305 chevy sbc

by abajc3 Sun Jan 02, 2011 3:19 pm

does any of you have experience with tweaking this unfortunate engine? i know 350 is much better, but is also hard to find where i live. so ill have to stick with 305.

im thinking flat tops, MSD hei, edelbrock performer dual plane intake, edelbrock 600cfm, porting (gasket matching), roller rocker arms (1:5 for now, maybe 1:6 some time in the future), headers... i probably forgot something

which cam do you recommend? id like more low end power, but nothing too radical. i read somewhere that it has to have at least .440 lift to make this restrictive heads breathe well.

it is a mercruiser boat engine and it says on the sticker that it had 200hp when new, which is quite a lot for 305, especially with dished pistons (i think it was 8:1 cr), 2bbl carburation and marine exhaust manifolds. so i believe that 300 or little less hp is not too much to expect. or is it?

by pila Sun Jan 02, 2011 7:41 pm

Nothing wrong with a 305, it does what it was built for OK. Going with flat top pistons will help compression. A mild cam wont kill off the compression you gained with the flat tops. Maybe a Comp Cams HE252, or a HE260 would work OK. Nothing to be gained by roller rockers, unless you get the 1.6 ratio, or maybe a bit can be gained with the Comp 1.52 ratio Magnum roller rockers. I replace all Chevy SB stock rockers with the Comp 1.52 types, even on a stock rebuild.
The distributor doesn't need to be exotic, a GM HEI works just fine for an engine like that.
The 600 CFM carb should work OK, along with that manifold. Port matching is OK, but no big gains found there, and don't take any metal out of the chambers, you need all the compression you can get. Cam should be degreed in with about 4 degrees advance for good low-end power.

by The Dude Mon Jan 03, 2011 11:22 am

I pretty much agree with Pila's thoughts, but with flat top pistons you could probably go a little bit larger on the cam to help bleed off some of that compression for pump gas. Depending on the heads used your static CR could be anywhere from 9:1 - 10:1 since there are 305 heads with chamber cc's ranging from 52-63. Post some numbers & I might be able to help out on that.

On one hand, the 52-58 cc heads are gonna give you plenty of compression to go with a Comp HE 268 or a Crane Energizer 274 (now sold under Summit brand). On the other hand, the earlier large cc heads are limited by a number of things & your best bet is to stick it out with one of the smaller cams & spend your money on PnP & larger valves. Also don't over look some of the newer grinds like the Lunati Voodoo 60101 or 60102.

I know I'm rambling & probably confusing you more than helping, but the main thing to remember is that a 305 is going to be more sensitive to camshaft changes than a 350 so caution needs to be taken & it becomes even more important to match the cam to the rest of the combo, especially compression ratio.

by pila Mon Jan 03, 2011 5:21 pm

Good information Rago. I wasn't aware that there were different CC heads on 305s.

by The Dude Mon Jan 03, 2011 6:01 pm

Yeah the early 434 castings that came on our cars were the "big" 63cc heads, hence the ultra low 8:1 factory compression ratios. Most 305 heads are 56-60cc including the desirable 416 castings. I've heard of the 601 castings having 52cc but never confirmed that. Still doesn't hurt to be mentioned just in case though.

I actually looked into building a 305 to be different but decided against it in the end. I might still do one for my S10 later on though. My plan was to basically get an 83-86 HO motor with the 416 heads & factory 9.8:1 CR. PnP the heads, add a Voodoo 02 cam, an RPM intake, & call it good. I'm willing to bet with headers & 2.25 duals, it would put out around 275-300hp & similar torque figures.

by abajc3 Mon Jan 03, 2011 7:29 pm

unfortunately i dont know which heads are on it, because there is a lack of information about them mercruiser inboards, especially that old. but if im not mistaken i read somewhere that they were 63cc. we can buy 100 octane fuel on about 50% of the gas stations here (98 octane on the rest of them), so i think there is a plenty of reserve before it starts detonating.

dont be worried about confusing me, that means i learn something new each time:) so if i understood you right, a cam with longer duration leaves valves oben a bit during the beginning of compression cycle, so it actually lowers compression?

im sory if my questions sound a bit dumb to you, but i have read quite a few books/web pages about chevy sbc and noone has mention this before. so this is a very valuable information for me! thanks!

by pila Mon Jan 03, 2011 9:24 pm

The numbers on the heads are between the valves. The heads could be just automotive heads. Some of the inboard boats used heads that were truck heads etc.

by bigredlaguna Mon Jan 03, 2011 10:27 pm

Here is what I found via google

http://www.73-87.com/chevy_ids/sbheads.htm

http://www.fastnuf.com/Headguide.html

http://outintheshop.com/faq/casting/heads.html

http://web.archive.org/web/20080506065350/www.mortec.com/castnum.htm

by abajc3 Tue Jan 04, 2011 3:55 am

im pretty sure that i have 415 castings,used on 267 and 305 engines from 80-85. 1.71/1.50 valves, 64cc.

