 am

Little Mouse wrote:
Fri Mar 27, 2020 1:11 am
How big of an intake valve could you put in the common 23 degree head without it being 60/40. Would putting in a smaller then 1.60 exhaust valve help do it to maximise the intake valve size.
I have seen a 2.125" intake with a 1.550" exhaust used on a turbo-charged application so, I know they will fit.
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Re: Bigger intake smaller exhaust valves

Post by Little Mouse » Fri Mar 27, 2020 11:20 am

I'm probably wrong but it just seems like a smaller volume head I will call it that normally has a 2.02
or 2.05 valve and you put that big of intake in it resize the throat to match a little work around the chamber by the intake valve and then with a smaller bore 4.030 do what Sherman once mentioned chamfer the top of the cylinder by the intake valve and a 2.125 is not past the 52.5 percent to bore size rule for a wedge head. That at the very least at the .300 to .400 lift that the valve and opening behind it is going through twice that it should be an advantage. On top of that I'd like to take a new head that already has a guide for 11/32 and just on the intakes remove them put in new guides and run smaller stem intake valve. Then try to improve if needed the exhaust side with the now smaller valve in it. I'm I all wet in thinking this.

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Re: Bigger intake smaller exhaust valves

Post by Little Mouse » Fri Mar 27, 2020 11:55 am

This is a long way from apple's to apple's but it seems like way a 4 valve head works is that it ends up with all kinds of valve area then the volume behind the valves would not be big and that would mean lots of air speed. Of course it has a big advantage on the exhaust side to.

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Re: Bigger intake smaller exhaust valves

Post by mag2555 » Fri Mar 27, 2020 12:32 pm

If your taking about any of the common factory production iron heads then there is no need for anything bigger then a 2.05" valve since you can't even fully port any of those heads to make full use of that valve size without blowing tru a port wall!

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Re: Bigger intake smaller exhaust valves

Post by Little Mouse » Fri Mar 27, 2020 2:24 pm

Not a chance I want modern combustion chamber aftermarket heads. As a for instance afr makes the comp version the 195 runner head they claim it does 300 cfm that's very good for that size and they went to a 2.08 intake valve. Most all other heads in the 195 range are 2.02 to sometimes 2.05 and there advertised flow number not near as good.

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Re: Bigger intake smaller exhaust valves

Post by Little Mouse » Fri Mar 27, 2020 2:28 pm

I want build smaller then normal engine but very high rpm for a street engine. The other problem I have with the short stroke trying to avoid a dome or at least a tiny dome.

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Re: Bigger intake smaller exhaust valves

Post by Little Mouse » Fri Mar 27, 2020 2:33 pm

If I did the 49cc if there even really that small dart chamber head with a 3.00 stroke with a flat top then the compression is still to low.

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Re: Bigger intake smaller exhaust valves

Post by FC-Pilot » Fri Mar 27, 2020 3:02 pm

Little Mouse wrote:
Fri Mar 27, 2020 2:24 pm
Not a chance I want modern combustion chamber aftermarket heads. As a for instance afr makes the comp version the 195 runner head they claim it does 300 cfm that's very good for that size and they went to a 2.08 intake valve. Most all other heads in the 195 range are 2.02 to sometimes 2.05 and there advertised flow number not near as good.
For small cubic inches how much valve do you think you need? Figure out what cubes you have and what rpm it will be making power at and then size your port and valve based off of the need. As a side note, most of AFR’s small block stuff uses 8mm stem valves, which also helps with flow numbers. Take that into consideration when comparing.

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Re: Bigger intake smaller exhaust valves

Post by Little Mouse » Fri Mar 27, 2020 3:59 pm

Yes I know afr does that and that's sort of my point trying to make the most out of a smaller size head without going mostly to big in runner and throwing in to big a cam. Just the stem part should lighten the valve a bit and help airflow. I want to do light valve springs, light valves, light tool steel retainers, a 400 open conical spring and high duration not real high lift solid flat tappet cam
With not real 1960s slow ramps but not very fast rate drag race stuff. Of course that type of cam will cost me some power, but then small engines do that to. The short stroke and wanting a flat top piston makes head choices very limited.

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Re: Bigger intake smaller exhaust valves

Post by Little Mouse » Fri Mar 27, 2020 5:31 pm

Besides it un-american to not have something at least big in your engine. I'll make it the intake valve, maybe. Lol

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Re: Bigger intake smaller exhaust valves

Post by cv67 » Fri Mar 27, 2020 7:08 pm

know its not what you want but worked over a set of chinese heads wtih a nice chamber they went over 300 easy with a 2.02 intake
Havent poured them but would be surpised if they were much over 200cc

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Re: Bigger intake smaller exhaust valves

Post by Little Mouse » Sat Mar 28, 2020 12:06 pm

Ok you talked me out of it just more money to have to spend, does seem like it could be an advantage though, but then maybe not or the companies would be doing it.

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Re: Bigger intake smaller exhaust valves

Post by 6.50camaro » Sat Mar 28, 2020 2:35 pm

By the numbers if the valve guides are at std. sbc dimension of 1.860" ctc then you can fit a 2.125 int and a 1.55 exhaust valve with .0225" between the valves . If the ctc of the guides is 1.890 like Dart uses you can fit 2.150 int with the same 1.55 ex . with .040" between the valves . Thats not considering valve shrouding or seat size and placement . Just what would fit with guide ctc dimensions . I could be all wrong but dimensionally itshould work . Dan

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Re: Bigger intake smaller exhaust valves

Post by Little Mouse » Sat Mar 28, 2020 10:23 pm

Thanks for that info on a dart iron head. 040 cutting it close would that work ? I'll just go for the biggest 306 size chevy intake valve award. All kidding aside it just seems like at lower lifts like I would want maybe .625 net at most. That having a bigger opening at lower lift points would be some advantage. I realize the bigger the head on the valve may be harder to pull air around it with a punny engine but I do want it to make power to 8000 rpm one way or the other what it does below 4500 I dont care. So what would I need to do tey to buy the dart 49 chamber head with just seats installed without machined for valves. No guide at least on the intake.

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