Ultimate Diesel Builder's Guide - - Contents -

Do it your­self or not? The ins and outs

The horse­power game is all about get­ting as much air into the cylin­der as pos­si­ble, adding the ap­pro­pri­ate amount of fuel and light­ing it off. Boom! The big­ger the boom, the more power and torque. But out­side of the­ory, it's not quite that sim­ple. The "boom" is lim­ited by how much the struc­ture of the en­gine can han­dle, but it's 100-per­cent true that the first part of the power equa­tion is air.

Any nat­u­rally as­pi­rated (NA) en­gine can only draw in as much air as at­mo­spheric pres­sure and re­stric­tions in the in­take tract al­low. Be­cause most diesels are un­throt­tled, they have an air­flow ad­van­tage over an NA gasser. Vir­tu­ally any modern NA diesel can make 85-per­cent vol­u­met­ric ef­fi­ciency, or bet­ter, over a broad rpm range. Only re­ally well-tuned NA gassers can do that and usu­ally in much nar­rower rpm bands. Vol­u­met­ric ef­fi­ciency (VE) is the static per­cent­age of air that a cylin­der of a par­tic­u­lar size can hold at stan­dard at­mo­spheric pres­sure ver­sus the ac­tual amount it can pull in dy­nam­i­cally in the face of in­take re­stric­tions, cam tim­ing and en­gine speed.

When it comes to mak­ing diesels breathe, forced in­duc­tion is the great equal­izer. With forced in­duc­tion, tur­bocharg­ing or me­chan­i­cal su­per­charg­ing, air is forced into the cylin­der by a com­pres­sor and filled to more than its static vol­ume of air. The cylin­der is ac­tu­ally un­der pres­sure when it starts the com­pres­sion stroke. De­pend­ing on the amount of air the com­pres­sor can de­liver, that can be any­where from 125 to 200 per­cent more than the static vol­ume of the cylin­der at sea level at­mo­spheric pres­sure (125 to 200 per­cent VE). With forced in­duc­tion, a 360 cu­bic-inch diesel with VE of 125 or 200 per­cent is tak­ing in the a vol­ume of air equiv­a­lent to 450 to 720 cu­bic inch NA diesels, and with the pro­por­tion­ally cor­rect amount of fuel added, the smaller en­gine can make ap­prox­i­mately the

same power and torque as the larger. Again, it all comes down to how much "boom" the en­gine can tol­er­ate.

On an NA en­gine of any type, one of the ma­jor re­stric­tions to vol­u­met­ric ef­fi­ciency is the in­take tract. That starts with the air horn on the air fil­ter assem­bly and ends at the in­take valve. Air fil­ter im­prove­ments are the ob­vi­ous start­ing place, fol­lowed by the in­take man­i­fold, but very quickly the bot­tle­neck will be­come the in­take ports and in­take valve. Each en­gine is dif­fer­ent, so the ex­act cause of bot­tle­necks will have to be de­ter­mined by ex­pe­ri­ence and knowl­edge. To the hand­ful of you out there still run­ning old-school NA diesels, you are go­ing to turn to cylin­der head in­take and ex­haust port im­prove­ments and in­take man­i­fold im­prove­ments a lot sooner than peo­ple run­ning forced in­duc­tion. You have to! It's re­ally the only an­swer left be­fore forced in­duc­tion. The ben­e­fits will be only mod­estly cost ef­fec­tive in most cases ver­sus go­ing with forced in­duc­tion.

To the ma­jor­ity of you out there with turbo diesels, im­prove­ments in the cylin­der head ports and valve pock­ets will be a ways down the "Round-to-it" list. Any hotrod­ded turbo diesel can even­tu­ally get to the point where the cylin­der head is an im­ped­i­ment to power out­put. One in­di­ca­tion of that is ex­ces­sively high boost pres­sure and power gains lev­el­ing off. Boost pres­sure is an in­di­ca­tor of a lack of flow and im­prove­ments in head flow can some­times ac­tu­ally re­duce boost pres­sure with no other changes while in­creas­ing power at the same time. How much? It's highly vari­able, but ac­cord­ing to var­i­ous sources, a 40-80 hp gain on a streetable B-series Cum­mins is not uncommon. With ex­tra tun­ing to make use of the ex­tra air­flow, even more is pos­si­ble. On top of that, the other ben­e­fits in­clude lower EGT, faster spool-up and less drive pres­sure (back­pres­sure).


Head port­ing is the art of re­duc­ing re­stric­tions in the in­take and ex­haust tracts. We say "art" be­cause it's one of those things in which the "eye" and the sense of touch com­bine with hard science to get the job done. While port­ing in­volves a cer­tain amount of "voodoo," elab­o­rate test equip­ment and trade se­crets, it's some­thing a shade­tree builder can play with too, as long as he un­der­stands some ba­sic lim­i­ta­tions.

To get some per­spec­tive on head port­ing, we turned to Gavin Knisely. He is a well-known South­ern Ohio puller that gained a lot of prac­ti­cal ex­pe­ri­ence port­ing his own Cum­mins heads, dis­cov­ered he had a knack for it and even­tu­ally started do­ing it for oth­ers. When the Univer­sity of North­west­ern Ohio de­cided to start build­ing a sec­ond en­gine for their pulling truck, they turned to Knisely for a mas­saged 24-valve head. We were able to fol­low him through the process and get some tips along the way. We also were able to do be­fore and af­ter flow tests on the UNOH Su­per­flow flow benches to see the im­prove­ments.