Not very encouraging info i suppose?

The thing that bugs me though is why does this engine have 55 more hp than mine. Could it be because it doesnt have any emission equipment?

by The Dude Tue Jan 04, 2011 10:59 am

No telling on the power rating difference, but real world there won't be much difference I'm assuming. Go ahead & pull the valve cover. Read the entire number on it to be sure.

Here's some good reading on the types of compression ratio:
http://www.enginebasics.com/Advanced%20Engine%20Tuning/Static%20vs%20Dynamic.html

Basically, static is what most people refer to. What most people don't realize is that dynamic is actually more important because THAT is what will determine whether or not you can run pump gas.... to an extent. Static will still come into effect higher in the RPM ranges due to not having enough time to bleed off all of the pressure. Basically a safe bet for you running 98-100 octane vs our typical 87-91 octane, I'd say you could easily squeeze 10:1 SCR & probably 8.5:1 DCR.

What you will need to do is get a piston/head combo that gives you the desired SCR & then look at your cam timing events to get you close to the DCR that you are wanting. Here's a good simple calculator for figuring it out:
http://kb-silvolite.com/calc.php?action=comp

Play around with different cams & you will notice the difference that camshafts will make on compression ratio. Even more intriguing is the effect that rod length has.

by abajc3 Tue Jan 04, 2011 11:13 am

the engine is currently completely disasembled, so its not that i dont want to pull the valve covers, the problem is im at a college in another city and i get home only for weekends. thats when i can give you the whole number.

thanks for those links, some very useful stuff there!

by pila Tue Jan 04, 2011 6:45 pm

The number and letter designator on the right-front of the engine block deck surface may shed some light on what it is also.

by abajc3 Tue Jan 04, 2011 7:06 pm

ill get all this numbers when i get home this week. i checked them all before, i just didnt write them down. but im 99% sure that the heads are 415 casting.

could you tel me just for example how do different block castings with the same bore differ one from another?

@ ragu: that was an awesome link on that dinamic compression, choosing the cam was the biggest mistery for me until now. i was planing on searching different opinions on the internet, but now i can calculate which is best for my aplication.

by ant7377 Tue Jan 04, 2011 7:49 pm

It would be a hell of a lot of fun with a turbo on it. If I had the motivation I would do it .

Something like this-

305 chevy sbc TurboCaprice

by abajc3 Tue Jan 04, 2011 8:02 pm

if i had enough money for turbo kit id rather import a 350 or even a LQ4. i think it would be a better investment

by ant7377 Tue Jan 04, 2011 9:09 pm

Or a 350 and a turbo kit, thats better . It would be fun to get stuff like that caprice I posted ,get it cheap and go kick some butt with it at the track (or street/drive in, not that I condone that stuff)

by abajc3 Wed Jan 05, 2011 6:41 am

ant7377 wrote:Or a 350 and a turbo kit, thats better . It would be fun to get stuff like that caprice I posted ,get it cheap and go kick some butt with it at the track (or street/drive in, not that I condone that stuff)

well, maybe sometime in the future, who knows:)?

by abajc3 Wed Jan 05, 2011 6:49 am

im about to order a rebuild kit from ebay. i checked it and its all name brands, so i suppose it should be good right?

but i have one question. how does truck rebuild kit differ from the ordinary rebuild kit. the seller on ebay said that its probably the gaskets. what is different with them?

by ant7377 Wed Jan 05, 2011 8:33 am

I dont know why there would be a difference ? He probably dosent either ,go with Summit racing or JEG'S if you arent sure they will sell you the right thing.

by bigredlaguna Wed Jan 05, 2011 10:35 am

This might be a good kit for you to check out

http://www.northernautoparts.com/ProductModelDetail.cfm?ProductModelId=717

by 1973 454 MONTE Wed Jan 05, 2011 4:36 pm

if you were not so far away,i have a nice 74-400 small bock.

by JB2wheeler Wed Jan 05, 2011 5:39 pm

Truck engines are built for low RPM/high torque operations. They would have parts made for lower compression and milder cams. JB

by ant7377 Wed Jan 05, 2011 6:22 pm

bigredlaguna wrote:This might be a good kit for you to check out

http://www.northernautoparts.com/ProductModelDetail.cfm?ProductModelId=717

I bought a few from them,very good, I didnt know they were still around.

by abajc3 Thu Jan 13, 2011 3:20 pm

the plans for a 305 have changed completely today. i went to check the casting #s of the heads which are currently in the car and found out that they were 350 heads. out of curiosity i checked block #s too and found out that it is actually a 1980 truck 350 4 bolt main! i dont know why i havent checked them before, but it sure feels good to be an idiot at the moment:)

so the 305 will get only a basic rebuild without much tweaking, and it will be put in the car while the 350 will be on the stand getting some more power!

by The Dude Thu Jan 13, 2011 3:52 pm

I guess that's great news huh? Sweet!

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