Gt-40 Heads and Max RPM

First to specifically address the compression issue noted by WROLL above and without getting argumentive, a very common head used on many 302-351 engines from 75-77 on to the advent of the GT-40 headed ProBoss, was indeed this head shown in this E-Bay listing: http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&rd=1&item=7961798817&category=33617

This same head was used on both 302 and 351 engines and used the small 5/8" or 18mm plugs, as stated before for purposes of identification. Notice the small valve and large chamber sizes, which is about 10cc larger than a P-head. Going from this head to a regular GT-40 or GT-40P head will indeed, with no question, significantly increase your compression ratio a full 1 to 2 points. As noted by Woody above, this change will surely show some improvements and the improvements will come from both the increased flow and compression when replacing this common head.

Thanks for the good advise Wroll. Your advise is consistant with others. While replacing the heads I also installed a SpeedPro cam and a Holley 480CFM 4V. ReidP or someone else on the forum with a 289/302 said they were using this particular cam with good results over a wide speed range. The old carb was a Holley 2V. I'm replacing the points with an SS conversion. It should be interesting. This stuff is fun huh?

Thanks for the nice offer Reid. I'll run it this way all summer. I think 64 Skier is right though, high compression IS the way to go. I wasn't aware of the differences in combustion chamber sizes and I do have the dished pistons. Heck I may just swap pistons next year, why not?

My old heads are marked 302 D50E A3B, according to your chart 58.2 cc. I checked the exhaust port and sure enough they have the air injector bump with a hole! I really wasn't ready to pull the engine apart in install new pistions. The piston walls look great with no ridge at the top. I guess I will look for a set of 54 cc heads. Man o man, shoot!.

Jim,
The formula for figuring the exact compression is a long one and not one that I've personally tried to use or calculate by. I'm not sure if the Ford Tech Line guys that I spoke to were using the exact formula or just going by the years-old Ford factory pieces and combinations thereof. I.e:

There were a number of 289 heads available in terms of chamber size, which in conjunction with piston design as you know, determines your compression for a specific bore x stroke engine. Look at this chart of old Ford head casting numbers and chamber size: http://web.ask.com/redir?u=http%3a%2f%2ftm.wc.ask.com%2fr%3ft%3dan%26s%3da%26uid%3d0EF2E89A502C65A04%26sid%3d18DEC7436BB30BE14%26qid%3dBEEB9B9D912D0247A6B525799909866C%26io%3d6%26sv%3dza5cb0de0%26o%3d0%26ask%3dford%2bC6OE%2bcylinder%2bhead%26uip%3d438cfebc%26en%3dte%26eo%3d-100%26pt%3dCylinder%2bHead%2bCasting%2bNumbers%26ac%3d31%26qs%3d1%26pg%3d2%26ep%3d1%26te_par%3d102%26te_id%3d%26u%3dhttp%3a%2f%2fwww.mustangii.com%2fhead.asp&bpg=http%3a%2f%2fweb.ask.com%2fweb%3fq%3dford%2bC6OE%2bcylinder%2bhead%26o%3d0%26page%3d2&q=ford+C6OE+cylinder+head&s=a&bu=http%3a%2f%2fwww.mustangii.com%2fhead.asp&qte=0&o=0&abs=Cylinder-Head+Casting+Numbers+...+ENGINE.++YEAR+CASTING+NUMBER.++COMBUSTION-+CHAMBER+VOLUME+(CC)+...+*C6OE-C%2c+E.++C6OE-M+*C7OE-A+Rail-type+rocker...&tit=Cylinder+Head+Casting+Numbers&bin=&cat=wp&purl=http%3a%2f%2ftm.wc.ask.com%2fi%2fb.html%3ft%3dan%26s%3da%26uid%3d0EF2E89A502C65A04%26sid%3d18DEC7436BB30BE14%26qid%3dBEEB9B9D912D0247A6B525799909866C%26io%3d%26sv%3dza5cb0de0%26o%3d0%26ask%3dford%2bC6OE%2bcylinder%2bhead%26uip%3d438cfebc%26en%3dbm%26eo%3d-100%26pt%3d%26ac%3d31%26qs%3d1%26pg%3d2%26u%3dhttp%3a%2f%2fmyjeeves.ask.com%2faction%2fsnip&Complete=1

I know your new heads aren't on this chart, but this is a good starting point by looking at comparable chamber sizes. But they mean little without knowing what piston you have, flat top or dished, etc.. I'm rambling I know so I'll try to get to something concrete:,...

Typical factory automotive '65-'67 289-4V engine used flat top pistons and 54+/- cc head (see chart) which resulted in the factory rated 10.0:1 cr. Check your original heads to see which ones you have. If they are of the 54 cc variety, then you probably have dished pistons to lower the cr as the marine engines like yours were typically 8.5-9:1. If that's the case, then the P heads are going to "drop" your compression by half a point or so. This based off a rule of thumb of 1cc equalling .15 to .2 points of compression. We purchased a 302-4V Interceptor a few years ago which had heads stamped "2V" and which had the large 63cc chambers and this similar head is shown for a late 289. If you buy chance had these heads, then the P's will give you a boost to the 10.0 range, assuming you've still got the original pistons.

77Stang can you this chart and compare his existing heads to what he might want to pursue in order to gain the max benefit.

Reid, I just finished bolting on my GT-40Ps (on my 289) and I hope to have my engine running and my boat in the water this week-end (weather permitting).

I have been curious as to the new compression ratio. You think it will around 10:1 with stock pistons? If so, what are you basing that on (if you don't mind me asking)?

How much have you advanced your timing? -jim

If you plan on buying heads take a look at this Web Site. The original 289 AND 351 4BBl from '69 were pretty weak. The Windsor Jr's are better than the GT 40's. If you have the money, it's hard to beat the Edelbrock.

Carefull with the 202 valves and piston clearance.

Great data base.

http://www.allfordmustangs.com/Detailed/630.shtml

who has any of these heads for sale they have been out of stock for over a year now. when they were available we used them on some engines and they made a very big improvement over stock heads. did not have to change prop only if it was in need of repair anyway. hop this helps.

I did this to my 1978 Ski Nautique w/351, it just seems to rev easier, I took the vales out and ground off some ridges and smog bumps, if you like the idea of way better breathing, there's a big difference in the heads.

I'm running a federal 13x14 before and after so can't compare the prop swap.

I just like knowing I have stuff under the hood, same cam and short block as stock.

I am considering bolting on some GT40-P heads on my stock ‘77 302. I’ve been reading articles around here and elsewhere and have about decided that I will move forward. However, I need a little help understanding “how” to interpret the expected increase in RPM’s. Specifically, since the new heads are suppose to add 400 RPM, does that mean –

1-Is the rest of my engine is already capable of pulling the additional RPMs?

2-Will I need to reprop the boat to turn 4800 at WOT?   Currently it is propped to turn the recommended 4400 at WOT.

3-Additionally, should I consider the maximum cruise RPM to increase from 3600 to 4200?

I realize this may seem a bit silly to some, but I really don’t know how I am to operate this old motor if I make this upgrade.