Bear in mind it's the valve work that's go­ing to make or break your head flow num­bers. Since most of us don't have a Serdi valve cut­ting ma­chine in the garage, the shop you pick to do the valve work could make or break your port­ing im­prove­ments. In many ways, the valve work is the most im­por­tant part of the job be­cause the valve is usu­ally the most re­stric­tive part of the in­take tract.

Good valve work will pay the high­est div­i­dends at the low lift parts of the valve open­ing event. The valve is only fully open for a short pe­riod in that event, and when you con­sider at high rpm the pe­riod of time the valve is open to feed the cylin­der is mea­sured in frac­tions of a sec­ond. At 3,000 rpm, an av­er­age in­take valve is only off its seat for 2 tenths of a sec­ond, and the time short­ens with in­creased rpm, so ev­ery mil­lisec­ond the valve is off the seat is im­por­tant to fill­ing the cylin­der.

In the process of test­ing the UNOH head, we had a graphic demon­stra­tion of this. We flow tested the head be­fore and af­ter with­out any spe­cial valve work done and then got num­bers af­ter a top-qual­ity, per­for­mance-ori­ented valve and seat work. There was a 15-per­cent in­crease in

flow af­ter the valve work. Don't for­get—even with­out port­ing, good valve work can un­lock a lot of air­flow.


Keep in mind that head port­ing is only one part of a big equa­tion. The valve work men­tioned here is a very closely re­lated fac­tor in that equa­tion, but there are oth­ers. The cam pro­file is a ma­jor one. You may not be able to un­lock all the air­flow po­ten­tial in a cylin­der head with­out a change in cam pro­file. The turbo is prob­a­bly of equal im­por­tance, as is the ex­haust sys­tem and the air fil­ter assem­bly. Guard against think­ing of head port­ing as a sin­gle el­e­ment and think about all the pe­riph­eral items be­fore and af­ter the cylin­der head ports.


So, is head port­ing a do-it-your­self job? Yes and no. It's a mat­ter of de­gree. With the right tools, pa­tience and a lit­tle re­search, even a first timer can un­lock some horse­power by port­ing his own head.

The first ba­sic ad­vice Knisely has for home head porters is don't try to rein­vent the wheel. Smooth­ing port walls, ra­dius­ing edges, port match­ing and so on are all ben­e­fi­cial, but rad­i­cally chang­ing the shape of a port or

run­ner should only be done with knowl­edge afore­thought. What looks good may ac­tu­ally flow worse than stock and in­tro­duce ad­verse ef­fects. Any shape chang­ing of the ports should be lim­ited to what has been tested to work and that's pretty hard in­for­ma­tion to come by for a home porter, un­less he has a stack of heads for trial and er­ror and/or a flow­bench and dyno handy. A ver­i­fied "recipe" from some­one who has done the R&D is an­other al­ter­na­tive but you will find few skilled head porters will­ing to share their top-se­cret recipes, and it may be dif­fi­cult for a first timer to match what a pro can do. Even Knisely was un­will­ing to show us all of his tricks.

Also, more than a few heads have been ru­ined by overzeal­ous ama­teurs who got car­ried away. Un­less you know the thick­ness of the port walls and run­ners in all lo­ca­tions, you should be very care­ful how much ma­te­rial you take out. Ev­ery head has a few ar­eas where too much grind­ing gets you so near to a water jacket that you later get a crack. The pros slice heads apart

to both learn where these vul­ner­a­ble ar­eas are and to ob­serve the shape and di­men­sions of the ports and run­ners. You need a safety fac­tor in this too, be­cause core shift dur­ing the cast­ing process can lead to variances in what's out there. The OE man­u­fac­turer ac­counts for this in pro­duc­tion. A 1/16-inch vari­ance may not mean much in a stock en­gine, but it could for a guy tak­ing 1/8-inch out of a port wall dur­ing a port job. Pro porters get bite marks in the ass over this too, but they usu­ally have a broader ex­pe­ri­ence base to draw from and their own safety fac­tors based on ex­pe­ri­ence with par­tic­u­lar heads.


A DIY guy has to face the in­evitable fact that it's un­likely he can match some­thing done by a pro with a proven recipe based on lots of ob­jec­tive test­ing on flow benches and dynos. Does it sound like we are try­ing to talk you out of DIY port­ing? Not re­ally, but we want to give you some re­al­is­tic ex­pec­ta­tions of what a novice can or should do, and the po­ten­tial re­sults.

So how much should you ex­pect to pay for a port­ing job? That's go­ing

to be highly vari­able ac­cord­ing to your re­gion, the shop and how much work you want done. Con­sider a thou­sand bucks the bare min­i­mum on a six cylin­der head, with the valve work ex­tra. You could eas­ily put $2,500 in a head with all the work done, more if you want to fight over ev­ery CFM of flow and ev­ery horse­power unit. UDBG

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